Rethinking the X− + CH3Y [X = OH, SH, CN, NH2, PH2; Y = F, Cl, Br, I] SN2 reactions†
Abstract
Moving beyond the textbook mechanisms of bimolecular nucleophilic substitution (SN2) reactions, we characterize several novel stationary points and pathways for the reactions of X− [X = OH, SH, CN, NH2, PH2] nucleophiles with CH3Y [Y = F, Cl, Br, I] molecules using the high-level explicitly-correlated CCSD(T)-F12b method with the aug-cc-pVnZ(-PP) [n = D, T, Q] basis sets. Besides the not-always-existing traditional pre- and post-reaction ion-dipole complexes, X−⋯H3CY and XCH3⋯Y−, and the Walden-inversion transition state, [X–CH3–Y]−, we find hydrogen-bonded X−⋯HCH2Y (X = OH, CN, NH2; Y ≠ F) and front-side H3CY⋯X− (Y ≠ F) complexes in the entrance and hydrogen-bonded XH2CH⋯Y− (X = SH, CN, PH2) and H3CX⋯Y− (X = OH, SH, NH2) complexes in the exit channels depending on the nucleophile and leaving group as indicated in parentheses. Retention pathways via either a high-energy front-side attack barrier, XYCH3−, or a novel double-inversion transition state, XH⋯CH2Y−, having lower energy for X = OH, CN, and NH2 and becoming submerged (barrier-less) for X = OH and Y = I as well as X = NH2 and Y = Cl, Br, and I, are also investigated.