Revealing a double-inversion mechanism for the F- + CH3Cl SN2 reaction

Stereo-specific reaction mechanisms play a fundamental role in chemistry. The back-side attack inversion and front-side attack retention pathways of the bimolecular nucleophilic substitution (S(N)2) reactions are the textbook examples for stereo-specific chemical processes. Here, we report an accurate global analytic potential energy surface (PES) for the F- + CH3Cl S(N)2 reaction, which describes both the back-side and front-side attack substitution pathways as well as the proton-abstraction channel. Moreover, reaction dynamics simulations on this surface reveal a novel double-inversion mechanism, in which an abstraction-induced inversion via a FH center dot center dot center dot CH2Cl- transition state is followed by a second inversion via the usual [F center dot center dot center dot CH3 center dot center dot center dot Cl](-) saddle point, thereby opening a lower energy reaction path for retention than the front-side attack. Quasi-classical trajectory computations for the F- + CH3Cl(upsilon(1) = 0, 1) reactions show that the front-side attack is a fast direct, whereas the double inversion is a slow indirect process.