| Summary: | Infrared overtone absorption spectra of vapour phase methyl fluoride, and methyl bromide-d<sub>3</sub> have been measured and these results, combined with existing near infrared and visible spectra, have been analysed in terms of a model of local mode C-H/D stretching vibrations including the effects of Fermi resonances with the overtones of the C-H/D bending modes. Comparable analyses of CH<sub>3</sub>Cl, CH<sub>3</sub>Br, CH<sub>3</sub>I and CHD<sub>2</sub>Cl, up to 6 quanta of C-H stretching, improve earlier studies through the more quantitative treatment of the stretch/bend interactions. The empirical vibration frequencies, anharmonicity constants and vibration interaction constants determined in these analyses are found to be in good agreement with literature ab initio results. In particular discrepancies between the effective empirical Fermi resonance parameters and the theoretical normal coordinate cubic force constants are rationalised by consideration of higher order effects within the framework of the traditional perturbation theory treatment. An important feature of this work is the development of the understanding of the relationship between the local mode and normal mode models of coupled X-H stretching vibrations. In particular the simple harmonically coupled local mode model has been extended to include the effects of anharmonic interactions. Explicit relationships have been derived between this extended local mode parameter set and those of the conventional mode description for the X-H stretching vibrations of ammonia, methane, allene, ethylene and benzene type systems. The complete general harmonic force field of methyl fluoride has been recalculated using the most recent literature frequency, Coriolis and centrifugal distortion data for <sup>12</sup>CH<sub>3</sub>F, <sup>13</sup>CH<sub>3</sub>F, <sup>12</sup>CD<sub>3</sub>F, <sup>12</sup>CHD<sub>2</sub>F and <sup>12</sup>CH<sub>2</sub>DF. The anharmonic corrections applied to the observed frequency data are considered to be more realistic than those used hitherto. There is excellent overall agreement between the fitted force constants and the highest quality ab initio force field currently available.
|
|---|
