CH stretching frequencies, bond lengths and strengths in acetone, acetaldehyde, propene and isobutene

Abstract

I.r. CH stretching bands have been studied in partly deuterated acetone, acetaldehyde, propene and isobutene, in both gaseous and crystalline phases. “Isolated” CH stretching frequencies are obtained both for the methyl groups, each of which contains two weak out-of-plane and one strong in-plane CH bond, and for the olefinic hydrogens, although the latter are more susceptible to Fermi resonances with levels 2 v CC or v CC + δ CH. Bond lengths, dissociation energies and force constants are predicted and comparison made with microwave bond lengths and ab initio calculated ones. Some insight is obtained by considering the out-of-plane methyl hydrogen atoms to be trans to the CO or CC bonding pairs, considered as two “banana” bonds, or bent σ ± π hybrids. The substitution effects of the methyl group on olefinic CH frequencies are similar to those found in saturated hydrocarbons and alkyl halides. The crystal spectra provide information about the site groups.