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下的CCSD(T)-F12b/aug-cc-pVnZ（n=D、T、Q）理论级别，对X– + CH3Y与X– + SiH3Y（X、Y分别为F、Cl、Br、I）体系的双分子亲核取代反应（SN2）中，沿瓦尔登反转（Walden inversion）、正面进攻（front-side attack）与双反转（double-inversion）路径的驻点进行了表征。在碳中心位点，瓦尔登反转占据反应主导地位，其反应过程经由被通常为潜势过渡态（transition state）(X–H3C–Y)−分隔的反应前（X–···H3CY）与反应后（XCH3···Y–）离子偶极势阱完成；正面进攻需跨越较高能垒，双反转是X=F时能量最低的构型保留路径，反应入口通道中存在氢键合（F–···HCH2Y）与卤素键合（X–···YCH3）复合物。在硅中心位点，瓦尔登反转经由单极小值点(X–SiH3–Y)−完成；正面进攻可通过通常为潜势的过渡态进行，该过渡态分隔了X-Si-Y键角接近90°的反应前与反应后极小值点；双反转需跨越正值且通常较高的能垒，且未发现氢键合与卤素键合复合物。除双分子亲核取代反应（SN2）通道（Y– + CH3X/SiH3X）外，本研究还报道了以下反应的反应焓：质子夺取反应（HX + CH2Y–/SiH2Y–）、氢化物取代反应（H– + CH2XY/SiH2XY）、XH···Y–复合物形成反应（XH···Y– + 1CH2/1SiH2），以及卤素夺取反应（XY + CH3–/SiH3–与XY– + CH3/SiH3）。