Resonance structures of polycyclic aromatic hydrocarbons can be associated with numerical formulas by assigning pi-electrons of C=C double bonds to individual benzenoid rings. Each C=C double bond in a resonance structure assigns two pi-electrons to a ring in a fused-benzenoid system if it is not shared by adjacent rings and one pi-electron when it is common to two rings, obtaining thus a "local" characterization of rings in polycyclic conjugated hydrocarbons. In the present contribution we extend this approach to the aromatic pi-sextet model of Clar, which offers an alternative description of benzenoid hydrocarbons. In this model local characteristics of individual benzenoid rings are based on partitioning of pi-electrons but only for those resonance structures (fewer in number) that contribute to Clar's formula of benzenoid hydrocarbons.
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Randić et al. (2005) studied this question.
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