K-Clar structure

As we can see by comparing the HH-Clar structures and k-Clar structures they offer a distinctive basis for representation of benzenoid hydrocarbons. Each of the two approaches have their own merits, and as we will see in the next section, although the structures arising in the two models are quite different, the two approaches can be related in some respect. Both approaches use the same basis Kekule structures and differ... 

Kekule valence structures all the combination of Kekule valence structures of interest. The same is also true for finding combinations of Kekule valence structures involved in HH-Clar structures. A better way to obtain the correct combinations of Kekule valence structures instead of construction of various superpositions is to first write down k-Clar structures (or HH-Clar structures) and then decompose them into underlying Kekule valence structures. The apparent difficulty that remains in such an approach, that cannot be avoided, is the case of benzenoids having a large number of Kekuld valence structures which results in large number of decomposition. 

Let us clarify what we mean as the derivative structure by returning to Fig. 15 in which we listed the six k-Clar structure of... 

The above illustration suggests an algorithm for construction of k-Clar structures that can be outfined as follows ... 

ILLUSTRATION OF k-CLAR STRUCTURES FOR SMALLER BENZENOID HYDROCARBONS... 

There are number of smaller benzenoids in which the set of HH-Clar structures and the set of k-Clar structures almost coincide. For example, this is the case with dibenzo[bc,kl]coronene, one of smaller benzenoid with large number of CC double bond fixation in its leading (i.e., the proper) Clar structure. Its Clar structure has three re-sextets but six CC double bonds. In Fig. 18 we illustrated seven out of nine HH-Clar structures of dibenzo[bc,kl]-coronene. Of the nine HH structures one has three resextets, six have two re-sextets (see Fig. 18) and two have a single re-sextet (shown in Fig. 19). Interestingly, there are also nine k-Clar structures, seven of which coincide with the first seven HH-Clar structures of Fig. 18. However, instead of the last two HH-Clar structures having a single re-... 

Fig. 17 Generalized k-Clar Structures for several smaller benzenoid hydrocarbons...

Fig. 18 The seven of the nine HH Clar structures of dibenzo[bc,kl]coronene which coincide with seven generalized k-Clar structures...

A coincidence between the number of HH-Clar structures and k-Clar structures is again evident. To each of the five HH-CIar structures of Fig. 21 corresponds a k-Clar structure having one jt-sextet less (shown in Fig. 22). We have indicated by asterisk sextets of HH Clar structures that have not survived more restrictive definition of k-Clar structures which... 

In Fig. 23 we have illustrated several HH-Clar structures of smaller benzenoids and have indicated their vulnerable rings. Each of the Kekule valence structure having a single vulnerable ring will produce the corresponding O-Clar structure (i.e., a Kekule valence structure that qualifies as Clar structure). In Fig. 24 are shown additional HH-Clar structures having besides vulnerable ring additional ji-sextets. These structures will reduce to k-Clar structures when Ji-sextet of the vulnerable... 

While in this contribution we focused attention to k-Clar structures, and to their relation to the Hemdon-Hosoya type Clar structures, one can anticipate a number of interesting problems to emerge from the proposed innovation for Jt-aromatic sextets. It would be of interest to see, for instance, how k-Clar structures as a basis for semi-empirical VB... 

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