  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.
