Rotational isomerism and barriers to internal rotation in methyl vinyl ether and 3-fluoropropene

Abstract

The electronic structures of methyl vinyl ether (MVE) and of 3-fluoropropene (3FP) are investigated through ab initio SCF calculations using minimal basis sets of AOs contracted from 7 s, 3 p(for C, O and F) and 3 s(for H) Gaussian functions. The predicted structures and dipole moments of the two conformers of 3FP and the computed barrier heights hindering the internal rotation in 3FP agree quite well with experiment. The most stable conformer of MVE is correctly found to be s-cis-staggered, while the second conformer is predicted to be s-trans, which is at variance with a gauche form suggested by electron diffraction studies. A barrier height in the range 5.7–6.6 kcal/mol is found by assuming C—O bond distances ranging from 1.36 to 1.42 Å. The conjugation in MVE is studied with reference to 3FP using the Mulliken population analysis and the bond dipole moments calculated over localized MOs. The computed energy maxima between the two stable forms of 3FP and MVE are interpreted in terms of steric and conjugation contributions.