Institut de Physique de Rennes, Equipe: Astrochimie Expérimentale, UMR 6251 du CNRS, Bât. 11c-Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France.
Phys Chem Chem Phys. 2010 Oct 21;12(39):12702-10. doi: 10.1039/c0cp00591f. Epub 2010 Aug 24.
We report the first measurements of rate constants for the reaction in which OH radicals associate with O(2) to form HO(3). Our recent measurements (Science, 2010, 328, 1258) have shown that the HO-O(2) bond dissociation energy is only (12.3 ± 0.3) kJ mol(-1). Consequently, above ca. 90 K under attainable experimental conditions, the rate of the reverse dissociation of HO(3) becomes comparable to, and then greater than, the rate of the forward association reaction. We have used the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method to access low temperatures and have explored the kinetics of OH + O(2) + M → HO(3) + M in two series of experiments. At temperatures between 55.9 and 79.2 K, the OH radicals, created by pulsed laser photolysis of H(2)O(2) and observed by laser-induced fluorescence, decayed by pseudo-first-order kinetics to effectively zero concentration at longer times. The third-order rate constants derived from these experiments fit the expression: k(3rd)(o) (T) = (4.2 ± 1.9) × 10(-34) (T/298 K)(-(3.5 ± 0.3)) cm(6) molecule(-2) s(-1). At temperatures between 87.4 and 99.8 K, rate constants for the association reaction were determined allowing for the significant occurrence of the reverse dissociation reaction. The values of the derived rate constants are consistent with those obtained in the lower temperature range, though the errors are larger. The experimental values of k(3rd)(o) (T) are compared with (a) those for other association reactions involving species of similar complexity, and (b) values of k(3rd)(o) (T) estimated according to both the energy transfer (ET) and the radical-complex (RC) mechanisms. We conclude that the RC mechanism probably makes the major contribution to the association of OH + O(2) at the low temperatures of our experiments.
我们报告了首例关于 OH 自由基与 O(2)结合形成 HO(3)的反应速率常数的测量结果。我们最近的测量结果（《科学》，2010，328，1258）表明，HO-O(2)键离解能仅为（12.3 ± 0.3）kJ mol(-1)。因此，在可达到的实验条件下，高于约 90 K 时，HO(3)的逆离解速率变得与前向结合反应相当，然后超过前向结合反应。我们使用 CRESU（均相超声射流动力学）方法来获得低温，并在两个系列的实验中探索了 OH + O(2) + M → HO(3) + M 的动力学。在 55.9 到 79.2 K 的温度范围内，通过脉冲激光光解 H(2)O(2)产生的 OH 自由基，并用激光诱导荧光进行观察，在较长时间内按准一级动力学衰减至有效零浓度。从这些实验中推导出的三阶速率常数符合以下表达式：k(3rd)(o) (T) = (4.2 ± 1.9) × 10(-34) (T/298 K)^(-(3.5 ± 0.3)) cm(6) molecule(-2) s(-1)。在 87.4 到 99.8 K 的温度范围内，确定了结合反应的速率常数，同时考虑到反向离解反应的显著发生。推导得到的速率常数的值与在较低温度范围内得到的值一致，尽管误差较大。实验得到的 k(3rd)(o) (T)值与（a）其他涉及类似复杂物种的结合反应的速率常数进行了比较，（b）根据能量转移（ET）和自由基-复合物（RC）机制估计的 k(3rd)(o) (T)值进行了比较。我们的结论是，RC 机制可能在我们实验的低温下对 OH + O(2)的结合做出了主要贡献。
