Quantitative orbital analysis of ab initio SCF-MO computations. Part 3. Torsional isomerisms in but-2-enes

Abstract

We describe the results obtained from quantitative orbital analysis, using ab initio SCF-MO computations, of the torsional isomerism of but-2-ene. The analysis is based on the use of σ fragment localized MOs and π fragment canonical MOs and the energy effects associated with their interactions are estimated using the quantitative Perturbational Molecular Orbital (PMO) and Total Energy approaches. It is found that conformational preferences in both cis- and trans-but-2-ene are determined by the π non-bonded interactions under matrix element control, while the trend in the relative stability of the two stable geometric isomers, t_ee > c_ee, is determined mainly by the difference in the steric repulsions between the σ_CH bond MOs of the two methyl groups.