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 (