The effects of symmetry within the local mode picture: A reanalysis of the overtone spectra of the dihalomethanes

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Abstract
The overtone spectra of dichloromethane, dibromomethane, and di-iodomethane are measured in the spectral region from 1770 to 850 mμ. The spectra in this region, and the CH stretching fundamentals, are analyzed in terms of a local mode picture. The spectra display the splittings predicted between the symmetrized combinations of local mode states arising from coupling of the two CH oscillators. We calculate the coupling matrix elements directly using a ’’ladder’’ approximation based on the properties of harmonic oscillator wave functions. The validity of this approximation is demonstrated by a comparison to matrix elements of Morse oscillators. Through a direct calculation of the kinetic energy coupling in dichloromethane from the observed bond angle, we show that kinetic energy provides the principal source of coupling in all of these molecules. Differences in overtone spectral structure between the three molecules are attributed to a Fermi resonance involving two quanta of HCH bending. Adiabatic changes in this bending frequency with increased local CH stretching excitation are discussed.