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 - + 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-pV n Z [ 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 ).