Aromaticity experimental criteria

This ring current in benzene is termed diamagnetic and is characteristic of aromatic compounds in general. The presence of a diamagnetic ring current provides a useful experimental criterion for the presence of aromaticity in a compound. Other examples of the use of this method are provided in Section 16.10. 

While the diatropic shift is consistent with all aromatic systems, there is no satisfactory experimental criterion for antiaromaticity 38>. The antiaromatic systems are all paratropic but the degree of paratropicity is confusing. It has therefore been suggested that the extent of the HOMO-LUMO gap (AE) correlates with the paratropicity of the system (see section 6.1) 35,36). A generalized approach can be obtained from the Ramsey shielding formula (Eq. 10)53> ... 

The signals in the region typical for benzenoid compounds indicate that a diamagnetic ring current is induced in the fiiran molecule. Thus, furan fulfils an important experimental criterion for aromaticity in cyclic conjugated systems. 

Most substances which appear in the examples of this chapter are analysed In Part Two and their enthalpy of decomposition determined experimentally. This is because most of them are considered hardly stable. This is one of the reasons for assigning no Tow risk in the suggested classifications. But it is also indisputable that criterion Cf overestimates the instability risk. It is the case for all aromatic compounds that are generally very stable. In the examples above, N-methylaniline, dichlorobenzene... 

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. 

Thus, the ring currents are directly related to the REs indicating compatibility of the model in question with the energy criterion of aromaticity. Moreover, the conclusions as to aromaticity drawn from calculated values of the ring currents are in accord with those derived from a set of experimental parameters. 

The NICS of each ring, as a criterion of aromaticity, has been used to explain the stability order of benzo[/)]thio-phene and its isomer. The results indicate that the benzene ring is aromatic in all the systems. The five-membered ring of benzo[. ]thiophene is also aromatic, whereas in benzo[r]thiophene it is nonaromatic. This could be an explanation of the stability of the former molecule. The MOS and the condensed Fukui functions derived from the electronic-structure calculations explain the expected electrophilic substitution of these compounds. The theoretical structure, ionization energies, order of aromaticity, stability, and reactivity are in good agreement with the experimental results <2003T6415>. 

Theoretical ionization energies are in good agreement with the experimental values. For all the molecules, the HOMO-LUMO gap is larger for the most stable isomers. This confirms previous results that claim that the stability of aromatic hydrocarbons depends on the HOMO-LUMO gap. The principle of maximum hardness establishes that the system would be more stable if the global hardness, related to the HOMO-LUMO gap, is a maximum. As shown in Table 61, the HOMO-LUMO gap correlates well with the expected stability of these molecules and the energy difference between the HOMO and HOMO-1 for benzo[3]thiophene is smaller than for benzo[c]thiophene (Figure 27). Therefore, it is possible to use hardness as a criterion of stability. 

To sum up what we have said so far, clearly, the best experimental path to a knowledge of what aromaticity is, is the study of the molecular characteristics in the ground state of the resting compound, i.e. the compound as free as possible of any external perturbation. Measuring the bond lengths, for example, will give us the opportunity to analyse the famous bond alternation aromaticity criterion. 

Criterion (iii) is associated with a few indices of aromaticity related to the magnetic properties of molecules. The earliest one was the exaltation of the magnetic susceptibility A—the quantity which can be estimated both experimentally or theoretically. As in the case of energetic criteria, its reliability depends on the assumed reference structure, since by definition... 

This selectivity factor has not been used very widely, however, perhaps because its experimental determination is difficult. The yield of -substitution products is often so low that accurate measurement of is difficult. Table 9.5 gives some data on the selectivity of some representative aromatic substitution reactions. The most informative datum in terms of substrate selectivity is fp, since the partial rate factors for ortho-substitution contain a variable steric component. Using/p as the criterion, halogenation and Friedel-Crafts acylation exhibit high selectivity, protonation and nitration are intermediate, and Friedel-Crafts alkylation shows low selectivity. 

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