Hybrid DFT study on the gas-phase S N2 reactions at neutral oxygen

Abstract

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(Δ H=−48.6, −47.2, −56.5, −69.0 kJ/mol for Y=F, Cl, Br, I, respectively) and lower reaction enthalpies than the corresponding S N2 reactions by about 60 kJ/mol. Central barrier heights (Δ H YX ≠) for S N2 reactions in the exothermic directions vary from 52.5 kJ/mol for Y=I, X=Br up to 76.6 kJ/mol for Y=Br, X=Cl. Overall barriers (Δ H YX b) for reactions in the exothermic direction are all negative (varying from −13.8 kJ/mol for Y=I, X=Br to −5.2 kJ/mol for Y=Br, X=Cl). Complexation energies (Δ H comp) of the ion–molecule complexes Y −⋯HOX vary from 66.3 kJ/mol for Y=I, X=Br to 95.5 kJ/mol for Y=Cl, X=Br. The central barrier heights Δ H YX ≠ and Δ H 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.