Multicenter bond indices as a measure of aromaticity

A MO multicenter bond index involving the sigma+pi electron population is p roposed 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 po lycyclic hydrocarbons with benzenoid rings, to hydrocarbons including non-b enzenoid rings, to monocyclic azines, benzoazines and other heterocyclic co mpounds with 5-membered rings. The index gives satisfactory values for mono cyclic molecules and is shown to be quite suitable in discriminating the re lative aromaticity of the different rings in polycyclic compounds.