Aromaticity structural criteria

The 3/(ch) values for the C-N-CH, C N = CH, and C = N-CH structural elements were used as a valuable criterion for differentiating aromatic from non-aromatic structures and applied for tautomerism studies in 4-hydroxypyrimidines (83OMR20). 

Hydrogen donors are, however, not the only important components of solvents in short contact time reactions. We have shown (4,7,16) that condensed aromatic hydrocarbons also promote coal conversion. Figure 18 shows the results of a series of conversions of West Kentucky 9,14 coal in a variety of process-derived solvents, all of which contained only small amounts of hydroaromatic hydrocarbons. The concentration of di- and polyaromatic ring structures were obtained by a liquid chromatographic technique (4c). It is interesting to note that a number of these process-derived solvents were as effective or were more effective than a synthetic solvent which contained 40% tetralin. The balance between the concentration of H-donors and condensed aromatic hydrocarbons may be an important criterion in adjusting solvent effectiveness at short times. 

In a review, Gorelik51 has shown that magnetic, structural, and energetic properties are determined by the electronic structure of cyclic conjugated systems, which are stabilized by a cyclic delocalization of electrons. Chemical reactivity cannot serve satisfactorily as a general criterion of aromaticity. 

Nitrogen. Pyridine is one of the most important heterocycles. The aromaticity of pyridine was intensively connected to structural considerations and chemical behavior. The relative difference between the aromaticity of benzene and pyridine is controversial generally calculations give similar orders of magnitude and differences depend on the criterion of aromaticity considered and the mode of calculation used. A comprehensive review on the theoretical aspects in connection with the aromaticity of pyridine was published.191 Pyridine is about as aromatic as benzene according to theoretical calculations and to experimental data, while quinoline is about as aromatic as naphthalene and more aromatic than isoquinoline.192193 The degrees of aromaticity of pyridine derivatives strongly depend on their substituents. 

In discussing various indices of aromaticity developed from different criteria (energetic, structural, magnetic), we noted correlations between indices based not only on one type of criteria, but also on different ones. Is, however, the interrelation among different criteria always clear-cut and convincing Will the aromaticity inferred from, say, a magnetic criterion be confirmed by an energetic one ... 

The material presented in Section II warrants, apparently, the conclusion that the main test of aromaticity and antiaromaticity is represented by the energetic criterion realizable within the framework of various schemes for calculating resonance energies. In most cases it correlates with structural and magnetic criteria moreover, it often accords well with a manifestation of numerous properties of compounds, which, being regarded as attributes of aromaticity, make its very concept substantially broader. Indeed, the concept of aromaticity claims an increasing number of types of compounds and requires a more and more sophisticated classification. 

Angular momentum arguments show that the Jt-electron-rich nature of (4 +2) SN heterocycles is the key to their diatropic current. Formal expansion of (4 +2)-Jt-carbocyclic systems by insertion of NSN motifs in every CC bond is predicted to lead to structures that support diatropic ring currents explicit ab initio calculation of magnetic response predicts the 24-center, 30n-electron heterocycle S6N12(CH)6, formally derived from benzene, to be aromatic on the basis of this criterion <2002JA11202>. 

An additional criterion, in our opinion a very important one, but which is not a property of the ground state, is reactivity. Generally, aromatic compounds undergo electrophilic substitution reactions (aromatic substitution) more easily than addition, which is often expressed as a typical tendency of these kinds of systems to retain their initial tr-electron structure [16, 17]. 

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