High-Level Systematic Ab Initio Comparison of Carbon- and Silicon-Centered SN2 Reactions.

We characterize the stationary points along the Walden inversion, front-side attack, and double-inversion pathways of the X- + CH3Y and X- + SiH3Y [X, Y = F, Cl, Br, I] SN2 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-···H3CY) and postreaction (XCH3···Y-) ion-dipole wells separated by a usually submerged transition state (X-H3C-Y)-, front-side attack occurs over high barriers, double inversion is the lowest-energy retention pathway for X = F, and hydrogen- (F-···HCH2Y) and halogen-bonded (X-···YCH3) complexes exist in the entrance channel. At the silicon center, Walden inversion proceeds through a single minimum (X-SiH3-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 SN2 channels (Y- + CH3X/SiH3X), we report reaction enthalpies for proton abstraction (HX + CH2Y-/SiH2Y-), hydride substitution (H- + CH2XY/SiH2XY), XH···Y- complex formation (XH···Y- + 1CH2/1SiH2), and halogen abstraction (XY + CH3-/SiH3- and XY- + CH3/SiH3).