High-Level Systematic Ab Initio Comparison of Carbon- and Silicon-Centered S_N2 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).本研究采用化学精度级别下的CCSD(T)-F12b/aug-cc-pVnZ [n = D, T, Q] 理论方法，对X⁻ + CH3Y与X⁻ + SiH3Y [X、Y = F, Cl, Br, I] 体系中双分子亲核取代（SN2）反应沿瓦尔登翻转（Walden inversion）、正面进攻（front-side attack）及双翻转（double-inversion）路径的驻点进行了表征。针对碳中心体系，瓦尔登翻转为主要反应路径，其经由反应前离子偶极势阱（X⁻···H3CY）与反应后离子偶极势阱（XCH3···Y⁻）进行，两势阱之间以通常为潜势过渡态（X⁻H3C–Y）⁻相隔；正面进攻需翻越较高能垒；双翻转是X=F时能量最低的构型保持路径；入口通道中存在氢键型（F⁻···HCH2Y）与卤素键型（X⁻···YCH3）络合物。针对硅中心体系，瓦尔登翻转经由单极小点（X⁻–SiH3–Y）⁻进行；正面进攻可通过通常为潜势的过渡态实现，该过渡态分隔开X–Si–Y键角接近90°的反应前、后极小点；双翻转需克服正值且通常较高的能垒；未发现氢键与卤素键型络合物。除双分子亲核取代反应通道（Y⁻ + CH3X/SiH3X）外，本研究还报道了以下反应的焓变：质子夺除反应（HX + CH2Y⁻/SiH2Y⁻）、氢负离子取代反应（H⁻ + CH2XY/SiH2XY）、XH···Y⁻络合物形成反应（XH···Y⁻ + ¹CH2/¹SiH2）以及卤素夺除反应（XY + CH3⁻/SiH3⁻与XY⁻ + CH3/SiH3）。