Reorientational motion in liquid MeCN and its relationship to the properties of the intermolecular torques

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Abstract
The relaxation of the orientational correlation functions found in a simulation of methyl cyanide (MeCN), which is determined by both ‘tumbling’ of the molecular symmetry axis and the ‘spinning’ about the axis, has been investigated in the cumulant expansion scheme. For the tumbling motion the truncated cumulant expansion may be used to relate the short-time, non-exponential behaviour of the correlation function of an appropriately chosen component of the angular velocity; the physical situation is similar to that which has been found in studies of linear and tetrahedral molecules. The cumulant expansion also permits the separation of correlation functions which reflect only the spinning motion; however the shapes of these functions cannot be described within the scheme, that is they are not completely determined by an angular velocity correlation function. The reasons for this failure are examined and it is shown that the non-gaussian nature of the torques acting about the symmetry axis is having a significant effect on the correlation functions for the spinning motion and so on the spectra of perpendicular vibrational transitions in MeCN. In order to understand the origin of these non-gaussian effects, the behaviour found in the simulation is compared with two different stochastic models for the spinning motion. It is concluded that the spectra of suitable perpendicular transitions of prolate symmetric top molecules are considerably more sensitive to details of intermolecular interactions than are those of parallel vibrations in the same molecule.