Mass effects on the applicability of a local mode description - an analysis of the high energy overtone spectra of difluoro-, dichloro-, dibromo-, and diiodomethane

Abstract

The overtone spectra of difluoro-, dichloro-, dibromo-, and diiodomethane are measured in the liquid phase from 6000 to 12000 A. The principal absorption bands in the three latter molecules are assigned to Δυ CH = 3, 4, 5 and 6 in a pure local mode CH-stretching oscillator. Combination bands between these CH overtones are identified with either local mode combinations or local mode—normal mode combinations. The harmonic local mode frequency and the diagonal local mode anharmonicity constant are obtained for all four molecules. The change in harmonic frequency with halogen substitution is discussed. Off-diagonal local mode and off-diagonal local mode—normal mode anharmonicity constants are derived and their relative magnitudes discussed. The effects of changing mass on the spectrum are pointed out. In particular, it is demonstrated that the Δυ CH = 3 transition in difluoromethane corresponds to what is expected on the basis of symmetry allowed normal mode components rather than what is expected from the multiple excitation of a local oscillator.