Reorientational molecular motion in liquids: a comparison of Raman and Rayleigh scattering

Abstract

The spectral distribution of Rayleigh and Raman anisotropy scattering has been measured for ten simple organic liquids at 20°C and the interpretation in terms of reorientational motions is discussed. All spectra approximated closely to a Lorentzian form. In most cases the spectral linewidths derived from Raman bands were greater (by up to 150 per cent) than those derived from Rayleigh measurements. Furthermore, the linewidths of Raman anisotropy scattering from different totally symmetric vibrations of the same molecule were found to differ by as much as 100 per cent. The Rayleigh-Raman linewidth differences correlate strongly with the Raman vibrational linewidths and it is suggested that there are significant interactions between rotational and vibrational energies as a result of molecular interactions in the liquids. These observations obscure any interpretation of linewidth differences in terms of cooperative reorientational motions in these liquids. Anisotropy linewidths were also studied as a function of concentration for solutions of nitrobenzene and methyl cyanide in carbon tetrachloride. The data are discussed in the light of the above conclusions.