Experimental manifestations of the local-mode description of high energy polyatomic overtone spectra

Abstract

The local-mode model for polyatomic vibrational overtone absorption is extended to include the effects of nonequivalent XH groups in a molecule, and to describe the major features in the more complex combination regions of the spectrum. The high energy overtones and combinations in the spectra of HCCH, HCCD and HCN are interpreted and used to derive selection rules for the combination regions of the overtone spectra of more complex molecules. CHCl 3 and CH 2Cl 2 combination bands between Δυ CH = 3 and Δυ CH = 4 are used to illustrate the rules. The overtone spectra of methane, ethane, benzene, toluene and o-, m-, and p-xylene are measured and interpreted in terms of the local-mode model. In particular the double band structure of the overtones in these molecules is assigned to the discrete excitation of aryl and alkyl CH oscillators.