CHARGE DEVELOPMENT AT THE TRANSITION-STATE - A 2ND-ORDER MOLLER-PLESSET PERTURBATION STUDY OF GAS-PHASE SN2 REACTIONS

A systematic analysis of the charge distribution at the transition state has been carried out for the model S(N)2 reactions N- + CH3X --> CH3N + X-, where X = H, NH2, OH, F, CCH, CN, NC, SH, and Cl for N = H and X = H, NH2, OH, F, CN, SH, and Cl for N = F. Second-order Moller-Plesset perturbation calculations indicate that for some, but not all, reactions the charges on the nucleophile and leaving group at the transition state are equal. In such cases the assumption that the transition state occurs in the vicinity of the crossing point of valence bond configurations psi-1 and psi-2 holds. However, in general, the contributions of the reactant and product wave functions to the transition state are not equal. With an electronegative nucleophile or leaving group, charge development on the nucleophile or leaving group at the transition state is small. A large contribution of the N:-R:X- configuration (in addition to its contribution to the reactant and product) to the transition state leads to large charge development at the transition state. All other factors being equal, a more exothermic reaction leads to less charge development at the transition state. Also the charge distribution calculated at different theoretical levels (with and without electron correlation) are compared.