Mo- and Vb-benzene characters : Analysis of the “clar's aromatic sextet” in polycyclic aromatic hydrocarbons

Abstract

In order to analyse the mode of distribution of π-electrons in a given hexagon in a polycyclic aromatic hydrocarbon, normalised VB- and MO- (HMO and PPP) benzene characters are defined and calculated for a number of molecules. Their relation to the topological structure of the molecule was studied in detail. By combining these quantities it was shown that the π-electrons in the three different classes (primary, secondary and tertiary) of hexagons have different distribution over the benzene ring. It was found that the value of the benzene character is determined by the local topological structure up to the third next hexagons. These findings qualitatively support and quantitatively figure out the concept of the Clar's aromatic sextet, which can be interpreted as the “higher order π-electronic correlation” that the π-electronic system of polycyclic aromatic hydrocarbons can be described by the cooperative interaction of the units of closely correlated six π-electrons.