Phase of essential single and essential double bonds in HOMO/LUMO of benzenoid hydrocarbons

Abstract

There are certain bonds that are necessarily single/double in all the Kekulé structures of a given benzenoid hydrocarbon (molecular graph); such a single/double bond is called essential or fixed in valence bond theory. We conclude by use of perturbation molecular orbital theory that the bond phase, i.e. the sign either plus for bonding or minus for antibonding, of essential single/double bonds, respectively, is negative/positive in HOMO of benzenoid hydrocarbons; the bond phase of essential single/double bonds alters in HOMO and LUMO.