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 (S_N2) 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.