Phosphorus-centered ion-molecule reactions: benchmark ab initio characterization of the potential energy surfaces of the X^- + PH₂Y [X, Y = F, Cl, Br, I] systems

In the present work we determine the benchmark relative energies and geometries of all the relevant stationary points of the X- + PH2Y [X, Y = F, Cl, Br, I] identity and non-identity reactions using state-of-the-art electronic-structure methods. These phosphorus-centered ion-molecule reactions follow two main reaction routes: bimolecular nucleophilic substitution (S(N)2), leading to Y- + PH2X, and proton transfer, resulting in HX + PHY- products. The S(N)2 route can proceed through Walden-inversion, front-side-attack retention, and double-/multiple-inversion pathways. In addition, we also identify the following product channels: H--formation, PH2-- and PH2-formation, (PH)-P-1- and (PH)-P-3-formation, H-2-formation and HY + PHX- formation. The benchmark classical relative energies are obtained by taking into account the core-correlation, scalar relativistic, and post-(T) corrections, which turn out to be necessary to reach subchemical (<1 kcal mol(-1)) accuracy of the results. Classical relative energies are augmented with zero-point-energy contributions to gain the benchmark adiabatic energies.