COMPUTER-SIMULATION OF VIBRATIONAL DEPHASING IN CARBON-DISULFIDE

Vibrational dephasing of the totally symmetric stretching mode of liqu id carbon disulfide at different pressures at room temperature is inve stigated by molecular dynamics simulation. The thermodynamic states si mulated correspond to the liquid at atmospheric pressure, 1 kbar, 4 kb ar and 8 kbar, all at room temperature (294 K). An effective vibration al-rototranslational coupling potential, which contains an exponential repulsive term and an attractive long range term, is assumed. The fun damental role of anisotropy in this potential is emphasized. The exper imental trend of average gas-to-liquid vibrational frequency shifts an d vibrational correlation functions are well reproduced by molecular d ynamics. The simulations show that correlation functions of vibrationa l frequency fluctuation of CS2 are not a single exponential, what inva lidates the well-known Kubo's formula for vibrational correlation func tion. The long time behaviour seen in the frequency fluctuation correl ation functions, present even if one assumes a purely repulsive, short range coupling potential, is due to the characteristic hindrance of t he rototranslational dynamics of CS2. The simulations also show a smal l correlation between vibrational and reorientational relaxations, eve n when an isotropic coupling potential is considered, what is an indic ation of coupling between vibrational and reorientational degrees of f reedom by translational motions.