Light scattering studies of transverse sound wave and molecular motion in benzonitrile

The zero-frequency shear wave dip appearing in the depolarized Rayleigh spectrum in benzonitrile has been studied as a function of concentration and temperature. The solution study was carried out at constant viscosity equal to the viscosity of liquid benzonitrile at each temperature. The result indicates that the presence of shear wave fine structure does not depend on the collective orientational fluctuations. The orientational and vibrational relaxation times of benzonitrile were measured at various concentrations and temperatures. The orientational relaxation times show no concentration dependence at any temperature, suggesting that the pair correlation is negligible at all concentrations. The orientational relaxation times obtained from the Raman measurements are in good agreement with the depolarized Rayleigh values at the same temperature and concentration, again indicating pair correlation is negligible in benzonitrile. Thus, both the Raman and depolarized Rayleigh scattering techniques measure the single particle relaxation time of benzonitrile. The single particle times were compared with the predictions of the hydrodynamic slip and stick models for rotational diffusion. In contrast to the results obtained for most small molecules, the stick model better approximated the experimental results.