Alternating Stereospecificity upon Central-Atom Change: Dynamics of the F^- +PH₂ Cl S_N 2 Reaction Compared to its C- and N-Centered Analogues

Central-atom effects on bimolecular nucleophilic substitution (S_N 2) reactions are well-known in chemistry, however, the atomic-level S_N 2 dynamics at phosphorous (P) centers has never been studied. We investigate the dynamics of the F^- +PH₂ Cl reaction with the quasi-classical trajectory method on a novel full-dimensional analytical potential energy surface fitted on high-level ab initio data. Our computations reveal intermediate dynamics compared to the F^- +CH₃ Cl and the F^- +NH₂ Cl S_N 2 reactions: phosphorus as central atom leads to a more indirect S_N 2 reaction with extensive complex-formation with respect to the carbon-centered one, however, the title reaction is more direct than its N-centered pair. Stereospecificity, characteristic at C-center, does not appear here either, due to the submerged front-side-attack retention path and the repeated entrance-channel inversional motion, whereas the multi-inversion mechanism discovered at nitrogen center is also undermined by the deep Walden-well. At low collision energies, 6 % of the PH₂ F products form with retained configuration, mostly through complex-mediated mechanisms, while this ratio reaches 24 % at the highest energy due to the increasing dominance of the direct front-side mechanism and the smaller chance for hitting the deep Walden-inversion minimum. Our results suggest pronounced central-atom effects in S_N 2 reactions, which can fundamentally change their (stereo)dynamics.