Use of the distance/similarity measure for estimating local aromaticity in benzeneoid hydrocarbons

The distance/similarity¹ between an individual benzene ring in a benzeneoid hydrocarbon and benzene itself, with respect to their electronic structure, is used to define a new local aromaticity index. The electronic structure in ground and excited state is assumed to be adequately represented by the charge-bond order matrix calculated in a LCAO-MO quantum chemical approximation. The overall correctness of the index is verified for the ground state, but it should be especially useful for estimating the (S₁)-excited-state-benzene character of benzeneoid rings in excited molecules.