Theoretical Studies on F– + NH2Cl Reaction: Nucleophilic Substitution at Neutral Nitrogen

The SN2 reactions at N center, denoted as SN2@N, has been recognized to play a significant role in carcinogenesis, although they are less studied and less understood. The potential energy profile for the model reaction of SN2@N, chloramine (NH2Cl) with fluorine anion (F(-)), has been characterized by extensive electronic structure calculations. The back-side SN2 channel dominates the reaction with the front-side SN2 channel becoming feasible at higher energies. The minimum energy pathway shows a resemblance to the well-known double-well potential model for SN2 reactions at carbon. However, the complexes involving nitrogen on both sides of the reaction barrier are characterized by NH---X (X = F or Cl) hydrogen bond and possess C1 symmetry, in contrast to the more symmetric ion-dipole carbon analogues. In the F(-) + NH2Cl system, the proton transfer pathway is found to become more competitive with the SN2 pathway than in the F(-) + CH3Cl system. The calculations reported here indicate that stationary point properties on the F(-) + NH2Cl potential energy surface are slightly perturbed by the theories employed. The MP2 and CAM-B3LYP, as well as M06-2X and MPW1K functionals give overall best agreement with the benchmark CCSD(T)/CBS energies for the major SN2 reaction channel, and are recommended as the preferred methods for the direct dynamics simulations to uncover the dynamic behaviors of the title reaction.