本文透過密度泛函理論（density functional theory）計算來探討鈦亞胺錯合物和二氧化碳反應來產生尿素的反應機制。目前已經有各種的鈦亞胺錯合物和二氧化碳反應的文獻，其中有些反應後會產生尿素，有些則不會。我們希望透過密度泛函理論計算來找出為什麼有些鈦亞胺錯合物和二氧化碳反應後沒有產生尿素。我們發現錯合物 dichloro t-butylimido bispyridine titanium與文獻上所預測的反應途徑不同，它會先轉換成一個能量較高的異構物再和二氧化碳反應。異構物和二氧化碳反應的活化能較低是因為它有著反位效應（trans effect）。此外，第二步脫離出異氰酸酯的活化能不能太高。因為如果沒有異氰酸酯產生，就沒辦法進行第二次的碳氮鍵耦合反應。另外，鈦錯合物第一步和二氧化碳反應與第三步和異氰酸酯反應為競爭反應。如果第一步的活化能較低，則鈦錯合物會優先和二氧化碳反應。鈦錯合物也就不會和異氰酸酯反應，造成沒有尿素形式的鈦錯合物產生。如果實驗環境是在有過量二氧化碳的條件下進行，就需要遵照這個規則。這些結論是透過理論計算後，分析文獻上沒有辦法產生尿素的五個鈦錯合物原因所得出的。

The present study investigates the reaction mechanism of titanium imido complexes with carbon dioxide to produce urea using density functional theory (DFT) calculations. In the process of urea formation, the separation of isocyanate is a crucial step as it determines the subsequent reactions.
We found that titanium complex B follows a reaction pathway different from the one predicted in the literature. It does not react with carbon dioxide in its most stable molecular structure. Instead, it undergoes conversion to a higher-energy isomer before reacting with carbon dioxide. The lower activation energy for the isomer in its reaction with carbon dioxide is attributed to the trans effect, which has been confirmed through the analysis of HOMO and LUMO energies.
For urea formation to occur, the highest activation energy among the steps in the reaction should not exceed 24.4 kcal/mol, as determined by the highest activation energy observed in titanium complex B, known to produce urea. Additionally, the activation energy for the second step, the separation of isocyanate, should not be too high. Without the generation of isocyanate, the second carbon-nitrogen coupling reaction cannot proceed.
It was observed that the first step, the reaction between titanium complex and carbon dioxide, competes with the third step, the reaction with isocyanate. If the activation energy for the first step is lower, the titanium complex will preferentially react with carbon dioxide.

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