High-energy overtone spectroscopy of some deuterated methanes

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Abstract
High-energy overtone photoacoustic spectroscopy of gas phase CHD3 (ΔvCH=5,6, and 7), CH2D2, CH3D, and CH4 (ΔvCH=6) is reported. The overtone and combination bands of CHD3 display partially resolved rotational structure with laser limited linewidths (∼0.5 cm−1). A combination sum analysis is used to generate excited state rotational constants B′. We present an analysis of the Fermi resonances of CHD3 which indicates strong interactions of the CH stretch with degenerate bending modes. The relative intensities of the Fermi interacting states are in agreement with those calculated from an analysis based on frequency shifts and a two or three level model. However, the rotational B′ constants are not explained by such simple models indicating further interactions with states as yet unobserved. An upper limit of 10 cm−1 is estimated for the splitting of the ‖6,0〉± local mode states for CH2D2, giving support to a description based on the local mode picture. For CH3D and CH4 the spectra are apparently congested by overlapping overtone and combination bands and perhaps other mechanisms not identified in this work. Generally, our results emphasize the importance of the interactions of CH stretching with CH bending motions.