Benzenoids, modeling

The greater intensity of the band of the metabolite at 220 mis probably due to the presence of a second, superimposed chromophore which could also account for the shift of the minimum. On the other hand, the band near 300 m/u. has the expected intensity. Its broadness and displacement towards longer wavelength are probably due to the presence of a substituent on the double bond or benzenoid ring. That the assignment to a coumaroyl chromophore is essentially correct is evidenced by the fact that both M and the model compound underwent the same type of reaction on irradiation in the near-ultraviolet (Figure 4). The observed isosbestic points imply that the photoreaction is a simple one, such as A -> B or A = B, and is obviously the well-known light-induced trans- to c/r-isomerization (7) of cinnamic acid derivatives. 

Gust D, Moore TA (1991) Photosynthetic Model Systems. 159 103-152 Gutman I (1992) Topological Properties of Benzenoid Systems. J62 1 -28 Gutman I (1992) Total n-Electron Energy of Benzenoid Hydrocarbons. 162 29 - 64 Guyon P-M, Gislason EA (1989) Use of Synchrotron Radiation to Study-Selected Ion-Molecule Reactions. 151 161-178... 

Figure 6.18 Structural formulas of two families of dendrimers containing a 1,3,5-trisubsti-tuted benzenoide core and 9 (A918 + and B918 + ) and 21 (B2142+ and A2142 + ) 4,4 -bipyridinium units in their branches.68 The -type dendrimers are terminated with tetraaryl-methane bulky moieties while the -type dendrimers contain in their periphery less bulky aryloxy groups. For comparison purposes the structural formulas of dendrons A24 + and B24 + and model compound DoV2 + are also reported.

The long-wavelength absorption bands exhibited by solutions of methiodides of aza analogues of benzenoid hydrocarbons have been attributed to the presence of charge-transfer complexes.01 There is a correlation between the excitation energies of the bands and the calculated electron affinity of the cations, in agreement with the delocalized rather than the localized model of the excited state in the charge-transfer complex.91... 

Like benzenoid hydrocarbons, pyridine-like heterocycles give well-developed two-electron waves on reduction at the dropping mercury electrode. The latter are polarographically much more reducible than the former. This can be explained easily in terms of the HMO theory It is assumed (cf. ref. 3) that the value of the half-wave potential is determined essentially by the energy of the lowest free 7r-molecular orbital (LFMO) of the compound to be reduced, and for models of hetero analogues this quantity is always lower than that for the parent hydrocarbons. Introduction of an additional heteroatom into the molecule leads to a further enhancement of the ease of polarographic reducibility.95 On the other hand, anodic oxidation of the heterocyclic compounds is so much more difficult in comparison with benzenoid hydrocarbons that they are not oxidizable under the usual polarographic conditions. An explanation in terms of the HMO theory is obvious. 

By H NMR monitoring of the oxidation of benzene oxide-oxepine with dimethyldioxirane (DMDO), a significant by-product, oxepine 4,5-dioxide, was identified <1997CRT1314>. This fact supports the hypothesis that the route from oxepine to muconaldehyde proceeds via oxepine 2,3-oxide with a minor pathway leading to symmetrical oxepine 4,5-oxide. The DMDO oxidations provide model systems for the cytochrome P450-dependent metabolism of benzene and atmospheric photooxidation of benzenoid hydrocarbons. 

Because the n-networks of benzenoid hydrocarbons are the classical prototypical example of delocalized bonding, they provide a crucial test for chemical-bonding theories. Here there is revealed a systematic organization for valence-bond views to describe such n-bonding. With an initiation near the ab initio realm a sequence of semiempirical valence-bond models is identified and characterized. The refinement from one model to the next proceeds via either a (perturbative) restriction to a smaller model space or orthogonalization of a suitable natural basis for the model space. The known properties of the models are indicated, and possible methods of solution are mentioned. A great diversity of work is outlined, related, systematized and extended. New research is suggested. 

Here we skim over the field of semiempirical VB theory of the Jt-systems of benzenoids. Primary focus is on a systematic derivational development of a hierarchical sequence of VB models. Different VB-based models are addressed in different sections (2, 3,5, 6) here, and the overall development is summarized in the diagram at the conclusion of Sect. 7. Section 4 serves as an interlude on quantum chemical computational methods, with emphasis on the VB basis and its relationship to chemical structure — this being crucial for the following sections. Along the way we indicate some of the history and general characteristics of the models. The unifying view which emerges not only incorporates many aspects of past work but reveals avenues for future research. 

A number of computational approaches to the (G) have been developed and there have been widespread applications of the conjugated-circuits model, motivated both from Herndon s and from Randic s approaches. The applications extend even much beyond benzenoids. This is reviewed elsewhere by Randic et al. [76],... 

Various reactivity indices have been derived for benzenoid hydrocarbons from the following purely topological approaches the Huckel model (HMO), first-order perturbation theory (PMO), the free electron MO model (FEMO), and valence-bond structure resonance theory (VBSRT). Since many of the indices that have been known for a long time (index of free valence Fr, self-atom polarizability ir , superdelocalizability Sr, Brown s index Z, cation localization energy Lr+, Dewar reactivity number Nt, Brown s para-localization energy Lp) have been described in detail by Streitwieser in his well-known volume [23] we will refer here only to some more recent developments. 

Scheme 6- Clar s n-sextet model and the endocyclic Diels-Alder reaction of benzenoid hydrocarbons...

Most of the more previous attempts to correlate carcinogenic potency with structural features of benzenoid hydrocarbons are related to the mechanism outlined above. The different quantitative models that have been developed can be distinguished with regard to the number of independent variables. One-variable theories (e.g. the so-called bay-region theory [94]) are normally inferior compared to two- [95] and three-variable theories which refer more explicitely to the different and partly competing metabolic reaction pathways. A particularely efficient model has been developed by v. Szentpaly [96]. In his so-called MCS model three important influences on carcinogenic potency are taken into account M, the initial epoxidation... 

From the efficiency of the MCS model, which is purely topological in nature, it may be concluded that even very complex chemical behaviour of benzenoid hydrocarbons is dominated by molecular topology. 

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