Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States.
J Phys Chem A. 2011 May 5;115(17):4381-7. doi: 10.1021/jp2001089. Epub 2011 Apr 8.
Ozonolysis is a key reaction in atmospheric chemistry, although important details of the behavior of the ozonolysis intermediates are not known. The key intermediate in ozonolysis, the Criegee intermeiate (CI), is known to quickly isomerize, with the favored unimolecular pathway depending on the relative barriers to isomerization. Stabilized Criegee intermediates (SCI), those with energy below any barriers to isomerization, may result from initial formation with low energy or collisional stabilization of high energy CI. Bimolecular reactions of SCI have been proposed to play a role in OH formation and nucleation of new particles, but unimolecular reactions of SCI may well be too fast for these to be significant. We present measurements of the pressure dependence of SCI formation for a set of alkenes utilizing a hexafluoroacetone scavenger. We studied four alkenes (2,3-dimethyl-2-butene (TME), trans-5-decene, cyclohexene, α-pinene) to characterize how size and cyclization (endo vs exo) affect the stability of Criegee intermediates formed in ozonolysis. SCI yields in ozonolysis were measured in a high pressure flow reactor within a range of 30-750 Torr. The linear alkenes show considerable stabilization with trans-5-decene showing 100% stabilization at ∼400 Torr and TME having 65% stabilization at 710 Torr. Extrapolation of the yields for linear alkenes to 0 Torr shows yields significantly above zero, indicating that a fraction of their CI are formed below the barrier to isomerization. CI from endocyclic alkenes show little to no stabilization and appear to have neglible stabilization at 0 Torr. Cyclohexene derived CI showed no stabilization even at 650 Torr, while α-pinene CI had ∼15% stabilization at 740 Torr. Our results show a strong dependence of SCI formation on carbon number; adding just 2 to 3 CI carbons in linear alkenes increases stabilization by a factor of 10. Stabilization for endocyclic alkenes, at atmospheric pressure, begins to occur at a carbon number of 10, with only modest yields of SCI.
臭氧分解是大气化学中的一个关键反应，尽管臭氧分解中间体的行为的一些重要细节尚不清楚。臭氧分解的关键中间体，即克里格中间体 (CI)，已知会迅速异构化，而有利于异构化的单一分子途径取决于异构化的相对势垒。稳定的克里格中间体 (SCI)，那些能量低于任何异构化势垒的，可能是由于初始形成的低能量或高能 CI 的碰撞稳定化而产生的。已经提出 SCI 的双分子反应在 OH 形成和新粒子的成核中起作用，但 SCI 的单分子反应可能由于速度太快而没有明显作用。我们利用六氟丙酮清除剂，对一组烯烃的 SCI 形成的压力依赖性进行了测量。我们研究了四种烯烃（2,3-二甲基-2-丁烯（TME）、反式-5-癸烯、环己烯、α-蒎烯），以表征大小和环化（内型与外型）如何影响臭氧分解中形成的克里格中间体的稳定性。在 30-750 托的高压流动反应器中测量了臭氧分解中的 SCI 产率。线性烯烃表现出相当大的稳定性，反式-5-癸烯在约 400 托时显示出 100%的稳定性，TME 在 710 托时显示出 65%的稳定性。线性烯烃的产率外推至 0 托时，产率明显高于零，表明其一部分 CI 是在异构化势垒以下形成的。来自中环烯烃的 CI 几乎没有稳定作用，在 0 托时似乎没有稳定作用。环己烯衍生的 CI 即使在 650 托时也没有显示出稳定作用，而 α-蒎烯的 CI 在 740 托时显示出约 15%的稳定作用。我们的结果表明 SCI 形成强烈依赖于碳原子数；在直链烯烃中增加 2 到 3 个 CI 碳原子，稳定作用增加 10 倍。中环烯烃的稳定作用，在大气压下，从碳原子数为 10 开始发生，SCI 的产量仅适度。
