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WIREs Comput Mol Sci
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Rotational barriers in alkanes

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Abstract 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 other methods 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. © 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 164‐171 DOI: 10.1002/wcms.22 This article is categorized under: Structure and Mechanism > Computational Materials Science

Illustration of the hyperconjugation effect. The red lines refer to nonoptimal bond orbitals.

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Rotational potential energy profiles for butane around the central CC bond with the MP2, HF, and block‐localized wavefunction methods.

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Rotation barrier along with the steric repulsion, hyperconjugation, electronic relaxation, and geometric relaxation energy changes with respect to the torsional angle.

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The e‐symmetric group‐localized orbitals of methyl groups in ethane. 1πx and 1πy are degenerate and occupied, whereas the degenerate 2πx and 2πy are unoccupied. Orbital interactions in ethane: (a) hyperconjugative interaction, (b) steric interaction, and (c) overall interactions.

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Major competing explanations for the ethane rotation barrier.

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