Bond resonance energies of polycyclic benzenoid and non-benzenoid hydrocarbons

The bond resonance energy represents the contribution of a given It bond in a molecule to the topological resonance energy. Bond resonance energies for 43 typical polycyclic conjugated hydrocarbons and five of their molecular ions have been evaluated and critically examined. Polycyclic benzenoid hydrocarbons do not have pi bonds with negative bond resonance energies. For every benzenoid hydrocarbon, relative magnitudes of the bond resonance energies reflect the locations of aromatic sextets in the Clar structure. A molecule is predicted to be chemically very reactive if it has one or more a bonds with large negative bond resonance energies. Many non-benzenoid hydrocarbons are kinetically unstable in this sense. All antiaromatic hydrocarbons have one or more a bonds with large negative bond resonance energies. This bond resonance energy model can be used to explore the utility and the limitations of the conjugated-circuit theory developed by Herndon, Randic and Gomes. Except for azulene, Platt's perimeter model is not compatible with the relative magnitudes of the bond resonance energies in a molecule.