  Radical recombination has been proposed to lead to the formation of complex
organic molecules (COMs) in CO-rich ices in the early stages of star formation.
These COMs can then undergo hydrogen addition and abstraction reactions leading
to a higher or lower degree of saturation. Here, we have studied 14 hydrogen
transfer reactions for the molecules glyoxal, glycoaldehyde, ethylene glycol,
and methylformate and an additional three reactions where \ce{CH_nO} fragments
are involved. Over-the-barrier reactions are possible only if tunneling is
invoked in the description at low temperature. Therefore the rate constants for
the studied reactions are calculated using instanton theory that takes quantum
effects into account inherently. The reactions were characterized in the gas
phase, but this is expected to yield meaningful results for CO-rich ices due to
the minimal alteration of reaction landscapes by the CO molecules.
  We found that rate constants should not be extrapolated based on the height
of the barrier alone, since the shape of the barrier plays an increasingly
larger role at decreasing temperature. It is neither possible to predict rate
constants based only on considering the type of reaction, the specific
reactants and functional groups play a crucial role. Within a single molecule,
though, hydrogen abstraction from an aldehyde group seems to be always faster
than hydrogen addition to the same carbon atom. Reactions that involve
heavy-atom tunneling, e.g., breaking or forming a C--C or C--O bond, have rate
constants that are much lower than those where H transfer is involved.
