A gas phase ab initio study of the Menshutkin reaction

The reaction between ammonia and methyl halides (Menshutkin reaction) has been studied using ab initio quantum-chemical methods. An examination of the stationary points in the reaction potential surface leads to a new possible interpretation of the detailed mechanism of this reaction in different media. In contrast to the experimentally observed formation of free alkyl ammonium and halide ions as a result of this reaction in polar solvents, the formation of a strongly hydrogen-bonded complex between these ions is predicted in the gas phase. Both the initial reaction complex and the product complex are characterized by a nonlinear arrangement of the nitrogen, carbon and halogen atoms. The calculated transition state in the gas phase corresponds to a linear arrangement of these atoms but the reaction is prohibited in low dielectric constant media not because of the difference in the energies of reactants and products but rather because of the difference in the reaction activation barriers. (C) 1997 Elsevier Science B.V.