Phosphorus-centered ion-molecule reactions: benchmark ab initio characterization of the potential energy surfaces of the X - + PH 2 Y [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 - + PH 2 Y [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 - + PH 2 X, 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, PH 2 - - and PH 2 -formation, 1 PH- and 3 PH-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.