Resonance energy aromaticity, relationship

The question to be answered in the first place is whether the ring current model criterion is compatible with the chief energy criterion of aromaticity and antiaromaticity. The answer will be positive if a relationship is revealed between ring currents and resonance energies. 

A linear relationship has been shown to exist between structural aromaticity indices and resonance energies (92T335). From this so-called unified aromaticity index, IA, has been proposed. It makes for a more appropriate comparison the aromaticity of heterocycles of different size. 

For organic solids the individual molecular properties are used. For explosives containing aromatic rings, there is a correlation between the sensitivity and the resonance energy. The smaller the resonance energy per electron (REPE), the more sensitive is the compound. All these examples show the expected relationship the softer the solid (in chemical terms), the more reactive it is. 

In the foregoing paragraphs a variety of apparently unconnected possible criteria for aromaticity has been considered, although theoretical calculations have suggested that relationships do in fact exist, for example between thermodynamic and kinetic criteria of aromatic character [37] and between resonance energies and induced ring currents [38]. 

Haddon and Fukunaga showed that a direct relationship exists between the resonance energies and the HOMO—LUMO gaps in [4n 2] annulenes and thereby, as one of the first, demonstrated the connection between the thermodynamic and kinetic criterion of the aromatic character. Indeed, they showed that... 

A number of perylene derivatives undergo cyclo addition reaction. Hern-don has shown that the free energies of activation of the reaction of MA with aromatic hydrocarbon has a linear relationship with calculated resonance energy differences. This approach has significant prediction value (see Table 4.5). Recently, kinetic studies of the DA reaction of maleic anhydride with polycyclic aromatics have been reported. 

Substituent effects on redox potentials can be rationalized by free energy relationships in many cases Although 1 can be related to aromatic compounds, a linear correlation with Hammett s a-constants fails. As can be seen from the substituents -N(CH3)2 Ik) and -OCH3 11) the electron attracting effect is the dominating one and not the resonance effect. Therefore Taft s a "-constants describe the substituent effects more correctly. According to (1)... 

Activated monosubstituted benzenes The major doubt concerning a general linear free-energy relationship for aromatic substitution is contained in the question whether resonance contributions to an electron-deficient transition state are sufficiently... 

In 1958, Taft and Lewis showed that a reasonably precise linear inductive free-energy relationship contributed uniformly to aromatic reactivity. The extension of the treatment to the consideration of resonance effects was explored by Taft and Lewis in 1959. The authors examined the application of the three-parameter expression... 

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