The electron localization function description of aromaticity in five-membered rings

The electron localization function (ELF) has been calculated at the B3LYP/6-31+G(d,p)//MP2/6-31+G(d,p) level for a series of substituted five-membered cyclopentadienyl systems. While the behavior of the ELF data for the formally double C=C bond is somewhat chaotic, that of the formally single C-C bond shows a strong and smooth correlation with resonance energies, E(H), calculated at the BLYP/6-311G(d,p)//BLYP/6-31G(d) level in the homomolecular homodesmotic approach. When appropriately scaled, the functional behavior of the C-C bond basin populations and pair numbers follow very closely that found empirically for the reduced vapor density of real liquid-vapor coexistence curves. (C) 2000 Elsevier Science B.V. All rights reserved.