Theoretical investigation of the SN2 mechanism of X− [X = SH, PH2] + CH3Y [Y = F, Cl, Br, I] reactions in water

We investigated the SN2 Walden-inversion mechanism of X− (X = SH, PH2) + CH3Y (Y = F, Cl, Br, I) reactions in water using multi-level quantum mechanics (ML-QM) and molecular mechanics (MM) methods. The potentials of the mean force were mapped using not only the density functional theory (DFT)/MM method but also a high-level, accurate CCSD(T)/MM method using the aug-cc-pVTZ basis set. In particular, for the PH2− + CH3I reaction, although the backside attack Walden-inversion mechanics were not observed in the gas phase, we found that this mechanism takes place in water. The atomic-level dynamics of the concerted SN2 mechanism and the stationary points along the reaction paths were characterized. For these reactions in water, their Walden-inversion barriers are higher than their corresponding ones in the gas phase, indicating that the aqueous solution hinders their reactivity. For the reactions with the same nucleophile X− in water, the reaction barrier heights with different leaving groups are in the order of F > Cl > Br > I. For the same leaving group Y with different nucleophiles SH− and PH2−, the reaction barrier with SH− is greater than that of PH2− due to the former having higher electronegativity than the latter.