STATISTICAL-MECHANICS OF SOLVENT-INDUCED FORCES AND VIBRATIONAL FREQUENCY-SHIFTS - LOW-DENSITY EXPANSIONS AND MONTE-CARLO SIMULATIONS

Theoretical expressions are presented for the solvent configuration averaged force on a diatomic solute throughout the vapor-liquid density range. Analytical low density expansions and solvent configurational space averages are used to predict solvent induced changes in solute vibrational frequency. Purely classical Monte Carlo simulation results for a system representing bromine (Br2) dissolved in argon agree quantitatively with previous coupled quantum-classical results of Herman and Berne, up to liquid densities. It is found to be impossible to obtain a red gas to liquid shift (such as that typically observed experimentally) in any realistic diatomic system with only binary solvent atom-solute atom interaction potentials. However, redshifts are predicted when a three-atom potential, in which the solute-solvent interaction depends on solute bond length, is introduced.