Speak Thomas H, Blitz Mark A, Medeiros Diogo J, Seakins Paul W
School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K.
National Centre for Atmospheric Science, University of Leeds, Leeds LS2 9JT, U.K.
JACS Au. 2023 Jun 6;3(6):1684-1694. doi: 10.1021/jacsau.3c00110. eCollection 2023 Jun 26.
Literature rate coefficients for the prototypical radical-radical reaction at 298 K vary by close to an order of magnitude; such variations challenge our understanding of fundamental reaction kinetics. We have studied the title reaction at room temperature via the use of laser flash photolysis to generate OH and HO radicals, monitoring OH by laser-induced fluorescence using two different approaches, looking at the direct reaction and also the perturbation of the slow OH + HO reaction with radical concentration, and over a wide range of pressures. Both approaches give a consistent measurement of ∼1 × 10 cm molecule s, at the lowest limit of previous determinations. We observe, experimentally, for the first time, a significant enhancement in the rate coefficient in the presence of water, = (2.17 ± 0.09) × 10 cm molecule s, where the error is statistical at the 1σ level. This result is consistent with previous theoretical calculations, and the effect goes some way to explaining some, but not all, of the variation in previous determinations of . Supporting master equation calculations, using calculated potential energy surfaces at the RCCSD(T)-F12b/CBS//RCCSD/aug-cc-pVTZ and UCCSD(T)/CBS//UCCSD/aug-cc-pVTZ levels, are in agreement with our experimental observations. However, realistic variations in barrier heights and transition state frequencies give a wide range of calculated rate coefficients showing that the current precision and accuracy of calculations are insufficient to resolve the experimental discrepancies. The lower value of is consistent with experimental observations of the rate coefficient of the related reaction, Cl + HO → HCl + O. The implications of these results in atmospheric models are discussed.
298K 时典型自由基 - 自由基反应的文献速率系数变化接近一个数量级；这种变化对我们对基本反应动力学的理解提出了挑战。我们在室温下研究了该标题反应，通过激光闪光光解产生 OH 和 HO 自由基，使用两种不同方法通过激光诱导荧光监测 OH，观察直接反应以及自由基浓度对缓慢的 OH + HO 反应的扰动，并在很宽的压力范围内进行研究。两种方法都给出了一致的测量结果，即约 1×10 cm³ molecule⁻¹ s⁻¹，处于先前测定的最低极限。我们首次通过实验观察到，在有水存在的情况下速率系数有显著增强，k = (2.17 ± 0.09)×10 cm³ molecule⁻¹ s⁻¹，其中误差为 1σ 水平的统计误差。该结果与先前的理论计算一致，并且这种效应在一定程度上解释了先前 k 测定中部分但并非全部的变化。使用在 RCCSD(T)-F12b/CBS//RCCSD/aug-cc-pVTZ 和 UCCSD(T)/CBS//UCCSD/aug-cc-pVTZ 水平计算的势能面进行的主方程计算与我们的实验观察结果一致。然而，势垒高度和过渡态频率的实际变化给出了广泛的计算速率系数范围，表明当前计算的精度和准确性不足以解决实验差异。较低的 k 值与相关反应 Cl + HO → HCl + O 的速率系数的实验观察结果一致。讨论了这些结果在大气模型中的意义。
