Molecular orbital calculations of aromatic reactivity. Part II. All-valence-electron calculations for monosubstituted benzenes: an analysis of the sigma inductive effect

Abstract

The CNDO/2 method has been used to calculate localisation energies, and σ- and π-electron distributions for the ground state and transition states for ortho-, meta-, and para-substitution in some monosubstituted benzenes. The results are in very good agreement with the orientating effect and effect upon reactivity as actually observed. The σ-charge densities show a regular alternation in magnitude around the benzene ring, in direct disagreement with the predictions of the classical σ-inductive effect. It is suggested that the σ-electron distribution is controlled by the alternation in π-charge density. The order of effectiveness of substituents as electron acceptors is different in ground and ortho-, meta-, and para-transition states: in particular the methyl group is an electron acceptor in the ground and in the meta-transition state.