High-Level Systematic Ab Initio Comparison of Carbon- and Silicon-Centered S(N)2 Reactions

[Image: see text] We characterize the stationary points along the Walden inversion, front-side attack, and double-inversion pathways of the X(–) + CH(3)Y and X(–) + SiH(3)Y [X, Y = F, Cl, Br, I] S(N)2 reactions using chemically accurate CCSD(T)-F12b/aug-cc-pVnZ [n = D, T, Q] levels of theory. At the carbon center, Walden inversion dominates and proceeds via prereaction (X(–)···H(3)CY) and postreaction (XCH(3)···Y(–)) ion-dipole wells separated by a usually submerged transition state (X–H(3)C–Y)(−), front-side attack occurs over high barriers, double inversion is the lowest-energy retention pathway for X = F, and hydrogen- (F(–)···HCH(2)Y) and halogen-bonded (X(–)···YCH(3)) complexes exist in the entrance channel. At the silicon center, Walden inversion proceeds through a single minimum (X–SiH(3)–Y)(−), the front-side attack is competitive via a usually submerged transition state separating pre- and postreaction minima having X–Si–Y angles close to 90°, double inversion occurs over positive, often high barriers, and hydrogen- and halogen-bonded complexes are not found. In addition to the S(N)2 channels (Y(–) + CH(3)X/SiH(3)X), we report reaction enthalpies for proton abstraction (HX + CH(2)Y(–)/SiH(2)Y(–)), hydride substitution (H(–) + CH(2)XY/SiH(2)XY), XH···Y(–) complex formation (XH···Y(–) + (1)CH(2)/(1)SiH(2)), and halogen abstraction (XY + CH(3)(–)/SiH(3)(–) and XY(–) + CH(3)/SiH(3)).