On Generalized Clar Structures

We will now investigate the role of the new definition of Clar structures for Clar-type structures that do not posses the maximal number of inscribed re-aromatic sextets. Already Clar, in discussing properties of benzo[a]coronene, evoked a structure that has less than the maximal number of re-sextets [10]. While the proper Clar structure of benzo[a]coronene has four re-sextets, which is the maximal number for benzo[a]coronene, Clar considered also a structure having three sextets (Fig. 12). Such imperfect Clar structures were introduced into Clar Sextet Theory systematically by Hosoyaand Yamaguchi [11]. 

Hosoyaand Yamaguchi generalized Clar structures by removing the restriction that Clar structures have to have the maximal number of rearomatic sextets. We will refer to these as HY-Clar structures, or briefly HY-structures (HY for Hosoya-Yamaguchi) in order to differentiate them from other generalizations of Clar structures that will be discussed later. In Fig. 13 we show the set of generalized HY-Clar formulas for 

Generalized Clar structures introduced by Herndon and Hosoya illustrated on benzol ghi]perylene. 

Definition Ciar structures are defined to be structural diagrams in which all the carbon atoms of an aromatic hydrocarbon are spanned uniquely either by an aromatic sextet (designated by a circle) or by a jt electron pair (designated as a double bond). 

We have inserted in their definition the word uniquely for added precision. In other words, once the positions of inscribed circles have been selected the rest of the molecule should have the residual degree of freedom equal one, that is, positions of all CC double bonds are unique. In Fig. 14 we illustrate the five HH-Clar structures of benzo[ghi]perylene. In the case of benzo[ghi]perylene the first structure is the proper Clar structure that has the maximal number of Jt-sextets. Of the four generalized HH-Clar structures three have two it-sextet and one has but a single Jt-sextet. 

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