Crystallographic studies of inter- and intramolecular interactions reflected in aromatic character of .pi.-electron systems

An old quantitative measureof aromaticcharacter named HOMA (harmonic oscillator measureof aromaticity) is reformulated and related to the Jug and Koester concept of resonance coordinate. In its new version, the HOMA index can be used to estimate an aromatic character of r-electron systems (molecules, ions, or their fragments) built up of the following bonds: CC, CN, CO, CP, CS, NN, and NO. It is shown that even if aromatic stabilization is due to a-electron structure inclined to maintain equal bond lengths, the aromatic character is mainly a result of r-electron properties of the system. Application of the HOMA index to benzene rings in various molecular and ionic systems showed that the aromatic character depends rather weakly on deformation from planarity whereas it may be strongly dependent on the intermolecular H-bonding net as well as on cooperative substituent interactions of push-pull type and on the topology of a mutual link between benzene rings in condensed benzenoid hydrocarbons,

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