A study of vibrational dephasing of the A₁ modes of CH₃CN in a computer simulation of the liquid phase

Abstract

The vibrational dephasing of the four A ₁ modes of CH₃CN has been calculated for a simulation of the liquid phase at 288 K assuming that it is caused by generalized forces interacting with the cubic anharmonicities of each mode. The forces along each mode were resolved into electrostatic and Lennard-Jones components and the mean values, probability distributions and time autocorrelation functions were calculated. Line shifts in the liquid state were evaluated from the mean values. Line shapes are related to the shift-shift time correlation functions and hence to force-force correlation functions using the cumulant approximation. This was found to be a good approximation and the vibrational correlation functions (Fourier transforms of the line shapes) were computed using two sets of anharmonicity constants. The time scale of the shift fluctuations was never slow, so that the line shapes are nearly lorentzian in this model.