Gas-Phase Identity S_N2 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^– + CH₃X → XCH₃ + X^–, where X = F, Cl, Br, and I. The reaction profiles all involve central barriers (ΔH⁺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) ≥ 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 (ΔH⁺ _ovr) 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 (ΔH_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 ΔH⁺ _cent and the ionization energy of X^– is observed. The significance of these results to our understanding of the energetics of gas-phase S_N2 reactions is discussed.