Isolated CH stretching frequencies, CH bond lengths and strengths in ethyl and isopropyl halides

Abstract

“Isolated” CH frequencies have been obtained for all CH bonds in the compounds CH 3CH 2 X and (CH 3) 2CH X, where X = F, Cl, Br, I, from i.r. spectra of appropriate partially deuterated species. From these, bond lengths ( r 0), dissociation energies (D 298 0) and force constants are predicted. Trends found previously in t-butyl compounds occur also in the present molecules. The depressive effect of α-fluorine substitution seen in methyl fluoride gives place to an elevation in isopropyl fluoride. The range of variation in the trans effect of halogen from fluorine to iodine is seen to increase with the number of methyl groups present. The gauche effect of halogen is an elevation roughly independent of the halogen concerned. A CH bond trans to methyl is consistently stronger than one trans to hydrogen, in the same methyl group, as previously found in propane.