Rate rules, branching ratios, and pressure dependence of the HO2 + olefin addition channels.
Rate rules, branching ratios, and pressure dependence of the HO2 + olefin addition channels.
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DOI：
10.1021/jp405262r
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发表时间：
2013-07
期刊：
影响因子：
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通讯作者：
Stephanie M. Villano;H. Carstensen;A. Dean
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文献类型：
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作者：
Stephanie M. Villano;H. Carstensen;A. Dean
在这项工作中，我们给出了过氧化氢与烯烃通过协同加成通道生成烷基过氧基(Ho2+烯烃→O2)和通过自由基加成通道生成β-羟基过氧烷基(Ho2+烯烃→β-QOOH.)的高压速率规律和支化比。这些速率规则是通过计算包含不同支化度的烯烃的一系列加成反应的速率常数来制定的。各个速率表达式是根据在CBS-QB3理论水平上进行的电子结构计算结合TST计算确定的。计算的速率常数与文献报道的速率常数符合得很好。接下来，我们使用能量粒子主方程(ME)方法和修正的强碰撞(MSC)近似QRRK计算了HO2加成到1-丁烯末端的表观压力和温度相关的速率常数。这两种方法对两个反应类给出了相似的结果。我们发现，对于自由基加成反应，稳定通道的高压极限直到异常高压(>；1000atm)才能达到。相反，这个反应会直接生成环氧乙烷+羟基。总体而言，主要通道的计算结果与以前的理论和实验数据基本一致。最后，为了明确检查压力的影响，我们比较了在10和100大气压下，使用高压和压力依赖机制获得的HO2和丁烯在空气中反应的浓度-时间分布。这些模拟表明，与通常的预期相反，在动力学模型研究中，压力衰减效应的表现可能在压力增加时更为普遍。这一行为归因于β-QOOH与O2的反应，反应速度随着空气压力的增加而增加。在衰减效应对该通道很重要的条件下，这种双分子反应与β-QOOH单分子反应竞争。
In this work, we present high-pressure rate rules and branching ratios for the addition of HO2 to olefins through the concerted addition channel to form an alkyl peroxy radical (HO2 + olefin → RO2) and through the radical addition channel to form a β-hydroperoxy alkyl radical (HO2 + olefin → β-QOOH). These rate rules were developed by calculating rate constants for a series of addition reactions involving olefins with varying degrees of branching. The individual rate expressions were determined from electronic structure calculations performed at the CBS-QB3 level of theory combined with TST calculations. The calculated rate constants were found to be in good agreement with those reported in the literature. Next, we calculated apparent pressure- and temperature-dependent rate constants for HO2 addition to the terminal site of 1-butene using an energy-grained master equation (ME) approach and QRRK calculations with a modified strong collision (MSC) approximation. The two methods gave similar results for both reaction classes. We found that, for the radical addition reaction, the high-pressure limit for the stabilization channel is not reached until unusually high pressures (>1000 atm). Instead, this reaction leads to the direct formation of an oxirane + OH. In general, the results for the major channels are in reasonable agreement with prior theoretical and experimental data. Finally, to explicitly examine the effect of pressure, we compared concentration-time profiles for the reactions of HO2 plus butene in air that were obtained using both high-pressure and pressure-dependent mechanisms at 10 and 100 atm. These simulations showed that, contrary to general expectations, the manifestation of pressure falloff effects in kinetic modeling studies might be more prevalent at increasing pressures. This behavior is attributed to the reaction of β-QOOH with O2, the rate of which increases with increasing pressure of air. This bimolecular reaction competes with the unimolecular reactions of β-QOOH under conditions where falloff effects are important for that channel.