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

Abstract

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_N2), leading to Y⁻ + PH₂X, and proton transfer, resulting in HX + PHY⁻ products. The S_N2 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, ¹PH- and ³PH-formation, H₂-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.