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

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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.

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