理論計算探討教(1)HFCO + nH2O(n=1,2)之反應機構(2)N-H鍵活化反應機構: Ni(0) + NH3 -> NiNH + H2

Abstract

本論文分為三大主題:

第一部分: 理論計算探討HFCO + H2O 的反應機構 在ab initio MP2/6-311++G(d,p) 的理論計算層級下，討論HFCO + H2O 反應的可能反應機構，並對反應物、反應中間體、過渡態及產物進行全面性的幾何優選。考慮了三條可能的反應途徑: 催化反應、協同和逐步的水解反應機構。其中，以催化反應直接生成HF 和CO 所需的活化能障29.6 kcal/mol為最低，且其所成的產物複體在熱力學上較為穏定的。在水解機構方面，協同反應所需的活化能為33.0 kcal/mol，較逐步反應的活化能要低些。水解逐步反應所需的活化能為42.1 kcal/mol，為此一研究中活化能障最高者。

第二部分: 理論計算探討HFCO + 2H2O 的反應機構 採用ab initio MP2/6-311++G(d,p)研究了Formyl Fluoride 與二分子H2O在氣相中的反應機構。考慮了三條可能的反應途徑: 催化反應、協同和逐步的水解反應機構。計算結果表明氣相中催化反應是最優途徑，其所需的反應活化能為24.9 kcal/mol，且其所產生的產物複合物能量為位能圖中能量最低者。而在水解反應部分，反應中的一分子H2O為反應物，而另一H2O分子為催化劑，活化能則是逐步反應較協同反應為低，但二者並無明顯的差異，分別為29.7 kcal/mol與30.6 kcal/mol。

第三部分: 密度泛函理論計算探討N-H鍵活化: Ni (0) + NH3 à NiNH + H2的反應機構 在密度泛函B3PW91/6-311++(2d,2p)及 B3PW91/6-311++G(3df,2p)的理論基礎上，探討了三重態與單重態鎳原子與氨分子的反應。除了結構外，我們也利用Wiberg鍵級說明了反應路徑上各個中間體與過渡態的鍵結傾向。結果顯示，單重態和參重態的Ni與NH3反應產生NiNH及H2反應在熱力學上均屬於吸熱反應，其反應熱相對於反應物Ni + NH3分別是單重態的10.61 kcal/mol及參重態的36.99 kcal/mol。

There are three major themes in this thesis.

I. A theoretical study on the reaction pathways of HFCO + H2O The optimized structure and harmonic frequencies for the intermediates, transition states and product complex on the ground state potential energy surfaces of the reaction formal fluoride with water, including catalytic pathway, concerted and stepwise hydrolysis pathways, were characterized at the molecular orbital theory MP2/6-311++G(d,p) level. Theoretical method was employed to calculate the corresponding energies with the zero-point energy corrections by the MP2/6-311++G(d,p) approach. The catalytic reaction, which is formyl fluoride follow dissociation pathway to yield HF and CO, was computed to be the lowest barrier with 29.6 kcal/mol. And the product complex is the most stable in this study. The concerted hydrolysis pathway has smaller barriers 33.0 kcal/mol than the stepwise one. The stepwise hydrolysis pathway has the highest barrier, 42.1 kcal/mol, in this work.

II. Calculated reaction mechanism of HFCO + 2H2O The ab initio MP2/6-311++G(d,p) theoretical method was employed to study the mechanism of reaction of HFCO with 2 molecular H2O in gas phase. Three reaction pathways, the catalytic, concerted hydrolytic and the stepwise hydrolytic pathways were considered. The result shows that the catalytic pathway has the lowest barrier of 24.9 kcal/mol, with the product complex being the most stable on the potential energy profile in this study. H2O molecule provides two functions in the hydrolysis reaction, one acts as a nucleophile, the other as a catalyst. The two active barriers of hydrolysis pathways are without significant difference. However, the barrier of the stepwise one, 29.7 kcal/mol, is a little smaller than the concerted counterpart, 30.6 kcal/mol.

III. N-H bond activation: Ni (0) + NH3 à NiNH + H2. A DFT study The reaction between Ni (d10, 1S, and s1d9, 3D) and NH3 have been carried out at the B3PW91/6-311++G(3df,2p) theoretical levels. The intermediates in the reaction pathway transfering from one to another via specific transition state is rationalized by their structures and Wiberg bond indices. The reaction of Ni (singlet and triplet ) + NH3 à NiNH + H2 is predicated to be endothermic (10.61 kcal/mol for singlet and 36.99 kcal/mol for triplet).