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

2018 
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 SN2 (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 Cs first-order saddle point, X–···H2NY, as well as a halogen-bonded front-side complex, X–···YNH2. SN2 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]−, respectivel...
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