Ion Pair SN2 Reactions at Nitrogen: A High-Level G2M(+) Computational Study

Abstract

The gas-phase ion pair SN2 reactions at nitrogen, LiY + NH2X → NH2Y + LiX (Y, X = F, Cl, Br, or I), with inversion mechanisms are investigated at the level of modification of G2 theory, and the results are compared with two related reactions (Y- + NH2X and LiY + CH3X). Calculated results show that the LiY + NH2X reactions are exothermic only when the nucleophile is a heavier lithium halide. The complexation enthalpies are found to depend primarily on the identity of nucleophile LiY and only to a small extent on the identity of NH2X, decreasing in the order LiI > LiBr > LiCl > LiF. Including a Li cation in the anionic SN2 model will raise the overall barriers for the LiY + NH2X (Y, X = F−Br) reactions and lower the overall barriers for the LiI + NH2X (X = F−I) reactions. Another interesting feature of the ion pair reactions at nitrogen is the good correlation between the reaction barriers with the geometrical looseness of Li−Y and N−X bonds in the transition-state structures. The data for the reaction barr...