E.P.A. Couzijn

Abstract: A novel deconvolution method for energy-resolved reaction cross sections is applied to determine intrinsic gas-phase dissociation energies for non-covalent α-cyclodextrin host–guest complexes. M06-2X//M06-L/6-31+G(d,p) calculations reproduce the experimental results and enable us to quantify the contribution of intermolecular hydrogen bonding.

Abstract: The non-covalent interactions in host–guest complexes of α-cyclodextrin (α-CD) with a series of benzoic acid derivatives (RBA) were investigated by electrospray ionization tandem mass spectrometry and density functional theory (DFT) calculations. The 1:1 stoichiometry of the anionic host–guest complexes was unequivocally confirmed by their mass-to-charge ratios (m/z) and isotope patterns. Collision-induced dissociation experiments revealed exclusive fragmentation into [α-CD]− and neutral RBA and afforded the gas-phase kinetic stability trend [α-CD·3,5-diMeBA]− < [α-CD·3-MeBA]− < [α-CD·BA]− < [α-CD·3-OHBA]− < [α-CD·3,5-diOHBA]−. This trend follows that of the gas-phase basicities of the guest anions used, indicating that host–guest pairs with more comparable basicities form more stable complexes. DFT calculations at the M06 L/6-31+G(d,p) level of theory provided detailed structural assignments and further elucidated the experimental observations, suggesting that the anionic [α-CD·RBA]− inclusion complexes are favored over the non-specific complexes in the gas phase and that hydrogen bonding constitutes the primary host–guest interaction. Additionally, the results provide an estimated gas-phase basicity ΔG0 = 325–327 kcal mol−1 for [α-CD]−.

Abstract: The activation reaction of the olefin polymerization precatalyst Cp*2ZrMe2 with a boron-C6F5 compound (B(C6F5)3, [Ph3C][B(C6F5)4]) and an aluminum alkyl species (Al2Me6, Al2iBu6) is studied by NMR spectroscopy in order to determine the nature of the ion pairs that are formed preferentially. We show that a mixture of ion pairs with general formula Cp*2Zr(Me)−μ-Me–E(C6F5)3–xRx (E = Al, B; x = 0, 1; R = Me, iBu) (1, 2a/b, 4) is generated due to a rapid transfer of pentafluorophenyl groups from boron to aluminum. Therefore, the molecular ratio of the activators determines the final composition of the ion pairs present in solution. When the pentafluorophenyl group transfer is suppressed, the ion pair Cp*2Zr–(μ-Me)2–AlMe2 (5) forms irrespective of the reagent ratio. The high dynamics of these solutions is demonstrated by DNMR studies. Gibbs free energies of activation were determined of 13.6(12) kcal mol–1 at 298 K for the cocatalyst exchange of ion pair Cp*2Zr(Me)−μ-Me–Al(C6F5)2Me (2a) and 13(2) kcal mol–1 at 298 K for the methyl exchange in Al2(C6F5)xMe6–x (x = 0, 1). Due to the stability of the ion pairs generated from the Cp*2ZrMe2 precatalyst at temperatures relevant for polymerization, correlations between activities in ethylene polymerization and the nature of the ion pairs can be established. All solutions containing the various ion pairs were found to be catalytically active in ethylene polymerization except that containing the ion pair 2a, which was attributed to the reduced Lewis acidity of the abstractor, as supported by DFT calculations.

Abstract: Energy-resolved collision-induced dissociation experiments using tandem mass spectrometry are reported for an phenylpalladium N-heterocyclic carbene (NHC) complex. Reductive elimination of an NHC ligand as a phenylimidazolium ion involves a barrier of 30.9(14) kcal mol-1, whereas competitive ligand dissociation requires 47.1(17) kcal mol-1. The resulting three-coordinate palladium complex readily undergoes reductive C–C coupling to give the phenylimidazolium pi complex, for which the binding energy was determined to be 38.9(10) kcal mol-1. Density functional calculations at the M06-L//BP86/TZP level of theory are in very good agreement with experiment. In combination with RRKM modeling, these results suggest that the rate-determining step for the direct reductive elimination process switches from the C–C coupling step to the fragmentation of the resulting sigma complex at low activation energy.

Abstract: Dimethylaminonitrene complexes of IMesM+ (IMes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; M = Cu, Ag, Au) were prepared in the gas phase and structurally characterized by high-resolution infrared spectroscopy of the cold species, ion-molecule reactions, and DFT computations. We measured the binding energies of the nitrene fragment to the IMesM+ moiety by energy-resolved collision-induced dissociation experiments in the gas phase, affording a trend in bond strength of M = Cu ≈ Au > Ag. This trend is explained in terms of a detailed metal-nitrogen bonding analysis, from which relativistic effects on the bonding were assessed. Various density functionals were evaluated for reproducing the observed thermochemical data and Truhlar’s M06 functional was found to give the best agreement.

Abstract: A general reaction mechanism describes the qualitative change in chemical topology along the reaction pathway. Based on this principle, we present a method to characterize intramolecular substituent permutation in pentavalent compounds. A full description of the geometry around five-coordinate atoms, using internal coordinates, enables analysis of the structural changes along the stereomutational Intrinsic Reaction Coordinate. The fluxional behavior of experimentally known pentavalent phosphoranes, silicates, and transition-metal complexes is investigated by Density Functional Theory calculations. The identified three principal mechanisms are the Berry pseudorotation, threefold cyclic permutation, and half-twist axial-equatorial interchange. The frequently cited Turnstile rotation is shown to be equivalent to the Berry pseudorotation. In combination with graph theory, this approach provides a means to systematically investigate the stereomutation of pentavalent molecules and potentially identify hitherto unknown mechanisms.

Abstract: How strong are you? It takes about 45 kcal mol-1 to form a gold dimethylaminonitrene complex in the gas phase via NP bond dissociation and 51 kcal mol-1 to break its AuN bond (see figure). A screening of density functionals identified the M06 family to be the best in reproducing the observed thermochemistry.

Abstract: Trapped! Transient electrophilic phosphinidenes [R-P-W(CO)5] have emerged as versatile intermediates that are highly valuable in the synthesis of a plethora of organophosphorus compounds, nevertheless their existence has never been unequivocally established. By employing electrospray ionisation tandem mass spectrometry (ESI-MS/MS), this low-valent species has now been detected and its gas-phase reactivity perfectly matches the well-established solution-phase data.

Abstract: The title compound, [-Si(C12H9)(2)OSi(CH3)(2)O-](2), was obtained unintentionally as the product of an attempted crystallization of caesium bis(biphenyl-2,2'-diyl)fluorosilicate from dimethylformamide. In the crystal, the molecule is located on an inversion center and the siloxane ring adopts a twist-chair conformation with the two dimethyl-substituted Si atoms lying 0.7081 (5) angstrom out of the plane defined by the two bis(biphenyl-2-yl)-substituted Si atoms and the four O atoms. In each Si(C12H9)(2) unit, the orientation of one terminal phenyl ring relative to the phenylene ring of the other biphenyl moiety suggests a parallel displaced pi-pi stacking interaction [centroid distance = 4.2377 (11) angstrom and dihedral angle = 15.40 (9) degrees].

Abstract: In this research report, we explore the emerging field of stable pentaorganosilicates, with the aim to show the development from transient species to stable and configurationally rigid analogues that may find application e.g. as novel chiral auxiliaries or in chiral ionic liquids.

Abstract: The intramolecular substituent interchange in recently reported pentaorganosilicates is investigated by B3LYP calculations, which show excellent agreement with the experimental thermochemical data. Two types of ligand permutation are discerned (A and B), which both lead to racemization of the helical, spirocyclic anions. IRC calculations show that stereomutation A bifurcates into two enantiomeric reaction paths, which are inhibited by ortho substitution of the bidentate ligands. The other pathway (8) proceeds through a trigonal bipyramidal transition state with one bisequatorial bidentate ligand and is disfavored by increasing the pi-electron density of the ligand. A more electronegative fifth, monodentate substituent increases the barrier of pathway A and lowers that of pathway B, as in bis(biphenyl-2,2'-diyl)fluorosilicate, which is the first tetraorganofluorosilicate to be isolated and fully characterized. These concepts enabled us to design and synthesize methyl- and ethylbis([2]naphthylpyrrol-2,1'-diyl)silicate as Si-chiral pentaorganosilicates that are configurationally rigid at room temperature.

Abstract: 1,2-Addition of transient W(CO)(5)-complexed phosphinidenes exo to hexamethyl Dewar benzene affords the novel 3-phosphatricyclo[3.2.0.0(2,4)]hept-6-ene complexes. The fused tricyclic phosphiranes are obtained as both the Z and the thermally less stable E isomers, the P-31 NMR chemical shifts of which differ by about 60 ppm. A computational investigation shows that the phosphorus pyramidalization and the presence of the gamma double bond are responsible for this effect. ne semiquantitative results contribute to a more systematic understanding of the structural influences on P-31 chemical shieldings. The congested double bond of the Z isomer can be epoxidized with m-chloroperbenzoic acid (MCPBA) to afford a fused tetracyclic P,O bis-adduct.

Abstract: The exceptional stability of recently reported pentaorganosilicates is investigated by bond energy analyses. Experimental coupling constants are used to probe their electronic structure, entailing bonds with mixed ionic-covalent character. Our analyses reconfirm that the axial bonds are more prone to heterolytic cleavage than are the equatorial bonds. Aryl substituents provide substantial electronic stabilization by charge delocalization, but cause steric crowding due to ortho-hydrogen repulsion. In contrast, silicates with two ax, eq biaryl groups are not congested. The remaining substituent is confined to an equatorial site, where it is insensitive to elimination. These concepts adequately explain the experimentally observed stability trends and are valuable for designing other stable pentaorganosilicates.

Abstract: Molecule with three faces: Two of the three possible configurations that a stable silicate with five carbon substituents can adopt have been observed by NMR spectroscopy and shown to be in dynamic equilibrium through a Berry pseudorotation. The thermodynamics and kinetics of their exchange are in good agreement with calculations. The major configuration (from the crystal structure of the nBu4N+ salt) is shown in the center of the picture.

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Abstract: This manual gives 10 Tips to efficiently perform DFT calculations in Gaussian and ADF.

Abstract: This manual describes an improved methodology to acquire collision-induced dissociation (CID) threshold curves on our customized Finnigan MAT TSQ-700 tandem mass spectrometer. An ICL script collects the collision offset DAC scans of parent and daughter channels in an interleaved fashion; the collision gas pressure is also monitored via the hot cathode gauge. This approach reduces inconsistencies between the channels due to fluctuations in spray quality and, especially, the collision gas pressure. Furthermore, the number of data files is significantly fewer than with the traditional acquisition method. A set of ICL scripts, C programs, and Excel sheets facilitate the data acquisition and processing.