GAS-PHASE 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 identity nucleophilic substitution reactions, X(-) + CH(3)X --> XCH(3) + X(-), where X = F, Cl, Br, and I. The reaction profiles all involve central barriers (Delta H-double dagger (cent)) which are found to lie within a surprisingly narrow range, decreasing in the order Cl (53.5 kJ mol(-1)) > F (46.1 kJ mol(-1)) greater than or equal to Br (45.0 kJ mol(-1)) > I (40.8 kJ mol(-1)) at 298 K; the value for X = Cl is in good agreement with a recent experimental determination (55.2 +/- 8.4 kJ mol(-1)). The overall barriers relative to the reactants (Delta H-ovr(double dagger)) are - 11.0(F), 9.8 (Cl), 4.5 (Br), and 5.5 (I) kJ mol(-1) at 298 K. Stabilization energies of the ion-molecule complexes (Delta H-comp) decrease in the order F (57.1 kJ mol(-1)) > Cl (43.7 kJ mol(-1)) > Br (40.5 kJ mol(-1)) > I (35.3 kJ mol(-1)) at 298 K and are found to correlate well with halogen electronegativities. A reasonably good correlation between Delta H-double dagger (cent) and the ionization energy of X(-) is observed. The significance of these results to our understanding of the energetics of gas-phase S(N)2 reactions is discussed.