OH radicals play a key role as an intermediate in the water formation chemistry of the interstellar medium. For example, the reaction of OH radicals with H₂ molecules is among the final steps in the astrochemical reaction network starting from O, O₂, and O₃. Experimentally, it was shown that, even at 10 K, this reaction occurs on ice surfaces. Because the reaction has a high activation energy, only atom tunneling can explain such experimental findings. In this study, we calculated reaction rate constants for the title reaction on a water-ice I_h surface. To our knowledge, low-temperature rate constants on a surface are not available in the literature. All surface calculations were performed using a quantum mechanics/molecular mechanics framework (BHLYP/TIP3P) after a thorough benchmark of different density functionals and basis sets to highly accurate correlation methods. Reaction rate constants are obtained using the instanton theory, which takes atom tunneling into account inherently, with reaction rate constants down to 110 K for the Eley–Rideal mechanism and down to 60 K for the Langmuir–Hinshelwood mechanism. We found that the reaction rate is nearly temperature-independent below 80 K. We give kinetic isotope effects for all possible deuteration patterns for both reaction mechanisms. For the implementation in astrochemical networks, we also give fit parameters to a modified Arrhenius equation. Finally, several different binding sites and binding energies of OH radicals on the I_h surface are discussed, and the corresponding rate constants are compared to the gas-phase case.

中文翻译：

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冰表面水形成反应中的原子隧穿H ₂ + OH→H ₂ O + H

OH自由基作为星际介质水形成化学过程中的中间体起着关键作用。例如，OH自由基与H ₂分子的反应是从O，O ₂和O ₃开始的星形化学反应网络中的最后步骤。实验表明，即使在10 K下，该反应也会在冰面上发生。因为该反应具有很高的活化能，所以只有原子隧穿可以解释这种实验结果。在这项研究中，我们计算了反应速率常数的水冰我的标题反应_{^ h}表面。据我们所知，文献中没有表面的低温速率常数。所有表面计算均使用量子力学/分子力学框架（BHLYP / TIP3P），在对不同密度泛函和基础集进行了彻底的基准测试后，得出了高精度的相关方法。反应速率常数是使用Instanton理论获得的，该理论固有地考虑了原子隧穿，对于Eley-Rideal机理，反应速率常数低至110 K，对于Langmuir-Hinshelwood机理，反应速率低至60K。我们发现，在80 K以下时，反应速率几乎与温度无关。对于两种反应机理，我们对所有可能的氘代模式给出了动力学同位素效应。为了在星化网络中实施，我们还为拟合的Arrhenius方程提供了拟合参数。最后，在I上的几个不同的结合位点和OH自由基的结合能讨论了_h面，并将相应的速率常数与气相情况进行了比较。