C-H and C-D interbond coupling in CHD2Cl

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Abstract
The absorption spectrum of vapour-phase chloromethane-d2 has been measured between 500 and 11400 cm-1 using FTIR spectroscopy, and between 12800 and 17100 cm-1 using laser photoacoustic spectroscopy. A total of 46 fundamental and overtone vibrational bands have been assigned, and for 11 of these bands the coarse rovibrational structure has been assigned and rotational parameters measured. The experimental data may be fitted successfully to a model local-mode Hamiltonian that includes the harmonic (1 : 1) resonance between the C-H and C-D bonds, but neglects the anharmonic (1 : 2) C-H stretch/bend Fermi resonance. The model suggests that the predominant extra structure in the high-overtone spectra can be ascribed to C-H/C-D interbond coupling. The ratio of the fitted interbond coupling parameters of the CHD and CD2 groups is consistent with the value calculated from the structural data with the assumption that kinetic coupling dominates over potential coupling. It is suggested that the weaker structure in the high-overtone spectra is due to C-H stretch/bend Fermi resonance, and an order-of-magnitude estimate of the relevant coupling parameter is given.

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