Overtone spectra as a probe of molecular conformations: local mode description of the CH-stretching overtone spectra of the haloethanes

Abstract

The overtone spectra of 1,1,2,2-tetrachloro- and tetrabromoethane in the liquid phase are measured and analyzed in the region of the CH-stretching local mode overtones corresponding to Δν CH = 3, 4, and 5. Overtone spectra are also measured and analyzed in the same region for pentachloroethane in the liquid phase and for pentabromoethane in carbon tetrachloride solution. The overtone spectra of tetrachloroethane and pentachloroethane are also measured in the gas phase in the region of Δν CH = 3. The principal absorption bands in all cases are assigned to CH-stretching local mode overtones. Asymmetry in the overtone bands of the pentahaloethanes is attributed to intermolecular interaction whereas the principal source of asymmetry in the tetrahaloethane overtone bands is attributed to the independent absorption of two rotational conformers. The CH-stretching local mode parameters, local mode frequency and diagonal local mode anharmonicity, are calculated from the spectra for the pentahaloethanes. The corresponding parameters are also obtained for the trans and gauche conformers of the tetrahaloethanes from a decomposition of their overtone spectra into contributions from the two rotational conformers. Combination peaks are identified in the spectra of all four molecules involving 2, 3, and 4 quanta of local CH-stretching and two quanta of CH-bending modes. The increased interaction of the combination state with the pure CH-stretching overtone state with increasing energy is described. An analysis of the combination bands yields the anharmonic coupling constants between the local CH-stretching mode and the CH-bending mode.