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J. Chern. SOC., Furuduy Truns. 2, 1988, 84(9), 1423-1442 Infrared Studies of CH and CD Stretching Anharmonicity J. Lindsay Duncan, Donald C. McKean,* Ivy Torto, Alan Brown and Allan M. Ferguson Chemistry Department, University of Aberdeen, Aberdeen AB9 2UE Progress is reported in the analysis of CH and CD stretching overtones and their accompanying Fermi resonances in C,H4, CLH3D, CHzCDz and C2HD3. CH stretching overtone data have been obtained for a number of partially deuterated alkanes, halogenated alkanes, MeCN, MeC2H, MeNH, , Me,", MeOH and Me20. Values of anharmonicity constants x,, obtained from these are compared with the results of local-mode studies. A provisional Fermi resonance analysis of CH(CD3)3 is reported. The possiblity of a relationship between x,,, w, and CH dissociation energy is explored. The purpose of this paper is to summarise recent and current work at Aberdeen on the anharmonicity associated with CH and CD stretching vibrations, at two levels of sophistication, and thereafter to discuss the implications of our results for the understand- ing of earlier correlations between isolated CH stretching frequencies and dissociation energies' or high-overtone, local-mode transitions.2 Recent work elsewhere'-' has emphasized the extent to which CH stretching energy levels may be affected by Fermi resonances involving levels associated with CH bending. The work described in Part 1, on isotopic species of ethylene, identifies several types of Fermi resonance effect in analyses based on both normal- and local-mode models. In Part 2, by contrast, we report CH stretching overtone data for a number of partially deuterated substances in situations where it is not yet possible to include Fermi resonance effects. These are of interest in relation both to similar analyses of local-mode spectra of the unlabelled compounds *-12 and also to possible differences of CH properties within a given methyl group. CH and CD Stretching Anharmonicities in Isotopic Ethylenes We are currently investigating the CH and CD stretching vibrational manifolds, up to 6 quanta in favourable cases, over the isotopic series H2CCH2, H2CCHD, H2CCD2, HDCCD2, D2CCD2. We present here our preliminary results, which demonstrate both a high degree of compatibility over the series and nice agreement with the predictions of theoretical models, once the effects of Fermi resonance perturbations are taken into account. It was anticipated that a study of ethylene spectra up to high vibrational excitation might be a complex task, because it is known that at the fundamental V = 1 CH stretching level considerable Fermi resonance perturbations are present in all isotopic species. The situation with the CD stretching fundamentals seems more straightforward. We therefore commenced our study with H2CCD2, which we considered might yield information on both CH and CD stretching anharmonicities, on the assumption that each end of the molecule can be treated separately as an independent XY, stretching entity. This study rapidly necessitated extension to C2H4 and C2D4 for corroborative information. Having obtained consistent results for these three isotopic ethylenes, we are now including the unsymmetrical C2HD3and C2H3Dspecies in our study. 1423

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