H-atom addition and abstraction reactions in mixed CO, H2CO and CH3OH
  ices: an extended view on complex organic molecule formation
  Complex organic molecules (COMs) have been observed not only in the hot cores
surrounding low- and high- mass protostars, but also in cold dark clouds.
Therefore, it is interesting to understand how such species can be formed
without the presence of embedded energy sources. We present new laboratory
experiments on the low-temperature solid state formation of three complex
molecules: methyl formate (HC(O)OCH3), glycolaldehyde (HC(O)CH2OH) and ethylene
glycol (H2C(OH)CH2OH), through recombination of free radicals formed via H-atom
addition and abstraction reactions at different stages in the CO-H2CO-CH3OH
hydrogenation network at 15 K. The experiments extend previous CO hydrogenation
studies and aim at resembling the physical&chemical conditions typical of the
CO freeze-out stage in dark molecular clouds, when H2CO and CH3OH form by
recombination of accreting CO molecules and H-atoms on ice grains. We confirm
that H2CO, once formed through CO hydrogenation, not only yields CH3OH through
ongoing H-atom addition reactions, but is also subject to H-atom-induced
abstraction reactions, yielding CO again. In a similar way, H2CO is also formed
in abstraction reactions involving CH3OH. The dominant methanol H-atom
abstraction product is expected to be CH2OH, while H-atom additions to H2CO
should at least partially proceed through CH3O intermediate radicals. The
occurrence of H-atom abstraction reactions in ice mantles leads to more
reactive intermediates (HCO, CH3O and CH2OH) than previously thought, when
assuming sequential H-atom addition reactions only. This enhances the
probability to form COMs through radical-radical recombination without the need
of UV photolysis or cosmic rays as external triggers.
