Resonance structures of benzenoid conjugated radicals

Abstract

For comparable size, the number of resonance structures (SC) in radical (non-Kekulean) benzenoid hydrocarbons is significantly larger than the number of resonance structures (K) in Kekulean (closed-shell) benzenoid hydrocarbons. For a given benzenoid size, as the radical multiplicity increases so does SC (resonance structure count). Analytical expressions for the SC of families of benzenoid radicals have been derived. Recent experimental evidence for the existence in flames of the first and second generation members to our constant-isomer benzenoid series is noted. The interconnection of deviations to Hund's rule, essentially disconnected pπ-electronic systems, and concealed non-Kekulean benzenoids is discussed and illustrated.