INTRINSIC BARRIERS OF SOME MODEL SN2 REACTIONS - 2ND-ORDER MOLLER-PLESSET PERTURBATION CALCULATIONS

Ab initio calculations including electron correlation are used to determine the factors which govern the height of the intrinsic barriers of S(N)2 reactions. It is shown that the reactions can be classified according to the hybridization of the leaving group and the electronic structure of the transition state. A sp-hybridized atom leads to a high intrinsic barrier. A large contribution of the N:-R+:X- configuration to the transition state (in addition to its contribution to the reactant and product) lowers the intrinsic barrier. Within each group, the intrinsic barrier is related to the electronegativity of the leaving group. As the electronegativity of the leaving group increases, the intrinsic barrier decreases. Within each group there is a linear relationship between DELTA-E-double-ended-dagger and DELTA-E-degrees-c. Results are presented for N- + CH3X --> CH3N + X-, where X = H, CCH, CN, NC, PH2, SH, Cl, NH2, OH, OOH, and F for N = H; X = H, CCH, CN, NC, PH2, SH, Cl, NH2, OH, and F for N = F; and X = N = CCH, CN, NC, PH2, SH, Cl, NH2, OH, and OOH.