Experimental and computational study of the OH-isoprene reaction: Isomeric branching and low-pressure behavior
Experimental and computational study of the OH-isoprene reaction: Isomeric branching and low-pressure behavior
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DOI：
10.1021/jp001163c
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发表时间：
2000-07-20
影响因子：
2.9
通讯作者：
North, SW
中科院分区：
文献类型：
--
作者：
McGivern, WS;Suh, I;North, SW
异戊二烯-OH 反应的动力学已通过实验和计算进行了研究。使用脉冲光解/激光诱导荧光检测 OH 自由基，在 295 K 下确定了 0.5-20 Torr 压力范围内的实验速率常数。在 20 Torr 的氩气中测定了 (0.99 +/- 0.05) x 10(-10) molecular(-1) cm(3) s(-1) 的速率常数，这与之前的高压限制速率常数结果一致。我们首次对该反应的衰减区域进行了实验观察，并使用树形式对速率的压力依赖性进行了建模。在最近的从头计算的基础上进行了规范变分过渡态理论计算，以确定高压极限下四种可能的异戊二烯-OH加合物之间的相对支化。我们发现外部碳位置的 OW 添加主导内部碳位置的 OH 添加。我们采用 RRKM/主方程计算来评估总速率和压力范围 0.25 - 1000 Torr 内各个异构体速率的压力依赖性。计算的衰减行为与实验的衰减行为之间的出色一致性为从头算能量学和 RRKM/ME 处理提供了独立的测试。研究结果揭示了对流层中异戊二烯氧化的机制。
The kinetics of the isoprene-OH reaction have been studied both experimentally and computationally. Experimental rate constants at pressures in the range 0.5-20 Torr have been determined at 295 K using pulsed photolysis/laser-induced fluorescence detection of the OH radical. A rate constant of (0.99 +/- 0.05) x 10(-10) molecules(-1) cm(3) s(-1) at 20 Torr in argon was determined, which is consistent with previous results for the high-pressure limiting rate constant. We present the first experimental observation of the falloff region for this reaction and have modeled the pressure dependence of the rates using the Tree formalism. Canonical variational transition state theory calculations were performed on the basis of recent ab initio calculations to determine the relative branching among the four possible isoprene-OH adducts in the high-pressure limit. We find OW addition to the outer carbon positions dominates OH addition to the inner carbon positions. We have employed RRKM/master equation calculations to evaluate the pressure dependence of the overall rate and the rates for the individual isomers in the pressure range 0.25 - 1000 Torr. The excellent agreement between the calculated and experimental falloff behavior provides an independent test of the ab initio energetics and RRKM/ME treatment. The results shed light on the mechanisms for oxidation of isoprene in the troposphere.