A study of vibrational anharmonicity, fermi resonance interactions, and local mode behavior in CH₃Cl

The infrared spectrum of CH₃Cl has been studied up to 16 500 cm^-1 and vibrational assignments made for up to six quanta of excitation in CH stretching. Analysis of a total of 66 vibrational levels has been made in terms of a joint local mode and normal mode model, local with respect to CH stretching vibrations and normal with respect to other vibrations. Fermi resonance interactions with CH₃ deformation overtones must be included in the analysis. Reproduction of the observed vibration levels with a rms error of 2.61 cm^-1 is achieved in terms of a 37-parameter model, with the four anharmonicity constants and two Darling-Dennison constants associated with CH₃ group stretchings constrained to be interrelated through local mode relations. The parameters obtained are in excellent accord with those available for the other methyl halides, and with established structural differences between the molecules.