Solution reaction path hamiltonian based on reference interaction site model self-consistent field method: application to menshutkin-type reactions

摘要

A reaction path Hamiltonian model is developed for describing chemical reaction dynamics in solution based on reference interaction site model self-consistent field (RISM-SCF) method. A systematic procedure is devised to determine all the quantities involved in the present Hamiltonian by the RISM-SCF electronic structure calculations for reaction systems combined with molecular dynamics simulations for bulk solvents. The present model is applied to Menshutkin-type reactions between NH_3 and methylhalides CH_3X (X = Cl, Br, and I) in aqueous solution. The barrier height becomes lower with the increase of the size of leaving halogen atom, and the calculated activation energy for the CH_3I reaction is in a good agreement with the experiment. The magnitude of the time dependent as well as the static solvent frictions is CH_3Cl < CH_3Br < Ch_3I in the interasing order. The frequency dependent solute-solvent coupling term is found to be almost a linear function of the reaction coordinate. The transmission coefficient is estimated to be 0.84, 0.81, and 0.60, respectively, for the reactions of CH_3Cl, CH_3Br, and CH_3I by trajectory calculations with the present reaction path Hamiltonial model. The calculated transmission coefficients are compared with those by the Grote-Hynes and Kramers' theory.