High-Level Systematic Ab Initio Comparison of Carbon- and Silicon-Centered S_N2 Reactions.

ABSTRACT: 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] S_N2 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 S_N2 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).

SUBMITTER: Dekany AA 

PROVIDER: S-EPMC8591615 | biostudies-literature | 2021 Nov

REPOSITORIES: biostudies-literature