Steric strain versus hyperconjugative stabilization in ethane congeners.

Rotation barriers in the group IVB ethane congeners H(3)X-YH(3) (X, Y = C, Si, Ge, Sn, Pb) have been systematically studied and deciphered using the ab initio valence bond theory in terms of the steric strain and hyperconjugation effect. Our results 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 (and vice versa) plays a secondary role, although indeed the hyperconjugation effect favors staggered structures. By the independent estimations of the hyperconjugative and steric interactions in the process of rotations, we found that the structural effect which mainly refers to the central X-Y bond relaxation makes a small contribution to the rotational barriers. Therefore, we conclude that both the rigid and fully relaxed rotations in the group IVB ethane congeners H(3)X-YH(3) observe the same mechanism which is governed by the conventional steric repulsion.