Gas-phase non-identity S(N)2 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(paragraph)(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 S(N)2 reactions X^- + CH₃X → XCH₃ + X^- (X = F-I). Overall barriers (ΔH(paragraph)(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 (ΔH(comp)) 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. Complexation enthalpies in the reaction series Y^- + CH₃X (Y = F-I, X = F, Cl, Br, I) are found to exhibit good linear correlations with halogen electronegativity. Both the central barriers and the overall barriers show good linear correlations with reaction exothermicity, indicating a rate-equilibrium relationship in the Y^- + CH₃X reaction set. The data for the central barriers show good agreement with the predictions of the Marcus equation, though modifications of the Marcus equation that consider overall barriers are found to be less satisfactory. Further interesting features of the non-identity reaction set are the good correlations between the central barriers and the geometric looseness (%L(paragraph)), geometric asymmetry (%AS), charge asymmetry (Δq(X-Y)), and bond asymmetry (ΔWBI) of the transition structures.