- 學位論文
雙分子親核取代 (SN2) 反應的動力學同位素效應 (KIEs) 與穿隧效應的理論研究
Theoretical Modeling of the Kinetic Isotope Effects (KIEs) and Tunneling Effects of the Bimolecular Nucleophilic Substitution (SN2) Reactions
摘要
本碩士論文共有四章，第一章和第二章我們探討 vinyl halide 和鹵素陰離子親核基的消去 (elimination) 及親核取代 (substitution) 反應，第三章和第四章則是以理論計算分析 SN2 反應的動力學同位素效應 (kinetic isotope effects, KIEs) 以及穿隧效應 (tunneling effects) 對反應速率常數的影響。 第一章我們探討在 X- + C2H3Y (X, Y = F, Cl, Br) 此類型反應進行 elimination 且為放熱的兩個反應 F- + C2H3Y (Y = Cl, Br) → HF + C2H2 + Y- 在氣態、微水合 (F-(H2O) + C2H3Y) 以及水溶液等不同環境中的性質。Elimination 反應中根據 nucleophile 所擷取 vinyl halide 上的氫原子不同可分成 anti-elimination on C2、syn-elimination on C2 和 elimination on C1 三種，而此兩個放熱的氣態反應其 anti-elimination 反應路徑是最容易進行的。在 microsolvation 和模擬水溶液的 PCM 模型中，水合作用分別提升了 F- + C2H3Cl 反應能障至 5.4 和 22.3 kcal/mol，大幅降低了其反應性。 第二章我們探討 X- + C2H3Y (X, Y = F, Cl, Br) 此類型反應進行substitution 且為放熱的 F- + C2H3Cl、F- + C2H3Br、Cl- + C2H3Br 和反應能量為零的 F- + C2H3F 四個系統。Vinyl halide 的 substitution 反應根據反應過渡態的不同可分成 out-of-plane SNπ 與in-plane SNσ 兩種反應路徑，在 F- + C2H3F、F- + C2H3Cl 以及 F- + C2H3Br 三個反應中，SNπ 的反應能障都比 SNσ 來的低，且都小於零，在 Cl- + C2H3Br 反應中，SNπ 與 SNσ 反應能障都很高 (24.1 與 17.0 kcal/mol)，但此反應進行 SNσ 反應路徑是較有利的。和第一章 elimination 反應做比較，我們認為 F- + C2H3F 和 Cl- + C2H3Br 會進行 substitution 路徑，而 F- + C2H3Cl 和 F- + C2H3Br 應仍以 elimination 反應為主。由我們計算的反應速率常數來看，F- + C2H3F (k = 1.41 × 10-11 cm3 molecule-1 s-1)、F- + C2H3Cl (k = 5.88 × 10-12 cm3 molecule-1 s-1) 和 F- + C2H3Br (k = 2.14 × 10-10 cm3 molecule-1 s-1) 有機會藉由實驗來觀察到反應發生，但 Cl- + C2H3Br 因反應能障太高要藉由實驗觀察並不容易 (k = 5.37 × 10-24 cm3 molecule-1 s-1)。 第三章我們介紹以理論模擬氣態 SN2 反應速率常數以及動力學同位素效應的各種方法，我們根據反應能障的大小分別使用 ion-molecule collision theory、canonical unified statistical (CUS) theory、transition state theory (TST)，對於高反應能障的系統我們也利用 canonical variational theory with small curvature tunneling correction (CVT/SCT) 來考慮穿隧效應對速率常數的影響。此外，我們依據十一個氣態 SN2 反應 deuterium KIEs 的實驗值來測試數種理論方法計算 deuterium KIEs 的準確度。研究結果顯示 MP2/aug-cc-pVDZ 和 MP2/6-31+G(d,p) 方法有較好的準確性 (MUE = 0.049 和 0.068)，而常被使用的 B3LYP/6-31+G(d,p) 方法誤差則大了許多 (MUE = 0.129)。對於高反應能障的 SN2 反應，我們的計算結果也顯示穿隧效應對速率常數以及 13C 和 14C KIEs 有明顯的貢獻，也就表示 alkyl halide 中心碳原子的穿隧效應在低溫下的反應中扮演重要的角色。 第四章我們利用雙層 VTST/MT (dual-level dynamics approach with variational transition state theory including multidimensional tunneling) 理論來研究具有不同程度水合狀態的三個放熱 SN2 反應的動力學性質：(1) 氣態 CN- + CH3F，(2) 氣態微水合 OH-(H2O) + CH3F，(3) 水溶液 OH- + CH3F。此三個反應的反應能障在 CCSD(T)/aug-cc-pVTZ 單點能量計算下分別為 11.7、5.1 和 16.2 kcal/mol。計算結果顯示三個反應在室溫下穿隧效應就已明顯地增加了 TST 反應速率常數 44%、38% 以及 65%。雖然水溶液中的 OH- + CH3F 有最高的反應能障，但因為水合作用也使得能障的寬度增加，所以穿隧效應對速率常數的影響並沒有比另外二個反應大幅增加。當以 13C 和 14C 取代 alkyl halide 中心碳原子時，我們發現三個SN2反應在低溫下皆有很明顯的動力學同位素效應，且與不考慮穿隧效應算出來的結果明顯不同。在反應能障最高的水溶液 OH- + CH3F 中，在35 K 計算所得到的 KIE(13C) 和 KIE(14C) 高達 7 和 40，這也告訴我們在 SN2 反應中alkyl halide 的中心碳原子明顯地參與了穿隧效應。
並列摘要
This thesis consists of four chapters. In chapter 1 and chapter 2, we studied the elimination and substitution reactions of vinyl halide with a nucleophile (halide anion). In the chapters 3 and 4, we investigated the kinetic isotope effects and tunneling effects of SN2 reactions. In chapter 1, we studied two exothermic E2-like reactions of the F- + C2H3Y (Y = Cl, Br) → HF + C2H2 + Y- in the gas phase, in microsolvation with a H2O (F-(H2O) + C2H3Y), and in bulk water solvent. Basis on the different orientation of the hydrogen abstraction by the nucleophile, elimination reactions of vinyl halide can be classified into three types: the anti-elimination on C2, the syn-elimination on C2, and the elimination on C1. The anti-elimination was the most feasible pathway for these two exothermic reactions and their barrier heights were quite small in the gas phase. In the reactions with microsolvation and in the solution modeled by the polarizable continuum model (PCM), the F- + C2H3Cl reaction were predicted to have barrier heights of 5.4 and 22.3 kcal/mol, respectively. Thus, the reactivity of the E2-like reaction would be significantly decreased due to the solvation effects. In chapter 2, we studied four substitution reactions which included three exothermic reactions of F- + C2H3Cl, F- + C2H3Br, and Cl- + C2H3Br, and the thermal neutral reaction of F- + C2H3F. According to the geometry of the transition states, the vinyl halide substitution reactions could be classified into two types: the out-of-plane SNπ and the in-plane SNσ. For the F- + C2H3F, F- + C2H3Cl, and F- + C2H3Br reactions, the barrier heights of the SNπ pathway were lower than the SNσ pathway, and were too small to become the reaction bottlenecks. For the Cl- + C2H3Br reaction, the barrier heights were both high for the SNπ and SNσ pathways (24.1 and 17.0 kcal/mol), but the SNσ pathway was favored over the SNπ pathway. Comparing to the elimination reactions studied in chapter 1, we concluded that the substitution reactions were favored over the elimination reactions for the F- + C2H3F and Cl- + C2H3Br reactions. However, the anti-elimination reactions of the F- + C2H3Cl and F- + C2H3Br were predicted to be very fast that the substitution reaction pathways cannot compete. In chapter 3, we detailed the theoretical methods that should be used for modeling the rate constants and kinetic isotope effects of gas-phase SN2 reactions. The methods included the ion-molecule collision theory, canonical unified statistical (CUS) theory, and transition state theory (TST) depending on the sizes of the barrier heights. For reactions with high barriers, we also used the canonical variational theory with small curvature tunneling correction (CVT/SCT) to estimate the tunneling effects. We have also benchmarked a few ab initio and DFT methods for their performance in predicting the deuterium KIEs against eleven experimental values. The results showed that the MP2/aug-cc-pVDZ and MP2/6-31+G(d,p) methods gave the most accurate prediction overall (MUE = 0.049 and 0.068). The popular B3LYP/6-31+G(d,p) method gave significant larger errors (MUE = 0.129). For the SN2 reactions with appreciable barrier heights, the tunneling effects were predicted to contribute significantly both to the rate constants and to the 13C and 14C KIEs, which suggested important carbon atom tunneling in the SN2 reactions. In chapter 4, we used the dual-level dynamics approach with variational transition state theory including multidimensional tunneling (VTST/MT) to investigate three exothermic SN2 reactions with different degrees of solvation: (1) gas-phase CN- + CH3F, (2) gas-phase microsolvated system OH-(H2O) + CH3F, and (3) OH- + CH3F in bulk water. The barrier heights of these three reactions were predicted to be 11.7, 5.1, and 16.2 kcal/mol, respectively, at the CCSD(T)/aug-cc-pVTZ level. The calculated results indicated that at room temperature the tunneling effects significantly raised the TST rate constants by 44%, 38%, and 65%, respectively, for those three reactions mentioned above. Even though the OH- + CH3F reaction in bulk water has the highest barrier, the solvation effects also made the barrier significantly wider, and the tunneling effects were predicted not to be dramatically larger than the other reactions. The calculated results also suggested that, regardless of the levels of solvation, the tunneling effects resulted mostly from the motion of the central carbon atom of the alkyl halide, not from the primary hydrogen atoms. For the OH- + CH3F reaction in bulk water, the predicted 13C and 14C KIEs at 35 K were as high as 7 and 40, respectively.