Multicenter bond indices as a measure of aromaticity

A MO multicenter bond index involving the sigma+pi electron population is proposed as a measure of aromaticity. It is related both to the energetical and to the magnetic criteria. The index is applied to linear and angular polycyclic hydrocarbons with benzenoid rings, to hydrocarbons including non-benzenoid rings, to monocyclic azines, benzoazines and other heterocyclic compounds with 5-membered rings. The index gives satisfactory values for monocyclic molecules and is shown to be quite suitable in discriminating the relative aromaticity of the different rings in polycyclic compounds.