利用時域解析傅氏紅外光譜儀研究硫代醋酸之光解反應：以氬氣碰撞活化分子內轉移路徑產生一氧化碳、羰基硫、甲基硫醇及甲烷等產物

Abstract

本論文使用時間解析傅立葉轉換紅外光光譜儀，偵測硫代醋酸（thioacetic acid，CH3COSH）分子於單光子248nm能量之下光解產生一氧化碳、亞甲基、羰基硫、甲基硫醇及甲烷等產物。吾人在1850至2200 cm-1中紅外光範圍偵測到一氧化碳及羰基硫（ν(C=O)，ν3）之基頻放光訊號，並且標定產物一氧化碳（1≦v≦3，J≦35）之振動轉動光譜譜線。藉由分析光解產物時間解析之振動及轉動分布及內能分配，我們可得到一氧化碳分子平均轉動能量為2.04±0.05 kcal/mol且平均振動能量為6.72±0.29 kcal/mol。此外在2400至3300 cm-1範圍亦觀察到甲基硫醇（ν1、ν2和ν3）、甲烷之伸縮振動態（ν3）及硫化氫之對稱（ν1）及非對稱伸縮（ν3）振動態之放光訊號，但由於硫化氫放光較弱，實驗中所觀察到之圖譜訊號主要由甲烷及甲基硫醇組成。實驗結果顯示以氬氣作為焠熄氣體具有促進硫代醋酸分子經由內轉移（internal conversion，IC）路徑而分解並增加其光分解產物訊號強度的功效。吾人亦藉由理論計算方法了解分子光解的途徑，其結果顯示主要有三種路徑可產生穩定分子產物。（1）硫代醋酸分子經由三中心過渡態分解為一氧化碳及甲基硫醇。（2）由C-S單鍵同向形式（s-syn form）轉為C-S單鍵反向形式（s-anti form）經四中心過渡態再分解為羰基硫和甲烷分子。（3）硫代醋酸分子分解生成硫化氫和乙烯酮，乙烯酮再經由二次斷鍵形成一氧化碳與亞甲基光解碎片。另外吾人藉由加入氧氣與乙烯酮二次分解碎片亞甲基（CH2）進行反應，觀察到二氧化碳、氫氧自由基（OH）及甲醛（H2C=O）等產物，由實驗數據計算出二氧化碳生成速率為1.21(±0.11) × 10-12 cm3 molecule-1 s-1。藉由觀察硫代醋酸分子在氣體狀態下的光分解研究，有助於人們對此類型氧族取代醋酸分子（CH3C(O)XH, X=O,S,Se,Te）有更進一步的了解。

Photodissociation dynamics of thioacetic acid at 248nm is studied by detecting photofragments with step-scan time-resolved Fourier-transform infrared emission spectroscopy. We can observe CO and OCS fundamental vibrational emission spectra in the mid-infrared range from 1850 cm-1 to 2200 cm-1. CH3SH, CH4, and H2S are also detected in the range of 2400 - 3300 cm-1; however, we proposed the signals are mainly contributed by CH3SH and CH4, due to the poor emission intensity of H2S. Information of internal energy distribution in photofragments can be obtained via spectral analysis of experimental data. Assignments of the CO spectra indicate that vibrational level is populated up to ν=3 and rotational level J up to J=35. In this study, the CO fragments are directly confirmed by high resolution rovibrational spectral observation. On the other hand, the CH2 product can only be indirectly confirmed by the reaction with O2 to form CO2. We found that the addition of Ar or O2 quenching gas enhance the collision-induced internal conversion process in the photodissociation of CH3COSH. In previous theoretical work, there are three predominant dissociation channels. The first pathway leads to the products of CO and CH3SH through a three-membered ring intermediate. The second pathway leads to OCS and CH4 products. The third pathway yields ketene and H2S, and ketene decomposes further into CO and CH2. The photodissociation study of thioacetic acid in gas phase allows us to gain further understanding of chalcogen substituted carboxylic acid.