利用時間解析傅立葉轉換紅外光譜法研究乙醛與甲酸甲酯在308/248 nm的光解反應: 漫遊機制 vs. 過渡態機制

Abstract

自從2004年甲醛的光分解反應中被報導存在漫遊機制(roaming mechanism) 後這個主題的論文就如雨後春筍般的被發表，如乙醛(CH3CHO)、丙醛 (C2H5CHO)、甲酸甲酯(HCOOCH3)等都被發現有漫遊機制的蹤跡。他們觀察到在 H2+CO這個分子通道，CO有雙轉動分布(bi-rotational distribution)，且隨著偵測轉動態的不同，兩個分布所貢獻的比例也不同，其中低轉動態的CO對應到的是一個非常低的移能。後來，經由半古典軌跡(quasi-classical trajectory)的計算也證實了如果經由ㄧ般 的途徑(過渡態機制)形成分子產物H2+CO，其CO會有較高的轉動態及移動能分 布。 本論文使用時間解析傅立葉轉換紅外光譜法研究乙醛與甲酸甲酯在308nm / 248nm下的光分解反應。其中，甲酸甲酯的研究我們有跟義大利佩魯賈大學prof. Aquilanti的研究團隊合作。在CH3OH+CO這個分子通道中，我們藉由取得1900-2200cm-1範圍的CO高解析(0.25cm-1)圖譜，並且配合光譜模擬來分析CO的內能分布。我們發現，在CO分子中v=1及2的振動態存在著雙轉動分布，我們排除其他通道產生CO的可能，並請prof. Aquilanti團隊執行半古典軌跡的計算，發現實驗與計算的結果相當吻合。而在乙醛的實驗中，我們得到CO在v=1的漫遊機制與過渡態機制的分支比，漫遊機制佔0.10

Since roaming mechanism have been reported in photolysis of formaldehyde at 2004, the papers on this topic have been published mushroomed, such as acetaldehyde (CH3CHO), propionaldehyde (C2H5CHO), methyl formate (HCOOCH3 ) etc. were found to have traces of the roaming mechanism. CO were found to be bi- rotational distribution and with the detection of different rotational states, the proportion contributed by the two mechanism are different. The low-J component of CO are correlated to a very low translational energy. Later, the quasi-classical trajectory calculations also confirmed that the conventional transition state mechanism to form molecular products H2 + CO, CO will have higher angular momentum and with substantial translational energy released. We use time-resolved Fourier transform infrared spectroscopy to study photolysis reaction of acetaldehyde and methyl formate at 308nm / 248nm. In project of methyl formate, we collaborate with prof. Aquilanti research team in Perugia university in Italy. We obtain the range of 1900-2200cm-1 CO high-resolution (0.25cm-1) spectrum to analyze the internal energy of CO and with the aid of spectral simulation We found that the CO molecule in v = 1 and 2 are bi-rotational distribution. Prof. Aquilanti team perform quasi-classical trajectory calculations, and the experimental results and calculations are quite consistent. In acetaldehyde experiments, we got branching ratio of roaming mechanism in v=1 is 0.10±0.02. Compare to Kable’s LIF results of 0.13, which represents that the time-resolved Fourier transform infrared spectroscopy is a promising method to observe roaming reaction in the internal energy aspect.