Double-Inversion Mechanisms of the X^– + CH₃Y [X,Y = F, Cl, Br, I] S_N2 Reactions

The double-inversion and front-side attack transition states as well as the proton-abstraction channels of the X^– + CH₃Y [X,Y = F, Cl, Br, I] reactions are characterized by the explicitly correlated CCSD­(T)-F12b/aug-cc-pVTZ­(-PP) level of theory using small-core relativistic effective core potentials and the corresponding aug-cc-pVTZ-PP bases for Br and I. In the X = F case the double-inversion classical­(adiabatic) barrier heights are 28.7(25.6), 15.8(13.4), 13.2(11.0), and 8.6(6.6) kcal mol^–1 for Y = F, Cl, Br, and I, respectively, whereas the barrier heights are in the 40–90 kcal mol^–1 range for the other 12 reactions. The abstraction channels are always above the double-inversion saddle points. For X = F, the front-side attack classical­(adiabatic) barrier heights, 45.8(44.8), 31.0(30.3), 24.7(24.2), and 19.5(19.3) kcal mol^–1 for Y = F, Cl, Br, and I, respectively, are higher than the corresponding double-inversion ones, whereas for the other systems the front-side attack saddle points are in the 35–70 kcal mol^–1 range. The double-inversion transition states have XH···CH₂Y^– structures with C_s point-group symmetry, and the front-side attack saddle points have either C_s (X = F or X = Y) or C₁ symmetry with XCY angles in the 78–88° range. On the basis of the previous reaction dynamics simulations and the minimum energy path computations along the inversion coordinate of selected XH···CH₂Y^– systems, we suggest that the double inversion may be a general mechanism for S_N2 reactions.