What is the origin of rotational energy barrier in ethane? What is the origin of rotational barrier in ethane?
In most organic chemistry textbooks, we learned that it's originated from steric repulsion between two methyl groups. But recently I heard that there also exists other perspective which explains rotation barrier using hyperconjugation model.
So I searched and read many papers about the origin of rotational barrier in ethane and I notice that there are two competing models for explaining this rotation barrier.
But there are no significant conclusions and it still remains controversy &quot;what is the primary energy term in rotation barrier?&quot;.
Because many authors suggested many calculations to prove what is mainly contributed to rotation barrier and there results for supporting there argument, but each calculation is based on different approximations and theories. (e.g. NBOs are not optimal --&gt; hyperconjugation energy is overestimated)
So my question is that &quot;Is there no conclusion? What factor mainly contributes to ethane rotation barrier?&quot;
As you say, this issue appears to be unsettled. I find the argument in favor of hyperconjugation compelling, but there is theoretical evidence that steric strain contributes as well.
This debate is about 20 years old. Here are two papers summarizing the opposing views:

Pophristic, V.; Goodman, L. (2001). &quot;Hyperconjugation not steric repulsion leads to the staggered structure of ethane&quot;. Nature. 411 (6837): 565–8. doi:10.1038/35079036.
Bickelhaupt, F.M.; Baerends (2003). &quot;The case for steric repulsion causing the staggered conformation of ethane&quot;. Angew. Chem. Int. Ed. 42 (35): 4183–4188. doi:10.1002/anie.200350947.

A more recent paper that has an interesting take:

Baranac-Stojanović, M. (2015). &quot;Theoretical analysis of the rotational barrier in ethane: cause and consequences&quot;. Structural Chemistry. 26 (4): 989–996. doi:10.1007/s11224-014-0557-5.

I cannot read the entire article, but the abstract offers the conclusion that hyperconjugation is responsible for the initial energy rise during rotation, but that steric factors are needed to fully explain the high energy of the eclipsed conformation.