| Abstract
| - Rotation barriers in the group IVB ethane congeners H3X−YH3 (X, Y = C, Si, Ge, Sn, Pb) have beensystematically studied and deciphered using the ab initio valence bond theory in terms of the steric strain andhyperconjugation effect. Our results show that in all cases the rotation barriers are dominated by the stericrepulsion whereas the hyperconjugative interaction between the X−H bond orbitals and the vicinal Y−Hantibond orbitals (and vice versa) plays a secondary role, although indeed the hyperconjugation effect favorsstaggered structures. By the independent estimations of the hyperconjugative and steric interactions in theprocess of rotations, we found that the structural effect which mainly refers to the central X−Y bond relaxationmakes a small contribution to the rotational barriers. Therefore, we conclude that both the rigid and fullyrelaxed rotations in the group IVB ethane congeners H3X−YH3 observe the same mechanism which is governedby the conventional steric repulsion.
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