Reduced HOMO-LUMO gap as an index of kinetic stability for polycyclic aromatic hydrocarbons

A reduced HOMO-LUMO gap, which is defined as the HOMO-LUMO energy separation of a molecule divided by that of the hypothetical polyene reference, can be used as an index of kinetic stability for a variety of polycyclic aromatic hydrocarbons (PAHs). The reduced HOMO-LUMO gap < 1.00 indicates that the HOMO contributes to the decrease in the topological resonance energy. In general, PAHs with reduced HOMO-LUMO gaps < 1.30 are chemically very reactive. Fully benzenoid hydrocarbons are kinetically very stable with very large reduced HOMO-LUMO gaps. Many of the PAH molecules with large reduced HOMO-LUMO gaps are closed-shell substructures of nonmetallic one-dimensional benzenoid polymers.