Fang Yi, Liu Fang, Klippenstein Stephen J, Lester Marsha I
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.
Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
J Chem Phys. 2016 Jul 28;145(4):044312. doi: 10.1063/1.4958992.
The unimolecular decay of carbonyl oxide intermediates, known as Criegee intermediates, produced in alkene ozonolysis is a significant source of OH radicals in the troposphere. Here, the rate of appearance of OH radical products is examined directly in the time-domain for a prototypical alkyl-substituted Criegee intermediate, CH3CH2CHOO, following vibrational activation under collision-free conditions. Complementary statistical Rice-Ramsperger-Kassel-Marcus calculations of the microcanonical unimolecular decay rate for CH3CH2CHOO are also carried out at energies in the vicinity of the barrier for 1,4 hydrogen atom transfer that leads to OH products. Tunneling through the barrier, derived from high level electronic structure calculations, contributes significantly to the decay rate. Infrared transitions of CH3CH2CHOO are identified in the CH stretch overtone region, which are detected by ultraviolet laser-induced fluorescence of the resultant OH products. The features observed are attributed to CH vibrational excitations and conformational forms utilizing insights from theory. Both experiment and theory yield unimolecular decay rates for CH3CH2CHOO of ca. 10(7) s(-1), which are slower than those obtained for syn-CH3CHOO or (CH3)2COO reported previously [Fang et al., J. Chem. Phys. 144, 061102 (2016)] at similar energies. Master equation modeling is also utilized to predict the thermal decay rate of CH3CH2CHOO under atmospheric conditions, giving a rate of 279 s(-1) at 298 K.
烯烃臭氧分解过程中产生的羰基氧化物中间体（即所谓的克里吉中间体）的单分子衰变是对流层中OH自由基的一个重要来源。在此，在无碰撞条件下振动激活后，针对典型的烷基取代克里吉中间体CH3CH2CHOO，在时域中直接研究了OH自由基产物的生成速率。还在导致OH产物的1,4氢原子转移势垒附近的能量下，对CH3CH2CHOO的微正则单分子衰变速率进行了补充性的统计Rice-Ramsperger-Kassel-Marcus计算。由高水平电子结构计算得出的隧穿势垒对衰变速率有显著贡献。在CH伸缩倍频区域识别出了CH3CH2CHOO的红外跃迁，这些跃迁通过所得OH产物的紫外激光诱导荧光进行检测。利用理论见解将观察到的特征归因于CH振动激发和构象形式。实验和理论都得出CH3CH2CHOO 的单分子衰变速率约为10(7) s(-1)，这比之前在类似能量下报道的顺式CH3CHOO 或(CH3)2COO 的衰变速率要慢[Fang等人，《化学物理杂志》144, 061102 (2016)]。还利用主方程模型预测了大气条件下CH3CH2CHOO 的热衰变速率，在298 K时为279 s(-1)。
