tokens: mode of HDO at 1475 cm is not clearly visible as a result of the signal-to-noise (S/N) level. The bending mode is at least four times weaker than the stretching mode, which is not detectable for the S/N achieved here ( Oba et al. 2014 ). This means that the produced OD radicals either form DO or abstract an H-atom from CH. The availability of D atoms on the surface results in the barrierless deuteration of a CH radical. Since the D atoms are abundant and diffuse quickly, other radical-radical reactions are prevented. The CHOD or CH yields lie below the sensitivity of our RAIRS technique, that is, reactions R7 and R8 are not relevant here. Note that the abstraction reaction between HO and CH has a barrier of K ( Aguilera-Iparraguirre et al. 2008 ), and we therefore exclude any CH radical production via this route. Furthermore, none of the control experiments (numbers 2–4) resulted in the detection of CHD. This means that the CHD detected in experiment 1 is not the cause of contamination (follows from experiment 2) nor the result of H-abstraction from CH by D atoms directly (follows from control experiments 3 and 4). In other words, in our experimental setup, the reaction between CH and the D atom does not proceed within our experimental sensitivity. Since the same setup is used for experiments 1, 3, and 4, essentially for identical conditions, we conclude that the reaction CH + OD is more efficient and the only one responsible for the formation of the observed CHD. This result can be rationalized by realizing that the reaction CH + H CH + H has a high barrier of 69407545 K ( Nyman et al. 2007 ; Corchado et al. 2009 ), making it highly unlikely to be competitive at the low temperatures considered here, even if it were to occur via tunneling. Indeed, the (bimolecular) rate constants calculated on several PES at 300 K are six orders of magnitude lower than the bimolecular rate constants calculated here for CH + OH (compare Corchado et al. 2009 ; Espinosa-García et al. 2009 ; and Li et al. 2013 with values mentioned in Appendix A ). The next question is whether or not the isotope-substituted reaction R5, CD + OH CD + HDO, leads to D-abstraction by OH radicals. In this case, the mass of the atom that is transferred is twice as high, the activation energy is also higher due to a ZPE effect, and if tunneling is important, the reaction rate should drop significantly. In Fig. 3 , and especially from the zoomed-in spectra in Fig. 4 , it is clear that no detectable amounts of CDH are produced during experiment 5. Since OH radicals are present, HO is formed and can be seen at its respective peak positions, around 3315, 2835, and 1385 cm ( Oba et al. 2014 ). Furthermore, the OH radicals that are produced are not being used for reactions with CD and therefore are available for other reactions, leading to the formation and detection of the HO bending mode at 1630 cm, while the stretching mode overlaps with that of HO. HO can also be formed via H + HO HO +
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