In 2001, Pophristic and Goodman¹ initiated a controversy over the nature of the rotational barrier in ethane. Most organic textbooks argue that the barrier is due to unfavorable steric interactions in the eclipsed conformation. The Nature paper argues rather that the staggered conformation is favored due to hyperconjugative interactions between the C-H bond orbital on one methyl interacting with the anti-disposed antibonding C-H orbital of the other methyl group. Schreiner² wrote a follow-up essay where he was surprised by the response to this paper since he thought that the hyperconjugative explanation had been well-accepted within the community.

Now we have a nice review article by Mo and Gao³ that summarizes their recent investigation of the rotational barrier of ethane. Their main approach is to take advantage of the block localization method. Essentially, the methyl e-orbitals are localized to each methyl group, forbidding any hyperconjugation with each other. The energy difference then between the fully relaxed ethane and the block localized energy accounts for hyperconjugation – and this is about 0.76 kcal/mol, or about 25% of the barrier. The most important contributing factor to the barrier is the steric component – this is estimated by comparing the energies of the staggered and eclipsed conformers while freezing the π-like orbitals and removing the hyperconjugation effects. The estimate for the steric component is 2.73 kcal/mol. Mo and Gao conclude that the simple, traditional explanation, namely that steric interactions destabilize the eclipsed conformation, is in fact correct.

References

(1) Pophristic, V.; Goodman, L., "Hyperconjugation not Steric Repulsion leads to the Staggered Structure of Ethane," Nature 2001, 411, 565-568, DOI: 10.1038/35079036.

(2) Schreiner, P. R., "Teaching the Right Reasons: Lessons from the Mistaken Origin of the Rotational Barrier in Ethane," Angew. Chem. Int. Ed. 2002, 41, 3579-3582, DOI: 10.1002/1521-3773(20021004)41:19<3579::AID-ANIE3579>3.0.CO;2-S

(3) Mo, Y.; Gao, J., "Theoretical Analysis of the Rotational Barrier of Ethane," Acc. Chem. Res., 2007, 40, 113-119, DOI: 10.1021/ar068073w