Gas-Phase Identity S_N2 Reactions of Halide Ions at Neutral Nitrogen: 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 at saturated nitrogen, X^– + NH₂X → XNH₂ + X^–, for X = F, Cl, Br, and I, and the results compared with data for the analogous reactions at saturated carbon, X^– + CH₃X → XCH₃ + X^–. Central barriers ΔH^‡ _cent for substitution at nitrogen are found to lie within a relatively narrow range, decreasing in the following order: Cl (58.5 kJ mol^–1) ≥ F (58.2 kJ mol^–1) > Br (46.9 kJ mol^–1) > I (39.1 kJ mol^–1). They are surprisingly similar to those for substitution at carbon, the barriers at nitrogen being slightly higher than the corresponding barriers at carbon for X = F and Cl and slightly lower for X = Br and I. The overall bathers relative to the reactants (ΔH^‡ _ovr) are negative for all halogens: –55.8 (F), –9.3 (Cl), –13.7 (Br), and –10.9 kJ mol^–1 (I), in contrast to the analogous reactions at carbon where the overall barrier is negative only for X = F. This suggests that nucleophilic substitution is likely to be more facile at nitrogen than at carbon. Stabilization energies of the ion-molecule complexes (ΔH_comp) decrease in the order F (114.0 kJ mol^–1) > Cl (67.8 kJ mol^–1) > Br (58.4 kJ mol^–1) > I (50.0 kJ mol^–1) and are found to correlate well with halogen electronegativities.