π-electron densities and resonance effects in benzene monoderivatives

The pi-electron densities according to Mulliken or Lowdin in monosubstituted benzenes C6H5X with 30 different substituents X were calculated at a variable level from STO-3G to B3LYP/6-311 + G(d,p). Sum of the density, Sigmaq(pi), at all C atoms or at all C and H atoms of the benzene ring was taken as an estimate of resonance ability of the substituent X and correlated with the standard scales of resonance constants sigma(R). The model is warrantable since Sigmaq(pi) at STO-3G obtained previously on monosubstituted ethenes correlate well with sigma(R) derived from experiments. However, at higher levels of calculations, the correlations become stepwise worse with the greatest deviations for substituents not coplanar with the benzene ring. We attribute this deviation to a shortcoming of the valence-split basis set and held as probable that the pi-electron density is represented better in the STO-3G basis set. For estimating the resonance constants, the present model has a merit compared to the substituted ethenes that it removes problems with ambiguous conformation. Nevertheless, comparison of various scales sigma(R) revealed the limits of this approach. (C) 2002 Elsevier Science B.V. All rights reserved.