We used a combined quantum mechanics and molecular mechanics (QM/MM) method to investigate the solvent effects and potential of mean force of the CH3F+CN-reaction in water.Comparing to gas phase,the water solution substantially affects the structures of the stationary points along the reaction path.We quantitatively obtained the solvent effects’ contributions to the reaction:1.7 kcal/mol to the activation barrier and-26.0 kcal/mol to the reaction free energy.The potential mean of force calculated with the density functional theory/MM theory has a barrier height at 19.7 kcal/mol,consistent with the experimental result at 23.0 kcal/mol;the calculated reaction free energy at-43.5 kcal/mol is also consistent with the one estimated based on the gas-phase data at-39.7 kcal/mol.