Quantum tunneling during interstellar surface-catalyzed formation of water: the reaction H + H${}_2$O${}_2$ $\to$ H${}_2$O + OH

Abstract

The final step of the water formation network on interstellar grain surfaces starting from the H + O${}_2$ route is the reaction between H and H${}_2$O${}_2$. This reaction is known to have a high activation energy and therefore at low temperatures it can only proceed via tunneling. To date, however, no rate constants are available at temperatures below 200 K. In this work, we use instanton theory to compute rate constants for the title reaction with and without isotopic substitutions down to temperatures of 50 K. The calculations are based on density functional theory, with additional benchmarks for the activation energy using unrestricted single-reference and multireference coupled-cluster single-point energies. Gas-phase bimolecular rate constants are calculated and compared with available experimental data not only for H + H${}_2$O${}_2$ $\to$ H${}_2$O + OH, but also for H + H${}_2$O${}_2$ $\to$ H${}_2$ + HO${}_2$. We find a branching ratio where the title reaction is favored by at least two orders of magnitude at 114 K. In the interstellar medium this reaction predominantly occurs on water surfaces, which increases the probability that the two reactants meet. To mimic this one, two, or three spectator H2O molecules are added to the system. Eley-Rideal bimolecular and Langmuir-Hinshelwood unimolecular rate constants are presented here. The kinetic isotope effects for the various cases are compared to experimental data as well as to expressions commonly used in astrochemical models. Both the rectangular barrier and the Eckart approximations lead to errors of about an order of magnitude. Finally, fits of the rate constants are provided as input for astrochemical models.

Accepted for publication by Phys. Chem. Chem. Phys. 2016. Supplementary information available: http://www.rsc.org/suppdata/c6/cp/c6cp06457d/c6cp06457d1.pdf

References in corpus (4)

• Simulation of the Formation and Morphology of Ice Mantles on Interstellar Grains
• Laboratory evidence for efficient water formation in interstellar ices
• Formation of hydrogen peroxide and water from the reaction of cold hydrogen atoms with solid oxygen at 10K
• Detection of interstellar hydrogen peroxide

Cited by in corpus (5)

• Formation of Glycerol through Hydrogenation of CO ice under Prestellar Core Conditions
• Atom Tunneling in the Water Formation Reaction H${}_2$ + OH $\to$ H${}_2$O + H on an Ice Surface
• Influence of surface and bulk water ice on the reactivity of a water-forming reaction
• Importance of tunneling in H-abstraction reactions by OH radicals: The case of CH4 + OH studied through isotope-substituted analogs
• Extension of the HCOOH and CO2 solid-state reaction network during the CO freeze-out stage: inclusion of H2CO