GAS-PHASE IDENTITY S(N)2 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(-) + NH(2)X --> XNH(2) --> X(-), for X = F, Cl, Br, and I, and the results compared with data for the analogous reactions at saturated carbon, X(-) + CH(3)X --> XCH(3) + X(-). Central barriers Delta H double dagger(cent) for substitution at nitrogen are found to he within a relatively narrow range, decreasing in the following order: Cl (58.5 kJ mol(-1)) greater than or equal to 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 barriers relative to the reactants (Delta H double dagger(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 (Delta H-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.