Mechanism of proton exchange with water molecules by NH3 and HF molecules during the formation of ion pairs

The semiempirical quantum-chemical MNDO/VS-PD method has been used to investigate the mechanism of the formation and dissociation of quaternary ammonium salts in aqueous solution. It is shown that dissociation involving proton exchange with the H₂O molecules of the hydration shell has a considerably lower activation barrier than direct proton transfer. The energetic preference for the exchange mechanism arises because the transition state for the synchronous migration of two protons occurs earlier than in the reaction taking place in a single solvent cage.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 817–822, April, 1991.