HYDRATION EFFECTS ON REACTION PROFILES - AN AB-INITIO DIELECTRIC CONTINUUM STUDY OF THE S(N)2 CL-+CH3CL REACTION

We present ab initio studies of the hydration effects on the reaction profile of the S(N)2 Cl- + CH3Cl reaction using our recently proposed generalized conductor-like screening model (GCOSMO) within the quantum mechanical and classical frameworks including both electrostatic and nonelectrostatic contributions to the hydration free energy. For the q uantum mechanical approach, we employed the Hartree-Fock (HF), second- order Moller-Plesset perturbation theory (MP2), and hybrid nonlocal de nsity functional theory (DFT) with the 6-31+G(d,p) basis set. Using th e GCOSMO model, we found that the calculated free energy reaction prof ile of the title S(N)2 reaction in aqueous solution is in good agreeme nt with experimental observations and previous studies. The solvent re action field was found to have a noticeable effect on the solute elect ronic density distribution. The DFT free energies of hydration agree v ery well with MP2 results and are in better agreement with experimenta l data than HF results. The DFT-GCOSMO approach provides a promising t ool for studies of mechanisms of chemical and biochemical reactions in solutions.