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

We report a comprehensive high-level explicitly correlated ab initio study on the X + NHY [X,Y = F, Cl, Br, I] reactions characterizing the stationary points of the S2 (Y + NHX) and proton-transfer (HX + NHY) pathways as well as the reaction enthalpies of various endothermic additional product channels such as H + NHXY, XY + NH, XY + NH, 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 C minima, X···HH'NY and X···H'HNY connected by a C first-order saddle point, X···HNY, as well as a halogen-bonded front-side complex, X···YNH. S2 reactions can proceed via back-side attack Walden inversion and front-side attack retention pathways characterized by first-order saddle points, submerged [X-NH-Y] and high-energy [HNXY], respectively. Product-like stationary points below the HX + NHY asymptotes are involved in the proton-transfer processes.