Separation of the energetic and geometric contributions to the aromaticity of π-electron carbocyclics.

Abstract

Separation of the aromaticity index HOMA, based on experimental geometry, into energetic and geometric contributions is proposed for carbocyclic π-electron systems. The decrease of the aromatic character may be due either to an increase of bond length alternation (geometric term) or to the lengthening of the mean bond lengths of the ring (energetic term). On the basis of 169 sets of data for benzene rings in benzenoid hydrocarbons, 116 in para-disubstituted benzene derivatives and 90 in TCNQ molecules in EDA complexes and salts, as well as 48 for five-membered rings in cyclopentadienyl complexes with Rh it is shown that these two terms are uncorrelated, supporting the view of Katritzky et al. 1–4 and Jug et al. 5 that aromaticity is a multidimensional phenomenon. The separation is applied to five-, six- and seven- membered rings in typical π-electron systems (benzenoid hydrocarbons, fullerenes, fulvene and heptafulvene derivatives). Aromaticity index (HOMA) = 1 - [dearomatization term due to bond length alternations (GEO) + energetic term (EN)]. All terms are accessible from experimental bond lengths.