Wheland Intermediates: An ab Initio Valence Bond Study

The traditional resonance model for electrophilic attacks on substituted aromatic rings is revisited using high level valence bond (VB) calculations. A large pi-donation is found in the X = NH2 case and a weaker one for the X = Cl case, not only for ortho and para isomers but also for the meta case, which can be explained by considering five (not three) fundamental VB structures. No substantial pi-effect is found in the X = NO2 case, generally viewed as pi-attractive.