Hybrid DFT study on the gas-phase S_N2 reactions at neutral oxygen

The hybrid DFT method MPW1K, in conjunction with 6-31+G(d,p) basis sets, has been examined for the gas-phase reactions, Y^-+HOX (Y, X=F, Cl, Br, I). Comparison of the results with the high-level G2(+) theory indicates that MPW1K/6-31+G(d,p) approach performs well in describing the potential energy surface for the identity S_N2 reactions X^-+HOX→HOX+X^- (X=Cl, Br, I). The corresponding non-identity reactions (Y≠X, Y, X=Cl, Br, I), are exothermic if the nucleophile is the heavier halide, in contrast to the corresponding reactions at carbon. The fluorine behaves different from the other halogens. The reactions Y^-+HOF (Y=F, Cl, Br, I) are predicted to form the energetically favorable products YO^-+HF with a large driving force(ΔH=-48.6, -47.2, -56.5, -69.0kJ/mol for Y=F, Cl, Br, I, respectively) and lower reaction enthalpies than the corresponding S_N2 reactions by about 60kJ/mol. Central barrier heights (ΔH_YX^≠) for S_N2 reactions in the exothermic directions vary from 52.5kJ/mol for Y=I, X=Br up to 76.6kJ/mol for Y=Br, X=Cl. Overall barriers (ΔH_YX^b) for reactions in the exothermic direction are all negative (varying from -13.8kJ/mol for Y=I, X=Br to -5.2kJ/mol for Y=Br, X=Cl). Complexation energies (ΔH^comp) of the ion-molecule complexes Y^-⋯HOX vary from 66.3kJ/mol for Y=I, X=Br to 95.5kJ/mol for Y=Cl, X=Br. The central barrier heights ΔH_YX^≠ and ΔH_XY^≠ correlate well with the degree of the O-X and O-Y bond elongation in the transition structures. Both central and overall barriers can be interpreted with the aid of Marcus equation.