Aromaticity theoretical methods

These cover the following topics (a) theoretical methods, with emphasis on the utility of such methods b) molecular dimensions, as determined by X-ray, electron diffraction and microwave spectra (c) molecular spectra, covering NMR, IR, UV, mass and photoelectron spectra [d) thermodynamic aspects, such as stability, ring strain, aromaticity, shape and conformation of saturated and partially saturated rings (c) tautomerism, including prototopic and ring-chain (/) betaine and other unusual structures. 

As with the methylbenzenes the methyl derivatives of condensed aromatic substances show an increase in basicity compared to the unsubstituted compound. Thus, the methyl groups exert a very profound influence on the electron distribution. The extension of these considerations by theoretical methods will be discussed in Section V. 

Anwander R (1996) Routes to Monomeric Lanthanide Alkoxides. 179 149-246 Anwander R, Herrmann WA (1996) Features of Organolanthanide Complexes. 179 1-32 Artymiuk PJ, Poirette AR, Rice DW, Willett P (1995) The Use of Graph Theoretical Methods for the Comparison of the Structures of Biological Macromolecules. 174 73-104 Astruc D (1991) The Use of p-Organoiron Sandwiches in Aromatic Chemistry. 160 47-96 Baerends EJ, see van Leeuwen R (1996) 180 107-168 Balbds LC, see Alonso JA (1996) 182 119-171... 

UV spectroscopy has revealed electron transitions close to those of established aromatics and the differences have been explained by theoretical methods. 

The application of theoretical methods to the question of aromaticity is discussed in Section 2.22.8. 

In this review we have tried to summarize our contribution to heteroaromaticity, which is essentially based on the use of theoretical methods. In the future it is expected that more and more elaborate methods will shed light on the properties of aromatic heterocycles, allowing the creation of a rational picture of them. At the limit, one can forecast a day where the teaching of this part of chemistry will be based on computed properties. 

Within the last decade, ab initio and hybrid quantum-chemical methods were in considerable use in tetrazole chemistry, and the level of calculations significantly improved with extended basis sets used for quite complex polyatomic molecules. During this time, theoretical methods were exploited in the study of several fundamental properties of the terazole ring, such as aromaticity and capability to be involved in various kinds of tautomerism, including the effects of substituents and media on these parameters. It was demonstrated that many physical and physicochemical characteristics of tetrazoles could be successfully estimated by these methods not only for the gas phase but also for the condensed state (solvents, crystals). 

The aromatic stabilization of the heteroaromatic systems detailed here has been probed extensively by theoretical methods (see Section 7.14.2) and experimental structural methods, particularly X-ray diffraction (see Section 7.14.3). 

The graph-theoretical methods are extensively used in the theory of conjugated systems. It is only natural since in this area of chemistry the HMO method was used most frequently. In the next chapters we shall dwell on some interesting results derived in the topological theory of conjugated and aromatic systems. 

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