ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model I-Model I-Model I-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
tokens: the low-temperature value for the rate constant of reaction FAR2 compared to FAR1 . For FAR2 a C–O bond is formed and as heavy-atom tunneling is less efficient than hydrogen atom tunneling, the rate constant is much lower than what would be expected from the barrier height only (compare for instance MF4 , GA6 , and FAR2 ). Similarly for the reaction of FA with , where a C–C bond is formed ( FAR3 ), again the low-temperature rate constant is very low. Note also the lower values for the T c of FAR2 and FAR3 compared to the hydrogen transfer reactions, indicating that tunneling also sets in at lower temperatures. Comparing the rate constant to the typical value assumed for radical–radical barrierless reactions, ∼10 12 s −1 , it is clear that these reactions are much less likely to contribute to COM formation. On that note we do wish to stress, however, to keep in mind that although radical–radical reactions may be able to proceed without a barrier, this does not mean that all reaction pathways are open, see for instance Lamberts ( 2018 ). 5 ASTROCHEMICALLY RELEVANT CONCLUSIONS Unimolecular reaction rate constants have been calculated and are provided for hydrogen addition and abstraction reactions from methylformate, glyoxal, glycoaldehyde, an ethylene glycol and are thus available to be implemented in both rate-equation and kinetic Monte Carlo models aimed at studying the formation of COMs at low temperatures. Our results are generally in agreement with experimental work, although some discrepancies exist on the efficiency of specific reaction paths, such as the formation of or after hydrogen abstraction from methanol, which impacts on the ease of methylformate formation (for which is needed) or ethylene glycol formation (for which is required). A microscopic model aiming to reproduce experiments may be able to provide a clear picture of how the reactions are intertwined with each other. The reaction could not be studied and thus deserves further attention. We found that one cannot predict average rate constants solely based on the type of the reaction. The spread in the low-temperature rate constant can be roughly seven orders of magnitude for a single reaction type (e.g. hydrogen addition to an aldehyde carbon) showing a strong dependence on the other functional groups that are attached to the carbon backbone. Within a single molecule, on the other hand, one can loosely say that hydrogen abstraction from an aldehyde group is faster than hydrogen addition to the same carbon. Both of these have a rate constant that is larger than hydrogen abstraction from a methyl group. Care should be taken with extrapolating rate constants based on the height of the barrier alone, as calculations show that reactions with narrow barriers can have rate constants at low temperature that are higher than those with a lower activation energy. Reactions that include the breakage or formation of a bond between two heavy atoms generally have low-temperature rate