RISM-SCF study of the free-energy profile of the Menshutkin-type reaction NH3+CH3Cl -> NH3CH3++Cl- in aqueous solution

The free-energy profile for the Menshutkin-type reaction NH3 + CH3Cl --> NH 3CH3+ + Cl- in aqueous solution is studied using the RISM-SCF method. The e ffect of electron correlation on the free-energy profile is estimated by th e RISM-MP2 method at the HF optimized geometries along the reaction coordin ate. Solvation uas found to have a large influence on the vibrational frequ encies at the reactant, transition state and product; these vibrational fre quencies are utilized to calculate the zero-point energy correction of the free-energy profile. The computed barrier height and reaction exothermicity are in reasonable agreement with those of experiment and previous calculat ions. The change of solvation structure along the reaction path is represen ted by radial distribution functions between solute-solvent atomic sites. T he mechanisms of the reaction ape discussed from the view points of solute electronic and solvation structures.