AN ABINITIO STUDY OF MODEL SN2 REACTIONS WITH INCLUSION OF ELECTRON CORRELATION-EFFECTS THROUGH 2ND-ORDER MOLLER-PLESSET PERTURBATION CALCULATIONS

A systematic analysis of electron correlation effects in model SN2 reactions shows large differences between the structures optimized at the Hartree-Fock (HF) and second-order Mϕller-Plesset (MP2) levels, but the relative energies critical to an understanding of gas-phase SN2 reactions are essentially unaffected by the effect of electron correlation on the optimized structures. Integrated charges obtained from the topological definition of an atom in a molecule indicate that the HF method overestimates the ionic character at the transition state and leads to large negative charges on the nucleophiles and leaving groups. Also, the HF method underestimates the electron density at the bond critical point and overestimates the critical radius. The model systems studied are N- + CH3X ⟶ CH3N + X-, where X = H, NH2, OH, F, CCH, CN, NC, SH, and Cl for N = H and where X = H, NH2, OH, F, CN, SH, and Cl for N = F, and also N = X = Cl. The 6-31G basis set supplemented with diffuse and polarization functions (standard notation 6-31+G⋆⋆) was used for all atoms, except for the three methyl hydrogens for which the 6-31G basis set was used. © 1990, American Chemical Society. All rights reserved.