Comparison of electronic structure theories for solvated molecules: RISM-SCF versus PCM

Two modern theories that can handle electronic structures of solvated molecules, RISM-SCF and PCM, are compared in a typical S(N)2 reaction: Cl- + CH3Cl --> ClCH3 + Cl-. The potentials of mean forces for the reaction in aqueous solution calculated with the two methods are very similar despite the different theoretical standpoint in dealing with solvation effects. However, a new energy-partitioning analysis sheds light on their differences. Both of the electronic structures computed by RISM-SCF and PCM are polarized compared to that from the standard gas-phase computation, but they are slightly different from each other in detail. The polarization of the C-H bond is emphasized in RISM-SCF, but that of the Cl-C bond is dominant in PCM. It is also shown that the change in the electronic structure of the carbon atom is a major cause of the reaction barrier.