Rethinking the X- + CH3Y [X = OH, SH, CN, NH2, PH2; Y = F, Cl, Br, I] SN2 reactions.

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 XCH3Y-, and the Walden-inversion transition state, [X-CH3-Y]-, we find hydrogen-bonded X-HCH2Y (X = OH, CN, NH2; Y ≠ F) and front-side H3CYX- (Y ≠ F) complexes in the entrance and hydrogen-bonded XH2CHY- (X = SH, CN, PH2) and H3CXY- (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, XHCH2Y-, 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.