Origin of the ethane rotational barrier: Steric repulsion or hyperconjugation?


A. Patrícia Bento, ap.bento@few.vu.nl, Department of Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, NL-1081 HV Amsterdam, Netherlands
It has recently been argued on the basis of natural bond orbital (NBO) analyses that the barrier to rotation around the C-C bond in ethane (see illustration) is not to be explained by the generally accepted picture of steric hindrance between vicinal C-H bonds in the eclipsed conformation but is caused by more favorable orbital interaction in the staggered conformation.

However, we show that an analysis based on canonical Kohn-Sham molecular orbital (MO) theory fully corroborates the classical interpretation of the barrier as caused by steric repulsion between vicinal C-H bonds and we conclude it is perfectly valid for organic chemists to adhere to this view. This has meanwhile been confirmed through an independent valence bond (VB) approach. We comment on the possible source of the difference between our analysis and the NBO one.