Hyperconjugative interactions in vinylic systems: the problem of the barrier to methyl rotation in acetone

Abstract

The origin of the barrier to methyl rotation in acetone has been studied through natural bond orbital theory. The analysis is divided in two parts, one involving the stability of bonds and lone pairs and other involving hyperconjugative donor–acceptor interactions. In the first part, we observed that the carbon–carbon bond of the rotor is destabilized upon rotation, and it represents the largest contribution to the barrier among bond energy and lone pair components, in accordance to studies of similar molecules. In addition, lone pairs were found to play an important role. The analysis of hyperconjugation effects showed that interactions involving the out-of-plane sigma CH orbitals and sigma and pi CO orbitals contribute to increase the rotational barrier, while analogous interactions involving the in-plane CH bond are either null or antibarrier forming. By excluding the mentioned donor–acceptor interactions during geometry optimization, it was possible to estimate their influence on bond and lone pair stabilities. From this analysis, it was observed that the destabilization of bonds and lone pairs upon rotation is determined by some of the considered hyperconjugative interactions, which led us to conclude that the latter are the primary source of the rotational barrier. Finally, a simple set of canonical structures is proposed to describe this effect. The model showed to be useful in the qualitative understanding of the rotational barrier in similar systems and even of conformational preferences.