Analysis of rotational barriers in molecules on the basis of exchange perturbation theory

Abstract

Exchange perturbation theory of the Rayleigh-Schrödinger type in first and second orders is applied to a calculation of barriers of internal rotation about single bonds in molecules. The barrier is assumed to arise as a result of exchange interactions between nonbonded atoms; the electrons of these atoms are replaced by one effective electron per atom, with a Gaussian charge distribution. Spins of effective electrons on different atoms are taken to be parallel. The method is applied to a large variety of molecules; calculated results for the barriers are in substantial agreement with experiment. A decomposition of the interaction energy for different conformers and for the barrier itself is given in terms of two- and many-atom exchange interactions.

A study of methyl group dynamics and barrier heights in a homologous series of unbranched ketones

“…These theoretical investigations have commonly employed a variety of ab initio and semi-empirical calculations. Broadly speaking the agreement with experiment is sufficiently good to confirm that the site B barrier heights observed in this investigation are understood [15,16]. The source of the intra-molecular contribution to the barrier for site B is confirmed to be steric in nature and repulsive dominant.…”