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 examinat ion of the stationary points in the reaction potential surface leads t o a new possible interpretation of the detailed mechanism of this reac tion in different media. In contrast to the experimentally observed fo rmation of free alkyl ammonium and halide ions as a result of this rea ction in polar solvents, the formation of a strongly hydrogen-bonded c omplex between these ions is predicted in the gas phase. Both the init ial reaction complex and the product complex are characterized by a no nlinear arrangement of the nitrogen, carbon and halogen atoms. The cal culated transition state in the gas phase corresponds to a linear arra ngement of these atoms but the reaction is prohibited in low dielectri c constant media not because of the difference in the energies of reac tants and products but rather because of the difference in the reactio n activation barriers. (C) 1997 Elsevier Science B.V.