Gas-Phase Non-Identity SN2 Reactions of Halide Anions with Methyl Halides: A High-Level Computational Study

High-level ab initio molecular orbital calculations at the G2(+) level of theory have been carried out for the six non-identity nucleophilic substitution reactions, Y- + CH3X → YCH3 + X-, for Y, X = F, Cl, Br, and I. Central barrier heights (ΔH⧧cent) for reaction in the exothermic direction vary from 0.8 kJ mol-1 for Y = F, X = I up to 39.5 kJ mol-1 for Y = Cl, X = Br (at 0 K), and are in most cases significantly lower than those for the set of identity SN2 reactions X- + CH3X → XCH3 + X- (X = F−I). Overall barriers (ΔH⧧ovr) for reaction in the exothermic direction are all negative (varying from −68.9 kJ mol-1 for Y = F, X = I to −2.3 kJ mol-1 for Y = Br, X = I), in contrast to the overall barriers for the identity reactions where only the value for X = F is negative. Complexation enthalpies (ΔHcomp) of the ion−molecule complexes Y-···CH3X vary from 30.4 kJ mol-1 for Y = F, X = I to 69.6 kJ mol-1 for Y = I, X = F (at 298 K), in good agreement with experimental and earlier computational studies. Complexati...