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Abstract: Recently assigned numerical expressions to qualitative benzenoid formulas of Clar have transformed Clar's theory into a quantitative model that allowed a structural characterization of aromaticity in benzenoid polycyclic hydrocarbons and construction of aromaticity maps for benzenoid polycyclic hydrocarbons. The key structural elements of the novel quantitative aromatic sextet theory of Clar are (1) Kekulé valence structures, (2) Pauling bond orders, and (3) Clar's aromatic sextet benzenoid rings. In the present contribution we go beyond Clar and consider local aromaticities of benzenoid rings in benzenoid polycyclic hydrocarbons. We have characterized each benzenoid ring by two descriptors: (1) their content of C = C bonds in the set of Kekulé valence structures, which is reflected in the ring bond orders, and by (2) the variations of CC bond orders within rings, which are zero for benzene. This allows us to introduce for any set of benzenoid hydrocarbons local aromaticity map, which allows one to graphically view similarities and dissimilarities of local aromaticity of different benzenoid rings. For such map, one can calculate the “distance” of each ring from the benzene, with coordinates (3.00, 0) for benzene RBO and variations in CC bonds, respectively. We show that the central ring in peropyrene is visibly less aromatic that the terminal sextets, as has been claimed by Gutman and Agranat, and similarly we show that the central ring of anthracene is unexpectedly more aromatic than the terminal rings, in having greater similarity in bond lengths to benzene ring than the terminal rings. This suggests limitations of the assumption of the Clar model that migrating sextets have equal local aromaticities. Finally, we examined a pair of non benzenoid hydrocarbons related to peropyrene, built from terminal benzenoid rings and naphthalene fragments. Surprisingly, we have found the terminal benzene rings in the two non benzenoid compounds to have unusually high ring bond orders, approaching that of benzene. [ABSTRACT FROM AUTHOR]

Comparison is made between three different indices that characterise the individual rings of a wide range of condensed benzenoid hydrocarbons. Two of these ("π-electron partitions" and the six-centre delocalisation-indices that have been called "Δ6-values") have been introduced only recently as potential indicators of what might be called "local aromaticity", whilst the third ("topological π-electron ring-currents") was suggested as a possible discriminator in this regard nearly fifty years ago. Whilst linear correlations between ring currents and π-electron partitions within certain restricted classes of ring types are good (with correlation coefficients of up to 0.998), agreement between the two indices over all classes of ring types is poor. Predictions arising from a consideration of π-electron partitions and Δ6-values seem, on the other hand, to be in somewhat better accord. It is therefore concluded that, despite its superficially intuitive appeal, the ring-current index is out of step both with π-electron partitions and Δ6-values as a general indicator of so-called "local aromaticity". [ABSTRACT FROM AUTHOR]

In a series of earlier studies, it was established that benzo-annelation in the angular (resp. linear) position relative to a ring R of a polycyclic conjugated π- electron system, increases (resp. decreases) the intensity of the cyclic conjugation in the ring R. Herein, it is shown how this regularity can be explained by means of a simple, Kekulé;-structure-based argument, itself based on an idea of Randić from the 1970s. [ABSTRACT FROM AUTHOR]

We revisited the π-electron ring partitions in catacondensed benzenoids and re-examined structural regularities reported for the ring partitions in these compounds, seeking the origin of the observed regularities. We examined the distribution of the π-electron ring partitions by counting the contributions arising from benzene rings having different assigned numbers of π-electrons. This has led to a better insight into the underlying structure of the π-electron ring partitions and also to a novel and unique π-electron “signature” of individual rings in benzenoid hydrocarbons. The physical interpretation of the local π-electron ring partition represents the local π-electron density of benzenoid molecules, and is not an alternative index of local aromaticity as one may erroneously assume. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]

Profiles two experts in the development of the aromaticity concept, Henry Edward Armstrong, professor at Imperial College London, England and Ernest C. Crocker, author of an article about the six aromatic electron structure of benzene, pyridine and five-membered ring heteroaromatics. Works and contributions to the theory of aromaticity; Pre-electronic and electronic theories of aromaticity; Development of a Benzene Model.

Discusses the aromaticity concept as a cornerstone of heterocyclic chemistry. Importance of heterocyclic chemistry; Tautomerism; Biological and technological properties; Extent and classification of heteroaromatic compounds.