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

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.