Two sources of the decrease of aromaticity: Bond length alternation and bond elongation. Part I. An analysis based on benzene ring deformations.

Decrease of aromaticity of the pi-electron system may be described by two d ifferent and independent mechanisms: (i) an increase of the bond length alt ernation and (ii) an extension of the mean bond length (bond elongation). T hese two mechanisms are described by two contributions to the HOMA index: G EO and EN, respectively. These contributions correlate very well with the t otal Hartree-Fock energy of the benzene ring deformed in two ways: (i) when all C-C bond lengths are set equal and their length is gradually increased and (ii) when the mean C-C bond length is fixed and the difference in leng th between adjacent C-C bonds gradually increases. Both changes in geometry bear the changes in the aromatic character of the ring, and the HOMA-value s as well as the Hartree Fock energies correlate excellently with Schleyer' s NICS values. (C) 1999 Published by Elsevier Science Ltd. All rights reser ved.