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

Separation of the aromaticity index HOMA based on experimental geometry, into energetic and geometric contributions is proposed for carbocyclic pi-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 Ph 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 pi-electron systems (benzenoid hydrocarbons, fullerenes, fulvene and heptafulvene derivatives).