Atom Tunneling in the Water Formation Reaction H₂ + OH → H₂O + H on an Ice Surface

Abstract

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 H 2 molecules is among the final steps in the astrochemical reaction network starting from O, O 2 , and O 3 . Experimentally it was shown that even at 10 K this reaction occurs on ice surfaces. As 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 done using a QM/MM 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 instanton theory which takes atom tunneling into account inherently, with 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 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.

H 2 formation on interstellar dust grains: The viewpoints of theory, experiments, models and observations

“…Most hydrogenation reactions on the surfaces involving H 2 have significant barriers so that, even though the abundance of H 2 is larger than that of atomic hydrogen, the reactions with H are considered to be much faster. There are a few of these reactions that are, however, exothermic and may proceed via tunneling; for example, H 2 + OH (Oba et al, 2012;Meisner et al, 2017).…”

Tunnelling dominates the reactions of hydrogen atoms with unsaturated alcohols and aldehydes in the dense medium

“…To model the reaction on a surface, the rotational partition function in the unimolecular case is assumed to be constant during the reaction. Such an implicit surface model (Meisner et al 2017a) covers the effect of the ice surface, where the rotation on the surface is suppressed. In principle, this approximation holds as long as the interaction between the surface and the admolecules is weak, as is the case for a CO-rich ice mantle and the C 3 H n O molecules studied here.…”

HOCO formation in astrochemical environments by radical-induced H-abstraction from formic acid

“…All rate constants reported in this work correspond to reactions on surfaces. Unimolecular rate constants are relevant for the Langmuir-Hinshelwood mechanism (Meisner, Lamberts & Kästner 2017a). To model the influence of the surface, the rotational partition function in the unimolecular case is assumed to be constant during the reaction.…”