Cata-condensed aromatics

Kearns, D. R. Determination of the assignment of triplet states in cata-condensed aromatic hydrocarbons. J. Chem. Phys. 36, 1608 (1962). 

The structural parameter Ham/Car, which gives the ratio of substitutable edge atoms to total aromatic atoms, is important because it is a measure of the average aromatic ring system which is independent of molecular weight measurements. Figure 2 shows the Ham/Car values for some typical 1-7 cata-condensed aromatic ring systems. When the Ham/... 

Figure 2. Comparison of Haru/ ar values for cata-condensed aromatic...

Herndon WC, Connor DA, Li P (1990) Structure-enthalpy relationships in polycyclic cata-condensed aromatic hydrocarbons. Pure Appl Chem 62 435 144... 

III. Polycyclic Aromatic Hydrocarbons A. Cata-Condensed Aromatic Hydrocarbons... 

The size of the potential well and the energy of the electron are related. For cata-condensed aromatic hydrocarbons, Platt [48] proposed a simple relation... 

The fused 3+ ring aromatics in petroleum include both cata- and peri-condensed stmctures (see Table 4, Fig. 8). The cata-condensed species are those stmctures where only one face is shared between rings, the peri-condensed molecules are those that share more than one face. The fused ring aromatics form the class of compounds known as polynuclear aromatic hydrocarbons (PAH) which includes a number of recognized carcinogens in the 4+ ring family (33). Because of the potential health and environmental impact of PAH, these compounds have been studied extensively in petroleum. 

For coals whose carbon contents are less than 90 weight %, it is reported (15,16) that the condensed aromatic rings are mostly of the cata-condensed type with two through five rings. 

Aromatic condensed rings are of cata-condensed type. 

Aromaticity. VIII). Resonance energies of cata-condensed benzenoid polycyclic hydrocarbons. Rev. Roum. Chim., 15, 1243—1250. 

Balaban, A.T. and Tomescu, 1. (1984) Chemical graphs. XL. Three relations between the Fibonacci sequence and the numbers of Kekule structures for non-branched cata-condensed polycyclic aromatic hydrocarbons. Croat. Chem. Acta, 57, 391-404. 

Benzoannelated triquinacenes [154],fenestranes [155], and centrohexaquinace-nes [156] are nonplanar aromatic hydrocarbons with an interesting molecular architecture. Extensive preparative work has been accomplished especially by Kuck and his coworkers [154-156] to explore different routes towards these compounds. Although the syntheses usually do not involve formation of aryl-aryl bonds or even condensed aromatic systems, we have chosen to depict three successful routes that all result in the centrohexaindane 116 in Scheme 54, whereas the probably most convenient route A is illustrated in more detail [156]. The final reaction steps along routes A and B are typical aromatic electrophilic alkylation reactions, and along route C a threefold cyclodehydrogenation cata-... 

Condensed polycyclic benzenoid aromatic hydrocarbons are customarily regarded as planar molecular structures because of the geometrical constraints of carbon atoms in a state of sp2 hybridization. A well-known exception is the class of compounds called the helicenes (18) for which the nonbonded overlap of two terminal benzenoid rings in a cata-condensed structure, as in structure 1, forces a molecule into a nonplanar helical structure. A second exceptional class of compounds is related to corannulene (2) and other an-nulenes of this type (19, 20). In corannulene, strain associated with the pericondensed five- and six-membered rings requires adoption of a bowlshaped structure (20, 21). For both structures 1 and 2 the aromatic character of the benzenoid rings is retained to an appreciable extent. 

PAHs are compounds consisting of two or more condensed aromatic rings, lineated together, either cata-annellated (linearly-, or angularly), or pcn-condensed. 

Nomenclature of Benzenoid Cata-Condensed Polycyclic Aromatic Hydrocarbons. 

A. T. Balaban and F. Harary, Chemical graphs V. Enumeration and proposed nomenclature of benzenoid cata-condensed polycyclic aromatic hydrocarbons. Tetrahedron 24 (1968) 2505-2516. 

Table 1. Enthalpies of Formation of Cata-Condensed Polycyclic Aromatic hydrocarbons (kj/mol) ...

Clar proposed that the maximum number of localized aromatic sextets (and thus the number of Kekule structures) that can be drawn for benzenoid hydrocarbons correlates well with several properties of the compounds. For example, phenanthrene contains two localized sextets (five Kekule resonance structures), while its isomer anthracene has only one localized sextet (four Kekule resonance structures), and so might be considered to be more aromatic . Of the 4-ring benzenoid isomers, naphthacene has the fewest sextets (one), triphenylene has the most (three), and the others have two apiece. Generally, the cata-condensed species which have more phenanthrene subunits, and thus have greater angularity , also have more localized Clar sextets. How well does the Clar model correlate with the enthalpies of formation ... 

Triphenylene, containing three phenanthrene subunits, should be the most aromatic of the four-ring cata-condensed polycyclic aromatic hydrocarbons. The enthalpies of reaction are endothermic upon successive benzoannelation of benzene in the series benzene to naphthalene (-1-67.7), naphthalene to phenanthrene (-1-54.4), and phenanthrene to triphenylene (-1-60.8). 

We now turn to the thermochemistry of cata-condensed polycyclic aromatic hydrocarbons with five or more rings. There is disappointingly little data. For five-ring species, we have only been able to find an unsigned and otherwise unpublished institute report that presents the enthalpies of formation of solid dibenz[a,(janthracene (29) and its [a,/ ]-isomer (30). 

To reveal the effect of the size and charge on azide photoactivity, the structures of linear cata-condensed heteroaromatic azides from azidopyridine to azidoazahexacene (the size of aromatic -system from 6 to 26 e) are calculated by semiempirical (PM3), ab initio (HF/6-31G ) and DFT (B3LYP/6-31G ) methods. Joint consideration of the experimental and quantum-chemical data results in the conclusion that the azide photoactivity depends on the nature of molecular orbital (MO) that is filled in the lowest excited singlet (Si) state. If the antibonding cnn -MO, which is localized on the azido group and is empty in the ground (So) state, is filled the Si state, the azide is photoactive (cp > 0.1). However, when the size of the Ti-system increases above a certain threshold, aromatic -MO is filled instead of the aNN -MO in the Si state, and the azide becomes photoinert (

positively charged azides, respectively. 

In this ehapter main especial attention will be devoted to a series of heterocyclic azides, 4-azidopyridine A1 and its higher cata-condensed analogues 4-azidoquinoline 2,9-azidoacridine A3,12-azido-benzo[b]acridine A4,13-azido-6-azapentacene (azidodibenzacridine) A5, and 15-azido-6-azahexacene A6 here the index is equal to the number of aromatic rings (Scheme 1). The first three eompoimds were studied both experimentally and theoretically, for the last three azides quantum-chemical calculations were performed. 

Thus, heteroaromatic azides proved to be convenient objects for studying general structure-reactivity relationship in aromatic azide photochemistry. On the example of the series of cata-condensed heteroaromatic azides from azidopyridine to azidoazahexacene (the size of aromatic 7t-system from 6 to 26 e), the size and charge effects, that is, the dependence of photodissociation quantum yield on the size of the azide 7t-system and its charge, were investigated both quantum-chemically and experimentally. These effects are interrelated the size effect depends on the charge of the azide molecule and vice versa, the charge effect depends on the size of the azide 7t-system. 

---