以紫外光吸收光譜法測量syn位向甲基取代克里奇中間體的熱分解速率

Thermal Decomposition Rate of Syn-CH3CHOO Measured with UV Absorption Spectroscopy

李鈺琳(Yu-Lin Li)

指導教授 ： 林志民

國立臺灣大學/理學院/化學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202000595

摘要

克里奇中間體在大氣化學中扮演重要的角色。其中，甲基取代的克里奇中間體(CH3CHOO)有兩種構形異構物:syn和anti。Syn-CH3CHOO已被預測會經由1,4-氫原子傳遞的路徑發生快速的單分子分解反應產生OH自由基，因此，此反應被視為syn-CH3CHOO在大氣中主要消散的路徑之一。在本篇論文中，使用紫外光吸收光譜法即時量測在不同溫度範圍(278-318 K)、不同壓力(100-700 Torr)下syn-CH3CHOO的單分子分解反應。由於兩個異構物(syn- & anti-CH3CHOO)對於水蒸氣的反應性十分不同，anti-CH3CHOO和水蒸氣反應十分快速，藉此可分辨兩個異構物的吸收訊號。在移除其他反應的貢獻(和自由基的反應、和水蒸氣的反應、壁損)後，可獲得syn-CH3CHOO的單分子反應速率常數:在278至318 K溫度範圍內為 (67 ± 15) s−1遞增至(288 ± 81) s−1，並得到大約為6 kcalmol−1的活化能(Arrhenius activation energy)。此反應在100至700 Torr的壓力範圍有微弱的壓力效應。相較先前的研究，本文探討了溫度效應，且在較高壓力下進行量測，並提供了較小誤差的結果。

關鍵字

反應動力學 ； 紫外光吸收光譜 ； 克里奇中間體 ； 熱分解反應

並列摘要

Criegee intermediates play an important role in atmospheric chemistry. Methyl Criegee intermediate, CH3CHOO, has two conformers, syn- and anti-conformers. Syn-CH3CHOO has been predicted to undergo fast unimolecular decomposition to form an OH radical via 1,4 H-atom transfer, thus this reaction was thought to be the main atmospheric sink of syn-CH3CHOO. In this work, unimolecular decomposition of syn-CH3CHOO was probed in real time with UV absorption spectroscopy at 278-318 K and 100-700 Torr. Because the reactivities of syn- and anti-CH3CHOO toward water vapor are different, we used water vapor as a scavenger of anti-CH3CHOO to distinguish the absorption signals of both conformers. After removing the contributions from the reactions with radicals and water vapor and wall loss, we obtained the unimolecular reaction rate coefficient of syn-CH3CHOO, which increases from (67 ± 15) s−1 at 278 K to (288 ± 81) s−1 at 318 K with an Arrhenius activation energy of ∼6 kcalmol−1. This reaction has a weak pressure effect from 100 to 700 Torr. Compared to previous studies, this work provides temperature-dependent thermal decomposition rates of syn-CH3CHOO with smaller error bars at higher pressures.

並列關鍵字

Reaction kinetics ； UV absorption spectroscopy ； Criegee intermediates ； thermal decomposition

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