Li Yu-Lin, Kuo Mei-Tsan, Lin Jim Jr-Min
Institute of Atomic and Molecular Sciences, Academia Sinica Taipei 10617 Taiwan
Department of Chemistry, National Taiwan University Taipei 10617 Taiwan.
RSC Adv. 2020 Feb 28;10(14):8518-8524. doi: 10.1039/d0ra01406k. eCollection 2020 Feb 24.
Criegee intermediates play important roles in atmospheric chemistry. Methyl Criegee intermediate, CHCHOO, has two conformers, - and -conformers. -CHCHOO would undergo fast unimolecular decomposition to form OH radical 1,4 H-atom transfer. In this work, unimolecular decomposition of -CHCHOO was probed in real time with UV absorption spectroscopy at 278-318 K and 100-700 torr. We used water vapor as the scavenger of -CHCHOO to distinguish the absorption signals of the two conformers. After removing the contributions from reactions with radical byproducts, reaction with water vapor and wall loss, we obtained the unimolecular reaction rate coefficient of -CHCHOO (at 300 torr), which increases from (67 ± 15) s at 278 K, (146 ± 31) s at 298 K, to (288 ± 81) s at 318 K with an Arrhenius activation energy of 6.4 kcal mol and a weak pressure dependence for 100-700 torr. Compared to previous studies, this work provides temperature dependent unimolecular rates of -CHCHOO at higher pressures, which are more relevant to atmospheric conditions.
克里吉中间体在大气化学中起着重要作用。甲基克里吉中间体CH₂CHOO有两种构象异构体，即反式和顺式构象异构体。反式-CH₂CHOO会通过1,4氢原子转移进行快速单分子分解形成OH自由基。在这项工作中，利用紫外吸收光谱在278 - 318 K和100 - 700托的条件下实时探测反式-CH₂CHOO的单分子分解。我们使用水蒸气作为反式-CH₂CHOO的清除剂来区分两种构象异构体的吸收信号。在去除自由基副产物反应、与水蒸气反应和壁面损失的贡献后，我们得到了反式-CH₂CHOO（在300托下）的单分子反应速率系数，其从278 K时的(67 ± 15) s⁻¹、298 K时的(146 ± 31) s⁻¹增加到318 K时的(288 ± 8 1) s⁻¹，阿累尼乌斯活化能为6.4 kcal mol⁻¹，在100 - 700托下压力依赖性较弱。与之前的研究相比，这项工作提供了更高压力下反式-CH₂CHOO的温度依赖性单分子速率数据，这与大气条件更相关。
