pi-Electrons in Three Dimensions.

In this review of our work for Accounts of Chemical Research, orbital orthogonality is offered as a unifying principle for the existence of pi- electrons (orbitals), which is applicable in one-, two- and three-dimensions. A pi-orbital is defined to be that hybrid orbital which is locally orthogonal to the sigma-orbitals. From this simple concept it has proved possible to provide an understanding of the structure and bonding in nonplanar conjugated organic molecules, and to develop a complete theoretical description of the phenomenon of homoconjugation [defined as: "... orbital overlap of a type intermediate between sigma and pi" (Winstein)].