Low frequency vibrational spectra, methyl torsional potential functions, and internal rotational potential of methyl vinyl ether and methyl-d3 vinyl ether

Abstract

The vibrational spectra (infrared and Raman) of gaseous methyl vinyl ether and methyl-d3 vinyl ether have been recorded below 500 cm−1. A large amount of torsional data are reported and the observed bands are assigned on the basis of an equilibrium between an s-cis conformer and a high energy conformer which is shown to be the gauche form. For methyl-d3 vinyl ether, the torsions are well separated in frequency and they are not greatly coupled, whereas for the ’’light’’ molecule, the torsional spectra are quite complicated, presumably due to coupling and/or a perturbation between the methyl and asymmetric torsional modes. Both series of transitions for the methyl and methoxy torsional modes were assigned for the methyl-d3 vinyl ether molecule and the assignments were confirmed by the observation of a large number of double jumps for both torsions. The potential function governing the asymmetric torsional mode was found to be V1=−413±6 cm−1 (1.18 kcal/mole), V2=1434±21 cm−1 (4.10 kcal/mole), V3=1177±10 cm−1 (3.36 kcal/mole), and V6=−96±3 cm−1 (0.27 kcal/mole) with the s-cis conformer being more stable than the gauche conformer (torsional angle 114°) by approximately 402 cm−1 (1.15 kcal/mole). The barriers to s-cis–gauche, gauche–gauche, and gauche-s-cis interconversion were found to be 2215 cm−1 (6.33 kcal/mole), 290 cm−1 (0.83 kcal/mole), and 1774 cm−1 (5.07 kcal/mole), respectively. The barriers to methyl rotation for the s-cis and gauche conformers were found to be slightly higher than 1610 cm−1 (4.60 kcal/mole) and 514 cm−1 (1.47 kcal/mole), respectively. Possible reasons for this large variation are discussed. Tentative assignments of the torsional modes for the light compound are also given and discussed.