Benchmark ab Initio Characterization of the Complex Potential Energy Surfaces of the X^– + NH₂Y [X, Y = F, Cl, Br, I] Reactions

Abstract

We report a comprehensive high-level explicitly correlated ab initio study on the X^- + NH₂Y [X,Y = F, Cl, Br, I] reactions characterizing the stationary points of the S_N2 (Y^- + NH₂X) and proton-transfer (HX + NHY^-) pathways as well as the reaction enthalpies of various endothermic additional product channels such as H^- + NHXY, XY^- + NH₂, XY + NH₂^-, and XHY^- + NH. Benchmark structures and harmonic vibrational frequencies are obtained at the CCSD(T)-F12b/aug-cc-pVTZ(-PP) level of theory, followed by CCSD(T)-F12b/aug-cc-pVnZ(-PP) [n = Q and 5] and core correlation energy computations. In the entrance and exit channels we find two equivalent hydrogen-bonded C₁ minima, X^-···HH'NY and X^-···H'HNY connected by a C_s first-order saddle point, X^-···H₂NY, as well as a halogen-bonded front-side complex, X^-···YNH₂. S_N2 reactions can proceed via back-side attack Walden inversion and front-side attack retention pathways characterized by first-order saddle points, submerged [X-NH₂-Y]^- and high-energy [H₂NXY]^-, respectively. Product-like stationary points below the HX + NHY^- asymptotes are involved in the proton-transfer processes.