VIBRATIONAL ANHARMONICITY IN DIDEUTEROMETHANE - A STUDY OF ITS INFRARED-SPECTRUM UP TO 17000 CM(-1)

Vibrational assignments for CH2D2 are made from a detailed study of it s gas-phase infrared spectrum below 17 000 cm(-1). up to six quanta of CH stretching excitation. Some 77 overtone and combination vibration levels are identified. These allow extraction of empirical values for approximately one-half of the 45 normal mode anharmonicity constants, and their comparison with recent ab initio predictions. Severe Fermi r esonance perturbations at low excitation levels in both CD2 stretching manifolds preclude the direct extraction of any of the 17 associated anharmonicity constants. However, observation of seven Fermi resonance diads below 4000 cm(-1) permits the accurate definition of the corres ponding cubic resonance parameters, and the determination of unperturb ed CD, stretching fundamental wavenumbers. A simple local mode interpr etation of the unperturbed stretching vibrational manifold in CH2D2 is attempted. This then yields the 10 corresponding normal mode CH and C D stretching anharmonicity constants consistent with the model applied . (C) 1997 Elsevier Science B.V.