Molecular-dynamics simulations of solvent effects on the C-H stretching Raman bands of cyclohexane-d(11) in supercritical CO2 and liquid solvents

Molecular-dynamics simulations are used to elucidate the molecular basis fo r the solvent effects on the isolated C-H stretching bands observed in the Raman spectrum of cyclohexane-d(11). The main focus is on modeling the dens ity dependence of the spectrum in supercritical CO2 recently reported by Pa n, McDonald, and MacPhail [J. Chem. Phys. 110, 1677 (1999)], but several li quid solvents (CCl4, CS2, and CH3CN) have also been examined. The frequency shifts and line shapes of the Raman spectrum are simulated using a rigid s olute and standard line shape theory in the limit of pure dephasing. Three models for the vibration-solvent coupling are considered. The simplest mode l, which is based on ground-state forces alone, provides a surprisingly goo d representation of the density dependence of the linewidths-line shapes bu t predicts the wrong sign for the gas-to-solution frequency shifts. This fa ilure is due to the neglect of changes in bond polarizability upon vibratio nal excitation. Allowing for this polarizability difference via a semiempir ical approach provides an accurate description of both the linewidths and f requency shifts with a physically reasonable vibrational difference potenti al. Interpretation of the instantaneous frequency shifts simulated with thi s model leads to the following general conclusions concerning the solvent e ffect on these spectra: (i) The relatively small gas-to-solution frequency shifts observed in experiment are the result of the near cancellation of mu ch larger positive and negative contributions from repulsive and attractive interactions. (ii) Fluctuations in the instantaneous frequency are suffici ently fast (correlation times similar to 100 fs) that the spectra are homog eneously broadened in all solvents examined. (iii) The dynamics of the solv ent-solute interactions that determine the Raman line shapes are quite well described by an isolated binary collision ("IBC") type picture. (iv) The s implicity of the dynamics, and the success of this IBC description, is due at least in part to the special, localized character of these isolated C-H stretching modes. (v) The linear density dependence of the linewidths obser ved in supercritical CO2 reflects the modest extent of local density augmen tation in the cyclohexane-CO2 system. (C) 1999 American Institute of Physic s. [S0021-9606(99)52702-5].