Gas-Phase Structures and Conformational Properties of tert-Butyl Vinyl Ether and (Z)-Methyl 1-Propenyl Ether

Abstract

The gas-phase structures of tert-butyl vinyl ether, t-BuOC(H)CH2 (t-BVE), and (Z)-methyl 1-propenyl ether, MeOC(H)C(H)Me (Z-MPE), have been studied by gas electron diffraction and ab initio calculations (HF/3-21G and MP2/6-31G*). For both compounds only a single conformer with a slightly nonplanar anti structure of the CC−O−C skeleton was observed. The vibrationally averaged dihedral angles φ(CC−O−C) are 167(5)° (t-BVE) and 161(5)° (Z-MPE), respectively. The experimental scattering intensities are fitted slightly better with dynamic models and double minimum potentials for the internal rotation around the O−C(sp2) bonds. The dihedral angles of the equilibrium structures φe(CC−O−C) are 170(5)° for t-BVE and 161(8)° for Z-MPE. The potential barriers at the exactly planar anti conformation (φ(CC−O−C)=180°) are V0 = 0.15(10) and 0.20(10) kcal mol-1, respectively. The experimental results are reproduced very well by MP2/6-31G* calculations (φe = 169.0° and 154.2° and V0 = 0.12 and 0.11 kcal mol-1 for t-BVE and Z-MPE, respectively). The HF/3-21G method, however, predicts a planar structure (φe= 180°) for t-BVE and an anticlinal structure (φe = 138.7°) for Z-MPE.