Leaving-Group Effects in S _N 2@P: Potential Energy Surface and Dynamics of the F <b> ^– </b> + PH ₂ I Reaction Compared to Its Cl <b> ^– </b> -Leaving-Group Analogue

We investigate the dynamics of the F– + PH2I reaction using the quasi-classical trajectory method on a newly developed ab initio analytical potential energy surface, with a special focus on stereoselectivity and leaving-group effects by comparing to previous F– + PH2Cl results. Besides the overall more exothermic energy profile, the SN2 pathway of the F– + PH2I reaction is found to be more indirect, with vibrationally highly excited products, and the retention of configuration more relevant due to a much more stable halogen-bonded prereaction complex supporting front-side attack, with respect to the Cl–-leaving-group process. Several further retention mechanisms of the title reaction are identified, as well, interestingly excluding multi-inversion, recently discovered at the nitrogen-center, while inversion mainly occurs through stripping. The proton-transfer channel, favoring also the stripping mechanism, is a primarily direct process with less internal excitation in the products, similarly distributed between rotation and vibration.