A critical analysis on the rotation barriers in butane.

As a textbook prototype for the introduction of steric hindrance in organic chemistry, the elucidation of the butane rotation barriers is fundamental for structural theory, and requires a consistent theoretical model to differentiate the steric and electronic effects. Here we employed the BLW method to probe the electronic (hyperconjugative) interactions. Results show that although there are stronger hyperconjugative interactions in the staggered anti and gauche conformers than the eclipsed structures, the energy curve and barriers are dominated by the steric repulsion.