Importance of tunneling in H-abstraction reactions by OH radicals: The case of CH4 + OH studied through isotope-substituted analogs

Abstract

We present a combined experimental and theoretical study focussing on the quantum tunneling of atoms in the reaction between CH4 and OH. The importance of this reaction pathway is derived by investigating isotope substituted analogs. Quantitative reaction rates needed for astrochemical models at low temperature are currently unavailable both in the solid state and in the gas phase. Here, we study tunneling effects upon hydrogen abstraction in CH4 + OH by focusing on two reactions: CH4 + OD -> CH3 + HDO and CD4 + OH -> CD3 + HDO. The experimental study shows that the solid-state reaction rate R(CH4 + OD) is higher than R(CD4 + OH) at 15 K. Experimental results are accompanied by calculations of the corresponding unimolecular and bimolecular reaction rate constants using instanton theory taking into account surface effects. From the work presented here, the unimolecular reactions are particularly interesting as these provide insight into reactions following a Langmuir-Hinshelwood process. The resulting ratio of the rate constants shows that the H abstraction (k(CH4 + OD)) is approximately ten times faster than D-abstraction (k(CD4 + OH)) at 65 K. We conclude that tunneling is involved at low temperatures in the abstraction reactions studied here. The unimolecular rate constants can be used by the modeling community as a first approach to describe OH-mediated abstraction reactions in the solid phase. For this reason we provide fits of our calculated rate constants that allow the inclusion of these reactions in models in a straightforward fashion.

arXiv commentAccepted by A&A

References in corpus (23)

• Observations of the Icy Universe
• Photochemistry and astrochemistry: photochemical pathways to interstellar complex organic molecules
• A three-phase chemical model of hot cores: the formation of glycine
• Interstellar water chemistry: from laboratory to observations
• Complex organic molecules in protoplanetary disks
• Laboratory evidence for efficient water formation in interstellar ices
• H-atom addition and abstraction reactions in mixed CO, H2CO and CH3OH ices: an extended view on complex organic molecule formation
• Constraining the abundances of complex organics in the inner regions of solar-type protostars
• The c2d Spitzer spectroscopy survey of ices around low-mass young stellar objects, III: CH4
• A Unified Monte Carlo Treatment of Gas-Grain Chemistry for Large Reaction Networks. II. A Multiphase Gas-Surface-Layered Bulk Model
• Water formation at low temperatures by surface O2 hydrogenation II: the reaction network
• Reprocessing of Ices in Turbulent Protoplanetary Disks: Carbon and Nitrogen Chemistry
• Water formation at low temperatures by surface O2 hydrogenation I: characterization of ice penetration
• Water Formation through a Quantum Tunneling Surface Reaction, OH + H2, at 10 K
• Derivation of instanton rate theory from first principles
• Cometary ices in forming protoplanetary disc midplanes
• CO2 formation in quiescent clouds; an experimental study of the CO + OH pathway
• Evolution of CO${}_2$, CH${}_4$, and OCS abundances relative to H${}_2$O in the coma of comet 67P around perihelion from Rosetta/VIRTIS-H observations
• Modeling Heat Dissipation at the Nanoscale: An Embedding Approach for Chemical Reaction Dynamics on Metal Surfaces
• Quantum tunneling during interstellar surface-catalyzed formation of water: the reaction H + H${}_2$O${}_2$ $\to$ H${}_2$O + OH
• Interstellar Simulations Using A Unified Microscopic-Macroscopic Monte Carlo Model with a full Gas-Grain Network including Bulk Diffusion in Ice Mantles
• Molecular Dynamics Simulations of CO2 Formation in Interstellar Ices
• Low-temperature chemistry between water and hydroxyl radicals: H/D isotopic effects

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