VIBRATIONAL DEPHASING IN LIQUID CARBONYL SULFIDE - COMPARISON BETWEENMOLECULAR-DYNAMICS OF RIGID AND FLEXIBLE MOLECULAR-MODELS

Vibrational dephasing of the v(1) and v(3) stretching modes of carbony l sulfide, OCS, was investigated by computer simulations of the liquid in the states corresponding to 183, 243 and 273 K. Perturbation theor y plus simulation of rigid molecules result in vibrational correlation functions which disagree with experimental ones. Systematic changes t o other intermolecular potential and anharmonicity constants proposed in the literature show that dephasing in this system is more sensitive to modifications in the inter than in the intramolecular potential. I n order to investigate possible flaws of the perturbation theory/rigid molecules simulation method, molecular dynamics considering fully fle xible OCS molecules was undertaken for the liquid at 273 K. Even then, the slow decay of the vibrational correlation function points to the effective intermolecular potential, and not to the approximations invo lved in the rigid simulation method, as responsible for disagreements with available experimental data. The calculated increasing of vibrati onal frequencies in the liquid as compared to the gas phase values is not confirmed by the experiments, and indicates that the effective Len nard-Jones potential proposed in the literature is too much repulsive in predicting vibrational frequency shifts in liquid OCS. (C) 1997 Els evier Science B.V.