Importance of tunneling in H-abstraction reactions by OH radicals

The case of CH₄ + OH studied through isotope-substituted analogs

¹ Institute for Theoretical Chemistry, University Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
e-mail: lamberts@theochem.uni-stuttgart.de
² Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

Received: 4 October 2016
Accepted: 15 December 2016

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. For 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.