Vibrational anharmonicity in dideuteromethane: A study of its infrared spectrum up to 17 000 cm^-1

Vibrational assignments for CH₂D₂ are made from a detailed study of its 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 resonance 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 corresponding cubic resonance parameters, and the determination of unperturbed CD₂ stretching fundamental wavenumbers. A simple local mode interpretation of the unperturbed stretching vibrational manifold in CH₂D₂ is attempted. This then yields the 10 corresponding normal mode CH and CD stretching anharmonicity constants consistent with the model applied.