CH stretching frequencies, bond lengths and strengths in halogenated ethylenes

Abstract New i.r. studies have been made of the CH stretching and other relevant bands for CH2CHF, CH2CHCl, CHDCHCl (cis/trans) CHDCDCl (cis/trans), CH2CHBr, CHDCHBr (cis/trans), CHDCDBr (cis/trans), CH2CF2, CHClCHCl (cis/trans), CHBrCHBr (cis/trans), CHClCCl2 and CHBrCBr2, in gaseous and/or condensed phases. Some Raman spectra have also been recorded for CH2CHF, CHClCHCl and CHBrCHBr. From these and previous data on CH2CX2, CHXCHX and CHXCX2 compounds, isolated CH stretching frequencies are obtained, from which bond lengths, dissociation energies and force constants are predicted. The substituent effects (S values) of CX bonds on the frequencies of CH bonds in cis, trans or α positions are obtained either directly from the CH2CHX isolated frequencies (X = Cl, Br) or indirectly from those of the X3 and X2 species (X = F, Cl, Br). The fair agreement between the two sets for Cl, Br shows that the effects are approximately additive; the incomplete results for the fluorides are promising in this respect. A pronounced trans effect of the CX bond (S tF> S tCl > S tBr > S tI) is discovered, comparable with similar effects in haloalkanes. S cF and SαF are much higher in the ethylenes than in alkanes. An unusual apparent case of Fermi resonance is discovered in vinyl bromide. One component of a dyad is almost unobservable in both i.r. and Raman spectra.

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