Disjoint molecular orbitals in nonalternant conjugated diradical hydrocarbons.
The concept of disjoint NBMOs in diradical alternant hydrocarbons (AHs) is reviewed and extended to diradical nonalternant hydrocarbons (nonAHs). A valence-bond (VB) method for recognizing disjoint versus nondisjoint NBMOs (nonbonding molecular orbitals) in diradicals is presented. When circumscribed with hexagonal rings, disjoint diradicals produce nonradical polycyclic successors and nondisjoint diradicals produce diradical polycyclic successors. The interconnection between the topological VB terms of cross-conjugated, disjoint NBMOs, and essentially disconnected polycyclic hydrocarbons is delineated.