Benzenoid hydrocarbons, and

Using the above procedure and the A j, values, Binsch has examined the second-order bond fixations in the ground states of linear, cyclic, and benzenoid hydrocarbons and nonalternant hydrocarbons... 

No such SDCI results are presently available for benzene, but taking advantage of the fact that the carbon atoms are evidently electroneutral in graphite and not so in benzenoid hydrocarbons, the results obtained for graphite support the approximate validity of bond energies deduced for polynuclear benzenoid hydrocarbons and of the net charge, 13.2 me (probably + 1 me), deduced for the carbon atom of benzene [44]. 

A simple relation between regioselectivity of maleic anhydride addition to benzenoid hydrocarbons and Claris n sextet formulae has already previously been observed [65]. In cases where the addition can lead to several isomeric adducts, the adduct whose Clar formula has the largest number of inherent it sextets is always the only one to be formed. Scheme 6 gives some examples. A necessary and, usually, also adequate condition for the endocyclic Diels-Alder reaction of benzenoid hydrocarbons with maleic anhydride is that at least one n sextet be gained by the addition, i.e. that the number of sextets in the product be at least one more than in the reactant. 

Phenylenyl monoradical (C13H9) is a strictly peri-condensed PAH6 with the methyl radical as an excised internal structure and both have e = 0. The diradical C22H12 isomer, triangulene, is a strictly peri-condensed benzenoid hydrocarbon and has trimethylenemethane diradical as an excised internal structure both triangulene and trimethylenemethane diradicals have two eigenvalues of s = 0. 

Table 1. Typical benzenoid hydrocarbons and their resonance-theoretical and chemical properties.

This is the central idea of the theory proposed by Herndon and Hosoya to simulate the resonance energy obtained from the SCFMO calculation by parametrizing the values of a, b, and k [23], They took the reference values from the PPP type calculation by Dewar and de Llano [26] for the set of ten typical benzenoid hydrocarbons, and... 

Fig. 3. All five Clar structures of a branched benzenoid hydrocarbon and the corresponding five colored Gutman trees. The indicated coloring generates F4, the fourth Fibonacci number...

In this article we will outline the investigations concerned with the total u-electron energy of benzenoid hydrocarbons and its dependence on molecular structure. This topic was one of the main themes examined within the project Topological Properties of Benzenoid Systems" (c.f. Sect. 9). We have excluded it from the survey [1] and decided to present it separately only because of a relative large number of results known in this area and because of the lack of any previous review. 

The definition of a benzenoid hydrocarbon/benzenoid system/benzenoid graph as well as a sufficient number of examples can be found in the preceding article [1] and elsewhere [12]. We shall not reintroduce the notation and terminology described in [1], except that for the readers convenience we list the most frequently employed symbols. Let BH be a benzenoid hydrocarbon and let B stand for the corresponding benzenoid system/benzenoid graph. Then ... 

Dias [16] has stretched the analogy between the periodic table for benzenoid hydrocarbons and the Mendeleev periodic table (for elements) rather far. It can be stretched still farther by comparing the position of benzene to the unique position of hydrogen in the Mendeleev table. 

First we will focus attention on selected topics relating to the equivalence between benzenoid hydrocarbons, and special types of graphs and other mathematical objects that we can associate with benzenoids- In particular we will explore relations involving caterpillar trees [3] associated with catacondensed benzenoids and their line graphs [17] called, as already mentioned, Clar graphs [4]. Also relations involving "boards" (known technically as polyominos) of special properties such as those associated with "king" and "rook" pieces of chess... 

It is one of the standard results of theoretical organic chemistry that benzenoid hydrocarbons (and polycyclic conjugated molecules in general) have no unique classical structural formula. The conjugation models of their 7r-electrons must be visualized by means of several so-called Kekule structures [22, 25, 32, 33], none of which correctly describes the actual situation. An attempt to overcome this century-long problem was proposed by Clar in the 1950 s and eventually elaborated in due detail in his seminal book [18]. 

Balasubramanian, K., Kaufmann, J.J., Koski, W.S. and Balaban, A.T. (1980). Graph Theoretical Characterization and Computer Generation of Certain Carcinogenic Benzenoid Hydrocarbons and Identification of Bay Regions. J.Comput.Chem., 1,149-157. 

Mekenyan, O., Balaban, A.T. and Bonchev, D. (1985a). Unique Description of Chemical Structures Based on Hierarchically Ordered Extended Connectivities (HOC Procedures). VII. Condensed Benzenoid Hydrocarbons and their H NMR Chemical Shifts. J.Magn.Reson., 63,1-13. 

Gutman, 1. and Klavzar, S. (1998) Relations between Wiener numbers of benzenoid hydrocarbons and phenylenes. Acta Chim. Hung. -Mod. Chem., 135, 45-55. 

Under mild reaction conditions and particularly over zeolite of low aluminiun content at high temperature, the products of methanol conversion are olefins. Subsequent, less facile reactions convert the sorbed olefins into cycloalkanes, and to benzenoid hydrocarbons and alkanes. These subsequent reactions involve hydride anion transfer reactions and cyclization reactions of larger sorbed olefins (ref. 27). Figure A shows the product sequence. 

El-Basil S (2000) Benzenoid Hydrocarbons and Modem Frontiers of Science. MatCh 42 233-259... 

Dias JR (1990) A Formula Periodic Table for Benzenoid Hydrocarbons and the Aujbau and Excised Internal Structure Concepts in Benzenoid Enumerations. J Math Chem 4 17-30... 

Fig. 3a-b. Distribution of Ring Energy Content for benzenoid hydrocarbons and polysubstituted benzene derivatives. The interquartile ranges for the distribution are 25.9 kcal/mol and 16.1 kcal/mol,... 

Both graphite and diamond in practice are not infinite, but finite nets and therefore have dangling bonds at the peripheries of single crystals normally, these bonds have hydrogen atoms attached to them. Consequently one may consider graphite to be an "honorary polycyclic peri-condensed benzenoid hydrocarbon", and diamond to be an "honorary adamantanoid or diamondoid hydrocarbon"... 

Other kinds of aromatic eom])ounds exist besides simple benzene derivatives. Two common types will be covered in this chapter polycyclic fused benzenoid hydrocarbons and other cyclic conjugated polyenes with either more or less than six carbons in the ring. In Chapter 25, a third common class, heterocyclic aromatic compounds, will be pre.sented. 

Friedel-Crafts type with benzenoid hydrocarbons and with phenols have been described. 

The weights of canonical structures estimated in this way correlated well with those obtained by other methods. A good correlation between the HOSE weights estimated for 63 ground-state canonical structures of seven benzenoid hydrocarbons and the data calculated by use of the Randic model has been found (the correlation coefficient R = 0.985). Similarly, a good but qualitative agreement was found between HOSE-calculated canonical structure weights for nitroaniline and those computed by use of valence bond theory. ... 

Computer Enumeration and Generation of Benzenoid Hydrocarbons and Identification of Bay Regions. 

Zhou and Navangul calculated MNDO hardnesses for 14 benzenoid hydrocarbons and showed it to be a good indicator for their aromaticity.Using the hardness criterion, kekulene, coronene, and the corannulene tetraanion were all predicted to be superaromatic, also confirmed by calculations of Babic and Trinajstic and Cioslowski, O Connor, and Fleischmann in the case of kekulene. " Jiao and Schleyer, however, concluded, based on geometrical, energetic, and magnetic criteria of aromaticity, that kekulene is not superaromatic. ... 

In a subsequent paper, Hess and Schaad compared their REPE values with those generated from Dewar s resonance energies. The major difference is that Dewar values are given in eV, whereas their own values are given in units of ji. Hess and Schaad finally furnished a comparison of HSRE per electron (HSREPE), DRE per electron (DREPE), and DE with p band frequencies of benzenoid hydrocarbons and found that HSREPE has a better linear relationship than DREPE and that DE totally fails to establish a linear relationship. Numerous applications followed. " ... 

Cyvin BN, BrunvoU J, Chen RS, Cyvin SJ (1993) Coronenic Coronoids — A Course in Chemical Enumeration [in] Topological Aspects of Benzenoid Hydrocarbons and Related Structures (Cyvin SJ, Edit). Max—Planck—Institut fur Strahlenchemie, Mulheim a d Ruhr Match 29 131... 

Cyvin BN, BrunvoU J, Cyvin SJ (1992a] Notes on FuUy Benzenoid Hydrocarbons and Their Constant-Isomer Series. J Chem Ini Comput Sd 32 72... 

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