Circumextremal single coronoid

A circumextremal (single) coronoid is defined in connection with a circumextremal benzenoid (Cyvin SJ, Zhang, Cyvin and Guo 1993), previously characterized as a pericondensed... 

In simple terms all the circumextremal single coronoids are obtained as circumscribed extremal single coronoids and their isomers. Once more, in other words a circumextremal single coronoid has the same formula as a drcumscribed extremal single coronoid. 

Let a circumextremal single coronoid be identified by P(n Then it is obtained from eqns. (51) and (53) ... 

Property 6.S An imperfect ground form or a circumextremal single coronoid, A, has neither an L — or P2—Taode hexagon. 

Definition 5.7 A single coronoid (P) is circumextremal if its associated benzenoid, P, is a drcumextremal benzenoid, and exds-P is the assodated benzenoid to a (smaller) extremal single coronoid. 

Some explanations should be attached to this somewhat complicated definition. It is dear that a drcumscribed extremal (single) coronoid is circumextremal write circum—A = P. Then it is also dear that excis-P = A. Assume now that the naphthalene hole of P is moved as near as possible to the outer perimeter, and call the new coronoid P. Then P and P are isomers, but P cannot be excised to give another coronoid excis—P becomes a degenerate system. Nevertheless, P falls under the definition of drcumextremal coronoids. An illustration is given below. 

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