Molecular-dynamics simulations of solvent effects on the C–H stretching Raman bands of cyclohexane-d11 in supercritical CO2 and liquid solvents

Abstract

Solvent-solute interactions and the Raman CH stretching spectrum of cyclohexane-d 11 . II. Density dependence in supercritical carbon dioxide Molecular-dynamics simulations are used to elucidate the molecular basis for 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 density dependence of the spectrum in supercritical CO 2 recently reported by Pan, McDonald, and MacPhail ͓J. Chem. Phys. 110, 1677 ͑1999͔͒, but several liquid solvents (CCl 4 , CS 2 , and CH 3 CN) have also been examined. The frequency shifts and line shapes of the Raman spectrum are simulated using a rigid solute and standard line shape theory in the limit of pure dephasing. Three models for the vibration-solvent coupling are considered. The simplest model, which is based on ground-state forces alone, provides a surprisingly good representation of the density dependence of the linewidths-line shapes but predicts the wrong sign for the gas-to-solution frequency shifts. This failure is due to the neglect of changes in bond polarizability upon vibrational excitation. Allowing for this polarizability difference via a semiempirical approach provides an accurate description of both the linewidths and frequency shifts with a physically reasonable vibrational difference potential. Interpretation of the instantaneous frequency shifts simulated with this model leads to the following general conclusions concerning the solvent effect on these spectra: ͑i͒ The relatively small gas-to-solution frequency shifts observed in experiment are the result of the near cancellation of much larger positive and negative contributions from repulsive and attractive interactions. ͑ii͒ Fluctuations in the instantaneous frequency are sufficiently fast ͑correlation times ϳ100 fs͒ that the spectra are homogeneously broadened in all solvents examined. ͑iii͒ The dynamics of the solvent-solute interactions that determine the Raman line shapes are quite well described by an isolated binary collision ͑''IBC''͒ type picture. ͑iv͒ The simplicity 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. ͑v͒ The linear density dependence of the linewidths observed in supercritical CO 2 reflects the modest extent of local density augmentation in the cyclohexane-CO 2 system.

Solvent–solute interactions and the Raman CH stretching spectrum of cyclohexane-d11. II. Density dependence in supercritical carbon dioxide

“…In order to provide a complementary, and possibly more critical and decisive test of the theories, we undertook the study of cyclohexane-d 11 in supercritical CO 2 described here. As described in the accompanying article, Frankland and Maroncelli 10 have carried out molecular-dynamics simulations of this same system which have provided molecular details critical to the proper interpretation and understanding of the experiments, particularly the linewidths.…”

A transferable electrostatic map for solvation effects on amide I vibrations and its application to linear and two-dimensional spectroscopy

“…In order to get a reasonable description of the solvent effects in CDCl 3 these forces cannot be neglected. In solvents with strong dispersion forces one might want to employ model types suggested earlier 25,[51][52][53][54] or possibly add the dispersion forces to the present map by using one of these perturbative schemes for the contribution of the dispersion forces to the frequency shift.…”

Dephasing of individual rotational states in liquids

“…However, we know that, barring unusual circumstances, vibrational lines do tend to be homogeneously broadened in liquids. 25,49,50 Since there are strong similarities in the basic friction governing both rotational and vibrational dynamics in liquids, 28,29 it is natural to wonder if we should therefore expect to see largely homogeneous broadening of the rotational Raman lines for species such as H 2 and D 2 . Vibration and rotation are very different kinds of motion though, so the question before us is how quantitative is the parallelism between the solvent influences on these disparate degrees of freedom?…”