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 substit ution 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 t he analogous reactions at saturated carbon, X(-) + CH(3)X --> XCH(3) X(-). Central barriers Delta H double dagger(cent) for substitution a t nitrogen are found to he within a relatively narrow range, decreasin g in the following order: Cl (58.5 kJ mol(-1)) greater than or equal t o F (58.2 kJ mol(-1)) > Br (46.9 kJ mol(-1)) > I (39.1 kJ mol(-1)). Th ey are surprisingly similar to those for substitution at carbon, the b arriers at nitrogen being slightly higher than the corresponding barri ers at carbon for X = F and Cl and slightly lower for X = Br and I. Th e overall barriers relative to the reactants (Delta H double dagger(ov r)) are negative for all halogens: -55.8 (F), -9.3 (Cl), -13.7 (Br), a nd -10.9 kJ mol(-1) (I), in contrast to the analogous reactions at car bon where the overall barrier is negative only for X = F. This suggest s that nucleophilic substitution is likely to be more facile at nitrog en than at carbon. Stabilization energies of the ion-molecule complexe s (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 foun d to correlate well with halogen electronegativities.