Are one-step aromatic nucleophilic substitutions of non-activated benzenes concerted processes?

Aromatic nucleophilic substitution (SAr) reactions of non-electrophilically activated benzenes have been studied within the Molecular Electron Density Theory (MEDT) at the B3LYP/6-311+G(d) computational level. These reactions, taking place through a one-step mechanism, present a high activation Gibbs free energy, ΔG = 31.0 kcal mol, which decreases to 22.1 kcal mol in the intramolecular process. A topological analysis of the electron localisation function along the reaction paths permits establishing the non-concerted nature of these SAr reactions. A series of unstable structures, with similar electronic structures to those of Meisenheimer intermediates, are characterised. The present MEDT study makes it possible to establish that even these one-step SAr reactions involving only two single bonds are non-concerted processes.