Studies of vibrational relaxation in liquid methyl iodide

Abstract

The intermolecular coupling contribution to the vibrational relaxation function for v 2 of methyl iodide has been deduced using the method of isotopic dilution. The band widths, correlation functions and frequency shifts of both isotropic and anisotropic spontaneous Raman spectra have been studied at 295 and 205 K for solutions of 0·1 mole fraction of CH3I in CD3I and in CS2. Although isotopic dilution leads to a considerable decrease in the rate of vibrational relaxation, neither the corresponding first moment changes nor the I vv-I VH frequency shifts very large. This implies that neither permanent dipole nor other intermolecular interactions lead to appreciable liquid phase ‘order’ in CH3I. This is understood in terms of a model of the intermolecular coupling potential which shows that, for weak ‘order’, the second moment is affected much more than the first moment. Our results for CH3I are different from those predicted by a recently published theory of intermolecular coupling in pure liquids (albeit for unspecified potentials) which predict large frequency shifts on isotopic dilution.