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- + CH(3)X --> YCH3 + X(-), for Y, X = F, Cl, Br, and I. Central barrier heights (Delta H double dagger 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(-) + CH(3)X --> XCH(3) + X(-) (X = F-I). Overall barriers (Delta H(double dagger)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 (Delta H-comp) of the ion-molecule complexes Y-... CH(3)X 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- + CH(3)X (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- + CH(3)X 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 double dagger), geometric asymmetry (%AS), charge asymmetry (Delta q(X-Y)), and bond asymmetry (Delta WBI) of the transition structures.