Alternating Stereospecificity upon Central‐Atom Change: Dynamics of the F−+PH2Cl SN2 Reaction Compared to its C‐ and N‐Centered Analogues

Abstract

AbstractCentral‐atom effects on bimolecular nucleophilic substitution (SN2) reactions are well‐known in chemistry, however, the atomic‐level SN2 dynamics at phosphorous (P) centers has never been studied. We investigate the dynamics of the F−+PH2Cl 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−+CH3Cl and the F−+NH2Cl SN2 reactions: phosphorus as central atom leads to a more indirect SN2 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 PH2F 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 SN2 reactions, which can fundamentally change their (stereo)dynamics.