On the Origin of the Rotational Barrier in Ethane

The origin of rotational barriers around CC single bonds is still vividly discussed and often referred to concepts like steric repulsion or hyperconjugation. In 1990, a paper was published in which the physical causes for the rotational barrier in ethane, that is, the well-known finding that the potential energy in the eclipsed form is slightly higher than in the staggered form, appears as a consequence of a lowering of the nuclear-electron attraction and not as a greater electronic repulsion in the eclipsed form. Surprisingly, this finding has practically not found its way neither into the textbook literature nor into the scientific discourse. Here we will show, by a careful analysis of the components to the total energy and their dependence on the geometry of the molecule, that the kinetic energy of the electrons and the virial theorem play the decisive role. This is very similar to their role for the origin of the chemical bond.