Fully benzenoid

Similar problems arise with the four isomeric dibenzazepines 4-7. since only 5//-dibenz-[6,d]azepine (4) and 5//-dibenz[/>,./]azepine (7) can be drawn as fully benzenoid ring structures. Even so, 5//-dibenz[/ ,t/]azepines are rare and are known only as the 7-oxo derivatives.4 In contrast, 5//-dibenz[6,e azepine (5) and 6//-dibenz[r,t>]azepine (6) exist only as the 11//- 5a and 5H- 6a isomers, respectively. In fact, there is no chemical or spectrosopic evidence for the isomerization of 5//-dibenz[e,e]azepine,5 or its 6-oxide,6 to the 6//-dibenz[r, e]azcpinc isomer (6). In addition, an X-ray crystal structure of 7-methoxy-5//-dibenz[e,e]azepine supports unequivocally the benzenoid rather than the quinonoid form.7 9//-Tribenz[6,d /]azepine (8) has only recently been prepared.8... 

As expected, annulation to one or two benzene rings increases the overall stability of the azepine system (Section 5.16.1.2). Even so, some systems, e.g. (4) and (9), are too unstable as the NH tautomers to permit isolation, and as yet are known only as the oxo or hydro derivatives or, as in the case of the dibenzazepine (9), as the more stable fully benzenoid 5//-tautomer. 

In Fig. 1 we illustrate Clar structures for several smaller benzenoids in which only inscribed circles appear. These are the unusually stable 6n Jt-electron benzenoids which Clar called fully benzenoid. In Fig. 2 we illustrate Clar structures for smaller benzenoids in which besides inscribed circles representing aromatic sextets also appear one or more CC double bonds. Finally in Fig. 3 we illustrate benzenoid hydrocarbons for which one can draw several Clar valence structures. In such cases the overall Clar structure is obtained as a superposition of the component valence structures in an analogous way as one represents molecules by superposition of several Kekul6 valence structures. Clar represented the final superposition of component structures by drawing a single structure to which one or more arrows are added to suggest delocalized Jt-sextets. 

In the book [3] the role of both Kekule and Clar structures in various (contemporary) chemical theories as well as their relevance for practical chemistry were outlined in detail. A recent book [46] by Cyvin and the present author is devoted to the enumeration of Kekule structures of benzenoid molecules. In addition to this, the first volume of Advances in the Theory of Benzenoid Hydrocarbons contains several review articles [47-51] dealing with topics of relevance for our considerations. In order to avoid repetition and overlapping we will just briefly mention the work on the elaboration and application of the John — Sachs theorem for the enumeration of Kekule structures [52-55], the search for concealed non-Kekulean benzenoid systems [2, 56-59], examination of fully benzenoid (=all-benzenoid) systems [60-62, 135] as well as the enumeration of Kekule structures in long and random benzenoid chains [63-65]. 

Especially good agreements between the Clar picture and ef are observed in the case of fully benzenoid systems [39]. (A benzenoid hydrocarbon is said to be fully benzenoid if it has a Clar formula without double bonds.) Two typical examples are provided by triphenylene (4) and dibenzo[fg,op]naphthacene (5). 

For fully benzenoid systems the following two rules were formulated [39]. 

Rule 1. The effect of empty rings on the total ir-electron energy of fully benzenoid hydrocarbons is nearly the same. The corresponding ef-values vary in the narrow interval (0.02, 0.03). 

The class of benzenoids called all-benzenoids (or fully benzenoids) is a subclass of the normal benzenoids. Hence each all-benzenoid is Kekulean and has A = 0. 

In contrast to benzene, the bond lengths in naphthalene are not all equal, as illustrated in 4. The resonance energy of naphthalene is 255 kJ mol", which is higher than, though not twice that of, benzene (151 kJ mol" ). In the canonical forms 5 and 7 that contribute to the valence bond structure for naphthalene, only one of the two rings is fully benzenoid. Naphthalene is less aromatic than benzene, which accounts for its higher reactivity towards electrophilic attack compared with benzene. 

The results of comparative analysis of the invariants values for fully-benzenoid hydrocarbons (fully-benzenoid systems) are presented in this section [35]. Benzenoid systems in which there are no double bonds in the Clar aromatic sextet formulas, i.e., in which all tt—electrons belong to aromatic sextets, are called fully-benzenoid [32]. One example of fully-benzenoid system is shown in Figure 26. 

For the class of the 16 benzenoid systems with two or three aromatic sextets (hyperedges), which are not fully-benzenoid, but which have a unique Clar aromatic sextet formula we found... 

The most efficient stabilization of the intermediate carbocation produced by electrophilic attack on naphthalene comes from those resonance forms which retain one fully benzenoid ring. In the case of 1-substitution, two such structures can be drawn, 8 and 9 (plus their Kekule forms). For 2-substitution there is only one structure, 10 (and its Kekule form). The intermediate from attack at the 1-position is the more stable and therefore the 1 -substituted product is favoured. 

As Aihara and Tamaribuchi recently pointed out, graphite can be classified as an infinite fully benzenoid. Some boundless toroidal polyhex surfaces also fall into this category, and examples are listed in Table 2. It was concluded that for a toroidal polyhex encoded (Section 8.9.1.1) as a-b-d, it will be fully benzenoid if and only if... 

Figure 23. A C50 toroidal polyhex, in fact TPH(30-8-1) (Section 8.9.1.1), that can be viewed as either fully benzenoid, with the vertices accounted for by ten hexagons (in this case 0, 3, 6.9, 12, 15, 18,21,24, and 27), or as fully naphthalenoid, with six hexagon-pairs (in this case 0-1, 5-6, 10-11, 15-16, 20-21, and 25-26). This concurrence does not occur among finite planar polyhexes. Note also that (again unlike the planar series) the "fuir -hexagon set is not unique. This example could equally well be described as fully benzenoid with the ten-hexagon set 1, 4, 7. 10, 13, 16, 19,22, 25, 28.

S. J. Cyvin, Chem. Phys. Lett., 181, 431 (1991). Note on the Series of Fully Benzenoid... 

An homologous series of not-fully benzenoid PAHs (35-38) decorated with solu-bihzing and phase-forming n-dodecyl chains was synthesized in order to investigate... 

Ring Partition of Ti-Electrons for Clar s Fully Benzenoid Systems ( Claromatic Benzenoids )... 

In Fig. 8.24 we show 11 smaller/uZZy benzenoid claromatic) systems that include triphenylene as the smallest fully benzenoid molecule (1/23), and hexabenzocoronene (6/23) the latter was described by Clar as an unusually stable compound which resisted the measurement of its melting point because in... 

The fully benzenoid hydrocarbons have 6n % electrons, n being an integer. It becomes obvious that the most stable hydrocarbons do not follow HUckeTs rule (2 + 4w) x electrons [19]. Therefore the latter must be strictly limited to monocyclic systems. 

Fully benzenoid hydrocarbons are those for which Clar s formulas contain only sextet rings and anpty rings and no CC double bonds. The smallest fully benzenoid hydrocarbon is triphenylene. 

In Figure 11.10, we show a few benzenoids having benzene rings with RBOs of similar magnitude (2.16 0.01). If one is to characterize the fully benzenoid sys-tans, which have a single Clar structure, as fully aromatic and benzenoids having... 

Use of synonyms to cover plagiarism has been known, particularly in social sciences [22]. In mathematics and natural sciences, use of synonyms is not so infrequent as it may offer alternative labels that may have distinct descriptive features. Thus, for example, Clar referred to benzenoid systems having only k electron aromatic sextets and empty rings as the fully benzenoid systems [23]. Polansky and Gutman have referred to the same as the all benzenoid systems [24], and Dias refers to these most stable benzenoids as the total resonant sextet benzenoids [25]. There are no problems with synonyms in chemistry/or informed persons, but uninformed chemists, as was the anonymous referee of our paper on ring currents, can be confused, which is, of course, their problem ... 

What keeps various graph theoretical and semi-empirical VB models alive is that the concepts involved in these models are expressed by a language that is commonly understood in chemistry, which includes such basic structural elements as atoms, bonds, rings, and various molecular fragments, such as the bay region or the pharmacophores, which are of interest in structure—bioactivity studies. In this article we will focus on aromaticity in hydrocarbons, and in particular the aromaticity of benzenoid hydrocarbons, and will see that there is some theoretical justification to expect the fully benzenoid 6n 7r-elec-tron systems of Clar to be the most aromatic polycyclic hydrocarbons. The reason that we have confined our attention to conjugated hydrocarbons is... 

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