Contribution Contributed Talk
Importance of tunneling in H-abstraction reactions by OH radicals: The case of CH4 + OH
We present a combined experimental and theoretical study on the importance of quantum tunneling of atoms in the reaction between CH4 and OH and isotope-substited analogs. Although this reaction has been studied previously both on the surface and in the gas phase, quantification of the reaction rates at low temperature is not trivial in either case. Quantitative rates, however, are needed for astrochemical models. Here, we are mainly concerned with two reactions: CH$_4$ +OD $\rightarrow$ CH$_3$ +HDO and CD$_4$ +OH $\rightarrow$ CD$_3$ +HDO. From the ultra-high vacuum surface experiments we can draw the conclusion that the reaction rates are related via R_CH$_4$ + OD >> R_CD$_4$ + OH at 15 K. This is further quantified by gas-phase calculations of the corresponding bi- and unimolecular reaction rate constants using instanton theory. Unimolecular reactions correspond to the Langmuir-Hinshelwood process and can as such be compared to the experimental findings. The ratio between the rate constants depends on the temperature and further quantifies the experimental result. The unimolecular rate constants presented here can be used by the modeling community as a first approach to describe OH-mediated abstraction reactions in the solid phase. Finally, the bimolecular rate constants can be used for the description of low-temperature gas-phase reactions as well as for the Eley-Rideal mechanisms.