Model calculations of kinetic isotope effects in the SN2 reaction of benzyl arenesulfonates with N,N-dimethyl-p-toluidine

Abstract

Model calculations of kinetic isotope effects in the Menschutkin-type S/sub N/2 reaction of benzyl arenesulfonates with N,N-dimethyl-p-toluidine have been carried out by use of the program BEBOVIB-IV. Transition-state models encompassing the region of n/sub 1/ + n/sub 2/ less than or equal to 1.0 were considered, where n/sub 1/ and n/sub 2/ are the bond orders of the breaking C/sub ..cap alpha../-O bond and the forming C/sub ..cap alpha../N bond, respectively. One or more interaction (off-diagonal) force constants were used in the transition-state force constant matrix to generate a reaction-coordinate frequency. Calculations with a single interaction force constant which generated an asymmetric stretching vibration as a reaction coordinate were unsuccessful in reproducing the experimentally observed large ..cap alpha..-carbon isotope effects and their variations with substituents on the leaving group. The experimental results were reasonably reproduced by calculations with multiple interaction constants, in which the Walden inversion motion was explicitly considered as a part of the reaction coordinate and the contribution of the Walden inversion motion was varied with the relative strength of the C/sub ..cap alpha../-O and C/sub ..cap alpha../-N bonds. The variation in this contribution was accomplished within the framework of program BEBOVIB-IV by using a new empirical equationmore » which relates the magnitude of the interaction force constants to the assumed transition-state geometry.« less