Abstract: Ab initio and density functional theory (DFT) calculations were performed for obtaining fundamental vibrational frequencies of methylamine, CH3NH2, and its deuterated variants CH3ND2, CD3NH2, and CD3ND2. The calculations were carried out using the CCSD(T) coupled cluster approximation with cc-pVTZ and cc-pVQZ basis sets, and by the DFT method with the semiempirical hybrid functional B97-1 with polarization consistent pc-2 and pc-3 basis sets. Reasonable performance of the DFT harmonic and ab initio harmonic calculations was found, which improved considerably upon combination of the harmonic fundamental frequencies with anharmonic corrections from the smaller, pc-2, basis. The computed anharmonic fundamental frequencies of methylamine isotopologues agree very well with the experimental values and represent a useful tool for assignment and analysis of the dominant resonances. (C) 2007 Wiley Periodicals, Inc.

Abstract: The crystal structure of the new cationic Rh-1 complex trans-[Rh(CO)(2)(L)(2)]BF4 (L=alpha(2)-(diisopropyl-phosphino)isodurene) wag found to exhibit a nonlinear OC-Rh-CO fragment and weak intramolecular C-H center dot center dot center dot Rh interactions. These interactions, which have also been shown to occur in solution, have been examined by density functional theory calculations and found to be inextricably linked to the presence of the distorted OC-Rh-CO fragment. This linkage has also been demonstrated by comparison with a highly similar Rh-1 complex, in which these C-H center dot center dot center dot Rh interactions are absent. Furthermore, the presence of these weak interactions has been shown to have a significant effect on the reactivity of the metal center.

Abstract: Activation of a strong aryl-Br bond of a halogenated vinylarene by nickel(0) is demonstrated in the presence of aryl-I containing substrates. eta(2)-Coordination of Ni(PEt3)(2) to the C=C moiety of halogenated vinylarenes is kinetically preferable and is followed by an intramolecular aryl-halide bond activation process. This "ring-walking" process is quantitative and proceeds under mild reaction conditions in solution. Mechanistic studies indicate that the metal insertion into the aryl-halide bond is not the rate-determining step. The reaction obeys first-order kinetics in the eta(2)-Coordination complexes with almost identical activation parameters for Br and I derivatives. The ring-walking process is kinetically accessible as shown by density functional theory (DFT) calculations at the PBE0/SDB-ccpVDZ//PBE0/SDD level of theory.

Abstract: We propose two new double-hybrid functionals, denoted B2K-PLYP and mPW2K-PLYP, which yield thermochemical performance comparable to existing double-hybrid functionals but offer superior performance for barrier heights of various kinds. We show that the new functionals yield excellent performance for all of the following: (a) main-group thermochemistry; (b) main-group thermochemical kinetics; (c) late transition metal reactions. In addition, B2K-PLYP performs well for weak interactions.

Abstract: A series of phenylethynyl pyridine derivatives 1-4 possessing both perfluorocarbon (PFC) and hydrocarbon (HQ moieties have been synthesized and used for the formation of halogen bonding (XB)-based networks. X-ray crystal structure analyses indicate the dominance of XB synthons, which represent the one-dimensional (1D) structure directing interaction, leading to the formation of supramolecular chains. The influence of structural/electronic factors (e.g., electron donor/acceptor strength, sterically demanding substituents) on XB formation of compounds 1-4 have been compared with structurally related stilbazole systems (I, II). The XB-bonded networks are formed in collaboration with other noncovalent interactions such as pi-pi stacking, hydrogen bonding, C-H center dot center dot center dot F and F center dot center dot center dot F. Molecular electrostatic potentials and atomic polar tensor (APT) charges of the donor and acceptor sites have been determined by density functional theory (DFT) calculations.

Abstract: Terminal oxo complexes of transition metals have critical roles in various biological and chemical processes(1,2). For example, the catalytic oxidation of organic molecules(3,4), some oxidative enzymatic transformations(5-7), and the activation of dioxygen on metal surfaces(8) are all thought to involve oxo complexes. Moreover, they are believed to be key intermediates in the photocatalytic oxidation of water to give molecular oxygen, a topic of intensive global research aimed at artificial photosynthesis and water splitting(9-13). The terminal oxo ligand is a strong pi-electron donor, so it readily forms stable complexes with high-valent early transition metals. As the d orbitals are filled up with valence electrons, the terminal oxo ligand becomes destabilized(2). Here we present evidence for a d(n) (n > 5) terminal oxo complex that is not stabilized by an electron withdrawing ligand framework. This d(6) Pt( IV) complex exhibits reactivity as an inter- and intramolecular oxygen donor and as an electrophile. In addition, it undergoes a water activation process leading to a terminal dihydroxo complex, which may be relevant to the mechanism of catalytic reactions such as water oxidation.

Abstract: Both experimental and theoretical evidence suggest that the proton exchange between water and the methyl group in [TpPt(CO)CH3] (1, Tp = hydridotripyrazolylborate) involves the formation and deprotonation of a "sticky" sigma-methane ligand. ne efficiency of this nontrivial process has been attributed to the spatial orientation of functional groups that operate in concert to activate a water molecule and then achieve a multistep proton walk from water to an uncoordinated pyrazolyl nitrogen atom, to the methyl ligand, and then back to the nitrogen atom and water. The overall proton-exchange process has been proposed to involve an initial attack of water at the CO ligand in I with concerted deprotonation by the uncoordinated pyrazolyl nitrogen atom. The pyrazolium proton is then transferred to the Pt-CH3 bond, leading to a a-methane intermediate. Subsequent rotation and deprotonation of the a-methane ligand, followed by reformation of 1 and water, result in scrambling of the methyl protons with the hydrogen atoms of water. An alternative two-step process that involves oxidative addition and reductive elimination has also been considered. The two competing mechanistic routes from 1 into [D-3]-1, as well as the conversion of 1 into [TpPt(CH3)H-2] (2), have been examined by density functional theory (DFT) using a variety of exchange-correlation methods, primarily PW6B95, which was recently shown to be highly accurate for evaluating reactions of late -transition-metal complexes. The key role played by the free pyrazolyl nitrogen atom, acting as a proton carrier that abstracts a proton from water and transfers the proton to the Pt-CH3 bond, is reminiscent of the dual functionality of histidine in the catalytic triad of natural serine proteases.

Abstract: We examine the applicability of density functional theory (DFT) to the polarizability of C-n(-) (n = 3-9) cluster anions. This was achieved by comparing DFT calculations using two different exchange-correlation functionals (the non-empirical local density approximation, LDA, and the semiempirical hybrid functional B97-1) to quantum chemical calculations using the coupled cluster method in the CCSD(T) "gold standard" approximation. We find that, unless the extra electron is not bound at all by DFT, both LDA and B97-1 agree with the CCSD(T) calculation to within 5-10%, allowing for a meaningful qualitative and semiquantitative analysis. Furthermore, the polarizability is found to increase monotonically with chain size, consistent with the trend inferred from electron detachment experiments.

Abstract: The potential triple-halogen-bond acceptor, sym-triiodo-trifluorobenzene IFB (1), has been co-crystallized with a series of bipyridyl derivatives (2-4) to gain insight to the factors controlling formation of multiple halogen bonds with a single aromatic system. Co-crystals 5-7 were obtained that consistently contained two N center dot center dot center dot I halogen bonds. The reluctance to the formation of a supramolecular assembly having a third N center dot center dot center dot I halogen bond does not depend on the size of the bispyridine donor systems (2-4). Apparently, there are limitations to the number of halogen bonds that can be formed with a single aromatic halogen donor. The solid-state structure of co-crystal (5) contains short I center dot center dot center dot F contacts of 2.96 and 3.05 angstrom. DFT calculations were performed at the PBE0/(apc1-aSDBDZ)//PBE0/(pc1-SDBDZ) level of theory to investigate the nature of the interaction between the pyridine nitrogen and IFB (1). These calculations reveal a weakening of N center dot center dot center dot I interactions as more pyridine moieties coordinate to the IFB (1), which might be a contributing factor to the consistent formation of two rather than three N center dot center dot center dot I halogen bonds.

Abstract: A benchmark calculation of the atomization energy of the 'simple' organic molecule C2H6 (ethane) has been carried out by means of W4 theory. While the molecule is straightforward in terms of one-particle and n-particle basis set convergence, its large zero-point vibrational energy (and anharmonic correction thereto) and nontrivial diagonal Born-Oppenheimer correction (DBOC) represent interesting challenges. For the W4 set of molecules and C2H6, we show that DBOCs to the total atomization energy are systematically overestimated at the SCF level, and that the correlation correction converges very rapidly with the basis set. Thus, even at the CISD/cc-pVDZ level, useful correlation corrections to the DBOC are obtained. When applying such a correction, overall agreement with experiment was only marginally improved, but a more significant improvement is seen when hydrogen-containing systems are considered in isolation. We conclude that for closed-shell organic molecules, the greatest obstacles to highly accurate computational thermochemistry may not lie in the solution of the clamped-nuclei Schrodinger equation, but rather in the zero-point vibrational energy and the diagonal Born-Oppenheimer correction. (c) 2007 Elsevier B.V. All rights reserved.

Abstract:

Abstract: Basis set convergence of correlation effects on molecular atomization energies beyond the coupled cluster with singles and doubles (CCSD) approximation has been studied near the one-particle basis set limit. Quasiperturbative connected triple excitations, (T), converge more rapidly than L-3 (where L is the highest angular momentum represented in the basis set), while higher-order connected triples, T-3-(T), converge more slowly-empirically, proportional to L-5/2. Quasiperturbative connected quadruple excitations, (Q), converge smoothly as proportional to L-3 starting with the cc-pVTZ basis set, while the cc-pVDZ basis set causes overshooting of the contribution in highly polar systems. Higher-order connected quadruples display only weak, but somewhat erratic, basis set dependence. Connected quintuple excitations converge very rapidly with the basis set, to the point where even an unpolarized double-zeta basis set yields useful numbers. In cases where fully iterative coupled cluster up to connected quintuples (CCSDTQ5) calculations are not an option, CCSDTQ(5) (i.e., coupled cluster up to connected quadruples plus a quasiperturbative connected quintuples correction) cannot be relied upon in the presence of significant nondynamical correlation, whereas CCSDTQ(5)(Lambda) represents a viable alternative. Connected quadruples corrections to the core-valence contribution are thermochemically significant in some systems. We propose an additional variant of W4 theory [A. Karton , J. Chem. Phys. 125, 144108 (2006)], denoted W4.4 theory, which is shown to yield a rms deviation from experimental atomization energies (active thermochemical tables, ATcT) of only 0.05 kcal/mol for systems for which ATcT values are available. We conclude that "3 sigma <= 1 kJ/mol" thermochemistry is feasible with current technology, but that the more ambitious goal of +/- 10 cm(-1) accuracy is illusory, at least for atomization energies. (C) 2007 American Institute of Physics.

Abstract: Benchmark total atomization energies (TAE(0) values) were obtained, by means of our recent W4 theory [Karton, A.; Rabinowitz, E.; Martin, J. M. L.; Ruscic, B. J. Chem. Phys. 2006, 125, 144108], for the molecules Be-2, BeF2, BeCl2, BH, BF, BH3, BHF2, B2H6, BF3, AlF, AlF3, AlCl3, SiH4, Si2H6, and SiF4. We were then able to deduce "semi-experimental" heats of formation for the elements beryllium, boron, aluminum, and silicon by combining the calculated TAE(0) values with experimental heats of formation obtained from reactions that do not involve the species Be(g), B(g), Al(g), and Si(g). The elemental heats of formation are fundamental thermochemical quantities that are required whenever a molecular heat of formation has to be derived from a calculated binding energy. Our recommended Delta H-f,H-0 degrees[A(g)] values are Be 76.4 +/- 0.6 kcal/mol, B 135.1 +/- 0.2 kcal/mol, Al 80.2 +/- 0.4 kcal/mol, and Si 107.2 +/- 0.2 kcal/mol. (The corresponding values at 298.15 K are 77.4, 136.3, 80.8, and 108.2 kcal/mol, respectively.) The Be value is identical to the CODATA recommendation (but with half of the uncertainty), while the B, Al, and Si values represent substantial revisions from established earlier reference data. The revised B and Si values are in agreement with earlier semi-ab initio derivations but carry much smaller uncertainties.

Abstract: The high-accuracy W4 computational thermochemistry protocol, and several post-W4 methods, have been applied to the P-2 and P-4 molecules. Contrary to previous studies, we find the experimental thermochemistry to be fundamentally sound. The reaction enthalpy for P4 -> 2P(2) has a very significant contribution from post-CCSD(T) correlation effects. We derive a gas-phase heat of formation for the phosphorus atom of Delta H degrees(f),(0)[P(g)] = 75.54 +/- 0.1 kcal mol(-1) and Delta H degrees(f),(298) [P(g)] = 75.74 +/- 0.1 kcal mol(-1), in the upper half of the CODATA uncertainty interval.

Abstract: Hydrogen bonds are of utmost importance in both chemistry and biology. As the applicability of density functional theory and ab initio methods extends to ever larger systems and to liquids, an accurate description of such interactions is desirable. However, reference data are often lacking, and ab initio calculations are only possible and done in very small basis sets. Here, we present high level [CCSD(T)] ab initio reference calculations at the basis set limit on a large set of hydrogen-bonded systems and assess the accuracy of second-order perturbation theory (MP2). The possibilities of using basis set extrapolations for geometries and dissociation energies are discussed as well as the results of R12 methods and density functional and local correlation methods.

Abstract: Pulse electron-electron double resonance distance measurements between two high spin Gd3+ ions in a novel bis-Gd3+ complex involving two pyridine-based gadolinium tetracarboxylate systems linked by a rigid aryl-alkyne unit were carried out at Ka- and W-band frequencies. The experimental distance found was 2.02 +/- 0.02 nm, and it was further compared with the Gd3+-Gd3+ distance (2.2126 nm) determined by density functional theory.

Abstract: The conductance of a single molecule transport junction is investigated in the Landauer-Imry regime of coherent tunneling transport. Utilizing aromatic systems with thiol end groups, we have calculated using density functional theory the expected conductance of junctions containing molecules with different levels of conjugation and of different lengths. The calculated variations in transport junction conductance are explained in terms of the continuity of the conjugation path between leads. Molecular conjugation describes this continuity within the molecule, and the interfacial terms (spectral densities or imaginary parts of the self-energy) describe its continuity at the molecule/metal interface. We compare the results from junction conductance calculations with isolated molecule electronic structure calculations These density functional theory calculations suggest that for these dithiol molecules, transport occurs mostly through the occupied orbital manifold. The decay of the transport with length is found to be exponential for poly-Ph dithiol molecules. We compare the calculated conductance of conjugated aromatic molecules with their molecular orbital calculations and with the Green's function formulation and evaluate the relative significance of different factors (such as energetic alignment and spectral density) that control the conductance of molecules.

Abstract: Rozen's epoxidation reagent, (CH3CNHOF)-H-., and a prototype epoxidation reaction employing it, have been subjected to an extensive ab initio and density functional study. Its anharmonic force field reveals a very strong red shift for the OH stretch and a strong blue shift for the HOF bend, in semiquantitative agreement with experiment. The very strong hydrogen bond (8.20 kcal/ mol at the W1 level) not only serves to stabilize the reactant but also considerably lowers the barrier height for epoxidation of ethylene. Moreover, the reaction byproduct HF is found to act autocatalytically. The OH moiety acquires HO+ character in the transition state. Our W1 benchmark data for the reaction profile allow the performance of various DFT functionals to be assessed. In general, "kinetics" functionals overestimate barrier heights, the BMK functional less so than the others. The B1B95 and TPSS33B95 meta-GGA functionals both perform very well, whereas general-purpose hybrid GGAs underestimate barrier heights. The simple PBE0 functional does reasonably well.

Abstract: Reaction of (PNP)Ir(COE)(+) PF6- (1) (PNP=2,6-bis(di-tert-butylphosphinomethyl) pyridine; COE=cyclooctene) with benzene yields a stable unsaturated square pyramidal Ir(III) hydrido-aryl complex, 2, which undergoes arene exchange upon reaction with other arenes at 50 degrees C. Upon reaction of 1 with haloarenes (chlorobenzene and bromobenzene) and anisole at 50 degrees C, selective ortho C-H activation takes place. No C-halogen bond activation was observed, even in the case of the normally reactive bromobenzene and despite the steric hindrance imposed by the halo substituent. The ortho-activated complexes (8a, 9a, and 10a) exhibited a higher barrier to arene exchange; that is, no exchange took place when heating at a temperature as high as 60 degrees C. These complexes were more stable, both thermodynamically and kinetically, than the corresponding meta- and para-isomers (8b, c, 9b, c, and 10b,c). The observed selectivity is a result of coordination of the heteroatom to the metal center, which kinetically directs the metal to the ortho C-H bond and stabilizes the resulting complex thermodynamically. Upon reaction of complex 1 with fluorobenzene under the same conditions, no such selectivity was observed, due to low coordination ability of the fluorine substituent. Competition experiments showed that the ortho-activated complexes 8a, 9a, and 10a have similar kinetic stability, while thermodynamically the chloro and methoxy complexes 8a and 10a are more stable than the bromo complex 9a. Computational studies, using the mPW1K exchange-correlation functional and a variety of basis sets for PNP-based systems, provide mechanistic insight. The rate-determining step for the overall C-H activation process of benzene is COE dissociation to form a reactive 14e complex. This is followed by formation of a eta(2)(C-C) intermediate, which is converted into an eta(2)(C-H) complex, both being important intermediates in the C-H activation process. In the case of chlorobenzene, bromobenzene, and anisole, eta(1)-coordination via the heteroatom to the 14e species followed by formation of the ortho eta(2)(C-H) complex leads to selective activation. The unobserved C-halide activation process was shown computationally in the case of chlorobenzene to involve the same Cl-coordinated intermediate as in the C-H activation process, but it experiences a higher activation barrier. The ortho C-H activation product is also thermodynamically more stable than the C-Cl oxidative addition complex.

Abstract: The performance of a wide variety of DFT exchange-correlation functionals for a number of late-transition-metal reaction profiles has been considered. Benchmark ab-initio reference data for the prototype reactions Pd + H-2, Pd + CH4, Pd + C2H6 (both C-C and C-H activation), and Pd + CH3Cl are presented, while ab-initio data of lesser quality were obtained for the catalytic hydrogenation of acetone and for the low-oxidation-state and high-oxidation-state mechanisms of the Heck reaction. "Kinetics" functionals such as mPWIK, PWB6K, BB1K, and BMK clearly perform more poorly for late-transition-metal reactions than for main-group reactions, as well as compared to general-purpose functionals. There is no single "best functional" for late-transition-metal reactions, but rather a cluster of several functionals (PBE0, B1B95, PW6B95, and TPSS25B95) that perform about equally well; if main-group thermochemical performance is additionally considered, then B1B95 and PW6B95 emerge as the best performers. TPSS25B95 and TPSS33B95 offer attractive performance compromises if weak interactions and main-group barrier heights, respectively, are also important. In the ab-initio calculations, basis set superposition errors (BSSE) can be greatly reduced by ensuring that the metal spd shell has sufficient radial flexibility in the high-exponent range. Optimal HF percentages in hybrid functionals depend on the class of systems considered. increasing from anions to neutrals to cations to main-group barrier heights; transition-metal barrier heights represent an intermediate situation. The use of meta-GGA correlation functionals appears to be quite beneficial.

Abstract: DIFT (density functional theory) anharmonic force fields with basis sets near the Kohn-Sham limit have been obtained for perchloric acid, HClO4, and perchloric anhydride, Cl2O7. Calculated fundamental frequencies are in very good agreement with available experimental data. Some reassignments in the vibrational spectra of Cl2O7 are proposed based on our calculations. HClO4 and Cl2O7 are particularly severe examples of the 'inner polarization' phenomenon. The polarization consistent basis sets pc-1 and pc-2 (as well as their augmented counterparts) should be supplemented with two (preferably three) and one (preferably two) high-exponent d functions, respectively, on second-row atoms. Complete anharmonic force fields are available as electronic supporting information [1]. (c) 2005 Elsevier B.V. All fights reserved.

Abstract: We demonstrate that a minor modification of the extrapolation proposed by Jensen [( 2005): Theor Chem Acc 113: 267] yields very reliable estimates of the Hartree-Fock limit in conjunction with correlation consistent basis sets. Specifically, a two-point extrapolation of the form E-HF,E-L = E(HF,infinity+)A(L+ 1) exp(- 9 root L) yields HF limits E-HF,E-infinity with an RMS error of 0.1 millihartree using aug-cc-pVQZ and aug-cc-pV5Z basis sets, and of 0.01 millihartree using aug-cc-pV5Z and aug-cc-pV6Z basis sets.

Abstract:

Abstract: The heats of formation of HClO4 and Cl2O7 have been determined to chemical accuracy for the first time by means of W1 and W2 theory. These molecules exhibit particularly severe degrees of inner polarization, and as such obtaining a basis-set limit SCF component to the total atomization energy becomes a challenge. (Adding high-exponent d functions to a standard spd basis set has an effect on the order of 100 kcal/mol for Cl2O7.) Wilson's aug-cc-pV(n + d)Z basis sets represent a dramatic improvement over the standard aug-cc-pVnZ basis sets, while the aug-cc-pVnZ + 2d1f sequence converges still more rapidly. Jensen's polarization consistent basis sets still require additional high-exponent d functions: for smooth convergence we suggest the {aug-pc1 + 3d,aug-pc2 + 2d,aug-pc3 + d,aug-pc4} sequence. The role of the tight d functions is shown to be an improved description of the Cl (3d) Rydberg orbital, enhancing its ability to receive back-bonding from the oxygen lone pairs. In problematic cases like this (or indeed in general), a single SCF/aug-cc-pV6Z + 2d1f calculation may be preferable over empirically motivated extrapolations. Our best estimate heats of formation are Delta H-f(0), (298)[HClO4(g)] = -0.6 +/- 1 kcal/mol and Delta H-f(0), (298)[Cl2O7(g)] = 65.9 +/- 2 kcal/mol, the largest source of uncertainty being our inability to account for post-CCSD(T) correlation effects. While G2 and G3 theory have fairly large errors, G3X theory reproduces both values to within 2 kcal/mol. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: In an attempt to improve on our earlier W3 theory [A. D. Boese , J. Chem. Phys. 120, 4129 (2004)] we consider such refinements as more accurate estimates for the contribution of connected quadruple excitations (T-4), inclusion of connected quintuple excitations (T-5), diagonal Born-Oppenheimer corrections (DBOC), and improved basis set extrapolation procedures. Revised experimental data for validation purposes were obtained from the latest version of the Active Thermochemical Tables thermochemical network. The recent CCSDT(Q) method offers a cost-effective way of estimating T-4, but is insufficient by itself if the molecule exhibits some nondynamical correlation. The latter considerably slows down basis set convergence for T-4, and anomalous basis set convergence in highly polar systems makes two-point extrapolation procedures unusable. However, we found that the CCSDTQ-CCSDT(Q) difference converges quite rapidly with the basis set, and that the formula 1.10[CCSDT(Q)/cc-pVTZ+CCSDTQ/cc-pVDZ-CCSDT(Q)/cc-pVDZ] offers a very reliable as well as fairly cost-effective estimate of the basis set limit T-4 contribution. The T-5 contribution converges very rapidly with the basis set, and even a simple double-zeta basis set appears to be adequate. The largest T-5 contribution found in the present work is on the order of 0.5 kcal/mol (for ozone). DBOCs are significant at the 0.1 kcal/mol level in hydride systems. Post-CCSD(T) contributions to the core-valence correlation energy are only significant at that level in systems with severe nondynamical correlation effects. Based on the accumulated experience, a new computational thermochemistry protocol for first- and second-row main-group systems, to be known as W4 theory, is proposed. Its computational cost is not insurmountably higher than that of the earlier W3 theory, while performance is markedly superior. Our W4 atomization energies for a number of key species are in excellent agreement (better than 0.1 kcal/mol on average, 95% confidence intervals narrower than 1 kJ/mol) with the latest experimental data obtained from Active Thermochemical Tables. Lower-cost variants are proposed: the sequence W1 -> W2.2 -> W3.2 -> W4lite -> W4 is proposed as a converging hierarchy of computational thermochemistry methods. A simple a priori estimate for the importance of post-CCSD(T) correlation contributions (and hence a pessimistic estimate for the error in a W2-type calculation) is proposed. (c) 2006 American Institute of Physics.

Abstract: The notoriously small X (3)Pi-a (1)Sigma(+) excitation energy of the BN diatomic has been calculated using high-order coupled cluster methods. Convergence has been established in both the one-particle basis set and the coupled cluster expansion. Explicit inclusion of connected quadruple excitations T-4 is required for even semiquantitative agreement with the limit value, while connected quintuple excitations T-5 still have an effect of about 60 cm(-1). Still higher excitations only account for about 10 cm(-1). Inclusion of inner-shell correlation further reduces T-e by about 60 cm(-1) at the CCSDT, and 85 cm(-1) at the CCSDTQ level. Our best estimate, T-e=183 +/- 40 cm(-1), is in excellent agreement with earlier calculations and experiment, albeit with a smaller (and conservative) uncertainty. The dissociation energy of BN(X (3)Pi) is D-e=105.74 +/- 0.16 kcal/mol and D-0=103.57 +/- 0.16 kcal/mol. (c) 2006 American Institute of Physics.

Abstract: This is the first part of a series of articles reporting critically evaluated thermochemical properties of selected free radicals. The present article contains datasheets for 11 radicals: CH, CH2(triplet), CH2(singlet), CH3, CH2OH, CH3O, CH3CO, C2H5O, C6H5CH2, OH, and NH2. The thermochemical properties discussed are the enthalpy of formation, as well as the heat capacity, integrated heat capacity, and entropy of the radicals. One distinguishing feature of the present evaluation is the systematic utilization of available kinetic, spectroscopic and ion thermochemical data as well as high-level theoretical results. (C) 2005 American Institute of Physics.

Abstract: The very good performance of modern density functional theory for molecular geometries and harmonic vibrational frequencies has been well established. We investigate the performance of density functional theory (DFT) for quartic force fields, vibrational anharmonicity and rotation-vibration coupling constants, and thermodynamic functions beyond the RRHO (rigid rotor-harmonic oscillator) approximation of a number of small polyatomic molecules. Convergence in terms of basis set, integration grid and the numerical step size for determining the quartic force field by using central differences of analytical second derivatives has been investigated, as well as the performance of various exchange-correlation functionals. DFT is found to offer a cost-effective approach with manageable scalability for obtaining anharmonic molecular properties, and particularly as a source for anharmonic zero-point and thermal corrections for use in conjunction with benchmark ab initio thermochemistry methods.

Abstract: The novel pi-accepting, pincer-type ligand, dipyrrolylphoshinoxylene (DPyPX), is introduced. This ligand has the strongest pi-accepting phosphines used so far in the PCP family of ligands and this results in some unusual coordination chemistry. The rhodium(i) complex, [(DPyPX)Rh(CO)(PR3)] (4, R=Ph, Et, pyrrolyl) is prepared by treating the relevant [(DPyPX)Rh(PR3)] (3) complex with CO and is remarkably resistant to loss of either ligand. X-ray crystallographic analysis of complex 4b (R = Et) reveals an unusual cisoid coordination of the PCP phosphine ligands. These observations are supported by density functional theory (DFT) calculations.

Abstract: The static and dynamic first hyperpolarizabilities for a series of substituted metallabenzene-based nonlinear optical (NLO) chromophores were determined by time-dependent density functional theory (TDDFT). The electronic excitation contributions to the first hyperpolarizability are rationalized in terms of the two-level model. The effects on the hyperpolarizabilities of (a) the metal center (Os, Ir, Pt); (b) the ligand environment (PH3, CO, Cl); (c) various donor and acceptor substituents (NH2, OH, Me, H, Cl, Br, I, COOMe, COOH, CN, NO2); and (d) the length of pi-conjugation were studied. Our calculations predict that metallabenzenes have significant second-order NLO susceptibilities, ranging from beta(tot)(0) = 10 x 10(-29) to 5.6 x 10(-28) esu and from u beta(tot)(0) = 3.0 x 10(-47) to 1.1 x 10(-44) esu, that can be tuned by changing the metal center and/or ligand environment.

Abstract:

Abstract: A variety of theoretical procedures, including the high-level ab initio methods G3, G3[CC](dir,full), and W2C//ACQ, have been used to predict the structures and heats of formation of several small calcium-containing molecules (CaH, CaH2, CaO, CaOH, Ca(OH)(2), CaF, CaF2, CaS, CaCl, and CaCl2) B3-LYP and CCSD(T) with both the (aug-)cc-pWCVQZ and (aug-)cc-pWCVQ+dZ basis sets are found to give molecular geometries that agree well with the experimental results. The CCSD(T)(riv)/(aug-)cc-pWCVQ+dZ results are found to be the most accurate, with a mean absolute deviation from experiment of just 0.008 angstrom. Zero-point vibrational energies (ZPVEs) and thermochemical corrections are found to be relatively insensitive to the level of theory, except in the case of molecules with highly anharmonic calcium-centered bending modes (CaH2, Ca(OH)(2), CaF2, CaCl2), where special procedures need to be employed in order to obtain satisfactory results. Several potential improvements to the W2C method were investigated, most of which do not produce significant changes in the heats of formation. It was observed, however, that for CaO and CaS the scalar relativistic corrections are unexpectedly large and highly basis set dependent. In these cases, Douglas-Kroll CCSD(T)/(aug-)cc-pWCV5Z calculations appear to give a converged result. The G3[CC](dir,full) and best W2C-type heats of formation are both found generally to agree well with experimental values recommended in recent critical compendia. However, in some cases (CaO, Ca(OH)(2), and CaF2), they differ from one another by more than their predicted error margins. The available experimental data are not sufficiently precise to distinguish definitively between the two sets of results although, in general, when discrepancies exist the W2C heats of formation are lower in energy and tend to be in better agreement with experiment. In the case of CaCl, the W2C heat of formation (20.7 kJ mol(-1)) is similar to 20 kJ mol(-1) lower than the G3[CC](dir, full) result and most of the experimental data. Extensive investigation of possible refinements of the W2C method has failed to reveal any weaknesses that could account for this discrepancy. We therefore believe that the heat of formation of CaO is likely to lie closer to the more recent direct experimental determination of 27 kJ mol(-1) than to the value of similar to 40 kJ mol(-1) recommended in recent thermochemical reviews.

Abstract: By means of a number of computationally more advanced methods the search for an acceptable overall calculated gas-phase geometry of Roesky's ketone (5-oxo-1,3,2,4-dithiadiazole) is continued. The results of CCSD, QCISD and MP4(SDQ) calculations are compared with the results of different CASSCF and DFT calculations. The results obtained with the wave-function-based methods are better than those generated by a large number of different DFT functionals, especially for the description of the carbon-sulfur bond. However, even at the CCSD and QCISD levels of theory no convergence is achieved: upon increasing the level of theory from CCSD to CCSD(T) the quality of the description actually becomes worse. (c) 2005 Elsevier B.V. All rights reserved.

Abstract: in a previous contribution, we established the suitability of density functional theory (I)FT) for the calculation of molecular anharmonic force fields. In the present work, we have assessed a wide variety of basis sets and exchange-correlation functionals for harmonic and fundamental frequencies, equilibrium, and ground-state rotational constants, and thermodynamic functions beyond the rigid rotor-harmonic oscillator (RRHO) approximation. The fairly good performance of double-zeta plus polarization basis sets for frequencies results from an error compensation between basis set incompleteness and the intrinsic error of exchange-correlation functionals. Triple-zeta plus polarization basis sets are recommended, with an additional high-exponent d function on second-row atoms. All conventional hybrid generalized gradient approximation (GGA) functionals; perform about equally well: high-exchange hybrid GGA and meta-GGA functionals designed for kinetics yield poor results, with the exception of of the very recently developed BMK functional, which takes a middle position along with the HCTH/407 (second-generation GGA) and TPSS (meta-GGA) functionals. Second-order Moller-Plesset perturbation theory (MP2) performs similarly to these functionals but is inferior to hybrid GGAs such as B3LYP and B97-1. (c) 2005 Wiley Periodicals, Inc.

Abstract: Halide abstraction from the 18 electron Ru(II) complex RuCl(CO)(2)[2,6-(CH2P'Bu-2)(2)C6H3] (2) with AgPF6 results in the exclusive formation of the cationic complex {Ru(CO)(2)[2,6-(CH2P'Bu)(2)C6H3]}+PF6-(3). The molecular structures of 2 and 3 were determined by complete single-crystal diffraction studies. X-ray crystallographic analysis of 3 reveals that the "open" coordination site is occupied by an agostic interaction between the metal center and an sp(3) C-H bond of a tert-butyl substituent. DFT gas phase calculations (B97-1/SDD) show the necessity of two sterically demanding tert-butyl substituents on one P donor atom for the agostic interaction to occur. The reaction of 3 with H-2 results in the quantitative conversion to {Ru(H)(CO)(2)[2,6-((CH2PBu2)-Bu-t)(2)C6H4]}+PF6- (4) where the aromatic C-ipso-H bond is eta(2)-coordinated to the metal center. Treatment of the agostic complex 4 with Et3N results in the formation of the neutral complex Ru(H)(CO)(2)[2,6-((CH2PBu2)-Bu-t)(2)C6H3] (5). The mechanistic details of 3 + H-2 --> 4 were investigated by DFT calculations at the B97-1/SDB-cc-pVDZ//B97-1/SDD level of theory. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: Anharmonic force fields and vibrational spectra of the azabenzene series (pyridine, pyridazine, pyrimidine, pyrazine, s-triazine, 1,2,3-triazine, 1,2,4-triazine, and s-tetrazine) and benzene are obtained using density functional theory (DFT) with the B97-1 exchange-correlation functional and a triple-zeta plus double polarization (TZ2P) basis set. Overall, the fundamental frequencies computed by second-order rovibrational perturbation theory are in excellent agreement with experiment. The resolution of the presently calculated anharmonic spectra is such that they represent an extremely useful tool for the assignment and interpretation of the experimental spectra, especially where resonances are involved.

Abstract: We are proposing a new computational thermochemistry protocol denoted W3 theory, as a successor to W1 and W2 theory proposed earlier [Martin and De Oliveira, J. Chem. Phys. 111, 1843 (1999)]. The new method is both more accurate overall (error statistics for total atomization energies approximately cut in half) and more robust (particularly towards systems exhibiting significant nondynamical correlation) than W2 theory. The cardinal improvement rests in an approximate account for post-CCSD(T) correlation effects. Iterative T-3 (connected triple excitations) effects exhibit a basis set convergence behavior similar to the T-3 contribution overall. They almost universally decrease molecular binding energies. Their inclusion in isolation yields less accurate results than CCSD(T) nearly across the board: It is only when T-4 (connected quadruple excitations) effects are included that superior performance is achieved. T-4 effects systematically increase molecular binding energies. Their basis set convergence is quite rapid, and even CCSDTQ/cc-pVDZ scaled by an empirical factor of 1.2532 will yield a quite passable quadruples contribution. The effect of still higher-order excitations was gauged for a subset of molecules (notably the eight-valence electron systems): T-5 (connected quintuple excitations) contributions reach 0.3 kcal/mol for the pathologically multireference X (1)Sigma(g)(+) state of C-2 but are quite small for other systems. A variety of avenues for achieving accuracy beyond that of W3 theory were explored, to no significant avail. W3 thus appears to represent a good compromise between accuracy and computational cost for those seeking a robust method for computational thermochemistry in the kJ/mol accuracy range on small systems. (C) 2004 American Institute of Physics.

Abstract: DFT calculations were employed to investigate the methylene transfer reaction mechanism in a model system related to the experimental Rh/PCP ligand system previously reported by us. The computationally established mechanism is in accordance with the experimental results. It was found to be a C-C reductive elimination/C-C oxidative addition sequence in which the C-C reductive elimination is the rate-determining step. The C-C activation reaction was found to take place by two different routes; both proceed through the initial formation of the eta(2)-arene complex 2. In one pathway, C-C activation takes place from an agostic C-H complex intermediate, and in the other, it occurs from the eta(2)-arene complex directly. In both intermediates the C-C bond is predirected to the metal center. The methylene transfer reaction outcome is governed by thermodynamic factors. However, changing the thermodynamic factors might lead to the reverse methylene transfer reaction becoming kinetically accessible. The reverse reaction is relevant to the design of a potential catalytic methylene transfer system.

Abstract: Various metallabenzene complexes, analogues of benzene where one CH unit has been replaced by an organometallic fragment, have been reported in the literature. A detailed theoretical investigation on the chemistry of these complexes is presented here. This includes an evaluation of their aromaticity, the mechanisms of formation of osmium, iridium, and platinum metallabenzene complexes, and one intriguing aspect of their chemistry, the formation of cyclopentadienyl (Cp) complexes. X-ray photoelectron spectroscopy (XPS) measurements on two osmabenzene examples are also presented. In addition, diffuse functions for use with the SDD and SDB-cc-pVDZ basis set-RECP combinations are presented for the transition metals.

Abstract: DFT calculations were employed to investigate the mechanism of formation and the interconversion of reduced-hapticity (eta(x)-C6H6)Cr(CO)(n) complexes (n = 1-5, x = 1-6) on both singlet and triplet energy surfaces. A variety of functionals (mPW1k, B3LYP, B97-1, and BP86) and basis sets (SDD, SDB-cc-pVDZ, and LANL2DZ + P) were used. The same structures were consistently found for all functional/basis set combinations. The structures of the different singlet and triplet complexes and comparisons between their energies are described. On the basis of the structures and energies of the intermediates and the transition states found for singlet reduced-hapticity complexes, a mechanism for the decomposition of the (eta(6)-C6H6)Cr(CO)(3) complex to Cr(CO)(6) is suggested. This mechanism, which is closely related to the arene exchange reaction mechanism, involves the formation of (eta(1)-C6H6)Cr(CO)(4) and (eta(2)-C6H6)Cr(CO)(5) complexes as ring-slippage intermediates along the pathway for the complete replacement of the benzene ring by carbonyl ligands. The barriers found for the ring-slippage process are accessible under mild conditions. Triplet reduced-hapticity complexes have different structures; however, they are similar in energy to the analogous singlet complexes. Formation of triplet reduced-hapticity complexes might take place in reactions of (eta(6)-C6H6)Cr(CO)(3) initiated by photolysis, which have been reported to show no observable decomposition to Cr(CO)(6).

Abstract: Heats of formation for a number of key C-1 and C-2 bromoalkanes and radicals have been calculated ab initio, both directly using an all-relativistic variant of W2 theory and indirectly using Douglas-Kroll relativistic CCSD(T)/Aug-VTZ reaction energies. For some of the bromoalkanes, our calculated values represent the first reliable data available. Bromine (3d) correlation contributes significantly to the molecular binding energies, but the effect of bromine (3s, 3p) correlation appears to be very small despite these orbitals lying above the carbon (1s) in energy. Thermodynamic functions have been obtained from molecular geometries and harmonic frequencies obtained at the B97-1/Aug-VTZ level and are given in the Supporting Information. These accurate thermodynamic parameters can be used to develop kinetic rate parameters.

Abstract: The mechanistic details of the Meerwein-Schmidt-Ponndorf-Verley (MSPV) reduction of ketones to the corresponding alcohols were investigated both experimentally and computationally. Density functional theory (DFT) was used to assess the energetics of several proposed pathways (direct hydrogen transfer, hydridic, and radical). Our results demonstrate that a direct hydrogen transfer mechanism involving a concerted six-membered ring transition state is the most favorable pathway for all calculated systems starting from a small model system and concluding with the experimentally investigated BINOLate/Al/PrOH/ MePhC=O system. Experimental values for the activation parameters of acetophenone reduction using the BINOLate/Al/(PrOH)-Pr-i system (DeltaG(#) = 21.8 kcal/mol, DeltaH(#) = 18.5 kcal/mol, DeltaS(#) = -11.7 au) were determined on the basis of kinetic investigation of the reaction and are in good agreement with the computational findings for this system. Calculated and experimental kinetic isotope effects support the concerted mechanism.

Abstract: A density functional theory exchange-correlation functional for the exploration of reaction mechanisms is proposed. This functional, denoted BMK (Boese-Martin for Kinetics), has an accuracy in the 2 kcal/mol range for transition state barriers but, unlike previous attempts at such a functional, this improved accuracy does not come at the expense of equilibrium properties. This makes it a general-purpose functional whose domain of applicability has been extended to transition states, rather than a specialized functional for kinetics. The improvement in BMK rests on the inclusion of the kinetic energy density together with a large value of the exact exchange mixing coefficient. For this functional, the kinetic energy density appears to correct "back" the excess exact exchange mixing for ground-state properties, possibly simulating variable exchange. (C) 2004 American Institute of Physics.

Abstract: PCP ligand (1,3-bis-[(diisopropyl-phosphanyl)-methyl]-benzene), and PCN ligand ({3-[(di-tertbutyl-phosphanyl)-methyl]-benzyl}-diethyl-amine) based rhodium dinitrogen complexes (11 and 2, respectively) react with phenyl diazomethane at room temperature to give PCP and PCN-Rh carbene complexes (3 and 5, respectively). At low temperature (-70 degreesC), PCP and PCN phenyl diazomethane complexes (4 and 6, respectively) are formed upon addition of phenyl diazomethane to 1 and 2. In these complexes, the diazo moiety is ill coordinated through the terminal nitrogen atom. Decomposition of complexes 4 and 6 at low temperatures leads only to a relatively small amount of the corresponding carbene complexes, the major products of decomposition being the dinitrogen complexes I and 2 and stilbene. This and competition experiments (decomposition of 6 in the presence of 1) suggests that phenyl diazomethane can dissociate under the reaction conditions and attack the metal center through the diazo carbon producing a eta(1)-C bound diazo complex. Computational studies based on a two-layer ONIOM model, using the mPW1K exchange-correlation functional and a variety of basis sets for PCP based systems, provide mechanistic insight. In the case of less bulky PCP ligand bearing H-substituents on the phosphines, a variety of mechanisms are possible, including both dissociative and nondissociative pathways. On the other hand, in the case of i-Pr substituents, the eta(1)-C bound diazo complex appears to be a critical intermediate for carbene complex formation, in good agreement with the experimental results. Our results and the analysis of reported data suggest that the outcome of the reaction between a diazoalkane and a late transition metal complex can be anticipated considering steric requirements relevant to eta(1)-C diazo complex formation.

Abstract: Core-valence basis sets for the alkali and alkaline earth metals Li, Be, Na, Mg, K, and Ca are proposed. The basis sets are validated by calculating spectroscopic constants of a variety of diatomic molecules involving these elements. Neglect of (3s, 3p) correlation in K and Ca compounds will lead to erratic results at best, and chemically nonsensical ones if chalcogens or halogens are present. The addition of low-exponent p functions to the K and Ca basis sets is essential for the smooth convergence of molecular properties. Inclusion of inner-shell correlation is important for accurate spectroscopic constants and binding energies of all the compounds. In basis set extrapolation/convergence calculations, the explicit inclusion of alkali and alkaline earth metal subvalence correlation at all steps is essential for K and Ca, strongly recommended for Na, and optional for Li and Mg, while in Be compounds an additive treatment in a separate 'core correlation' step is probably sufficient. Consideration of (1s) inner-shell correlation energy in first-row elements requires the inclusion of (2s, 2p) 'deep core' correlation energy in K and Ca for consistency. The latter requires special CCVnZ 'deep core correlation' basis sets. For compounds involving Ca bound to electronegative elements, additional d functions in the basis set are strongly recommended. For optimal basis set convergence in such cases, we suggest the sequence CV(D + 3d)Z, CV(T + 2d)Z, CV(Q + d)Z, and CV5Z on calcium.

Abstract:

Abstract: When developing and assessing density functional theory methods, a finite basis set is usually employed. In most cases, however, the issue of basis set dependency is neglected. Here, we assess several basis sets and functionals. In addition, the dependency of the semiempirical fits to a given basis set for a generalized gradient approximation and a hybrid functional is investigated. The resulting functionals are then tested for other basis sets, evaluating their errors and transferability. (C) 2003 American Institute of Physics.

Abstract: DFT calculations were used to reveal the unexpected reactivity and mechanism of the addition of acetone to metallabenzene, metallapyrylium and metallathiabenzene complexes of iridium.

Abstract: Zinc-dependent enzymes play important roles in many cellular processes. Assignment of their reaction mechanisms is often a subject of debate because the zinc ion is silent in several spectroscopic techniques. We have combined time-resolved X-ray absorption spectroscopy, pre-steady state kinetics and computational quantum chemistry to study the active site zinc ion of bacterial alcohol dehydrogenase during single substrate turnover. We detect a series of alternations in the coordination number and structure of the catalytic zinc ion with concomitant changes in metal-ligand bond distances. These structural changes are reflected in the effective charge of the metal ion. The present work emphasizes the flexibility of catalytic zinc sites during catalysis and provides novel mechanistic insights into alcohol dehydrogenase catalysis.

Abstract: The ammonia dimer (NH3)(2) has been investigated using high-level ab initio quantum chemistry methods and density functional theory. The structure and energetics of important isomers are obtained to unprecedented accuracy without resorting to experiment. The global minimum of eclipsed C-s symmetry is characterized by a significantly bent hydrogen bond which deviates from linearity by as much as approximate to20degrees. In addition, the so-called cyclic C-2h structure, resulting from further bending which leads to two equivalent "hydrogen bonding contacts," is extremely close in energy on an overall flat potential energy surface. It is demonstrated that none of the currently available [generalized gradient approximation (GGA), meta-GGA, and hybrid] density functionals satisfactorily describe the structure and relative energies of this nonlinear hydrogen bond. We present a novel density functional, HCTH/407+, which is designed to describe this sort of hydrogen bond quantitatively on the level of the dimer, contrary to, e.g., the widely used BLYP functional. This improved generalized gradient approximation functional is employed in Car-Parrinello ab initio molecular dynamics simulations of liquid ammonia to judge its performance in describing the associated liquid. Both the HCTH407+ and BLYP functionals describe the properties of the liquid well as judged by analysis of radial distribution functions, hydrogen bonding structure and dynamics, translational diffusion, and orientational relaxation processes. It is demonstrated that the solvation shell of the ammonia molecule in the liquid phase is dominated by steric packing effects and not so much by directional hydrogen bonding interactions. In addition, the propensity of ammonia molecules to form bifurcated and multifurcated hydrogen bonds in the liquid phase is found to be negligibly small. (C) 2003 American Institute of Physics.

Abstract:

Abstract: High-level ab initio calculations, including variants of the Wn and G3 procedures, have been used to determine the structures and heats of formation of the alkali metal and alkaline earth metal oxides and hydroxides (M2O, MOH: M = Li, Na, and K; MO, M(OH)(2): M = Be, Mg, and Ca). Our best structures were obtained at the CCSD(T)(riv,rv)/aug'-cc-pWCVQZ level and are in uniformly close agreement with available experimental data, with a mean absolute deviation from experimental metal-oxygen bond lengths of just 0.007 Angstrom. Structures obtained with CCSD(T)/cc-pWCVQZ, B3-LYP/cc-pVTZ, B3-LYP/6-31 G(2df,p), and MP2(full)/6-311+G(3df,2p) are also in good agreement with experiment. Zero-point vibrational energies and enthalpy temperature corrections are found to be relatively insensitive to the various procedures employed. However, the heats of formation for these molecules are challenging targets for high-level ab initio procedures. In the Wn-type procedures, it is found that expanding the correlation space on the metal atoms from the normal relaxed valence (rv) (or frozen-core) specification to relaxed inner valence (riv) requires the use of newly developed core-valence basis sets (cc-pWCVnZ) in the extrapolation calculations to obtain reliable results. Our best calculated heats of formation (DeltaH(f) (298)) come from a procedure designated W2C//ACQ, while oG3[CC](dir,full) is the best of the G3-type procedures. Recommended DeltaH(f) (298) values, weighted toward the W2C//ACQ results, are -157 +/- 5 (Li2O), -25 +/- 5 (Na2O), -60 +/- 5 (K2O), +130 +/- 12 (BeO), +142 +/- 10 (MgO), +26 +/- 17 (CaO), -239 +/- 5 (LiOH), -189 +/- 5 (NaOH), -223 +/- 5 (KOH), -632 +/- 7 (Be(OH)(2)), -547 +/- 5 (Mg(OH)(2)), and -604 +/- 19 (Ca(OH)(2)) kJ mol(-1).

Abstract: The mechanistic details of 1,2- and 1,4-cycloaddition reactions of acetone, CO2, and CS2 to isostructural iridiabenzene, iridiapyrylium, and iridiathiabenzene complexes, as well as their rhodium analogues, were elucidated by density functional theory (DFT) at the PCM/mPW1K/SDB-cc-pVDZ//mPW1K/ SDD level of theory. The calculated reaction profiles concur with reported experimental observations. It was found that the first complex reacts via a concerted reaction mechanism, while the latter two react by a stepwise mechanism. Several factors affecting the reaction mechanisms and outcome were identified. They include the composition and size of the metal-aromatic ring, the length of the substrate C=X (X = O, S) bond, the geometry of the product, the symmetry of the frontier molecular orbitals, and the type of reaction mechanism involved.

Abstract: Milstein, and co-workers reported that the complex [(bpy)Rh(hd)]+PF6- (bpy = substituted bipyridine ligand, hd = 1,5-hexadiene) shows catalytic activity in the hydrogenation of acetone (Tollner, K. et al. Science 1997, 278, 2100). The activity in an ordered monolayer was found to be dramatically greater than in solution. We used the DFT functional mPW1K (Lynch, B. J. et al. J. Phys. Chem. A 2000, 104, 4811) to investigate the mechanism of the homogenous reaction. The suitability of the mPW1K functional was verified by coupled cluster calculations on a model system. Bulk solvent effects were considered. Various alternative catalytic cycles were evaluated, and we found that one potential mechanism involves metal-catalyzed keto-enol tautomerization to form [(bpy)Rh(enol)](+) that adds hydrogen yielding a complex with axial and equatorial hydride ligands. The reaction continues via transfer of the hydrides to the enolic C=C bond thereby forming 2-propanol and regenerating the catalyst. Another potential catalytic cycle involves formation of [(bpy)Rh(acetone)(2)(H)(2)](+), which has a spectator solvent ligand, and initial transfer of the equatorial hydride to the carbonyl carbon of acetone. Other mechanisms involving hydrogen transfer to the acetone tautomer involved higher barriers. With an eye toward modeling multi-center catalysis, various model systems for the bpy ligand were considered. It was found that diimine (HN=CH-CH=NH) compares very well with bpy, whereas cis-1,2-diiminoethylene (H2C=N-CH=CH-N=CH2) yields a reaction profile very close to that of bpy. Finally, the system with two rhodium centers, [(diimine)Rh](2)(2+), was investigated. The results strongly suggest that an enol-type catalytic cycle occurs and that cooperativity between the two metal centers is responsible for the acceleration of the reaction in the monolayer system.

Abstract: The Lewis acid-base reaction between Sn(CH3)(3)X and Y- (with X, Y = F, Cl, Br and I) has been studied using quantum chemical calculations. Complexation energies were calculated at the Density Functional Theory (DFT) level and rationalized on the basis of a local application of the hard and soft acids and bases principle. It was observed that smaller differences in the local softness of the interacting sites in the Lewis acid and base correspond to stronger interactions. Moreover, the calculated sequences in complexation energies can be reproduced using equations containing chemical concepts introduced within the framework of conceptual density functional theory and rooted in the hard and soft acids and bases principle and referring only to the reactants. A method of treating the electronegativity and softness of the halide anions is presented based on a Taylor expansion of the electronegativity of the neutral halogens and the softness-polarizability proportionality. Experimental evidence for the calculated sequences was gathered from measured Sn-117 chemical shifts and (1)J (C-13-Sn-119/117) coupling constant changes upon complexation. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Abstract: The aryl-PC type ligand 3, benzyl(di-tert-butyl)phosphane, reacts with [Rh(coe)(2)(solv)(n)]BF4 (coe = cyclooctene, sov = solvent), producing the C-H activated complexes 4a-c (solv (a) acetone, (b) THF, (c) methanol). Complexes 4a-c undergo reversible arene C-H activation (observed by NMR spin saturation transfer experiments, SST) and H/D exchange into the hydride and aryl ortho-H with ROD (R = D, Me). They also promote catalytic H/D exchange into the vinylic C-H bond of olefins, with deuterated methanol or water utilized as D-donors. Unexpectedly, complex 2, based on the benzyl-PC type ligand 1 (analogous to 3), di-tert-butyl(2,4,6-trimethylbenzyl)phosphane, shows a very different reversible C-H activation pattern as observed by SST. It is not active in H/D exchange with ROD and in catalytic H/D exchange with olefins. To clarify our observations regarding C-H activation/reductive elimination in both PC-Rh systems, density functional theory (DFT) calculations were performed. Both nucleophilic (oxidative addition) and electrophilic (H/D exchange) C-H activation proceed through eta(2)-C,H agostic intermediates. In the aryl-PC system the agostic interaction causes C-H bond acidity sufficient for the H/D exchange with water or methanol, which is not the case in the benzyl PC-Rh system. In the latter system the C-H coordination pattern of the methyl controls the reversible C-H oxidative addition leading to energetically different C-H activation processes, in accordance with the experimental observations.

Abstract: The heats of formation of haloacetylenes are evaluated using the recent W1 and W2 ab initio computational thermochemistry methods. These calculations involve CCSD and CCSD(T) coupled cluster methods, basis sets of up to spdfgh quality, extrapolations to the one-particle basis set limit, and contributions of inner-shell correlation, scalar relativistic effects, and (where relevant) first-order spin-orbit coupling. The heats of formation determined using W2 theory are: DeltaH(f)(298) (HCCH) = 54.48 kcal mol(1), DeltaH(f)(298) (HCCF) = 25.15 kcal mol(1), DeltaH(f)(298) (FCCF) = 1.38 kcal mol(1), DeltaH(f)(298) (HCCCl) = 54.83 kcal mol(1), DeltaH(f)(298) (ClCCCl) = 56.21 kcal mol(1), and DeltaH(f)(298) (FCCCl) = 28.47 kcal mol(1). Enthalpies of hydrogenation and destabilization energies relative to acetylene were obtained at the W1 level of theory. So doing we find the following destabilization order for acetylenes: FCCF > ClCCF > HCCF > ClCCCl > HCCCl > HCCH. By a combination of W1 theory and isodesmic reactions, we show that the generally accepted heat of formation of 1,2-dichloroethane should be revised to -31.8 +/- 0.6 kcal mol-(1), in excellent agreement with a very recent critically evaluated review. The performance of compound thermochemistry schemes, such as G2, G3, G3X and CBS-QB3 theories, has been analysed.

Abstract: The CCSD(T) method has been used to compute a highly accurate quartic force field and fundamental frequencies for all N-14 and N-15 isotopomers of the high energy density material TdN4. The computed fundamental frequencies show beyond doubt that the bands observed in a matrix isolation experiment by Radziszewski and coworkers [Chem. Phys. Lett. 328 (2000) 227] are not due to different isotopomers of TdN4. The most sophisticated thermochemical calculations to date yield a 2N(2) --> N-4 heat of reaction of 182.22 +/- 0.5 kcal/mol at 0 K (180.64 +/- 0.5 at 298 K). It is hoped that the data reported herein will aid in the ultimate detection of TdN4. (C) 2002 Published by Elsevier Science B.V.

Abstract: The reactivity of TpPtMe(H)(2) (Tp = hydrido-tris (pyrazolyl)borate) was investigated. This complex is remarkably resistant to methane loss; heating it in methanol at 55 degreesC does not lead to either methane or hydrogen loss. When CD3OD is used, reversible H/D scrambling of the hydrides and the methyl hydrogens occurs. This reactivity was investigated by density functional theory (DFT) methods at the mPW1k/ LANL2DZ+P//mPW1k/LANL2DZ level. It was found that methane loss cannot occur due to the rigidity of the Tp ligand, which does not allow the trans geometry which would be required for the product of methane elimination, TpPtH. The resulting complex is very high in energy, and therefore the loss of methane is unfavorable. On the other hand, H/D scrambling of the methyl ligand is relatively facile. It proceeds through an eta(2-CH)-CH4 complex, even though methane loss is not observed. The model system, [(NH3)(3)PtMe(H)(2)](+) was examined to verify that the cause of the observations is the rigidity of the Tp system. The reaction was investigated at a number of levels of DFT. It was concluded that investigations of similar sized systems should be examined at the above level of theory or the mPW1k/SDB-cc-pVDZ//mPW1k/SDD level for improved accuracy of the energetic calculations.

Abstract: The anharmonic force field up to quartic terms has been calculated for both CF2 and CCl2 at the CCSD(T) level of theory with large basis sets. The calculated spectroscopic parameters are in excellent agreement with the available experimental data. The equilibrium structure of CCl2 has been calculated at the CCSD(T) level of theory with the cc-pV5Z basis set ( including core correlation corrections). It is in excellent agreement with the semi-experimental equilibrium structure derived from the experimental rotational constants and the ab initio rovibrational interaction parameters. The heats of formation of CF2 and CCl2 have been calculated using the W2 method and are -193.43 and 227.96 kJ mol(-1), respectively.

Abstract: The unimolecular gas phase chemistry of the title ion, (CH3O)(2)P(H) = O.+, (1(.+)) and its tautomer dimethyl phosphite, (CH3O)(2)P-OH.+, (2(.+)) was investigated using mass spectrometry based experiments in conjunction with computational quantum chemistry. A facile tautomerization of the "keto" ion 1(.+) into its "enol" isomer 2(.+) is prevented by a high 1,2-H shift barrier. Instead, 1(.+) readily isomerizes via a 1,4-H shift to the very stable distonic ion CH2O-(CH3O)P(H)OH.+ (1a(.+)) and related ion-dipole complexes which serve as precursors for the low energy loss of CH2=O. Loss of CH2=O is also the major dissociation of the enol ion 2(.+), which is more stable than 1(.+) by 31 kcal/mol. The reaction involves a 1,3-H shift leading to 1a(.+) and, at a marginally higher energy, a competing 1,4-H shift leading to the ion-dipole complex CH2O-P(OH)-O(H)CH3.+ (1b(.+)). The resulting product ions, viz (CH3O)P(H)OH.+ and P(OH)-O(H)CH3.-, are separated by a high 1,2-H shift barrier (44kcal/mol). However, the CH2O moiety in 1a(.+) and 1b(.+) is calculated to reduce this barrier significantly by a mechanism coined as proton-transport catalysis. The identity of the ions was probed by tandem mass spectrometry methods. These include MI (metastable ion) or CID (collision induced dissociation) spectra, consecutive MI/CID and CID/CID spectra, NRMS (neutralization-reionization mass spectra), NR/CID and CIDI (collision induced dissociative ionization) spectra, time-resolved CID spectra and deuterium labelling. The energetics of the CH2=O loss from 1(.+) and 2(.+) was derived from ionization and appearance energies determined by VUV photoionization. The experimental results agree quite well with the computational findings. Heats of formation, isomerization barriers and dissociation energies of the various ionic and neutral species were obtained by the wavefunction-based CBS-QB3 method. Essentially identical energy profiles on the C2H7O3P.+ surface were obtained with the computationally less demanding novel MPW1K empirical DFT method in conjunction with the aug-cc-pVTZ basis set. Theory and experiment yield a consistent potential energy profile that describes the isomerization and low energy dissociation chemistry of ions 1(.+) and 2(.+) (Scheme 4). In the mus timeframe ions 1(.+) have completely isomerized into distonic ions 1a(.+) which do not significantly communicate with their more stable enol counterparts. However, ion-molecule reactions of 1(.+) with benzonitrile lead to a complete enolization, This is by virtue of a dipole-assisted lowering of the 1,3-H shift barrier separating isomers 1a(.+) and 2(.+).

Abstract: The performance of two recent ab initio computational thermochemistry schemes, W1 and W2 theory [J. M. L. Martin and G. de Oliveira, J. Chem. Phys. 111, 1843 (1999)], is assessed for an enlarged sample of thermochemical data consisting of the ionization potentials and electron affinities in the G2-1 and G2-2 sets, as well as the heats of formation in the G2-1 and a subset of the G2-2 set. We find W1 theory to be several times more accurate for ionization potentials and electron affinities than commonly used (and less expensive) computational thermochemistry schemes such as G2, G3, and CBS-QB3: W2 theory represents a slight improvement for electron affinities but no significant one for ionization potentials. The use of a two-point A+B/L-5 rather than a three-point A+B/C-L extrapolation for the self-consistent field (SCF) component greatly enhances the numerical stability of the W1 method for systems with slow basis set convergence. Inclusion of first-order spin-orbit coupling is essential for accurate ionization potentials and electron affinities involving degenerate electronic states: Inner-shell correlation is somewhat more important for ionization potentials than for electron affinities, while scalar relativistic effects are required for the highest accuracy. The mean deviation from experiment for the G2-1 heats of formation is within the average experimental uncertainty. W1 theory appears to be a valuable tool for obtaining benchmark quality proton affinities. (C) 2001 American Institute of Physics.

Abstract: We propose large-core correlation-consistent (cc) pseudopotential basis sets for the heavy p-block elements Ga-Kr and In-Xe. The basis sets are of cc-pVTZ and cc-pVQZ quality, and have been optimized for use with the large-core (valence-electrons only) Stuttgart-Dresden-Bonn (SDB) relativistic pseudopotentials. Validation calculations on a variety of third-row and fourth-row diatomics suggest them to be comparable in quality to the all-electron cc-pVTZ and cc-pVQZ basis sets for lighter elements. Especially the SDB-cc-pVQZ basis set in conjunction with a core polarization potential (CPP) yields excellent agreement with experiment for compounds of the later heavy p-block elements. For accurate calculations on Ga (and, to a lesser extent, Ge) compounds, explicit treatment of 13 valence electrons appears to be desirable, while it seems inevitable for In compounds. For Ga and Ge, we propose correlation consistent basis sets extended for (3d) correlation. For accurate calculations on organometallic complexes of interest to homogenous catalysis, we recommend a combination of the standard cc-pVTZ basis set for first- and second-row elements, the presently derived SDB-cc-pVTZ basis set for heavier p-block elements, and for transition metals, the small-core [6s5p3d] Stuttgart-Dresden basis set-relativistic effective core potential combination supplemented by (2f1g) functions with exponents given in the Appendix to the present paper. (C) 2001 American Institute of Physics.

Abstract: The energetics of the gas-phase S(N)2 reactions Y- + CH3X- --> CH3Y + X- (where X,Y = F,Cl,Br), were studied using (variants on) the recent Wi and W2 ab initio computational thermochemistry methods. These calculations involve CCSD and CCSD(T) coupled cluster methods, basis sets of up to spdfgh quality, extrapolations to the one-particle basis set limit, and contributions of inner-shell correlation, scalar relativistic effects, and (where relevant) first-order spin-orbit coupling. Our computational predictions are in excellent agreement with experimental data where these have small error bars; in a number of other instances reexamination of the experimental data may be in order. Our computed benchmark data (including cases for which experimental data are unavailable altogether) are used to assess the quality of a number of compound thermochemistry schemes such as G2 theory, G3 theory, and CBS-QB3, as well as a variety of density functional theory methods. Upon applying some modifications to the level of theory used for the reference geometry (adding diffuse functions, replacing B3LYP by the very recently proposed mPW1K functional [Lynch, B. J.; Fast, P. L.; Harris, M.; Truhlar, D. G. J. Phys. Chem. A 2000, 104, 4811]), the compound methods appear to perform well. Only the "half-and-half" functionals BH&HLYP and mPWH&HPW91 and the empirical mPW1K functional, consistently find all required stationary points; the other functionals fail to find a transition state in the F/Br case. BH&HLYP and mPWH&HPW91 somewhat overcorrect for the tendency of B3LYP land, to a somewhat lesser extent, mPW1PW91) to underestimate barrier heights. The Becke97 and Becke97-1 functionals perform similarly to B3LYP for the problem under study, while the HCTH and HCTH-120 functionals both appear to underestimate central barriers. HCTH underestimates complexation energies; this problem is resolved in HCTH-120. mPW1K appears to exhibit the best performance of the functionals considered, although its energetics are still inferior to the compound thermochemistry methods. mPW1K, however, appears to be very suitable for generating reference geometries for more elaborate thermochemical methods in kinetics applications.

Abstract: The sensitivity of computed DFT (Density Functional Theory) molecular properties (including energetics, geometries, vibrational frequencies, and infrared intensities) to the radial and angular numerical integration grid meshes, as well as to the partitioning scheme, is discussed for a number of molecules using the Gaussian 98 program system. Problems with typical production grid sizes are particularly acute for third-row transition metal systems, but may still result in qualitatively incorrect results for a molecule as simple as CCH. Practical recommendations are made with respect to grid choices for the energy (+ gradient) steps, as well as for the solution of the CPKS (Coupled Perturbed Kohn-Sham) equations. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract: A benchmark study has been carried out on the ground-state potential curve of the hydroxyl anion, OH-, including detailed calibration of both the 1-particle and n-particle basis sets. The CCSD(T) basis set limit overestimates omega (e) by about 10 cm(-1), which is only remedied by inclusion of connected quadruple excitations in the coupled cluster expansion - or, equivalently, the inclusion of the 2 pi orbitals in the active space of a multireference calculation. Upon inclusion of scalar relativistic effects (-3 cm(-1) on omega (e)), a potential curve of spectroscopic quality (sub-cm(-1) accuracy) is obtained. Our best computed EA(OH), 1.828 eV, agrees to three decimal places with the best available experimental value. Our best computed dissociation energies, D-0(OH-) = 4.7796 eV and D-0(OH) = 4.4124 eV, suggest that the experimental D-0(OH)= 4.392 eV may possibly be about 0.02 eV too low. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract: A long-standing controversy concerning the heat of formation of methylenimine has been addressed by means of the W2 (Weizmann-2) thermochemical approach. Our best calculated values, DeltaH(f,298)degrees (CH2NH) = 21.1 +/- 0.5 kcal/mol and DeltaH(f,298)degrees (CH2NH2+) = 179.4 +/- 0.5 kcal/mol, are in good agreement with the most recent measurements but carry a much smaller uncertainty. As a byproduct, we obtain the first-ever accurate anharmonic force field for methylenimine: upon consideration of the appropriate resonances, the experimental gas-phase band origins are all reproduced to better than 10 cm(-1). Consideration of the difference between a fully anharmonic zero-point vibrational energy and B3LYP/cc-pVTZ harmonic frequencies scaled by 0.985 suggests that the calculation of anharmonic zero-point vibrational energies can generally be dispensed with, even in benchmark work, for rigid molecules. (C) 2001 John Wiley & Sons, Inc.

Abstract: Bifunctional conjugated molecules, consisting of electron donating or accepting groups that are connected, via a conjugated bridge, to a carboxylic acid group, were adsorbed as monomolecular carboxylate films on n-GaAs (100) and characterized by reflection FTIR, ellipsometry, and contact angle techniques. The way the donors and acceptors affected the electronic properties of the semiconductor was investigated. In agreement with theory, we find a linear relation between the calculated dipole moment of the molecules and the change in electron affinity of the moleculary modified surface, as well as between the barrier height of Au/molecule on n-GaAs junctions, extracted from their current-voltage characteristics and the dipole moment. The experimental results show little effect of the nature of the conjugated bridge in the molecules. Comparison with earlier work shows a clear decrease in the effect of the dipole of the free molecule on the semiconductor surface and interface behavior, notwithstanding the strongly conjugated link between the donor or acceptor groups of the molecule and the semiconductor surface. The simplest way to understand this is to consider the higher polarizability of the intervening bonds. Such effect needs to be considered in designing molecules for molecular control over devices.

Abstract: In this theoretical study on the Heck reaction we explore the feasibility of an alternative pathway that involves a Pd-II/Pd-IV redox system. Usually, the catalytic cycle is formulated based on a Pd-0/Pd-II mechanism, We performed quantum chemical calculations using density functional theory on a model system that consisted of diphosphinoethane (DPE) as a bidentate ligand and the substrates ethylene and phenyl iodide to compare both mechanisms. Accordingly, the Pd-II/Pd-IV mechanism is most likely to occur in the equatorial plane of an octahedral Pd-IV complex. The energy profiles of both reaction pathways under consideration are largely parallel. A major difference is found for the oxidative addition of the C-I bond to the palladium centre,This is a rate-determining step of the Pd-II/Pd-IV mechanism, while it is facile for a Pd-0 catalyst. The calculations show that intermediate ligand detachment and reattachment is necessary in the course of the oxidative addition to Pd-II. Therefore, we expect the Pd-II/Pd-IV mechanism to be only feasible if a weakly coordinating ligand is present.

Abstract: C-H bond activation was observed in a novel PCO ligand 1 (C6H(CH3)(3)(CH2OCH3)(CH2P(t-Bu)(2))) at room temperature in THF, acetone, and methanol upon reaction with the cationic rhodium precursor, [Rh(coe)(2)(solv)(n)]BF4 (solv = solvent; coe = cyclooctene). The products in acetone (complexes 3a and 3b) and methanol (complexes 4a and 4b) were fully characterized spectroscopically. Two products were formed in each case, namely those containing uncoordinated (3a and 4a) and coordinated (3b and 4b) methoxy arms, respectively. Upon heating of the C-H activation products in methanol at 70 degreesC, C-C bond activation takes place. Solvent evaporation under vacuum at room temperature for 3-4 days also results in C-C activation. The C-C activation product, ((CH3)Rh(C6H(CH3)(2)(CH2OCH3)(CH2P(t-Bu)(2))BF4), was characterized by X-ray crystallography, which revealed a square pyramidal geometry with the BF4- anion coordinated to the metal. Comparison to the structurally similar and isoelectronic nonchelating Rh-PC complex system and computational studies provide insight into the reaction mechanism. The reaction mechanism was studied computationally by means of a two-layer ONIOM model, using both the B3LYP and mPW1K exchange-correlation functionals and a variety of basis sets. Polarization functions significantly affect relative energetics, and the mPW1K profile appears to be more reliable than its B3LYP counterpart, The calculations reveal that the electronic requirements for both C-C and C-H activation are essentially the same (14e intermediates are the key ones). On the other hand, the steric requirements differ significantly, and chelation appears to play an important role in C-C bond activation.

Abstract: The mechanism of competitive intramolecular C-C and C-H bond activation in PCN pincer complexes of rhodium(I) has been studied computationally. Experimentally this system exhibits exclusive cleavage of an aryl-methyl C-C bond; no products corresponding to C-K bond cleavage in the methyl group have been observed. The calculated relative energies indicate that the C-C activation product is the most stable one (DeltaE < -50 kJ<bullet>mol(-1) relative to the C-H activation product) and that its formation is irreversible. C-K activation is not kinetically forbidden, but it is fast and reversible. Because of the weak Rh-N bond several intermediates on the C-H activation paths are accessible; therefore low concentrations of these species are a plausible reason for the experimental result. Further, this study reveals that the unique preference of C-C bond activation in the PCN system results from insufficient stabilization of the C-H activation product due to strain. To model the complete ligand including all bulky substituents used in the experiment, the ONIOM method has been employed, using the B3LYP/lanl2dz level for the inner layer and the HF/lanl1mb level to model substituent effects.

Abstract: The total atomization energy at absolute zero, (TAE(0)) of benzene, C6H6, was computed fully ab initio by means of W2h theory as 1306.6 kcal/mol, to be compared with the experimentally derived value 1305.7 +/-0.7 kcal/mol. The computed result includes contributions from inner-shell correlation (7.1 kcal/mol), scalar relativistic effects (-1.0 kcal/mol), atomic spin-orbit splitting (-0.5 kcal/mol), and the anharmonic zero-point vibrational energy (62.1 kcal/mol). The largest-scale calculations involved are CCSD/cc-pV5Z and CCSD(T)/cc-pVQZ; basis set extrapolations account for 6.3 kcal/mol of the final result. Performance of more approximate methods has been analyzed. Our results suggest that, even for systems the size of benzene, chemically accurate molecular atomization energies can be obtained from fully first-principles calculations, without resorting to corrections or parameters derived from experiment. (C) 2001 American Institute of Physics.

Abstract: The electronic structure and properties of the silabenzenes series have been investigated using basis sets of spdf quality and many-body perturbation theory, hybrid density functional theory, and coupled cluster methods. Basic measures of aromatic character derived from structure, molecular orbitals, isodesmic and homodesmotic bond separation reactions, and a variety of magnetic criteria (magnetic isotropic and anisotropic susceptibilities, magnetic susceptibility exaltations, NICS) are considered. Energetic criteria suggest that 1,3,5-trisilabenzene and, to a lesser extent, 1,3-disilabenzene and its complement 1,2,3,5-tetrasilabenzene enjoy conspicuous stabilization. However, by magnetic criteria, these systems are among the least aromatic of the family: population and bond order analyses reveal that they derive part of their stability from ionic contributions to the bonding. Within their isomer series, 1,2-disilabenzene, 1,2,3-trisilabenzene, and 1,2,3,4-tetrasilabelizene are the most aromatic using magnetic criteria: overall, "magnetic aromaticity" decreases with increasing number of Si atoms. The different magnetic aromaticity criteria are fairly consistent within an isomer series: over the complete set of silabenzenes, the magnetic susceptibility exaltations correlate fairly well with the magnetic susceptibility anisotropies. Second-order Jahn-Teller effects cause deviations from planarity to occur in all systems with at least four silicon ring atoms, except for 1,2,4,5-tetrasilabenzene. The relative energetics (isomers, deviation from planarity) at our highest level of theory, CCSD(T)/cc-pVTZ, are better reproduced by the B3LYP/cc-pVTZ density functional method than by any of the less accurate wave function methods (HF, MP2, CCSD) considered. In general, the use of high levels of theory with large basis sets removes some ambiguities in previously reported studies.

Abstract: Core correlation and scalar relativistic contributions to the atomization energy of 120 first- and second-row molecules have been determined using coupled cluster and averaged coupled-pair functional methods and the MTsmall core correlation basis set. These results are used to parametrize an improved version of a previously proposed bond order scheme for estimating contributions to atomization energies. The resulting model, which requires negligible computational effort, reproduces the computed core correlation contributions with 88%-94% average accuracy (depending on the type of molecule), and the scalar relativistic contribution with 82%-89% accuracy. This permits high-accuracy thermochemical calculations at greatly reduced computational cost. (C) 2000 American Institute of Physics. [S0021-9606(00)30328-2].

Abstract: Accurate quartic anharmonic force fields for CF4 and SiF4 have been calculated using the CCSD(T) method and basis sets of spdf quality. Based on the ab initio force field with a minor empirical adjustment, the vibrational energy levels of these two molecules and their isotopomers are calculated by means of high order Canonical Van Vleck Perturbation Theory (CVPT) based on curvilinear coordinates. The calculated energies agree very well with the experimental data. The full quadratic force field of CF4 is further refined to the experimental data. The symmetrization of the Cartesian basis for arbitrary combination bands of T-d group molecules is discussed using the circular promotion operator for the doubly degenerate modes, together with tabulated vector coupling coefficients. The extraction of the spectroscopic constants from our second order transformed Hamiltonian in curvilinear coordinates is discussed, and compared to a similar procedure in rectilinear coordinates. (C) 2000 American Institute of Physics. [S0021-9606(00)01403-3].

Abstract: A theoretical study of the oxidative addition of C-C vs C-H bonds to a rhodium(I) complex with PCP-type ligands has been carried out. Special attention has been paid to the effect of different bulky substituents at the phosphorus atoms of the chelate ligand. Therefore, B3LYP/lanl2dz+p:B3LYP/lanl2dz and ONIOM(B3LYP/lanl2dz+p:B3LYP/lanl2dz)//ONIOM(B3LYP/lanl2dz:HF/lanl1mb) methods have been utilized. According to the calculations, C-H activation is always the kinetically favored process (Delta Delta E-double dagger 20 kJ . mol(-1)), though the C-C activation product is more stable (Delta Delta E 20 KJ . mol(-1)). C-H addition is a reversible process; the product of the C-H activation can interconvert to the C-C activation product via an intermediate structure. Bulky substituents are found to increase the barrier for C-H activation relative to that for C-C activation. With additional ligands (e.g., phosphines), hexacoordinate complexes are formed. This is more favorable for the C-C activation products. Our calculations show that the activation reaction proceeds via complexes with a pentacoordinated rhodium atom. Thus, in the presence of donor ligands, the activation reaction is inhibited.

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Abstract: Although the tetrakis(trimethylphosphine)iridium(III) hydridochloride cation, [HIrCl(PMe3)(4)](+), and similar complexes would intuitively be expected to adopt a trans geometry on electronic and sterical grounds, experimentally the cis geometry is found to prevail. Quantum chemical calculations suggest that the trans and cis structures are nearly isoenergetic, such that the cis:trans equilibrium is dominated by the higher entropy of the lower symmetry cis structure.

Abstract: A search of the gas phase conformational space of methyl alpha-D-lyxofuranoside was carried out at the B3LYP/cc-pVDZ level. Relative energies of the local minima were computed using the larger aug-cc-pVDZ basis set and the recent mPW1LYP exchange-correlation functional. Local minima include structures with zero, one, two, three, or four internal hydrogen bonds and conformational energies of 12-2 kcal/mol relative to the "strain-free" methyl alpha-D-lyxopyranoside. The two most stable structures, with three and four internal hydrogen bonds, have conformational energies of approximately 2 kcal/mol. The relevance of these results to the conformational behavior of furanosides in the crystalline state and in solution is discussed. An efficient and accurate general procedure for searching the conformational space of furanosides is proposed.

Abstract: To resolve a significant uncertainty in the heat of vaporization of silicon-a fundamental parameter in gasphase thermochemistry-Delta H degrees(f,o) [Si(g)] has been determined from a thermochemical cycle involving the precisely known experimental heats of formation of SiF4(g) and F(g) and a benchmark calculation of the total atomization energy (TAE(o)) of SiF4 using coupled-cluster methods. Basis sets up to [8s7p6d4f2g1h] on Si and [7s6p5d4f3g2h] on F have been employed, and extrapolations for residual basis set incompleteness applied. The contributions of inner-shell correlation (-0.08 kcal/mol), scaler relativistic effects (-1.88 kcal/mol), atomic spin-orbit splitting (-1.97 kcal/mol), and anharmonicity in the zero-point energy (+0.04 kcal/mol) have all been explicitly accounted for. Our benchmark TAE(o) = 565.89 +/- 0.22 kcal/mol leads to Delta H degrees(f,o) [Si(g)] = 107.15 +/- 0.38 kcal/mol (Delta H degrees(f,298) [Si(g)] = 108.19 +/- 0.38 kcal/mol): between the JANAF/CODATA value of 106.5 +/- 1.9 kcal/mol and the revised value proposed by Grev and Schaefer [ J. Chem. Phys. 1992, 97, 8389], 108.1 +/- 0.5 kcal/mol. The revision will be relevant for future computational studies on heats of formation of silicon compounds. Among standard computational thermochemistry methods, G2 and G3 theory exhibit large errors, while CBS-Q performs relatively well, and the very recent W1 theory reproduces the present calibration result to 0.1 kcal/mol.

Abstract: The quartic force field of SO3 was computed fully ab initio using coupled cluster (CCSD(T)) methods and basis sets of up to spdfgh quality. The effect of inner-shell correlation was taken into account. The addition of tight d functions is found to be essential for accurate geometries and harmonic frequencies. The equilibrium geometry and vibrational fundamentals are reproduced to within 0.0003 Angstrom and (on average) 1.15 cm(-1), respectively. We recommend the following revised values for the harmonic frequencies: omega(1) = 1082.7, omega(2) = 502.6, omega(3) = 1415.4, omega(4) = 534.0 cm(-1). In addition, we have shown that the addition of inner polarization functions to second-row elements is highly desirable even with more approximate methods like B3LYP, and greatly improves the quality of computed geometries and harmonic frequencies of second-row compounds at negligible extra computational cost. For larger such molecules, the B3LYP/VTZ + 1 level of theory should be a very good compromise between accuracy and computational cost. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: Highly accurate and precise crystal structures of methyl alpha-D-arabinofuranoside, methyl beta-D-ribofuranosidc, methyl alpha-D-lyxofuranoside, and methyl alpha-D-xylofuranoside have been determined at 100 K by X-ray crystallography. The structures of methyl alpha-D-arabinofuranoside and methyl beta-D-ribofuranoside have also been determined at 15 K by neutron diffraction. Equilibrium (r(e)) geometries of the same compounds were computed by means of density functional methods using a variety of exchange-correlation functionals and a sequence of basis sets. The validity of the computed results was assessed by several criteria including agreement between computed and observed bond distances and bond angles, agreement between computed and observed ring conformations, and basis set convergence of the computed geometrical parameters. Particular reference was made to computed internal hydrogen bond parameters, which are especially sensitive to the quality of the theoretical treatment. Because of the intrinsic sensitivity of the conformation of the five-membered ring to bond lengths and bond angles, molecular mechanics and small basis set SCF treatments are wholly inadequate. Local density functional theory also fails because of a tendency to strongly underestimate internal hydrogen bond distances. When the B3LYP exchange-correlation functional is used, bond lengths and bond angles agree with the neutron diffraction values to within their experimental uncertainty and the ring conformation is qualitatively correct, as long as a basis set of at least double-zeta plus polarization quality (such as cc-pVDZ) is used. Further expansion of the basis set leads to more accurate equilibrium bond lengths: and bond angles but does not appreciably affect the ring conformation. For methyl alpha-D-arabinofuranoside, methyl beta-D-ribofuranoside, and methyl alpha-D-xylofuranoside, there is very good correspondence between the best computed and observed ring conformations, even though some intermolecular hydrogen bonds in the crystal give way to internal hydrogen bonds in the predicted gas-phase structures. On the other hand, in the case of methyl alpha-D-lyxofuranoside, an O2H ... O4 internal hydrogen bond between the ring oxygen O4 and the hydroxyl hydrogen of a ring carbon (O2H) in the computed structure leads to a very large change of ring conformation from the northeast corner of the pseudorotation pathway (P = 28 degrees, crystal) to the southeast corner (P = 130 degrees, computed).

Abstract: Two new schemes for computing molecular total atomization energies (TAEs) and/or heats of formation (Delta H-f(degrees)) of first- and second-row compounds to very high accuracy are presented. The more affordable scheme, W1 (Weizmann-1) theory, yields a mean absolute error of 0.30 kcal/mol and includes only a single, molecule-independent, empirical parameter. It requires CCSD (coupled cluster with all single and double substitutions) calculations in spdf and spdfg basis sets, while CCSD(T) (i.e., CCSD with a quasiperturbative treatment of connected triple excitations) calculations are only required in spd and spdf basis sets. On workstation computers and using conventional coupled cluster algorithms, systems as large as benzene can be treated, while larger systems are feasible using direct coupled cluster methods. The more rigorous scheme, W2 (Weizmann-2) theory, contains no empirical parameters at all and yields a mean absolute error of 0.23 kcal/mol, which is lowered to 0.18 kcal/mol for molecules dominated by dynamical correlation. It involves CCSD calculations in spdfg and spdfgh basis sets and CCSD(T) calculations in spdf and spdfg basis sets. On workstation computers, molecules with up to three heavy atoms can be treated using conventional coupled cluster algorithms, while larger systems can still be treated using a direct CCSD code. Both schemes include corrections for scalar relativistic effects, which are found to be vital for accurate results on second-row compounds. (C) 1999 American Institute of Physics. [S0021-9606(99)30429-3].

Abstract: A benchmark ab initio study on the thermochemistry of the trans-HNNH, cis-HNNH, and H2NN isomers of diazene has been carried out using the CCSD(T) coupled cluster method, basis sets as large as [7s6p5d4f3g2h/5s4p3d2flg], and extrapolations towards the I-particle basis set limit. The effects on inner shell correlation and of anharmonicity in the zero-point energy were taken into account: thus accurate geometries and anharmonic force fields were obtained as by-products. The best computed Delta H(f0)degrees for trans-HNNH, 49.2 +/- 0.3 kcal mol(-1), is in very good agreement with a recent experimental lower limit of 48.8 +/- 0.5 kcal mol(-1) CCSD(T) basis set limit values for the isomerization energies at 0 K are 5.2 +/- 0.2 kcal mol(-1) (cis-trans) and 24.1 +/- 0.2 kcal mol(-1) (iso-trans). Our best computed geometry for trans- HNNH, r(e)(NN)= 1.2468 Angstrom, r(e)(NH) = 1.0283 Angstrom, and theta(e) = 106.17 degrees, reproduces the precisely known ground state rotational constants of trans-HNNH to within better than 0.1%. The rotation-vibration spectra of both cis-HNNH and H2NN are dominated by very strong Coriolis and Fermi resonances. In addition, the NH stretches in H2NN are so strongly anharmonic that vibrational perturbation theory breaks down, and the molecule appears to be an excellent test case for variational treatments of the vibrational Schrodinger equation.

Abstract: Extensive ab initio calibration calculations combined with infinite-basis limit extrapolations lead to a ground-state dissociation energy of Be-2, D-e = 944 +/- 25 cm(-1), substantially higher than the accepted experimental value. Our best computed spectroscopic observables (experimental values in parentheses) are G(1) - G(0)= 223.7 (223.8), G(2)- G(1)= 173.8(169 +/- 3), G(3)- G(2) = 125.4(122 +/- 3), and Bo = 0.6086 (0.609) cm(-1) revised spectroscopic constants are proposed. Multireference calculations based on a full valence CAS(4/8) reference space suffer from unbalanced description of angular correlation; for higher accuracy, the (3s,3p) orbitals should be added to the reference space. The quality of coupled-cluster results depends crucially on the description of connected triple excitations. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: A benchmark ab initio and density-functional theory (DFT) study has been carried out on the electron affinities of the first- and second-row atoms. The ab initio study involves basis sets of spdfgh and spdfghi quality, extrapolations to the one-particle basis set limit, and a combination of the coupled cluster with all single, double land triple) excitations [CCSD(T)], CCSDT, and full configuration-interaction electron correlation methods. Scalar relativistic and spin-orbit coupling effects were taken into account. On average, the best ab initio results agree to better than 0.001 eV with the most recent experimental results. Correcting for imperfections in the CCSD(T) method improves the mean absolute error by an order of magnitude, while for accurate results on the second-row atoms inclusion of relativistic corrections is essential. The latter are significantly overestimated at the self-consistent-held level; for accurate spin-orbit splitting constants of second-row atoms, inclusion of 2s,2p correlation is essential. In the DFT calculations it is found that results for the first-row atoms are very sensitive to the exchange functional, while those for second-row atoms are rather more sensitive to the correlation functional. While the Lee-Yang-Parr (LYP) correlation functional works best for first-row atoms, its PW91 counterpart appears to be preferable for second-row atoms. Among "pure DFT" (nonhybrid) functionals, G96PW91 (Gill 1996 exchange combined with Perdew-Wang 1991 correlation) puts in the best overall performance, actually slightly better than the popular hybrid B3LYP functional. B3PW91 outperforms B3LYP, while the recently proposed one-parameter hybrid functionals such as B1LYP seem clearly superior to B3LYP and B3PW91 for first-row atoms. The best results overall are obtained with the one-parameter hybrid modified Perdew-Wang (mPW1) exchange functionals of Adamo and Barone [J. Chem. Phys. 108, 664 (1998)], with mPW1LYp yielding the best results for first-row, and mPW1PW91 for second-row atoms. Indications are that a hybrid of the type a mPW1LYP+(1 - a) mPW1PW91 yields better results than either of the constituent functionals. [S1050-2947(99)10908-9].

Abstract: The energetics of isomerization between the azulene and naphthalene radical cations have been investigated using the hybrid density functional method B3LYP with the cc-pVDZ basis sets. CCSD/cc-pVDZ energy Calculations were also carried out for selected points along the reaction coordinate. The transition state barrier energies for isomerization are lower than the dissociation limit of C8H6.+' (benzocyclobutadiene(.+)) + C2H2 deduced earlier. A key intermediate is a hydrogen shifted naphthalene isomer analogous to the intermediate suggested in the Dewar-Becker isomerization mechanism for neutral azulene. The norcaradiene isomer of the Dewar-Becker mechanism was found to be a transition structure in the ionic system. Results of the present density functional theory (DFT) and coupled cluster calculations are discussed in the light of recent experimental evidence.

Abstract: From large basis set coupled cluster calculations and a minor empirical adjustment, an anharmonic force field for silane has been derived that is consistently of spectroscopic quality (+/-1 cm(-1) on vibrational fundamentals) for all isotopomers of silane studied. Inner-shell polarization functions have an appreciable effect on computed properties and even on anharmonic corrections. From large basis set coupled cluster calculations and extrapolations to the infinite-basis set limit, we obtain TAE(0) = 303.80 +/- 0.18 kcal mol(-1), which includes an anharmonic zero-point energy (19.59 kcal mol(-1)), inner-shell correlation (-0.36 kcal mol(-1)), scalar relativistic corrections (-0.70 kcal mol(-1)) and atomic spin-orbit corrections (-0.43 kcal mol(-1)). In combination with the recently revised Delta H<((f))over circle>,(0)[Si(g)], we obtain Delta H<((f))over circle>,(0)[SiH4(g)] = 9.9 +/- 0.4 kcal mol(-1) in between the two established experimental values.

Abstract: The one-electron Douglas-Kroll approach, and perturbation theory including only the mass-velocity and Darwin terms, are used to compute the scalar relativistic contribution to the atomization energies of BS. Both approaches predict an approximately 0.7 kcal/mol reduction in the atomization energy. In combination with improved one-particle extrapolation techniques, this leads to a revised estimate for the heat of formation of gaseous boron, Delta H(f,0)degrees[B(g)] = 135.1 kcal/mol and Delta H(f,298)degrees[B(g)] = 136.3 kcal/mol, with error bars +/-0.75 kcal/mol or less.

Abstract: Calibration ab initio (direct coupled-cluster) calculations including basis set extrapolation, relativistic effects, inner-shell correlation, and an anharmonic zero-point energy predict the total atomization energy at 0 K of SO, to be 335.96 (observed 335.92 +/- 0.19) kcal/mol. Inner polarization functions make large (40 kcal/mol with spd, 10 kcal/mol with spdfg basis sets) contributions to the SCF part of the binding energy. The molecule presents an unusual hurdle for less computationally intensive theoretical thermochemistry methods and is proposed as a benchmark for them. A slight modification of Weizmann-1 (W1) theory is proposed that appears to improve performance significantly for second-row compounds.(C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: The geometry of crambin, a protein with 46 residues, was determined by ab initio HF/4-21G geometry optimization. The results are compared with the crystal structure of the compound and with HF/4-21G phi, psi-conformational geometry maps calculated for the model dipeptide N-acetyl-N'-methylalaninamide. Root-mean-square (rms) deviations between calculated and crystallographic backbone structural parameters are 1.5 degrees for N-C(alpha)-C' and 0.013 and 0.017 Angstrom, respectively, for N-C(alpha) and C(alpha)-C'. In the case of N-C(alpha)-C' the rms deviations are small compared to the observed range of values, which is from <108 degrees to >118 degrees, confirming a definite conformational dependence of peptide backbone structural parameters on phi and psi. In contrast, the deviations in bond lengths are of the same magnitude as the overall variations. The considerable nonplanarity of the peptide units found in the crystal structure is well reproduced by the calculations. When the calculated and crystal structures are superimposed, the rms positional deviation is 0.6 Angstrom for the heavy atom framework and 0.4 Angstrom for the backbone chain. The phenomenon of helix compression is confirmed that is found in elongated helical chains compared to isolated residues or smaller oligomers.

Abstract: The heat of formation, geometry, harmonic frequencies, and quartic force field of the fluoroacetylene (FCCH) molecule have been computed using coupled cluster methods and large basis sets. Inner-shell correlation was included explicitly, as were both cubic and quartic resonances, Our final force field, which does not involve any empirical adjustment, reproduces 36 vibrational band origins with a mean absolute error of 1.6 cm(-1). The bending anharmonicities exhibit a marked basis set sensitivity, due almost entirely to the CCF bend. Evidence was found for the existence of an additional quartic resonance, v(2) approximate to v(3) 2v(4). OUT best estimates, with conservative uncertainties, for the heat of formation and the geometry are 25.3 +/- 0.3 kcal/mol and r(e)(CF) = 1.2768 +/- 0.0005, r(e)(CC) = 1.1964 +/- 0.0005, and r(e)(CH) = 1.0601 +/- 0.0005 Angstrom.

Abstract: In order to assess the relative importance of the intrinsic error in high-level conventional ab initio methods and errors inherent in the Born-Oppenheimer approximation, benchmark quality potential curves and spectroscopic constants have been computed for the light diatomic hydrides H-2, LiH, BeH, and BH. The intrinsic error is found to be comparable to or smaller than the nonadiabatic effects. Revised spectroscopic constants are proposed for BeH. BH is predicted to have an unusually large nonadiabatic contribution of about 0.0025 Angstrom to its bond length. Previous arguments favoring the use of atomic rather than nuclear masses are further corroborated. (C) 1998 Elsevier Science B.V.

Abstract: The quartic force field of acetylene was determined using the CCSD(T) method (coupled cluster with all single and double substitutions and quasiperturbative inclusion of connected triple excitations) with a variety of one-particle basis sets of the atomic natural orbital, correlation consistent, and augmented correlation consistent types. The harmonic pi(g) bending frequency omega(4) and the corresponding anharmonicity omega(4)-nu(4) are both found to be extremely sensitive to the basis set used, in particular to the presence of a sufficient complement of diffuse functions, (Due to symmetry cancellation, the corresponding effect on the pi(u) mode, i.e., omega(5) and omega(5)-nu(5), is much weaker.) Similar phenomena are observed more generally in bending modes for molecules that possess carbon-carbon multiple bonds. Tentative explanations are advanced, Our best computed quartic force field, which combines CCSD(T)/[6s5p4d3f2g/4s3p2d1f] anharmonicities with a geometry and harmonic frequencies that additionally include inner-shell correlation effects, reproduces the observed fundamentals for HCCH, HCCD, DCCD, (HCCH)-C-13, and (HCCH)-C-13-C-13 with a mean absolute error of 1.3 cm(-1), and the equilibrium rotational constant to four decimal places, without any empirical adjustment. Anharmonicity and quartic resonance constants are in excellent agreement with the recent determination of Temsamani and Herman [J. Chem. Phys. 103, 6371 (1995)], except for the vibrational l-doubling constant R-45, for which an adjustment to the computed force field is proposed. (C) 1998 American Institute of Physics.

Abstract: The energetics, structure, and anharmonic force fields of the isomers H2SiO, cis-HSiOH, and trans-HSiOH have been studied using coupled cluster methods and basis sets of up to spdfgh quality;and including inner-shell correlation. Inner polarization functions on Si were found to be of crucial importance for the correct reproduction of computed properties. At the basis set limit at 0 K, the H2SiO isomer is more stable than the two others by about 0.5 kcal/mol. cis-HSiOH is marginally more stable than trans-HSiOH (0.05 kcal/mol); deuteration shifts the equilibrium toward the trans form. The computed fundamentals for the (H,D) and (O-16,O-18) isotopomers closely track the experimental ones in an argon matrix, except for a few reassignments which impose themselves. Our best computed geometry for H2SiO, r(SiH) 1.4733 Angstrom, r(SiO) = 1.5140 Angstrom, and theta(HSiH) = 111.97 degrees, agrees within experimental error with the most recent determination. Using a recently proposed Schwartz-type extrapolation, we obtain the following Sigma D-0 values (including core correlation, atomic spin-orbit splitting, and anharmonic zero-point energy): SiO 189.9, SiH2 (a B-3(1)) 124.5 kcal/mol, SiH2 (X (1)A(1)) 145.80 kcal/mol, and H2SiO 292.6 kcal/mol, which we expect to be accurate to about 0.3 kcal/mol. The former three agree with experiment to within the latter's uncertainty but have smaller error bars. As a byproduct, we obtain a basis set limit for the singlet-triplet splitting in silylene, T-0 = 21.35 +/- 0.1 kcal/mol. To assist future gas-phase infrared and microwave studies on silanone and hydroxysilylene, computed anharmonic spectroscopic constants for all species concerned have been presented, and complete anharmonic force fields are available as Supporting Information.

Abstract: To determine the heat of formation of boron atom, a fundamental parameter in gas-phase thermochemistry, from a thermochemical cycle involving BS, the total atomization energy (TAE(0), Sigma D-0) of this molecule was accurately determined ab initio in an exhaustive convergence study using coupled cluster [CCSD(T)] methods. Basis sets up to [8s7p6d5f4g3h2i] quality have been considered, and an extrapolation for further basis set incompleteness was applied. Inner-shell correlation, anharmonicity in the zero-point energy, and atomic spinorbit splitting have all been taken into account. Our best computed TAE(0) for BF3, 462.6 +/- 0.3 kcal/mol, leads to a best heat of formation for gaseous boron at 0 K of 136.0 +/- 0.4 kcal/mol, in excellent agreement with an experimental determination of 136.2 +/- 0.2 kcal/mol and definitively confirming recent suggestions that the established reference value, 132.7 +/- 3.0 kcal/mol, should be revised. This revision will affect most known gas-phase thermochemical data for boron compounds. As a byproduct, we obtain a dissociation energy D-0 of 180.13 +/- 0.2 kcal/mol for the BF diatomic, in perfect agreement with experiment but with a much smaller uncertainty.

Abstract: The potential energy curves of the SiN radical for the low-lying electronic states correlating up to the fourth dissociation channel have been calculated at the internally contracted multireference configuration interaction (CMRCI) level with Dunning's correlation-consistent polarization valence triple zeta (cc-pVTZ) basis set. The equilibrium bond length (r(e)), harmonic frequency (w(e)), first-and second-order anharmonicity constants (w(e)x(e), w(e)y(e)), rotational constant (B-e), centrifugal distortion constant (D-e), and rotation-vibrational and centrifugal coupling constants (alpha(e) and beta(e) respectively), as well as the dipole moment (mu(0)), excitation (T-e), and dissociation energies (D-0) for the 11 bound doublet states X-2 Sigma(+), A(2) Pi, B-2 Sigma(+), C-2 Pi, D-2 Sigma(-), E-2 Delta, F-2 Pi, G(2) Delta, H-2 Sigma(+), I-2 Phi, and J(2) Pi and 10 bound quartet states a(4) Sigma(+), b(4) Pi, c(4) Delta, d(4) Sigma(-), e(4) Sigma(+), f(4) Sigma(-), h(4) Delta, i(4) Sigma(+), j(4) Pi, and k(4) Delta have been calculated at the CMRCI level with Dunning's correlation-consistent polarization valence quadruple zeta (cc-pVQZ) basis set. Our calculations imply that the D-2 Pi and L-2 Pi states in the literature should be reassigned as C-2 Pi and F-2 Pi, respectively. Where available, our results are found to be in good agreement with the experimental data. (C) 1998 Academic Press.

Abstract: Using sequences of Dunning's correlation consistent basis sets, cc-pVnZ (n = 3,4,5), convergence of molecular properties is much slower for second-row compounds than for their first-row counterparts, both at the Hartree-Fock and at correlated levels, due to core polarization effects. By adding a single high-exponent d function to the standard cc-pVnZ basis sets, convergence is greatly accelerated. After correcting for core correlation, computed D-0, r(e), and omega(e) values for a number of diatomics generally agree with experiment to better than 0.02 eV, 0.001 Angstrom, and 5 cm(-1), respectively. (C) 1998 Elsevier Science B.V.

Abstract: Reaction of the ligand 1,3-bis((di-tert-butylphosphino)methyl)benzene (la) with the [RhCO](+) fragment in THF resulted in clean formation of the crystallographically characterized bis-chelated complex 2a which contains an eta(2) agostic Rh C-H bond. Both the NMR data and the X-ray crystal structure show strong interaction between the metal center and the agostic C-H bond, which results in high acidity of the agostic proton. Reaction of 2a with a weak organic base (NEt3 Or collidine) affords the known cyclometalated complex 3. Reaction of the new ligand 1,3-bis((di-tert-butylphosphino)methyl)-4,5,6-trimethoxybenzene (Ib) with the [RhCO](+) fragment in THF gives the analogous to 2a agostic complex 2b. Analysis of the NMR data and the reactivity of both 2a and 2b showed that there is very little, if any, contribution of a metal arenium structure. This interpretation is supported by B3LYP/LANL2DZ density functional calculations on model compounds. Thus, deprotonation of eta(2) aromatic C-H agostic complexes can be proposed as an alternative route,:to electrophilic metalation of aromatic compounds.

Abstract: The binding energy of various Pd and Pt complexes to silanones has been studied using the B3LYP density functional method and relativistic effective core potentials. Pt complexes systematically show stronger binding to silanones than their Pd counterparts, particularly with bis-diphosphinoethane ligands. Electron withdrawing substituents on the silanone lead to stronger binding. The metal-silanone interaction appears to be a pure three-center bond, with electron donation from metal to silicon and back-donation from oxygen to metal. The (CF3)(2)SiOPt(DMPE) complex appears to be a good synthetic target for the generation of protected silanones. (C) 1998 Elsevier Science B.V. All right reserved.

Abstract: A calibration study has been carried out on the ground-state potential curves of CH, NH, OH and HF. Using uncontracted spdfgh or spdfghi basis sets in conjunction with treatments of core correlation and imperfections in the CCSD(T) method, r(e) and omega(e) of the first-row diatomic hydrides can on average be reproduced to within 0.0002 Angstrom and 1 cm(-1), respectively, as can the first 4-6 vibrational quanta. The CCSD(T) basis set limit systematically overestimates omega(e) by 4-9 cm(-1) and underestimates r(e) by 0.0003-0.0006 Angstrom. Higher anharmonicities exhibit pronounced basis set sensitivity, which is markedly reduced upon uncontracting the basis set. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract: To estimate the exact amount of the loss of material during the plasma disruption or during slow transients is critically important to design and component lifetime analysis. Over the years, a number of simulation experiments, using as heat sources electron beams, and laser beams on investigation of the influence of heat loads on the amount of sublimated carbon materials were carried out. In addition, computer programs and analytical techniques have been developed to estimate the toss of material from sublimation. For purpose of component life time assessment and comparison between theoretical calculation and experimental results, a detailed knowledge on sublimation behaviours of carbon materials at high temperature has therefore become indispensable. The sublimation behaviour of carbon at high temperature is a complex, because of the formation of stable carbon clusters. At present, in most of the literature, only the species C-1, C-2, C-3 and C-4 were taken into account to assess the sublimation rates. In this work, based on theoretical investigation, an analysis of the formation of larger clusters C-5, C-6, C-7, C-8, C-9 and C-10, as a function of temperature were performed and the total rate of sublimation of carbon materials was deduced. The consequence of this results on rates of carbon sublimation at elevated temperature is discussed. reserved.

Abstract: The association reactions undergone by 12-crown-4-ether, 12c4H(+), with NH3, CH3OH, CH3NH2, (CH3)(2)NH, and (CH3)(3)N have been studied using the B3LYP density functional method and a variety of basis sets. For comparison purposes the insertion reactions for the same bases into protonated dimethoxyethane ("glyme"), Gl.H+, and protonated glyme dimer, (Gl)(2)H+, have also been modeled. The B3LYP/aug-cc-pVDZ//B3LYP/4-21G(*) level of theory was found to be a particularly favorable compromise between accuracy and computational expense for the calculation of proton affinities of medium-sized species. The protonated glyme, Gl.H+ the protonated glyme dimer, (Gl)(2)H+, and the protonated crown ether, 12c4H(+), form two internal hydrogen bonds with NH3, CH3OH, CH3NH2, and (CH3)(2)NH, except for (Gl)(2)H+. NH3 which has four O ... H bonds. In Gl.NH(CH3)(3)(+), there is a single O ... H bond and the protons of the methyl groups assist weakly in O ... HC bonding. The insertion energy of methanol, ammonia, and the series of amines into 12c4H(+) increases with increasing proton affinity of the inserting base. A similar trend is observed for insertion into (Gl)(2)H+. Trimethylamine does not follow the expected trend because it forms proton-bound complexes that have a single O ... HN bond instead of two. The association energy of CH3OH2+, NH4+, etc., with 12c4 or Gl(2) decreases with increasing proton affinity (of methanol, ammonia, etc.).

Abstract: In an attempt to design a transition-metal complex capable of stabilizing an Si=O bond, the interaction of various Rh/lr complexes with silanones was investigated using the B3LYP density functional method and relativistic effective core potentials. Ir complexes are systematically more strongly bound than their Rh counterparts. Contrary to Pd/Pt complexes, phosphine chelate ligands do not improve bonding, while a strong trans effect exists. Ir(trans-(PH3)(2))Cl((CF3)(2)SiO) is calculated to exhibit strong three-center M-Si-O binding, while a CO ligand trans to the oxygen leads to an almost linear two-center M-Si=O binding, which is calculated to be strongest in Ir(trans-(PH3)(2))(CO)(iPr(2)SiO). (C) 1998 Elsevier Science B.V. Ail rights reserved.

Abstract: The total atomization energies, geometries, and anharmonic force fields of the SO and SO2 molecules have been studied at the augmented coupled cluster [CCSD(T)] level near the one-particle basis set limit. The effect of core correlation has been accounted for. The addition of high-exponent d and f "inner polarization functions" to the sulfur basis set was found to be essential for obtaining reliable molecular geometries. The differential effect of core correlation on computed properties is in fact much less important. The CCSD(T) one-particle basis set limit for the symmetric stretching frequencies appears to be higher than the exact value, while the antisymmetric stretching frequency benefits from an error cancellation. The basis set extension effects of diffuse functions and inner polarization functions appear to be nearly perfectly additive. Our best computed atomization energies and geometries agree to within 0.1 kcal/mol, 0.0004 Angstrom, and 0.03 degrees with experiment: The errors in the fundamentals of SO2 are +3.9, -0.4, and +0.4 cm(-1). The best computed quartic force field for SO2 should provide a good starting point for a new experimental refinement. (C) 1998 American Institute of.Physics.

Abstract: Successive coupled-cluster [CCSD(T)] calculations in basis sets of spdf, spdfg, and spdfgh quality, combined with separate Schwartz-type extrapolations A + B/(l + 1/2)(alpha) of the self-consistent field (SCF) and correlation energies, permit the calculations of molecular total atomization energies (TAEs) with a mean absolute error of as low as 0.12 kcal/mol. For the largest molecule treated, C2H4, we find Sigma D-0=532.0 kcal/mol, in perfect agreement with experiment. The aug'-cc-pV5Z basis set recovers on average about 99% of the valence correlation contribution to the TAE, and essentially the entire SCF contribution. (C) 1997 American Institute of Physics.

Abstract: Unimolecular fragmentations of the 9-bromoanthracene radical cation were studied by time-resolved photoionization mass spectrometry (TPIMS) in the vacuum-UV and by RRKM/QET calculation. The vibrational frequencies required in the RRKM calculation were obtained by density functional calculations. The major reactions observed are Br . loss and consecutive H . loss from C14H9+. Appearance energies (AEs) were determined for the microsecond and millisecond time ranges. Activation parameters were deduced for the reactions. The Br loss reaction is characterized by a loose transition state. The following heats of formation were deduced: Delta H(f0)degrees(C14H9+ anthracenyl) = 288.8 +/- 3 kcal mol(-1), Delta H(f0)degrees(C14H8 .(+)) less than or equal to 297.2 +/- 3 kcal mol(-1). The C-Br bond energy in the bromoanthracene radical cation is 3.30 +/- 0.08 eV. The effect of the ring system size on the bond energies of polycyclic aromatic hydrocarbons (PAHs) is discussed. (C) 1997 Elsevier Science B.V.

Abstract: Insertion complexes of various bases with the protonated acetonitrile and acetone dimers have been studied using density functional methods including exact exchange contributions. The insertion mechanism has been investigated using an intrinsic reaction coordinate calculation. For some thermochemical quantities, calibration studies using larger basis sets and coupled cluster methods have been carried out. We find that B3LYP/cc-pVDZ will somewhat overestimate association energies due to basis set incompleteness error, which is partially compensated by an opposite error in the correlation treatment. B3LYP/4-21G will yield qualitatively correct structures, which PM3 and HF/4-21G generally do not, yielding instead asymmetric insertion complexes. The insertion energy increases with increasing proton affinity of the inserting base, while the association energy between the protonated central base and the ligands decreases. For the insertion complexes of the acetone dimer, the conformational equilibrium shifts from syn-syn to syn-anti with increasing proton affinity. The r(e) geometries of protonated acetone dimer and its complexes are found to exhibit slight deviations from their intuitive symmetry that the calculations predict will be absent in the ro geometries. Computed association and switching enthalpies are in very good agreement with experiment, while proton transfer enthalpies fare less well due to the change in hydrogen bond number involved. Geometries and vibrational frequencies for all structures considered are available as Supporting Information to the paper.

Abstract: The energetics of acetylene loss from anthracene and phenanthrene radical cations and the relative stabilities of the six possible C12H8.+ fragments have been investigated using the hybrid density functional methods B3LYP and B3PW91. Isodesmic reactions involving the well-known fragmentation of the benzene radical cation were employed to derive the dissociation energies for the fragmentations of naphthalene, anthracene, and phenanthrene radical cations. CCSD(T) calculations were also carried out for the acetylene loss from benzene and naphthalene radical cations. The B3LYP and B3PW91 methods appear to bracket the CCSD(T) result. The result for the naphthalene radical cation agrees with the previous suggestion that the benzocyclobutadiene radical cation is the most plausible product of acetylene loss (Chem. Phys. 1995, 191, 165). Computational results for anthracene and phenanthrene will be discussed in the light of experimental data. Acetylene losses from anthracene and phenanthrene radical cations will be shown to form the same fragment, biphenylene(.+), which indicates that the isomerization barrier between the two isomers is lower than the dissociation limits. Scaled B3LYP/cc-pVDZ vibrational frequencies for naphthalene, anthracene, and phenanthrene radical cations are compared with available experimental data. Frequencies at the same level of theory are provided for C4H4.+, C8H6.+, and C(12)H8(.+) isomers as well as for the benzene radical cation and should facilitate further experimental work.

Abstract: The quartic force field of trans-HNNH (trans-diazene, Irans-diimide) has been computed ab initio at the CCSD(T)/cc-pVTZ level. A comprehensive set of formulas for 1-1, 2-2, and 1-3 resonance constants of asymmetric tops in the contact transformed vibrational Hamiltonian is presented, and an automatic procedure for setting up and solving the relevant resonance eigensystems is presented. Resonance polyads are found to be very important in trans-diazene: the fundamentals of HNNH and DNND each involve both a resonance tetrad and a triad, while a triad plus a separate Fermi type 1 resonance exist in HNND. For HNNH, HNND, and DNND, agreement be,tn:een computed and observed fundamentals is within the customary accuracy of 10 cm-l or better at this level of theory, except for the NN stretch where significant basis set and core correlation effects make for a slightly larger error. The experimentally proposed nu(1) value of 3128 cm(-1) is a misassignment and actually corresponds to nu(2) + nu(3), although strong mixing between these two levels and 2 nu(3) makes the distinction largely semantic. Performance of the resonance treatment is found to be quite satisfactory. (C) 1997 Elsevier Science B.V.

Abstract: Ab initio basis-set-limit total atomization energies Sigma D-0 have been calculated for the first-row hydrides and hydride radicals. Agreement to within about 0.1 kcal/mol with experiment is found for those species where experimental data of this accuracy are available. For the other species, the following new or revised Sigma D-0 values are proposed: BeH 47.7, BeH2 140.9, BH 81.5, BH2 160.8, BH3 264.6, CH2 179.9, NH 78.4, NH2 170.7, and OH 101.7 kcal/mol. These values are expected to be correct to 0.2 kcal/mol or better. (C) 1997 Published by Elsevier Science B.V.

Abstract: A number of analogs of the photosensitive molecule TMAE (tetrakis-dimethylamino-ethylene) have been investigated computationally with the goal of lowering the vertical ionization potential (IPv). This is of importance in UV-photodetectors based on gas photoionization and multiplication. The low IPv of TMAE, 6.11 eV, is shown to be due to resonance stabilization of the cation. N-substitution of phenyl groups and p-substitution of tertiary amines on the latter are found to be efficient ways of lowering IPv. The compound trans-bis-(p-dimethylaminophenyl, methyl)aminoethylene is calculated to have a IPv considerably lower than TMAE (similar to 5.5-5.7 eV) and appears to be a promising synthetic target. (C) 1997 Elsevier Science B.V.

Abstract: The coupling between improvement of the basis set and the valence electron correlation method has been studied quantitatively for the total atomization energies (TAEs) of a number of small molecules, using basis sets of up to [7s6p5d4f3g2h/5s4p3d2f1g] quality. Very significant coupling is found to exist. Using a scaled basis set extrapolation beyond [6s5p4d3f2g/4s3p2d1f] at the MP2 or CCSD level, mean absolute errors of 0.18 and 0.15 kcal/mol, respectively, can be obtained for the TAEs of a number of small polyatomic molecules, compared to 0.12 kcal/mol using CCSD(T) throughout.

Abstract: Energetically low-lying states of the C2P and CNP molecular systems and their cations are studied. Geometries, harmonic frequencies, and dipole moments have been obtained at the density functional (using the Becke three-parameter Lee-Yang-Parr B3LYP functional) and coupled cluster theoretical levels in conjunction with correlation consistent basis sets. Single-point coupled cluster calculations for B3LYP geometries have been used to compute energy differences for the different states. For the C2P radical, the global minimum is linear CCP((2) Pi) followed by cyclic C2P(B-2(2)) at 5.5 kcal/mol, linear CCP((4) Sigma(-)) (21.2 kcal/mol), cyclic C2P((4)A(2)) (38.4 kcal/mol), and linear CPC((2) Pi(g)) (88.0 kcal/mol). For the C2P+ cation the following sequence of states has been established: linear CCP+(1 Sigma(+)), cyclic C2P((1)A(1)) (8.7 kcal/mol), cyclic C2P+(B-3(2)) (24.1 kcal/mol), linear CCP+((3) Pi) (29.1 kcal/mol), and linear CPC+((1) Sigma(g)(+)) (104.3 kcal/mol). For neutral PCN and PNC, the following sequence of states has been computed: linear PCN((3) Sigma(-)); linear PNC((3) Sigma(-)) (14.2 kcal/mol), linear PCN(1 Delta) (25.2 kcal/mol), cyclic CPN((3)A'') (36.2 kcal/mol), cyclic CPN((1)A'') (37.7 kcal/mol), and linear PNC(1 Delta) (38.8 kcal/mol). The ground state of the single ionized species is linear PNC+((2) Pi) followed by linear PCN+((2) Pi); the energy difference is very small: 0.8-1.4 kcal/mol. Cyclic CPN+((2)A') lies much higher in energy (13.9 kcal/mol) and was found to be a local energy minimum at the doublet surface. Predictions for IR spectra of the species are given. The computed total atomization energies are 261.7 +/- 1 kcal/mol for CCP((2) Pi) and 276.0 +/- 1 kcal/mol for PCN((3) Sigma(-)). Adiabatic ionization potentials (eV) areas follows: linear CCP((2) Pi) 9.05, cyclic C2P(B-2(2)) 9.18, linear PCN((3) Sigma(-)) 10.39, and linear PNC((3) Sigma(-)) 9.73.

Abstract: The experimentally very accurately known total atomization energies (TAEs; Sigma D-e values) of 14 small polyatomic molecules have been computed at the CCSD(T) level using basis sets of up to [7s6p5d4f3g2h/5s4p3d2flg] quality. Core correlation was corrected for through separate calculations using a special core-correlation basis set. A comparative study of extrapolations to the infinite-basis set limit and empirical correction treatments for basis set incompleteness was made. Up to basis sets of [spdfg] quality, the Martin 3-parameter empirical correction formula significantly outperforms extrapolation techniques based on the asymptotic l-dependence of the correlation energy: for still larger basis sets, the two approaches yield comparable results. The effect of core correlation can be fairly well absorbed in the parameters of the correction, but for very large valence basis sets, it becomes worthwhile to treat it explicitly. 3-parameter-corrected CCSD(T)/aug'-cc-pVQZ calculations represent a particularly good compromise between computational cost and accuracy (mean absolute error 0.23 kcal mol(-1); maximum absolute error 0.63 kcal mol(-1)). (C) 1997 Elsevier Science B.V.

Abstract: The geometry and harmonic force field of benzene have been computed at the CCSD(T) level with basis sets of spdf quality. Two out-of-plane modes, omega(4) and omega(5) (and to a lesser extent omega(17)), exhibit a pathological basis set dependence due to basis set superposition error. Using an atomic natural orbital (ANO) basis set of [4s3p2d1f/4s2p] quality, the best available experimentally derived harmonic frequencies can be reproduced with an RMS error of 6 cm(-1) without any empirical corrections. We strongly recommend the use of ANO basis sets for accurate frequency calculations on unsaturated and aromatic systems. Our best estimate for the equilibrium geometry is r(e)(CC)=1.392(2), r(e)(CH)=1.081 Angstrom. (C) 1997 Published by Elsevier Science B.V.

Abstract: The vibrational spectrum of the linear carbon clusters C-2-C-9 and of cyclic C-4, C-6, C-8, and C-10 has been investigated using the augmented coupled-cluster, CCSD(T), method and correlation-consistent basis sets, Particular emphasis is placed on assignment of bands in matrix infrared spectra of trapped graphite vapor, The relative energetics of linear and cyclic isomers of C-6, C-8, and C-10 were investigated at the CCSD(T) level using basis sets of [4s3p2d1f] quality. Our best calculations predict that the ground state structure of C-6 is a cumulenic D-3h ring and that of C-8 a polyacetylenic C-4h ring. Linear (3) Sigma(g)(-) states are close in energy (13 +/- 2 and 8 +/- 2 kcal/mol, respectively), and both forms should be observable. The ground state structure of cyclic C-10 clearly has D-5h rather than D-10h symmetry, at least for the r(e) geometry. A linear regression of computed CCSD(T)/[3s2p1d] harmonic frequencies vs observed fundamentals for the previously assigned stretching frequencies of linear C-n clusters has a very high correlation coefficient (up to r = 0.99975 at 10 data points), From the regression line and the calculated frequencies, band origins for linear C-8 are predicted at 2063 +/- 18 and 1711 +/- 14 cm(-1) (99% confidence intervals) and ancillary bands for linear C-9 at 2093 +/- 18 and 1604 +/- 16 cm(-1). From the calculated infrared-active CCSD(T) frequencies for the cyclic clusters and anharmonicity considerations, we suggest that bands near 1695, 1818, and 1915 or 1921 cm(-1) belong to cyclic C-6, C-8, and C-10, respectively, while bands near 1590 and 1715 cm(-1) are probably due to linear C-9 and C-8, respectively. Finally, a broad feature at 2070-2090 cm(-1), which upon deconvolution appears to consist of three bands, is suggested to contain bands of both linear C-8 and C-9.

Abstract: The geometry and harmonic frequencies of linear and cyclic structures of C-5, C-7, C-9, C-11, C-13, and C-15 have been calculated using a hybrid density functional method (B3LYP), and the linear-cyclic isomerism considered using coupled cluster methods. It was established unambiguously that, from C-11 onwards, these molecules prefer cyclic structures. Structures with a blunt apical angle are considerably more stable than the elongated structures found at the MP2 level, which gives a qualitatively incorrect description of the potential surface. The B3PW9 1 (Becke 3-parameter-Perdew-Wang) functional appears to reproduce the CCSD(T) isomerization energies better than its B3LYP (Becke 3-parameter-Lee-Yang-Parr) counterpart. No indication appears to exist for the presence of either cyclic C-7 and C-9, or linear C-11 and C-13, in matrix-trapped graphite vapor.

Abstract: The geometry and vibrational spectra of pyridine, pyridazine, pyrimidine, pyrazine, s-triazine, 1,2,3-triazine, 1,2,4-triazine, and s-tetrazine have been computed using the B3LYP density functional method and correlation consistent basis sets of spd and spdf quality. Empirical corrections have been applied to the computed bond distances. Computed harmonic frequencies are in very good to excellent agreement with the available experimental data; the only real deficiency seen is for the Kekule-type vibration, which appears to have some multireference character. Significant basis set effects are seen for the Kekule modes and for ''pinwheel''-type hydrogen motions. Empirically corrected B3LYP/cc-pVDZ and B3LYP/cc-pVTZ bond distances are in essentially complete agreement for CH distances, excellent agreement (0.001 Angstrom) for CC distances, and good agreement (0.002 Angstrom) for CN distances, but differ markedly (0.003-4 Angstrom) for NN distances. Computed B3LYP/cc-VTZ dipole moments are in excellent agreement with the available experimental values (about 0.05-0.1 D too low). The calculations are found to be valuable in verifying a number of less certain experimental vibrational assignments.

Abstract: The quartic force field of cyclic C-4 has been computed using basis sets of spdf quality and augmented coupled cluster methods. The effect of core correlation and further basis set extension has been investigated. Vibrational energy levels have been obtained using perturbation theory and two different variational approaches. A severe Fermi resonance exists between the most intense vibration, nu(6), and nu(3) + nu(5) through an exceptionally large k(356) = -258.2 cm(-1); a large k(1356) = -54.8 cm(-1) causes significant higher-order anharmonicity, including a shift in nu(6) of +9 cm(-1). C-4 appears to be an excellent test case for methods for solving the vibrational Schrodinger equation, since perturbation theory breaks down even when the above resonances are accounted for. Our best estimate for nu(6), 1320 +/- 10 cm(-1), may suggest its assignment to a feature detected at 1284 cm(-1) in argon and 1302 cm(-1) in krypton matrix, but this would imply an unusually large matrix red shift in argon. (C) 1996 American Institute of Physics.

Abstract: The bowl, ring, and cage structures of C-20 have been studied using the B3LYP (Becke 3-parameter-Lee-Yang-Parr) method. Fingerprint-quality infrared spectra have been computed for all species. The computed geometries differ drastically from those obtained using the local density approximation (LDA). Combining coupled cluster results at LDA and SCF geometries with the present B3LYP results, and including zero-point energies, we find the bowl structure to be the lowest in energy, two distinct cage structures with C-i and C-2h symmetry both about 11 kcal/mol higher, and a C-10h ring about 47 kcal/mol higher. Statistical thermodynamical calculations reveal that the bowl will dominate up to about 1200 K, and the ring above 1900 K. Despite being much lower in energy than the ring, the cage structure is not present in appreciable proportion due to the extra vibrational entropy the ring derives from many extremely low-lying vibrations.

Abstract: The structure and harmonic vibrations of BnNn clusters (n = 3-10) have been investigated using density functional theory with exact-exchange contributions. All structures are found to be cumulenic D-nh rings (equal bonds, alternating angles), with one intense out-of-plane mode and three infrared-active degenerate modes, of which the top one is extremely intense and asymptotically goes to about 2040 cm(-1) for high n. SCF calculations perform relatively well (especially for n odd), contrary to the isoelectronic C-2n clusters. As n increases, the NBN angle approaches 180 degrees. The isoelectronic principle is seen to be followed only partially (odd n) for the similarity between BnNn and C-2n clusters.

Abstract: The equilibrium bond length, harmonic frequency, first and second order anharmonicity constants, rotational and centrifugal distortion constants, as well as the rotation-vibrational and centrifugal coupling constants for the ground X(2) Sigma(+) and first excited A(2) Pi states of the SiN radical have been calculated at the complete active space SCF (CASSCF), multireference CI (MRCI) and coupled cluster (CCSD(T)) levels using Dunning's correlation-consistent basis sets. The excitation energy of the A(2) Pi State has also been computed at these theoretical levels. Dipole moments of SiN in the X(2) Sigma(+) and A(2) Pi states are given. Our study shows that core correlation must be considered in order to obtain satisfactory accuracy for the spectroscopic constants.

Abstract: The anharmonic part of a recently calculated ab initio quartic force field for ethylene has been combined with geometries and harmonic frequencies at higher levels of theory, including expansion to spdfg basis sets and inclusion of core correlation. Resulting fundamentals and ground-state rotational constants have been compared with experiment, Our best estimate for the r(e) geometry is r(e)(CC) = 1.3307(3) Angstrom, r(e)(CH) = 1.0809(3) Angstrom, theta(e)(CCH) = 121.44(3)degrees, which reproduces the experimental rotational constants to 0.01%. The experimental fundamentals and main resonance partners are calculated with a mean absolute error of 2.3 cm(-1). Our best calculated total atomization energy, 531.7(5) kcal/mol, falls within the error bar of the experimental value 531.9(3) kcal/mol.

Abstract: Atomic populations according to the Mulliken, electrostatic, natural population, and atomic polar tensor (APT) definitions were evaluated for first- and second-row compounds using different correlated ab initio techniques, DFP methods, and basis sets. All definitions except Mulliken exhibit modest basis set sensitivity. B3LYP predicts partial charges in agreement with high-level ab initio results. Exact-exchange corrections are more important than gradient corrections for this property. B3LYP with at least sdpf basis sets usually predicts dipole moments and infrared intensities in agreement with more accurate calculations, while semiquantitative IR intensities are obtained even with the modest cc-pVDZ basis set.

Abstract: Several structures of C-24 have been studied using different density functional methods. Using the B3LYP (Becke 3-parameter Lee-Yang-Parr) method, fingerprint-quality infrared spectra have been computed for all species. The computed geometries differ substantially from those obtained at the SCF level due to delocalization effects. Both ''exact-exchange'' corrections to the exchange functional and the use of different correlation functionals have significant effects on the relative energetics. Our best calculations indicate that a dodecadehydrocoronene planar sheet and a fullerene cage are comparable in energy, followed by a polyacetylenic ring and a bowl structure, At high temperatures, the ring is expected to prevail due to vibrational entropy from many low-lying vibrations.

Abstract: Total atomization energies (TAEs) of about a dozen small polyatomic molecules have been calculated at the CCSD(T) level using correlation consistent basis sets of up to [7s6p5d4f3g2h] quality, and including core correlation. Single bond energies are close to convergence with such basis sets: for multiple bonds, extrapolation remains mandatory. 'Augmented' basis sets specifically designed for anions yield improved atomization energies for highly polar molecules. An extrapolation of the form A + B/(l + 1/2)(4) + C/(l + 1/2)(6), with l the maximum angular momentum in the basis set, is found to yield TAEs accurate, on average, to 0.5 kcal/mol using at most [spdfgh] basis sets. Using [spdfgh] basis sets and a small additivity correction for triple bonds, this can be reduced to 0.2 kcal/mol.

Abstract: The vibrational spectra of corannulene and coronene have been calculated using the B3LYP exchange-correlation functional and a basis of split-valence plus polarization quality. The vibrational assignments for these molecules have been reviewed and occasionally amended. Scaled B3LYP/cc-pVDZ frequencies are in excellent agreement with experimental data. B3LYP calculations with a basis set of only split-valence quality perform surprisingly well, suggesting that correlation effects on the vibrational frequencies of polycyclic aromatic hydrocarbons are dominated by internal and radial correlation, as distinct from angular correlation effects.

Abstract: The relative energies of different structures of C-28 have been studied by density functional theory using different exchange-correlation functionals, including two with exact exchange contributions. A fullerene structure with T-d symmetry ((5)A(2) state) is found to be by far the most stable structure, followed at 4-6 eV by a graphite sheet fragment and at 5-7 eV by a C-14h ring. Other structures lie still higher in energy, An infrared spectrum of fingerprint quality has been computed for the fullerene and should enable its experimental identification.

Abstract: An exhaustive ab initio convergence study, using coupled cluster methods and basis sets of up to [7s6p5d4f3g2h/5s4p3d2flg] quality, of the total atomization energy of HCN(g), both directly and via the isogyric reaction 2 HCN double left right arrow C2H2 + N-2, leads to a best estimate of Sigma D-0=303.08 +/- 0.25 kcal/mol, in between thermochemical and spectroscopic experimental values. The best estimate for Delta H-f(0) = 31.04 +/- 0.25 kcal/mol, or 129.9 +/- kJ/mol.

Abstract: The geometry and harmonic frequencies of naphthalene, azulene, phenanthrene, and anthracene have been computed using the Becke three-parameter Lee-Yang-Parr functional (B3LYP) and basis sets of spd and spdf quality. A simple scaling procedure for the harmonic frequencies is proposed that brings computed harmonics to within 10-20 cm(-1) or better of experimental fundamentals without resorting to internal coordinate transformations. A complete reassignment of the vibrational spectrum of phenanthrene is proposed, and several reassignments are due for anthracene as well. The assignments of Sellers, Pulay, and Boggs (J. Am. Chem. Sec. 1985, 107, 6487) for naphthalene, and of Kozlowski, Rauhut, and Pulay (J. Chern. Phys. 1995, 103, 5650) for azulene, are largely confirmed.

Abstract: The quartic force field of NH: has been computed using advanced coupled cluster methods and a basis set of spdf quality. Effects of further basis set extension and of the inclusion of core correlation on the geometry and harmonic frequencies have been investigated. Our best computed values are: r(e) = 1.02029 Angstrom, v(1) = 3236.6, v(2) = 1690.1, v(3) = 3345.1, and v(4) - 1447.2 cm(-1), in excellent agreement with the experimental data. Complete sets of fundamentals and anharmonicity constants are reported for NH3D+, NH2D2+ NHD3+, and ND4+.

Abstract: The performance of different density functional methods in the calculation of molecular electrostatic potentials and Fukui functions, i.e. two reactivity indices based on the electron density, is investigated. It turns out, as a whole, that the exact exchange functionals B3LYP and B3PW91 yield results close to accurate electron correlation methods if basis sets of sufficient quality are used.

Abstract: The total atomization energy and proton affinity of NH3 have been subjected to an extensive convergence study involving basis sets of up to spdfgh quality. Our best extrapolated Sigma D-o = 276.5 kcal/mol lies only 0.2 kcal/mol below the experimental value. Our recommended value for PA(298), 203.9+/-0.3 kcal/mol, is in excellent agreement with the most recent experimental value but has a smaller error margin. It is found that augmented basis sets are desirable when determining proton affinities using the empirical correction for further basis set expansion proposed in J. Chem. Phys. 97 (1992) 5012.

Abstract: The harmonic frequencies and infrared intensities of C-9, C-11 and C-13 have been calculated using SCF and complete active space SCF (CASSCF) methods. The ordering of the harmonic frequencies in C-9 is predicted wrongly unless at least the pi HOMO and LUMO are included in the active space. Infrared intensities depend crucially on the size of the active space. For linear odd-numbered clusters C-13 and larger, the computed SCF spectrum is qualitatively wrong. The recent observation of a band near 1809 cm(-1) in the gas phase is explained using our CASSCF results on C-13.

Abstract:

Abstract: The mixed boron-nitrogen clusters B3N3 and B4N4 have been studied using augmented coupled-cluster methods. Both molecules have monocyclic ground state structures (1A'1 and 1A(lg), respectively) with cumulenic-type equal bond lengths and alternating bond angles (BNB sharp, NBN blunt). The second most stable structures are linear BNBNBN(BN), with a 3PI ground state and alternating polyacetylenic-type bond lengths. B3N3 and B4N4 have intense infrared bands predicted to fall in the 1550-1650 and 1700-1800 cm-1 regions, respectively. Both B3N3 and B4N4 are stable, with SIGMAD(e) = 697.0 and 995.6 kcal/mol, respectively.

Abstract: The structures of B3N2, B2N3, and BN4 have been investigated using complete active space SCF (CASSCF) and augmented coupled cluster (CCSD(T)) methods. B3N2 has a symmetry-broken linear ground state structure BNBNB((2) Sigma(+)) and is very stable. B2N3 has a linear ground state structure BNBNN((II)-I-2) and is nearly isoenergetic with its (thermodynamically preferred but symmetry forbidden) dissociation products BNB((2) Sigma(+)) + N-2((1) Sigma(g)(+)). BN4 has the ground state structure NNBNN((IIu)-I-2) that is symmetry broken at the CCSD(T), but symmetric at the CASSCF level. Predictions for the infrared spectrum of the species involved have been given. The best computed total atomization energies Sigma D-e (error bar +/- 4 kcal mol(-1)) are: B3N2, 504.6 kcal mol(-1); B2N3, 479.1 kcal mol(-1); and BN4, 460.8 kcal mol(-1).

Abstract:

Abstract: The quartic force fields of the H2S, CS2, OCS, and CS molecules have been computed using the CCSD(T) augmented coupled cluster method and spdf and spdfg basis sets. Neglect of core correlation results in fairly noticeable error in computed geometries, but does not seem to affect other spectroscopic constants substantially. Computed results for H2S are in excellent agreement with experiment; of the available experimental force fields, our calculations favor the very recent one by Kozin and Jensen. Good agreement is also seen for CS2, except for the sigma(g) stretch. After correction for multiple Fermi resonances, excellent results are also obtained for OCS. Residual error in CS can be traced back almost entirely to overestimation of the bond distance. (C) 1995 Academic Press, Inc.

Abstract: The effect of core correlation on computed properties of a set of experimentally well studied first-row compounds has been investigated using augmented coupled cluster (CCSD(T)) methods and specially tailored one-particle basis sets. Core correlation accounts for virtually all the remaining error in bond lengths, but simple additivity corrections based on the bond order absorb essentially all such effects starting from [4s3p2d1f] basis sets. There are nontrivial effects on harmonic frequencies, but these do not lead to improved agreement with experiment due to an error compensation between neglect of core correlation and residual n-particle space inadequacies. Finally, while significant (up to 2.5 kcal/mol) effects on total atomization energies are seen, these are essentially completely absorbed in the three-term basis set incompleteness correction proposed by the author.

Abstract: The quartic force field of ethylene, C2H4, has been calculated ab initio using augmented coupled cluster, CCSD(T), methods and correlation consistent basis sets of spdf quality. For the C-12 isotopomers C2H4, C2H3D, H2CCD2, cis-C2H2D2, trans-C2H2D2, C2HD3, and C2D4, all fundamentals are reproduced to better than 10 cm(-1), except for three cases where the error is 11 cm(-1). Our calculated harmonic frequencies suggest a thorough revision of the accepted cm experimentally derived values. Our computed and empirically corrected r(e) geometry differs substantially from experimentally derived values: Both the predicted r(z) geometry and the ground-state rotational constants are, however, in excellent agreement with experiment, suggesting revision of the older values. Anharmonicity constants agree well with experiment for stretches, but differ substantially for stretch-bend interaction constants, due to equality constraints in the experimental analysis that do not hold. Improved criteria for detecting Fermi and Coriolis resonances are proposed and found to work well, contrary to the established method based on harmonic frequency differences that fails to detect several important resonances for C2H4 and its isotopomers. Surprisingly good results are obtained with a small spd basis at the CCSD(T) level. The well-documented strong basis set effect on the v(8) out-of-plane motion is present to a much lesser extent when correlation-optimized polarization functions are used. Complete sets of anharmonic, rovibrational coupling, and centrifugal distortion constants for the isotopomers are available as supplementary material to the paper via the World-Wide Web. (C) 1995 American Institute of Physics.

Abstract: The XCN and XNC (X = F, Cl) isomers have been investigated using the CCSD(T) method in conjunction with correlation consistent basis sets. Equilibrium geometries, harmonic frequencies, anharmonic constants, fundamental frequencies, and heats of formation have been evaluated. Agreement with experiment for the fundamental frequencies is very good, even for nu(2) for ClCN, which is subject to a strong Fermi resonance with 2 nu(3). It is also shown that a second-order perturbation theory approach to solving the nuclear Schrodinger equation gives results in excellent agreement with essentially exact variational calculations. This is true even for nu(2) Of ClCN, provided that near-singular terms are eliminated from the perturbation theory formulas and the appropriate Fermi interaction energy matrix is then diagonalized. A band at 615 cm(-1), tentatively assigned as the Cl-N stretch in ClNC in matrix isolation experiments, is shown not to be due to ClNC. Accurate atomization energies are determined and are used to evaluate accurate heats of formation (3.1 +/- 1.5, 33.2 +/- 1.5, 72.6 +/- 1.5, and 75.9 +/- 1.5 kcal/mol for FCN, ClCN, FNC, and ClNC, respectively). It is expected that the theoretical heats of formation for FCN, FNC, and ClNC are the most accurate available.

Abstract: The performance of the Becke three-parameter Lee-Yang-Parr (B3LYP) method for geometries and harmonic frequencies has been compared with other density functional methods and accurate coupled cluster calculations, and its basis set convergence investigated. In a basis of [3s2p1d] quality, B3LYP geometries are more accurate than CCSD(T) due to an error compensation. Using simple additivity corrections, B3LYP/[4s3p2d1f] calculations allow the prediction of geometries to within 0.002 Angstrom, on average. Except for certain special cases where frequencies are especially sensitive to the basis set, B3LYP/[4s3p2d1f] frequencies do not represent a clear improvement over B3LYP/[3s2p1d], while the latter are of nearly the same quality as CCSD(T)/[3s2p1d]. Applications to ethylene, benzene, furan and pyrrole are presented. For the latter three molecules, our best structures and harmonic frequencies are believed to be the most accurate computed values available.

Abstract: Pulsed laser ablated boron atoms were reacted with ammonia during condensation at 10 K with excess argon. Reaction products were identified from matrix infrared spectra by isotopic substitution (B-10, B-11 (NH3)-N-15, ND3), photolysis behavior, and comparison to ab initio calculated frequencies. The major primary reaction is insertion to form the [HBNH2]* intermediate, which either dissociates to BH and NH2, eliminates H or H-2 to give products, or relaxes in the matrix. The major primary product is iminoborane, HBNH, a reactive molecule isoelectronic with acetylene, and a minor primary product is the photosensitive BNH radical. A second boron atom can react with primary products, and the major secondary reaction products are cyclic B2N radical and linear BNBH. These studies show that atomic boron reacts readily with ammonia.

Abstract: Geometries and harmonic frequencies of linear C-n and cyclic C-2n (n = 2-9) clusters have been studied using the B3LYP (Becke 3-parameter-Lee-Yang-Parr) density functional method and compared with ab initio coupled cluster calculations. For C-2 through C-10, results are of nearly the same quality as those obtained at the CCSD(T)/[3s2p1d] (coupled cluster with all single, double, and quasiperturbative triple substitutions) level, except for relative energies. C-4n clusters (n = 2-4) are polyacetylenic rings with C-2nh symmetry, and C-4n+2 (n = 1-4) clusters are cumulenic rings with C((2n+1)h) symmetry. They have intense infrared absorptions in the 1800-2000 cm(-1) region, in addition to intense vibrations around 500 cm(-1) Tentative assignments of some bands in the 1900-1950 cm(-1) region to cyclic C-14 and C-18 are proposed.

Abstract: Ab initio molecular calculations carried out at the UQCISD(T)/6-311 + +G (3df, 2p) and RCCSD(T)/cc-pVTZ level shave been used to re-examine the heats of formation of the [C, H-3, N]+. radical cations and related species. The following values have been obtained: DELTAH(f,298)0 (H2CNH)+. = 1046 +/- 10 kJ/mol, DELTAH(f,298)0 (HCNH2)+. = 1030 +/- kJ/mol, DELTAH(f,298)0 (HCNH2) = 236 +/- 10 kJ/mol, PA(H2CNH) = 864+/-10 kJ/mol, PA(HCNH2) = 1012 +/- 10 kJ/mol, IE(a)(H2CNH) = 9.9 +/- 0.2 eV and IE(a)(HCNH2) = 8.2 +/- 0.2 eV. Ionized aminocarbene (HCNH2)+. is definitely more stable than ionized methylenimine (H2CNH)+..

Abstract: The potential energy surface for the B2C molecule and the potential energy curve for the ground state of BC have been investigated using full-valence complete active space SCF (CASSCF), augmented coupled cluster [CCSD(T)] and multireference treatments. The ground state of B2C is an extraordinarily stable ring (Sigma D-e=261.6+/-1 kcal/mol) with two 2-electron pi systems. The first excited state is linear BCB (1 Sigma(g)(+)), which is essentially biconfigurational due to a (4 sigma(g))-(3 sigma(u)) near degeneracy. Anharmonic spectroscopic constants were obtained from quartic force fields at the CCSD(T) level with a correlation-consistent basis set of [4s3p2dlf] quality. A severe Fermi resonance exists between the bending and symmetric stretching modes. All computed intensities are fairly weak. Spectroscopic constants for BC using elaborate multireference techniques were very well reproduced using the CCSD(T) method with a spin-restricted Hartree-Fock reference configuration, but not with an unrestricted Hartree-Fock reference. This suggests that even moderate levels of spin contamination that do not significantly affect relative energies may have a detrimental effect on computed spectroscopic constants.

Abstract: The potential energy surface of the BN3 molecule has been explored using coupled cluster methods. It is shown unambiguously that the ground state is linear NNBN(1SIGMA+). Harmonic frequencies for B3N and BN3 have been computed using extended basis sets and coupled cluster methods including triple excitations (CCSD(T)). Computed frequencies and geometry for BN3 are very sensitive to the electron correlation method. CCSD(T)/TZ2P frequencies and isotope shifts for BN, are in excellent agreement with experiment. Best estimates for the total atomization energies (SIGMAD(e)) of B3N and BN3 are 337.6 and 378.8 kcal mol-1, respectively. Preferred fragmentation channels are into B + B2N and BN + N2, respectively.

Abstract: Several low-lying structures and electronic states of the BN2 molecule have been studied using complete active space SCF, multireference averaged coupled-pair functional (ACPF), and coupled-cluster (CCSD(T)) methods. BNN(2PI) and BN2(2A1) are very close together in energy: at the ACPF level, the separation is 2.8 kcal/mol. The potential surface for the transition between these structures appears to have a significant energy barrier, so it is surprising that the 2A1 state has not yet been observed. Conversely, the low-lying BN2(2B2) state has a very low barrier towards dissociation, which explains why it has not been observed. The symmetric linear species NBN(2PI), on the other hand, is substantially higher in energy, but has a very large barrier towards dissociation and is therefore observed experimentally. At our highest level of theory, the ground BNN(2PI) state is nearly isoenergetic with B(2P)+N2(X 1SIGMA+).

Abstract: The total atomization energies (Sigma D-e values), geometries, and harmonic frequencies for a number of experimentally well-described molecules have been calculated at the CCSD(T) (coupled cluster) level using Dunning's correlation-consistent cc-pVDZ([3s2p1d]), cc-pVTZ([4s3p2d1f]), and cc-pVQZ([5s4p3d2f1g]) basis sets. Additivity correction are proposed for binding energies and geometries. Using a three-term additive correction of the form proposed by Martin [J. Chem. Phys. 97, 5012 (1992)] mean absolute errors in Sigma D-e are 0.46 kcal/mol for the cc-pVQZ, 0.93 for the cc-pVTZ, and 2.59 for the c-pVDZ basis sets. The latter figure implies that, although unsuitable for quantitatively accurate work, three-term corrected CCSD(T)/cc-pVDZ binding energies can still be used for a rough estimate when the cost of larger basis set calculations would be prohibitive. CCSD(T)/cc-pVQZ calculations reproduce bond lengths to 0.001 Angstrom for single bonds, and 0.003 Angstrom for multiple bonds; remaining error is probably partly due to core-core and core-valence correlation. CCSD(T)/cc-pVTZ calculations result in additional overestimates of 0.001 Angstrom for single, 0.003 Angstrom for double, and 0.004 Angstrom for triple bonds. CCSD(T)/cc-pVDZ calculations result in further overestimates of 0.01 Angstrom for single bonds, and 0.02 Angstrom for multiple bonds. CCSD(T)/cc-pVDZ harmonic frequencies are in surprisingly good agreement with experiment, except for pathological cases like the umbrella mode in NH3. Both CCSD(T)/cc-pVTZ and CCSD(T)/cc-pVQZ harmonic frequencies generally agree with experiment to 10 cm(-1) or better; performance of cc-pVQZ is somewhat superior on multiple bonds or the umbrella mode in NH3. Again, a source of remaining error appears to be core correlation. The use of MP2/6-31G* reference geometries in the Sigma D-e calculation can result in fairly substantial errors in the uncorrected Sigma D-e values fbr systems with cumulated multiple bonds. These errors however appear to be largely absorbed by the three-term correction. Use of CCSD(T)/cc-pVDZ reference geometries appears to have no detrimental effect on computed Sigma D-e values and is recommended for cases where only single-point calculations in the cc-pVTZ basis set are possible.

Abstract: The ground states of BNC, BCN, and C-3 were studied using the full-valence CASSCF (complete active space SCF) and CCSD(T) (augmented coupled cluster) methods and basis sets of spdf and spdfg quality. Full quartic force fields were obtained. r(c), and stretching fundamentals for C-3 are found to be in excellent agreement with experiment; r(0) and the bending fundamental reveal shortcomings in the bending part of the potential. The BNC ground state ((1) Sigma(+)) is only slightly less strongly bound than C-3 and is 9.5+/-0.5 kcal/mol lower in energy than the ground state of BCN. No other low-lying structures or states were found. Stretching fundamentals in BNC are affected by a severe Fermi resonance. Best estimates for the molecular constants are as follows. BNC: Sigma D-0 = 300.5+/-1 kcal/mol, r(c)(CN) = 1.167 Angstrom, r(c)(NC) = 1.416 Angstrom, omega(1) = 2075 cm(-1), omega(2) = 998 cm(-1), omega(3) = 131 cm(-1), v(1) = 2076 cm(-1), v(2) = 991 cm(-1). BCN: Sigma D-0 = 291.0+/-1 kcal/mol, r(c)(CN) = 1.135 Angstrom, r(c)(BC) = 1.597 Angstrom, omega(1) = 2197 cm(-1), omega(2) = 812 cm(-1), omega(3) = 204 cm(-1), v(1) = 2166 cm(-1), v(1) = 810 cm(-1). Both stretching fundamentals are computed to have high infrared intensities for both BNC and BCN. Spectroscopic constants for isotopic forms are also given. The best computed Sigma D-0 for C-3 is 314.7+/-1 kcal/mol, in very good agreement with experiment but with a smaller uncertainty.

Abstract: The quartic force fields of FNO and CINO have been computed at the CCSD(T)/cc-pVTZ level. Using an ''augmented'' basis set dramatically improves results for FNO but has no significant effect for CINO. The best computed force field for FNO yields harmonic frequencies and fundamentals in excellent agreement with experiment. Overall, the force fields proposed in the present work are probably the most reliable ones ever published for these molecules. Total atomization energies have been computed using basis sets of spdfg quality: our best estimates are Sigma D-0 = 208.5 +/- 1 and 185.4 +/- 1 kcal/mol for FN0 and CINO, respectively. The computed value for FNO suggests a problem with the established experimental heat of formation. Thermodynamic tables in JANAF style at 100-2000 K are presented for both FNO and CINO.

Abstract: The quartic force field of thioformaldehyde has been calculated ab initio using large basis sets and augmented coupled cluster methods. Calculated fundamentals are in excellent agreement with experiment, as is the most important Coriolis coupling constant. Computed values for the anharmonicity, rovibrational coupling, and centrifugal distortion constants of the four isotopomers (H2CS)-S-32, (H2CS)-S-34, (HDCS)-S-32, and (D2CS)-S-32 have been reported. Predictions have been made for all vibrational transitions from the ground state to excited states with at most two quanta for these isotopomers, both using second-order perturbation theory corrected for Darling-Dennison resonance and using vibrational SCF-CI calculations. For (D2CS)-S-32, perturbation theory performs quite well; for the other isotopomers, performance is poorer for states involving excitation of the out-of-plane bend and, for the (H2CS)-S-32 and (H2CS)-S-34 isotopomers, also for the antisymmetric bend that is in severe Coriolis resonance with it. A possible explanation has been suggested. (C) 1994 Academic Press, Inc.

Abstract: A flowing argon stream containing N-2 was Seeded with P-4 vapor, subjected to microwave discharge, and condensed at 12 K. Weak infrared absorptions due to PN (1327.7 cm((-)1)), P2O (1270.7 cm(-1)), P2O (1240.8 cm(-1)), SiO (1226.0 cm(-1)), P-4 (465 cm(-1)), and PO (1218 cm(-1)) and a strong new 1179.3 cm(-1) band were observed. The new absorption is assigned to the linear PNP radical on the basis of N-15 substitution, annealing and photolysis behavior, and complete-active-space self-consistent field (CASSCF) structure and frequency calculations. A 353 nm band system with a 560 +/- 20 cm(-)1 vibronic interval is assigned to the A(2) Sigma(u)(+)-X(2)IIg transition of PNP with the help of large-scale multireference calculations.

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Abstract: Several structures and electronic states of the C2N and CN2 molecules have been studied using complete active space SCF (CASSCF), multireference configuration interaction (MRCI), and coupled cluster (CCSD(T)) methods. Both molecules are very stable. Our best computed total atomization energies SIGMAD(e) are 288.6 +/- 2 kcal/mol for CN2, and 294.1 +/- 2 kcal/mol for C2N. The CNC and CCN structures for C2N are nearly isoenergetic. CNN(3PI) lies about 30 kcal/mol above NCN(3PI(g)), but has a high barrier towards interconversion and is therefore observed experimentally. Computed harmonic frequencies for CNN are sensitive to the correlation treatment: they are reproduced well using multireference methods as well as the CCSD(T) method. High spin contamination has a detrimental effect on computed harmonic frequencies at the CCSD(T) level.

Abstract: The geometries and harmonic frequencies of H2O and HF have been computed using a systematic sequence of 'correlation consistent' basis sets. Even basis sets of spdfgh quality fail to reproduce the omega, of HF unless they are augmented with special anion functions. CCSD(T) calculations with an 'augmented' spdfg basis set are in close agreement with experiment; addition of h functions does not substantially affect results, indicating convergence with respect to angular momentum. Core correlation increases stretching frequencies by 4-7 cm-1; accounting for residual inaccuracies in the correlation treatment and contraction error lowers them by about 6 and 3 cm-1, respectively. The remaining small errors are due to relativistic and nonadiabatic effects, in that order of importance. Calculations on BH indicate that, as expected, the augmenting functions can safely be omitted on elements that are not highly electronegative.

Abstract: The impact of spectroscopical data, obtained via quantum chemical methods, on the interpretation of infrared spectra is illustrated for small carbon clusters C(n) (n less-than-or-equal-to 11). It is shown that the assignments of Thompson, DeKock, and Weltner should be completely revised. A survey of the new assignments is presented.

Abstract: The quartic force fields of N2O and CO2 have been computed ab initio using large basis sets of spdf and spdfg quality and augmented coupled-cluster (CCSD(T)) methods. The CCSD(T)/spdf frequencies for N2O, and the CCSD(T)/spdfg ones for CO2, are in excellent agreement with experiment. g functions appear to be important for cumulenic double bond stretches and bends. Improving the basis from spdf to spdfg appears to shorten double bonds by almost-equal-to 0.003 angstrom; the effect on single bonds is smaller and too unsystematic to be quantified. SCF level anharmonicities for N2O are qualitatively incorrect: this is not the case for CO2 where reasonable agreement with experiment is reached even at this level. Of the various published experimental force fields for N2O and CO2, the ones obtained by algebraic contact transformation appear to be the most reliable.

Abstract: Using the example of a GAUSSIAN 90 computation of the linear BBNB molecule in its singlet state, it is shown that the calculated entropy value can be considerably different from that deduced from seemingly equivalent computations performed on a quasi-linear species. This problem is interpreted in terms of the limiting behaviour of the conventional rotational partition function of a polyatomic molecule. The problems originate from a routine application of quantum-chemical programs to such linear cases. The results are shown to be important for linear systems optimized (owing to, for example, better convergence properties) as quasi-linear systems. The finding explains why some published computed entropies of essentially linear species cannot be reproduced.

Abstract: Reactions of pulsed laser produced B and N atoms at high dilution in argon favored diboron species. At low laser power with minimum radiation, the dominant reaction with N2 gave BBNN (3PI). At higher laser power, reactions of N atoms contributed the B2N (2B2), BNB (2SIGMA(u)+), NNBN (1SIGMA+), and BNBN (3PI) species. These new transient molecules were identified from mixed isotopic patterns, isotopic shifts, and ab initio calculations of isotopic spectra.

Abstract: The protonation of N2O and the intramolecular proton transfer in N2OH+ have been studied using large basis sets in conjunction with second-order many-body perturbation theory (MP2), singles and doubles coupled cluster (CCSD), the augmented coupled cluster method [CCSD(T)], and complete active space self-consistent field (CASSCF) methods. It is shown that MP2 is inadequate even for HNNO+, which has a minor nondynamical correlation effect; for the transition state only CCSD(T) produces a reliable geometry due to serious nondynamical correlation effects. Harmonic frequencies accurate to 50 cm-1 or better are predicted for both protonated species. The proton affinity at 298.15 K is found to be 137.6 kcal/mol, in excellent agreement with the recent experimental redetermination of 137.3 +/- 1 kcal/mol; the HNNO+ isomer is found to be 4.4 kcal/mol above the HONN+ isomer, with an interconversion barrier of approximately 89 kcal/mol, herewith confirming recent experimental evidence that both species occur together with an energy difference of 6 +/- 1. 5 kcal/mol. Comparison of the traditional double-zeta plus polarization (DZP) basis and the newer correlation consistent polarized valence double zeta (cc-pVDZ) basis set appears to indicate that the latter might lead to more accurate geometries and harmonic frequencies, although a more detailed investigation would be needed before any definitive conclusions.

Abstract: The structure, energetics and harmonic vibrations of B3N have been computed at a correlated ab initio level. The lowest potential energy structure found exhibits a linear BBNB arrangement, being followed by a D3h structure separated by about 60 kJ/mol. C2v and linear BBBN configurations are still higher in energy so that they are located above triplet minimum-energy structures. The predicted IR lines only partly match the available observed matrix data.

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Abstract: The lowest doublet and quartet reaction surfaces of the title reaction have been studied using complete active space SCF, multireference CI, and augmented coupled cluster (CCSD (T) ) methods. Several additional intermediates and transition states have been found beyond those considered in previous work. It is found that not one but three intersystem crossings are present along one suggested ''reaction coordinate'', which also has a high barrier to reaction. It therefore seems possible that the reaction mechanism is different from what has been previously been suggested. At the highest level of theory, the reaction enthalpy at absolute zero is found to be + 3.1 kcal/mol, in excellent agreement with experiment.

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Abstract: The two recently elucidated isomers of B3N lowest in energy, the singlet linear ((1) Sigma(+)) and triangular ((1)A(1)') structures, are studied further. The all-electron limited configuration interaction method with an single and double excitations (CISD), quadratic configuration interaction with all singles and doubles (QCISD), and with a quasiperturbative treatment of higher excitations (QCISD(T)) are used together with a number of basis sets of the Pople family. Using standard G1 theory, the total atomization energy Sigma D-0 for the linear structure is found to be 1397 Kj/mol, which means the molecule is quite stable with respect to fragmentation. The X (1) Sigma(+) state has two intense IR bands in the 1100-1200 and 2000-2100 cm-(1) ranges and should be observable; isotopic substitution data have been given. The inter-isomeric thermodynamics is described using the best computed parameters, and it is shown that even at high temperatures the triangular form should represent at most about 3% of the B3N system.

Abstract: Pulsed laser evaporated boron atoms react with C2H2 to form new organoborane products during condensation with excess argon. Isotopic frequencies have been obtained for the B-11, B-10, C2H2, (C2H2)-C-13, and C2D2 reactions. Several structures of the HBC2 and BC2H2 molecules have been studied ab initio using large basis sets and augmented coupled cluster methods. Computed isotopic shifts are in excellent agreement with experiment for two BC2H2 SpeCies (bent HBCCH insertion and ring BC2H2 addition products) and three HBC2 Species (linear HBCC, linear HCCB, and C2v HBC2). The BC2H2 borirene radical is photosensitive, forms spontaneously on annealing above 18 K, and exhibits calculated bond lengths appropriate for delocalized pi bonding in the BC2 ring system. The linear HBCC and HCCB species are formed by the reaction of hyperthermal B atoms and C2H2 during matrix deposition. Thermochemistry has been addressed by ab initio calculations.

Abstract:

Abstract: Pulsed laser evaporation of pellets pressed from boron and graphite powder gave a new 1:4 doublet at 1232.5 and 1194.6 cm-1 in addition to the carbon cluster absorptions reported previously. The 1232.5 cm-1 band dominated boron-10 experiments. The new bands increased as carbon cluster bands decreased with increasing B/C ratio in the pellet and with increasing laser power. Augmented coupled cluster and full-valence complete active space SCF (CASSCF) calculations predict the global minimum BC2 Structure to be an asymmetric triangle: however, the vibrationally averaged structure will be an isosceles triangle with a strong symmetric B-C2 stretching frequency near 1200 cm-1. The calculated boron-10/boron-11 frequency ratio (1.0323) is in excellent agreement with the observed ratio (1.0317), and confirms assignment of the 1194.6 cm-1 band to the BC2 ring. Calculations predict linear BCC to be less stable by 6.2 +/- 2 kcal/mol and to absorb in the 2000-2050 cm-1 range: the barrier towards rearrangement to the cyclic structure is very low (1.1 kcal/mol). Linear BCC was not detected in these experiments. Computed energetics explain why BC2 is abundant in B/C experiments, but absent in B + C2H2 experiments.

Abstract: The quartic force fields of HCO+ and HOC+ have been computed using augmented coupled cluster methods and basis sets of spdf and spdfg quality. Calculations on HCN, CO, and N2 have been performed to assist in calibrating the computed results. Going from an spdf to an spdfg basis shortens triple bonds by about 0.004 angstrom, and increases the corresponding harmonic frequency by 10-20 cm-1, leaving bond distances about 0.003 angstrom too long and triple bond stretching frequencies about 5 cm-1 too low. Accurate estimates for the bond distances, fundamental frequencies, and thermochemical quantities are given. HOC+ lies 37.8 +/- 0.5 kcal/mol (0 K) above HCO+; the classical barrier height for proton exchange is 76.7 +/- 1.0 kcal/mol.

Abstract: The structure, infrared spectrum, and heat of formation of B2N, B2N-, BO, and B2O have been studied ab initio. B2N is very stable; B2O even more so. B2N, B2N-, B2O, and probably B2N+ have symmetric linear ground-state structures; for B2O, an asymmetric linear structure lies about 12 kcal/mol above the ground state. B2N+, B2N- and B2O have intense asymmetric stretching frequencies, predicted near 870, 1590 and 1400 cm-1, respectively. Our predicted harmonic frequencies and isotopic shifts for B2O confirm the recent experimental identification by Andrews and Burkholder. Absorptions at 1889.5 and 1998.5 cm-1 in noble-gas trapped boron nitride vapor belong the BNB and BNBN (3-PI), respectively; a tentative assignment of 882.5 cm-1 to BNB+ is proposed. Total atomization energies SIGMA-D(e) (SIGMA-D0) are computed (accuracy +/- 2 kcal/mol) as: BO 193.1 (190.4), B2O 292.5 (288.7), B2N 225.0 (250.3) kcal/mol. The ionization potential and electron affinity of B2N are predicted to be 8.62 +/- 0.1 and 3-34 +/- 0.1 eV. The MP4-level additivity approximations involved in G1 theory results in errors on the order of 1 kcal/mol in the SIGMA-D(e) values.

Abstract: The anharmonic potential surface of NH2 has been computed ab initio using the quadratic configuration interaction method and large basis sets. The all-electron augmented coupled cluster surface with the [5s4p2d1f,3s2p] basis set reproduces the experimental fundamentals to within a few cm-1. For this quantitative agreement with experiment, both f functions and core correlation seem to be prerequisite. From our best computed force field, a set of spectroscopic constants has been derived for all important isotopomers of NH2. Using a hybrid analytic/direct summation method recently developed by the present authors, the thermodynamic functions gef(T), hcf(T), S0, and C(p) are computed including exact accounts for anharmonicity and rovibrational coupling, and very good analytical approximations to centrifugal distortion and quantum rotation effects. Thermodynamic tables in JANAF style from 100-3000 K, as well as a full set of rovibrational spectroscopic constants, are presented for NH2 and all important isotopomers. The effect of the A2A1 state is found to become significant around 2000 K.

Abstract: The anharmonic potential energy surface of water has been computed ab initio using an augmented coupled cluster method and various basis sets. Whereas the Pople 6-311 G family is manifestly unsatisfactory, Huzinaga-Dunning basis sets perform quite well. The [5s4p2d 1f,3s2p] surface reproduces harmonic frequencies and anharmonicity constants to better than about 2 and 1 cm-1, respectively. For quantitative agreement with experiment, both f functions on oxygen and inclusion of core correlation seem to be prerequisite. Comparison with various experimentally derived force fields reveals that the ab initio force field is of comparable quality. From the best computed force field, a set of spectroscopic constants has been derived for all important isotopomers of water. Using a hybrid analytic/direct summation method recently developed by the present authors, the thermodynamic functions gef(T), hcf(T), S0, and C(p) are computed including exact account of anharmonicity and rovibrational coupling, and very good analytical approximations to centrifugal distortion and quantum rotation effects. The computed functions substantially revise previous literature results at high temperatures. Differences between thermodynamic functions from various computed force fields are an order of magnitude smaller than these errors. Thermodynamic tables in JANAF style from 100 to 3000 K, as well as a full set of rovibrational spectroscopic constants, are presented. It is concluded that obtaining force fields of near-spectroscopic accuracy, and thermodynamic tables of very high accuracy, is presently feasible for small polyatomic molecules.

Abstract: The potential energy surface of B4 has been studied ab initio using extended basis sets and coupled-cluster methods. The ground state (1A1g) is a singlet square that undergoes pseudo-Jahn-Teller distortion to a rhombic structure (1A(g)), but the energy gain is so small that the effective structure will be square at all practical temperatures. Total atomization energies SIGMA-D0 (kcal/mol) for B2 (62.2 +/- 2), B3 ( 189.1 +/- 2-192.6 +/- 2), and B4 (312.2 +/- 2) are computed by a modification of GAUSSIAN-1 (G1) theory. The validity of scaling and bond-additivity schemes for computing binding energies of higher clusters is discussed.

Abstract: The total atomization energies of a number of molecules have been computed using an augmented coupled-cluster method and [5s4p3d2 f1g] and [4s3p2d1f] atomic natural orbital (ANO) basis sets, as well as the correlation consistent valence triple zeta plus polarization (cc-pVTZ) and correlation consistent valence quadrupole zeta plus polarization (cc-pVQZ) basis sets. The performance of ANO and correlation consistent basis sets is comparable throughout, although the latter can result in significant CPU time savings. Whereas the inclusion of g functions has significant effects on the computed SIGMAD(e) values, chemical accuracy is still not reached for molecules involving multiple bonds. A Gaussian-1 (G1) type correction lowers the error, but not much beyond the accuracy of the G1 model itself. Using separate corrections for sigma bonds, pi bonds, and valence pairs brings down the mean absolute error to less than 1 kcal/mol for the spdf basis sets, and about 0.5 kcal/mol for the spdfg basis sets. Some conclusions on the success of the Gaussian-1 and Gaussian-2 models are drawn.

Abstract: The quartic force field of ammonia is computed using basis sets of spdf/spd and spdfg/spdf quality and an augmented coupled cluster method. After correcting for Fermi resonance, the computed fundamentals and nu4 overtones agree on average to better than 3 cm-1 with the experimental ones except for nu2. The discrepancy for nu2 is principally due to higher-order anharmonicity effects. The computed omega1, omega3, and omega4 confirm the recent experimental determination by Lehmann and Coy but are associated with smaller error bars. The discrepancy between the computed and experimental omega2 is far outside the expected error range, which is also attributed to higher-order anharmonicity effects not accounted for in the experimental determination. Spectroscopic constants are predicted for a number of symmetric and asymmetric top isotopomers of NH3.

Abstract: The quartic force fields of BH3 and BeH2 have been computed ab initio using an augmented coupled cluster (CCSD(T)) method and basis sets of spdf and spdfg quality. For BH3, the computed spectroscopic constants are in very good agreement with recent experimental data, and definitively confirm misassignments in some older work, in agreement with recent ab initio studies. Using the computed spectroscopic constants, the rovibrational partition function for both molecules has been constructed using a modified direct numerical summation algorithm, and JANAF-style thermochemical tables are presented.

Abstract: The lowest 1SIGMA+ and 3PI states of the BN molecule have been studied using multireference configuration interaction (MRCI) and averaged coupled-pair functional (ACPF) methods and large atomic natural orbital (ANO) basis sets, as well as several coupled cluster methods. Our best calculations strongly support a 3PI ground state, but the a 1SIGMA+ state lies only 381 +/- 100 cm-1 higher. The a 1SIGMA+ state wave function exhibits strong multireference character and, consequently, the predictions of the perturbationally-based single-reference CCSD(T) coupled cluster method are not as reliable in this case as the multireference results. The best theoretical predictions for the spectroscopic constants of BN are in good agreement with experiment for the X3PI state, but strongly suggest a misassignment of the fundamental vibrational frequency for the a 1SIGMA+ state.

Abstract: The lowest 1-SIGMA+ and 3-PI states of the BN molecule have been studied using the quadratic configuration interaction method and (spdf) basis sets. The lowest 1-SIGMA+ and 3-PI states lie extremely closely (T(e) almost-equal-to 100 cm-1) together; it is not clear which is the ground state. The very small separation should form a useful benchmark for basis sets and electron correlation methods. The dissociation energy D0 is computed to be 103.9 +/- 2 kcal/mol. A self-consistent set of spectroscopic constants is derived from a combination of ab initio and experimental data. JANAF-style thermodynamic functions in the range 100-6000 K, including anharmonic, rovibrational coupling, centrifugal stretching, and spin-orbit coupling effects are computed using direct numerical summation over the 25 lowest electronic states. A modified procedure for the latter is outlined that reduces computer time by one or two orders of magnitude without compromise in accuracy.

Abstract: The lowest-lying 1A(g) and 3-SIGMA-g- states of C4 have been studied using basis sets including f functions, quadratic configuration interaction, and full fourth-order Moller-Plesset perturbation theory with multiple spin projection. Basis set effects on the linear-cyclic separation have been found to be fairly significant. Use of the 6-311G* and [5s3p1d] basis sets leads to qualitatively different conclusions; this problem is remedied by expansion of the polarization space. At the highest level of theory considered, the cyclic structure lies about 1 kcal/mol below the linear structure. A G1-type estimate of its heat of formation at 0 K of 249.6 kcal/mol is proposed. This value is in disagreement with the generally quoted experimental values, but in good agreement with third-law values by Drowart et al. [J. Chem. Phys. 31, 1131 (1959)] corrected for improved partition functions. Geometries and harmonic frequencies are reported. It is tentatively suggested that a matrix ir band at 1284 cm-1 may belong to cyclic C4; to aid experimentalists in confirming or rejecting this assignment, theoretical isotope shifts are reported.

Abstract: Dissociation energies and potential energy surface features for the carbon clusters C2 to C-10 are compared with ab initio or experimental results for the semiempirical methods MINDO/3, MNDO, AM1 and PM3. Quite surprisingly, MINDO/3 gives a rather good account of the various structures and electronic states, unlike the other three methods. MINDO/3 tends towards systematic overestimate of binding energies, the other methods to systematic gross underestimates. Reparametrization of the diatomic parameters alpha, beta-s, beta-p for exact reproduction of the experimental data for C3 results in much improved values for binding energies, but fails to correct the state splittings. Also reparametrizing U(ss), U(pp), xi-s, and xi-p to reproduce the ab initio linear-rhombic energy difference in C4 results in a much improved description of the other states. For the linear structures, computed harmonic frequencies with the latter parameters are in surprisingly good agreement with experimental or correlated ab initio data, where available; experimental values are consistently overestimated by about 40 cm-1. Other results are comparable in quality to good ab initio treatments. The experimental IR bands at 2128 and 1892 cm-1, formerly assigned to C-9, should be reassigned to linear C7. The intense 1997 cm-1 feature almost certainly belongs to C-9; bands at 1952 and 1197 cm-1 both belong to linear C6. Tentative assignments of bands in the 1600-1850 cm-1 region to various cyclic structures of C6, C-8, and C-10 have been made. As such, this suggests a new and promising procedure for the theoretical study of large molecules in general, and of large clusters in particular.

Abstract: The energy difference between the linear 2-SIGMA-u+ and cyclic B-2(2) structures of C3+ has been investigated by using large [5s3p2dlf] basis sets and multireference electron correlation treatments, including complete active space SCF (CASSCF), multireference CI (MRCI), and averaged coupled-pair functional (ACPF) methods, as well as the single-reference quadratic configuration interaction [QCISD(T)] method. Our best estimate, including a correction for basis set incompleteness, is that the linear form lies above the cyclic form by 5.2(1.0)+1.5 kcal/mol. The 2-SIGMA-u+ state is probably not a transition state but a local minimum. Reliable computation of the cyclic/linear energy difference in C3+ is extremely demanding of the electron correlation treatment used: of the single-reference methods previously considered, CCSD(T) and QCISD(T) perform best. The MRCI+Q-(0.01)/[4s2pld] energy separation of 1.68 kcal/mol should provide a comparison standard for other electron correlation methods applied to this system.

Abstract: Correction terms (up to third order in temperature) for the effect of centrifugal distortion on the rotational partition function of linear molecules, spherical, symmetric, and asymmetric tops are evaluated by means of the classical partition function. It is shown that for the linear, spherical, and symmetric cases, the expressions thus obtained differ from the exact quantum expressions only in the absence of a very small constant correction term. It is then proposed that the partition function for a nonrigid asymmetric top, for which no exact expression has as yet been derived, be evaluated as the product of Watson's asymptotic expansion for the rigid rotor and the centrifugal correction factor derived in the present work. Numerical comparison with direct numerical summation shows that, even for troublesome cases, this approximation holds very well even at 2000 K. Similar performance is observed for the heat content function, except for pathological cases at high temperatures. In the rigid-rotor case, Watson's asymptotic series holds very well. As an example application, pilot calculations have been performed on the thermodynamic functions of water and formaldehyde. The present method yields functions in excellent agreement with those obtained by direct rovibrational summation, whereas computer-time requirements are reduced by 3 or 4 orders of magnitude.

Abstract: Linear, cyclic and three-dimensional structures of C-11 have been studied with ab initio methods. At the MP2/6-31G(d) level, a cyclic structure is clearly the ground state, about 5 kcal/mol below the lowest linear structure. The three-dimensional structure proposed by van Vechten and Keszler is less stable by at least 200 kcal/mol. Arguments are presented in favor of assignment of the 1804 and 1844 cm-1 matrix IR bands to cyclic C-11, and of the 1818 cm-1 band to cyclic C-10.