13C frequency shifts and the general harmonic force fields of methyl chloride, bromide and iodide

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Abstract
General harmonic and hybrid orbital force fields have been calculated for methyl chloride, bromide and iodide using the best available frequency, Coriolis coefficient, and centrifugal distortion data, including the recent 13C frequencies for the chloride and iodide. Anharmonicity corrections were made to the fundamental frequencies using Dennison's rule and x values of 0·04 for CH stretching, 0·02 for CH bending, and 0·01 for CX stretching modes. These were the most reasonable values which did not seriously over-compensate for the product rule deficiencies in the 12C and 13C frequencies. The 13C and CD3X frequency information was input in the form of frequency shifts from the 12CH3X frequencies. All 12 parameters in the most general harmonic force fields are determined with significance for the first time, although the previously uncertain parameters F12 and F13 are only determined with good precision when the 13C frequency data are utilized in addition to the other information. The signs of F12, F45 and F46 are in agreement with the predictions of the hybrid orbital model, which are quantitatively obeyed more closely in methyl iodide than in methyl bromide or chloride. There is a slight incompatibility between the centrifugal distortion data and the 13C frequency shifts in the A1 species, attributed here to the unknown effects of higher-order terms on the distortion constants. The effect of using the best present estimate of the equilibrium CH bond length, r e(CH) = 1·085 å, instead of the previous estimate of r e = 1·095 å, is considered and the changes in the force constants listed.