Steric strain versus hyperconjugative stabilization in ethane congeners

The rotation barriers in the group IVB ethane congeners H3X-YH3 (X, Y = C, Si, Ge, Sn, Pb) are systematically studied and deciphered using the ab initio valence bond theory in terms of the steric strain and hyperconjugation effect. The results obtained show that in all cases the rotation barriers are dominated by the steric repulsion whereas the hyperconjugative interaction between the X-H bond orbitals and the vicinal Y-H antibond orbitals plays a secondary role.