Calculation of Raman intensities for the torsional vibrations of ethyl halides

Abstract

Raman intensities are calculated for the torsional vibrations of CH3CH2Cl, CH3CH2Br, CH3CH2I, CH3CHCl2, and CH3CHBr2 using an anisotropic atom-point dipole interaction model to calculate the elements of the molecular polarizability tensor. The calculated relative intensities for the members of the Δv = 2 torsional overtone progression of each of the ethyl halides are in good agreement with experiment. It is predicted that electrically anharmonic terms contribute substantially to the Raman intensities of these transitions. The Δv = 1 torsional transitions of the five molecules are predicted to be 20–30 times more intense than the overtones (although these transitions are not observed because of broadband contours and interference from other vibrational modes). Electrically anharmonic terms in the polarizability expansions also contribute substantially to the intensity of the fundamentals.