Rotational barriers in alkanes

Rotational barriers in alkanes play a fundamental role in the stereochemistry and dynamics of alkanes and others such as proteins. Yet, the proper understanding of their origin which is central to chemical theory remains controversial. Currently, there are two major competing models to interpret the barriers, one is the steric repulsion model and the other is hyperconjugation model. No consensus has been reached. It is thus important to critically examine the quantum mechanical approaches producing conflicting data which lead to these models, as various approximations must be introduced to derive either the steric or hyperconjugative interaction energies in these approaches. The hyperconjugation model is largely based on the popular natural bond orbital (NBO) analysis which can estimate individual interactions between occupied bond orbitals and vicinal unoccupied antibond orbitals. But the concern is that these localized bond orbitals are projected out from a delocalized wavefunction and thus nonoptimal. Alternatively, recent studies with othermethods notably the ab initio valence bond theory where localized orbitals are self-consistently optimized reinstate the conventional steric repulsion model, although the NBO method correctly predicts that there is stronger hyperconjugative interaction in staggered structures than in eclipsed structures. After all, it is the steric effect rather than the hyperconjugation effect that plays a dominating role in rotational barriers in alkanes. (C) 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 164-171 DOI: 10.1002/wcms.22