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 CH 2CHF, CH 2CHCl, CHDCHCl ( cis/trans) CHDCDCl ( cis/trans), CH 2CHBr, CHDCHBr ( cis/trans), CHDCDBr ( cis/trans), CH 2CF 2, CHClCHCl ( cis/trans), CHBrCHBr ( cis/trans), CHClCCl 2 and CHBrCBr 2, in gaseous and/or condensed phases. Some Raman spectra have also been recorded for CH 2CHF, CHClCHCl and CHBrCHBr. From these and previous data on CH 2CX 2, CHXCHX and CHXCX 2 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 CH 2CHX isolated frequencies (X = Cl, Br) or indirectly from those of the X 3 and X 2 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 t F> S t Cl > S t Br > S t I) is discovered, comparable with similar effects in haloalkanes. S c F and S α F 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.