Conformations of some αβ-unsaturated carbonyl compounds. Part II. Infrared and Raman spectra of methyl and ethyl acrylates and trans-crotonates

Abstract

I.r. spectra of methyl and ethyl acrylates and trans-crotonates in solution and vapour states and Raman spectra in the liquid state are reported and vibrational assignments are suggested. Intensity measurements, at temperatures between –110° and 200°, of i.r. bands are interpreted in terms of conformational equilibria between high- and low-energy forms. The additional enthalpy (ΔH) of the high-energy forms are, for methyl acrylate 315 ± 45 cal. mole^–1 in solution in CS₂ and 400 ± 100 cal. mole^–1 in the vapour, and for methyl trans-crotonate 520 ± 25 cal. mole^–1 in the vapour state. Assignments of bands which show temperature-dependence in the regions 900–1000 cm.^–1 and 1150–1350 cm.^–1 are discussed, and the reasons for the lack of observed temperature variation of bands in the 1600–1800 cm.^–1 region are explained in terms of comparisons with related compounds. It is concluded that the low-energy form is probably the s-trans conformation and the high-energy form is possibly the s-cis-conformation or a non-planar conformation.