As seen in the trans-2-butene model and in the mediumsized
trans cycloalkenes, dihedral angles markedly smaller
than 180 result in a twisting of one lobe of the π bond with
respect to the other, an unequal distribution of electron density
on either face of the double bond, and an increase in the
interaction between the σ and π bonds in the HOMO. According
to Fukui, this σπ mixing causes the unsymmetrical
distribution of the π electron density on both faces of the
double bond, an effect that he termed “nonequivalent orbital
extension” (15).