On Superposition of Kekule Structures

Because Clar structures can be viewed as a superpositions of selected Kekule valence structures we will briefly examine pair-wise superposition of the five Kekule valence structures of phenanthrene (shown in Fig. 5). In all we can construct ten combinations illustrated in Fig. 8. First, 

All pair-wise combinations of the five Kekule valence structures of phenanthrene. 

Definition 1-Sextet structure is valence structure obtained by 

Superposition of 1-sextet structures results in 2-sextet structures, which in the case of phenanthrene represents the Clar s structure of phenanthrene. For benzenoids having larger number of benzene rings this process is continued till one arrives at k-sextet structures which have the maximal number of disjoint re-sextets. We can define k-sextet structure a follows  

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