Unusual solvent effect on a SN2 reaction. A quantum-mechanical and kinetic study of the Menshutkin reaction between 2-amino-1-methylbenzimidazole and iodomethane in the gas phase and in acetonitrile.

The quaternization reaction between 2-amino-1-methylbenzimidazole and iodomethane was investigated in the gas phase and in liquid acetonitrile. Both experimental and theoretical techniques were used in this study. In the experimental part of this work, accurate second-order rate constants were obtained for this reaction in acetonitrile from conductivity data in the 293-323 K temperature range and at ambient pressure. From two different empirical equations describing the effect of temperature on reaction rates, thermodynamic functions of activation were calculated. In the theoretical part of this work, the mechanism of this reaction was investigated in the gas phase and in acetonitrile. Two different quantum levels (B3LYP/[6-311++G(3df,3pd)/LanL2DZ]//B3LYP/[6-31G(d)/LanL2DZ] and B3LYP/[6-311++G(3df,3pd)/LanL2DZ]//B3LYP/[6-31+G(d)/LanL2DZ]) were used in the calculations, and the acetonitrile environment was modeled using the polarized continuum model (PCM). In addition, an atoms in molecules (AIM) analysis was made aiming to characterize possible hydrogen bonding. The results obtained by both techniques are in excellent agreement and lead to new insight into the mechanism of the reaction under examination. These include the identification and thermodynamic characterization of the relevant stationary species, the rationalization of the mechanistic role played by the solvent and the amine group adjacent to the nucleophile nitrogen atom, the proposal of alternative paths on the modeled potential energy surfaces, and the origin of the marked non-Arrhenius behavior of the kinetic data in solvent acetonitrile. In particular, the AIM analysis confirmed the operation of intermolecular hydrogen bonds between reactants and between products, both in the gas phase and in solution. It is also concluded that the unusual solvent effect on this Menshutkin reaction stems from the conjunction of a nucleophile possessing a relatively complex chemical structure with a dipolar aprotic solvent that is protophobic.