Molecular structures of the rotational isomers of methyl vinyl ether from infrared and raman spectra and non-empirical calculations

Abstract

The IR spectrum of the vapour phase and the Raman spectra of the solid, liquid and vapour phases of methyl vinyl ether have been recorded. All of the fundamentals of the s-cis isomer have been assigned and approximately described; for the s-trans isomer new bands have been detected in the Raman spectra and now eleven Fundamentals are known. Reassignment of the low-frequency region leads to a value for the barrier to methyl internal rotation of the s-cis isomer in close agreement with that derived from microwave data: V 3 = 1427 cm −1 (4.08 kcal mol −1) and V 6 = −40 cm −1 (−0.11 kcal mol −1). Ab initio calculations at the minimal basis SCF level including electron correlation have been performed in order to study the two possible internal rotations in the molecule. The correlation effects lower the energy difference between the s-trans and the s-cis isomers to 1.3 kcal mol −1 and enhance the barrier to interconversion, whereas they have very little effect on the barrier to methyl internal rotation; in the s-trans isomer this barrier is calculated to be as low as 0.67 kcal mol −1. A comparison is made between the first two measured and computed ionization potentials for the two isomers. The choice of the best set of structural parameters for the s-cis form is discussed. Repulsive overlap interactions between bond orbitais and conjugation may explain the stability of the s-cis form of methyl vinyl ether and other molecules.