Møller Kristian H, Bates Kelvin H, Kjaergaard Henrik G
Department of Chemistry , University of Copenhagen , Universitetsparken 5 , DK-2100 Copenhagen Ø , Denmark.
Center for the Environment , Harvard University , 29 Oxford Street , Cambridge , Massachusetts 02138 , United States.
J Phys Chem A. 2019 Jan 31;123(4):920-932. doi: 10.1021/acs.jpca.8b10432. Epub 2019 Jan 23.
With an annual emission of about 500 Tg, isoprene is an important molecule in the atmosphere. While much of its chemistry is well constrained by either experiment or theory, the rates of many of the unimolecular peroxy radical hydrogen-shift (H-shift) reactions remain speculative. Using a high-level multiconformer transition state theory (MC-TST) approach, we determine recommended temperature dependent reaction rate coefficients for a number of the H-shift reactions in the isoprene oxidation mechanism. We find that most of the (1,4, 1,5, and 1,6) aldehydic and (1,5 and 1,6) α-hydroxy H-shifts have rate constants at 298.15 K in the range 10 to 1 s, which make them competitive with bimolecular reactions in the atmosphere under typical atmospheric conditions. In addition, we find that the rate coefficients of different diastereomers can differ by up to 3 orders of magnitude, illustrating the importance of chirality. Implementation of our calculated reaction rate coefficients into the most recent GEOS-Chem model for isoprene oxidation shows that at least 30% of all isoprene molecules emitted to the atmosphere undergo a minimum of one peroxy radical hydrogen-shift reaction during their complete oxidation to CO and deposited species. This highlights the importance of peroxy radical H-shifts reactions in atmospheric oxidation.
异戊二烯的年排放量约为500太克，是大气中的一种重要分子。虽然其大部分化学反应已通过实验或理论得到很好的限制，但许多单分子过氧自由基氢转移（H转移）反应的速率仍具有推测性。我们使用高级多构象体过渡态理论（MC-TST）方法，确定了异戊二烯氧化机制中一些H转移反应的推荐温度依赖性反应速率系数。我们发现，大多数（1,4、1,5和1,6）醛式和（1,5和1,6）α-羟基H转移在298.15 K时的速率常数在10到1秒的范围内，这使得它们在典型大气条件下与大气中的双分子反应具有竞争力。此外，我们发现不同非对映异构体的速率系数相差可达3个数量级，这说明了手性的重要性。将我们计算得到的反应速率系数应用于最新的GEOS-Chem异戊二烯氧化模型表明，排放到大气中的所有异戊二烯分子中，至少30%在完全氧化为CO和沉积物种的过程中至少经历一次过氧自由基氢转移反应。这突出了过氧自由基H转移反应在大气氧化中的重要性。
