Nonaromatic structure

A comparison between aziridine 4, as a model of nonaromatic structure, l//-azirine lb, l//-diazirine 2b, and triazirine 3 using 6-31G /MP3 calculations leads to the following values for the N-H inversion barriers 4, 86.2 (experimental value 80 kJ mol ) lb, 190.4 2b, 160.2 and 3, 246 kJ mol [89JCC468].Tlie difference in inversion barrier values between lb and 2b was attributed to a decrease in the antiaromaticity of the latter. Tire antiaromaticity of lb was examined subsequently by the same authors [89JST(201)17]. 

Benzoquinones owe their unusual properties as a,/3-unsaturated ketones to the ease by which they are transformed to stable aromatic systems. How would these properties change if the quinone were derived from nonaromatic structures, such as cyclobutadiene, cyclooctatetraene, or pentalene There is no final answer to this question because few such substances have been prepared, the best known so far being the mono- and diphenylcyclobulenediones ... 

In an attempt to prepare the lophine (2,4,5-triphenylimidazole) derivative 55, we instead obtained 56 [70], This corresponds to a [l,5]-sigmatropic migration of a methoxy group and proves that the nonaromatic structure 56 is more stable than the aromatic one 55. This prompted us to examine theoretically the problem and study the related case of 57/58 [71]. At the G3B3 level the relative stabilities of 57, 58a, and 58b (in kJ mol1) are 202.3, 41.3, and 0.0, respectively. These studies were extended to other substituents (like fluorine) and to other azoles (like pyrazoles). 

In summary, we emphasize that homoaromatic stabilization appears to be more important for radical cations than for their neutral diamagnetic precursors. There are several reasons why radical cations may assume cyclic conjugated structures with 4n + 17i-electrons, one electron shy of the magic 4n + 2 7i-electrons which achieve aromaticity, when the parent molecules opt for a less delocalized nonaromatic structure. The principal cause might lie in the strength of the carbon-carbon single bond, which generally disfavors biradical or zwitterionic... 

We have so far looked into the relative importance of benzenoid and quinoid structures in conjugated polymer chains as part of a more general discussion of aromatic versus nonaromatic structures. It is interesting, therefore, that structural concepts of organic chemistry parallel those of condensed matter physics. For a more detailed discussion, however, the reader is recommended to consult the specialized literature. 

When a hydroxyazole can tautomerize to a nonaromatic structure, oxidation at an annular sulfur atom becomes easy, e.g., 213 giving 214. 

The XH NMR spectrum shows bands in the region 4.0-6.0 ppm (See Table 6). Following the discussion of priority of paths of delocalization of aceheptylene dianion 232 and acenaphthylene dianion 82 also 332 and 342 show that specific paths of delocalization are favoured. While in the neutral structure 33 and 34 the competition is between aromatic and nonaromatic structures, in the respective dianions the competition is between nonaromatic and antiaromatic structures (Fig. 9). From the spectroscopic parameters, i.e., chemical shifts and coupling constants of the bridge protons it can be concluded that the neutral systems are best represented by structures with an aromatic skeleton connected to a virtually isolated double bond. In the charged systems, viz. 332 and 342 it seems that a nonaromatic path of conjugation is preferred to an antiaromatic path (Fig. 9). These considerations are also reflected in the carbon chemical shifts and in their HOMO-LUMO gap (AE) (vide infra) 122). It can be concluded from all these observations that there is a tendency of aromatic systems to remain so and to avoid as much as possible paratropic antiaromatic contributions. 

There have been numerous studies of the radicals resulting from oxidation of ascorbic acid and related species. Combining in situ radiolysis and ESR, Laroff et al. have shown definitively that the principal radical has the highly conjugated but nonaromatic structure 22, thus resolving... 

All the peaks are somewhat upfield of the aromatic region, suggesting polyene character. This structure would also be consistent with the observed reactivity since the polyene has a quinodimethane stincture (see Section 11.3). The implication of a nonaromatic structure is that the combination of ring strain and the antiaromaticity associated with the four-membered ring results in a localized system. 

Some nonaromatic structures which have absorption bands in the 1600-1450 cm region include the NH2 group, which has a band near 1600 cm the NH and NH groups, which have bands... 

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