Abstract: The polarized IVV and depolarized IVH Raman profiles of the Fermi dyad (1285 cm-1 and 1388 cm-1) of supercritical (SC) CO2 have been measured along the isotherms 307, 309, 313, and 323 K in the reduced density range 0.04 < rho* = rho/rhoC < 2.04 (rhoC is the critical density). A band shape analysis of the dyad component shows that each one can be decomposed in two well-defined Lorentzian profiles in all of the temperature and density ranges investigated. These profiles have been assigned with the transitions of CO2 probing two kinds of environments. In each dyad peak, the Lorentzian profiles centered at higher frequency is associated with CO2 interacting through usual Van der Waals interactions with its nearest neighbors. The Lorentzian profiles centered at lower frequency in each dyad peak have been related to the transition of CO2 involved in a transient (CO2)2 dimer. The evolution with the density of the band center positions and bandwidths of the Lorentzian profiles shaping the lower and upper dyad components exhibits a nonlinear behavior along the near critical isotherm (307 K) for rho* ranging from 0.4 to 1.7. This behavior, although less pronounced, is still detected at higher temperatures. The deviation from linearity was interpreted as being due to an enhancement of the density that leads to a reduced local density excess Deltarho* = rho*loc - rho*bulk. Even if the spectroscopic observables involved probe the interactions in SC CO2 differently, we emphasize that the scaled spectral features are straightforwardly related to the Local Density Enhancement (LDE) phenomenon taking place in SC fluids (SCF). We show that the LDE effect can also be put in evidence from the band shape analysis of the weak satellite band situated at 1370 cm-1 associated with the upper Fermi dyad transition of the 13C16O2 molecule (1% isotopic natural abundance). The asymmetric shape of the evolution with density of the Deltarho* found from our spectroscopic observables shows some similitude with that obtained in a recent MD simulation [Skarmoutsos, I.; Samios, J. J. Chem. Phys. 2007, 126, 44503.] and the possible inter-connection and difference with these calculations are discussed.

Abstract: Polarized and depolarized Raman spectra of CO2-acetone mixtures have been measured along the isotherm 313 K as a function of CO2 concentration (0.1-0.9 molar fractions in CO2) by varying the pressure from 0.2 up to 8 MPa. Upon CO2 addition, a new band appears at about 655 cm-1 and is assigned to the lower frequency nu2(1) component of the bending mode after degeneracy removal due to the formation of a 1:1 electron donor acceptor (EDA) CO2 complex. The equilibrium constant associated with the complex formation was estimated and found close to those of contact charge transfer complexes. The main modifications of the Fermi dyad of CO2 in the mixtures compared with that of pure CO2 at equivalent density have been assessed. The band-shape analysis revealed that each dyad component is described by two Lorentzian profiles, showing that a tagged CO2 molecule probes two kinds of environment in its first shell of neighbors. The first one involves nonspecific interactions of CO2 with surrounding acetone whereas the second is assigned to the signature of 'transient' CO2 complexes formed with acetone. An upper bound life time of the complex has been estimated to be 8 ps. In addition, a broad band has been detected between the Fermi dyad peaks at about 1320 cm-1 and its origin interpreted as a further evidence of the CO2-acetone heterodimer formation. Finally, the values of the equilibrium concentration of the heterodimer versus the total concentration of CO2 deduced from the analysis of the nu2(1) band and from the Fermi dyad have been compared, and the difference is interpreted as due to a lack of theoretical approach of Fermi resonance transitions associated with species existing in different environments.

Abstract: We have performed molecular-dynamics simulations of CO(2) system along the gas-liquid coexistence curve and on the isochore 94.22 cm(3) mol(-1) (which corresponds to the critical isochore). The calculation has been carried out in order to analyze the diffusion of CO(2) and particularly to figure out how the diffusion coefficient may be decomposed along the molecular axes. This makes it possible to analyze the anisotropy of the diffusion along these axes and to shed light on the microscopic changes which accompany such behavior. This anisotropy is traced back to the effect of the translation-rotation coupling (TRC) along the molecular axes. Along the liquid-gas coexistence curve, the pseudolongitudinal diffusion is found to be more rapid than the transverse one. The opposite trend is found along the isochore 94.22 cm(3) mol(-1). The role of the local structure was explored by calculating intermediate scattering function and the autocorrelation functions for the forces acting along the molecular axes. It is shown that the strength of the TRC effect is correlated to the difference between the relaxation times of the local structure, that of the reorientation along the molecular axes, and that of the translational motion. The analysis of the correlation time and the average mean square force along the longitudinal and transverse directions confirms the anisotropy of the local environment that determines the translational dynamics of a molecule.

Abstract: The IHV and IHV Raman profiles of the v2 bending mode of CO2 diluted in acetone and in ethanol have been measured at pressures up to 10 MPa at 313 K. Upon the addition of CO2, the spectra exhibit a new polarised band interpreted as the spectral signature of transient electron donor-acceptor complexes formed between CO2and acetone or ethanol. This band is assigned to the lower frequency v(1)2 component of the bending mode after the degeneracy removal predicted in our previous work.

Abstract: We report the results of the low-frequency Raman experiments on CO(2) which were carried out in a wide density range, along the liquid-gas coexistence curve in a temperature range of 293-303 K, and on the critical isochore of 94.4 cm(3) mol(-1) in a temperature range of 304-315 K. In our approach, the qualitative behavior of the diffusion coefficient D is predicted, assuming the following: first, that the low-frequency Raman spectra can be interpreted in terms of the translation rotation motions; second, that the random force could be replaced by the total force to calculate the friction coefficient; and finally, that the Einstein frequency is associated with the position of the maximum of the low-frequency Raman spectrum. The results show that the diffusion coefficient increases along the coexistence curve, and its values are almost constant on the critical isochore. The predicted values reproduce qualitatively those obtained by other techniques. The values of D were also calculated by molecular-dynamics simulation and they qualitatively reproduce the behavior of D.