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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Mechanism of the Gas-Phase HO + H_2O -> H_2O + OH Reaction and Several Associated Isotope Exchange Reactions: A Canonical Variational Transition State Theory Plus Multidimensional Tunneling Calculation
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Mechanism of the Gas-Phase HO + H_2O -> H_2O + OH Reaction and Several Associated Isotope Exchange Reactions: A Canonical Variational Transition State Theory Plus Multidimensional Tunneling Calculation

机译：气相HO + H_2O-> H_2O + OH反应及几种相关的同位素交换反应的机理：典范变分过渡态理论加多维隧穿计算

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Hydrogen abstraction from a molecule by OH is an important step in several reaction mechanisms of a key relevance in the chemistry of the atmosphere. The upper limit at 300 K for the rate constant of one of the simplest hydrogen abstraction reactions. HO + HOH -> HOH + OH (1), has been experimentally established. This reaction is intrinsicaly interesting because the associated isotope exchange reactions, H~(18)O + HOH -> H~(18)OH + OH (2) and DO + HOH -> DOH + OH (3), could affect the isotopic composition of the stratospheric water. The rate constants for those two reactions have also been experimentally measured in the interval 300-420 K (reaction 2) and at 300 K (reaction 3). In addition, the upper limit at 300 K for the rate constant of the reaction HO + DOD -> HOD + OD (4) has also been given. In this paper, we have theoretically calculated the rate constants and their temperature dependence for the above-mentioned four hydrogen (deuterium) transfer reactions by means of ab initio electronic structure calculations on their corresponding potential energy surfaces, followed by dynamical calculations based on the canonical unified statistical theory (CUS). The only available experimental activation energy (4.2 +- 0.5 kcal/mol for reaction 2 over the range 300-420 K) is in very solid agreement with our theoretical value of 4.27 kcal/mol obtained from first principles. In addition, our results confirm the experimental finding that these reactions have preexponential factors clearly lower than other typical hydrogen abstractions by HO. These low values for the two Arrhenius parameters come from a noticeable curved theoretical Arrhenius plot that, in turn, is a consequence of the large tunneling effects present in all these hydrogen (deuterium) abstraction reactions. No curvature was detected in the experimental Arrhenius plot for reaction 2, due to the small temperature range studied (300-420 K). The main cause for such large tunneling effects is the existence of an association complex, in which the two reactants are hydrogen bonded, which forms before the abstraction process itself takes place. Then, the adiabatic energy profile (effective potential for the tunneling calculation) is much thinner than that in a more typical bimolecular hydrogen (deuterium) transfer reaction. Finally, the kinetic isotope effects have been calculated, and a comparison with experimental rate constants rations has also been made.
机译：通过OH从分子中提取氢是与大气化学关键相关的几种反应机理中的重要步骤。最简单的氢提取反应之一的速率常数的上限为300K。 HO + HOH-> HOH + OH（1）已通过实验建立。该反应本质上很有趣，因为相关的同位素交换反应H〜（18）O + HOH-> H〜（18）OH + OH（2）和DO + HOH-> DOH + OH（3）可能会影响同位素平流层水的组成。这两个反应的速率常数也已通过实验在300-420 K（反应2）和300 K（反应3）之间进行了测量。此外，还给出了反应HO + DOD-> HOD + OD的速率常数在300 K处的上限（4）。在本文中，我们从理论上计算了上述四个氢（氘）转移反应的速率常数及其对温度的依赖性，方法是在其相应的势能面上进行从头算电子结构的计算，然后根据正则表达式进行动力学计算。统一统计理论（CUS）。唯一可用的实验活化能（反应2在300-420 K范围内为4.2±0.5 kcal / mol）与我们从第一原理获得的理论值4.27 kcal / mol非常一致。此外，我们的结果证实了实验发现，即这些反应的指数前因子明显低于HO提取的其他典型氢。这两个Arrhenius参数的较低值来自引人注目的弯曲理论Arrhenius曲线，而这又是所有这些氢（氘）提取反应中均存在较大隧穿效应的结果。由于研究的温度范围较小（300-420 K），因此在实验2的实验Arrhenius曲线中未检测到曲率。产生如此大的隧穿效应的主要原因是存在缔合络合物，其中两种反应物是氢键结合的，这是在提取过程本身发生之前形成的。然后，绝热能量分布（用于隧穿计算的有效电位）比更典型的双分子氢（氘）转移反应中的绝热能量分布薄得多。最后，计算了动力学同位素效应，并与实验速率常数的定量比进行了比较。

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