Department of Chemistry, University of Copenhagen, Universitetsparken 5, Copenhagen Ø DK-2100, Denmark.
J Phys Chem A. 2020 Apr 9;124(14):2885-2896. doi: 10.1021/acs.jpca.0c01079. Epub 2020 Apr 1.
Monoterpenes are a group of volatile organic compounds (VOCs) emitted to the atmosphere in large amounts. Studies have linked the autoxidation of monoterpenes to the formation of secondary organic aerosols, which impact Earth's climate and human health. Here, we study the hydroxy peroxy radicals formed by OH- and O-addition to the six atmospherically relevant monoterpenes α-pinene, β-pinene, Δ-carene, camphene, limonene, and terpinolene. The six monoterpenes all have a six-membered ring but differ in the binding pattern of the four remaining carbon atoms and the position of the double bond(s). We use a multiconformer transition state theory approach to calculate the rate coefficients of the peroxy radical hydrogen-shift (H-shift) and endoperoxide formation reactions of these peroxy radicals. Our results suggest that primarily the isomers with a carbon-carbon double bond remaining after OH- and O-addition undergo unimolecular reactions with rate coefficients large enough to be of atmospheric importance. This greatly limits the number of potentially important unimolecular pathways. Specifically, we find that the ring-opened α- and β-pinene isomers as well as isomers of limonene and terpinolene have unimolecular reactions that are fast enough to likely dominate their reactivity under most atmospheric conditions.
单萜是大量排放到大气中的挥发性有机化合物 (VOC) 之一。研究表明，单萜的自动氧化与次生有机气溶胶的形成有关，而次生有机气溶胶会影响地球的气候和人类健康。在这里，我们研究了 OH- 和 O- 加成到 6 种大气相关单萜（α-蒎烯、β-蒎烯、Δ-蒈烯、莰烯、柠檬烯和萜品烯）后形成的羟基过氧自由基。这 6 种单萜都有一个六元环，但剩余的四个碳原子的结合方式和双键的位置不同。我们使用多构象过渡态理论方法来计算这些过氧自由基的氢转移（H-转移）和环氧化物形成反应的速率系数。我们的结果表明，主要是 OH- 和 O-加成后剩余碳-碳双键的异构体经历具有足够大的速率系数的分子内反应，这些速率系数对大气重要性很大。这大大限制了潜在重要的分子内途径的数量。具体来说，我们发现开环的 α-和 β-蒎烯异构体以及柠檬烯和萜品烯的异构体具有足够快的分子内反应，在大多数大气条件下可能主导它们的反应性。
