A bond-orbital analysis of the rotation barrier in borazane and its comparison with ethane

Abstract

Rotational barriers in BH₃NH₃ and CH₃CH₃ have been calculated using second-order perturbation theory by means of an ab initio bond-orbital method using experimental geometries and the Pople STO-3G basis. Although the electronic structure is quite different in the two molecules, comparative component analysis of the results (2.82 and 3.04 kcal mol^–1 favouring staggered conformers) shows that for both molecules the barrier is due chiefly to vicinal interactions arising from exchange-overlap repulsion (penetration) between localized bonds having a closed-shell structure, supplemented for ca. 1/3 by electron delocalization between polarized bonds. The latter effect appears to be 10% larger in borazane than in ethane.