Basic single coronoid

It is also needed to consider corona hole benzenoids which cannot be circumscribed. Such corona holes are associated with the non-primitive basic single coronoids (Vol. 1-8, especially 1-8.3). The well known smallest non-primitive basic coronoid (I-8.2.2 and I—Fig. 8.2) has h = 12. Its corona hole (benzo[c]phenanthrene) has ho = 4, 71 0 = 0. Here again h = h, where h is defined in (2), but n-= 1. The next-smallest non-primitive basic coronoid, which has h = 13, is also well known (references as above). Its corona hole (pentahelicene) has ho = 5, nfl = 0. In this case h = h — 1 and n = 0. The two smallest non—primitive basic coronoids are depicted in the following (cf. also the top row of I-Fig.8.1). 

Fig. 7.1. Corona holes (represented as benzenoids) of basic single coronoids with h < 14. All these benzenoid forms are found in Brunvoll, Cyvin BN and Cyvin (1992b). Correction for C28H14 (corresponding to = 8, = 6), two forms in the cited reference should be interchanged in...

These two cases exemplify corona hole benzenoids with a cove and a fjord, respectively. They are representative for all the non—primitive basic single coronoids with h = 12, 13 and 14 (I-Fig. 8.2), which are those of the prime interest in this section. For the sake of clarity we repeat the relevant rules in strict formulations. For 12 < h < 14 the non-primitive basic single coronoids are of two kinds, (a) The corona hole benzenoid possesses exactly one cove then h = h n = 1. (b) The corona hole possesses exactly one fjord then h = h — 1, = 0. 

The corona holes, represented as benzenoids, of all basic single coronoids for 8 < h < 14 (primitive and non-primitive) are depicted in Fig. 1. The encircled numerals identify the different classes with characteristic combinations of nfi and h. Table 3 includes a listing of the corona holes in question or the appropriate basic single coronoids, which amounts to the same. 

The situation is not so simple for the non—primitive basic single coronoids with h > 15. Already for h = 15 there is one instance not covered by the cases (a) and (b) above. It is depicted below (left-hand drawing). 

The occurrence of corona holes is cumulative in the following sense. All the holes which occur in the basic (primitive and non-primitive) single coronoids with h hexagons each, are also found in single coronoids with more than h hexagons. A detailed account on the numbers of single coronoid isomers with the different classes of holes (identified by endrded numerals), is furnished by Table 4. 

---