Formation of interstellar propanal and 1-propanol ice: a pathway
  involving solid-state CO hydrogenation
  1-propanol (CH3CH2CH2OH) is a three carbon-bearing representative of primary
linear alcohols that may have its origin in the cold dark cores in interstellar
space. To test this, we investigated in the laboratory whether 1-propanol ice
can be formed along pathways possibly relevant to the prestellar core phase. We
aim to show in a two-step approach that 1-propanol can be formed through
reaction steps that are expected to take place during the heavy CO freeze-out
stage by adding C2H2 into the CO + H hydrogenation network via the formation of
propanal (CH3CH2CHO) as an intermediate and its subsequent hydrogenation.
Temperature programmed desorption-quadrupole mass spectrometry (TPD-QMS) is
used to identify the newly formed propanal and 1-propanol. Reflection
absorption infrared spectroscopy (RAIRS) is used as a complementary diagnostic
tool. The mechanisms that can contribute to the formation of solid-state
propanal and 1-propanol, as well as other organic compounds, during the heavy
CO freeze-out stage are constrained by both laboratory experiments and
theoretical calculations. Here it is shown that recombination of HCO radicals,
formed upon CO hydrogenation, with radicals formed upon C2H2 processing - H2CCH
and H3CCH2 - offers possible reaction pathways to solid-state propanal and
1-propanol formation. This extends the already important role of the CO
hydrogenation chain in the formation of larger COMs (complex organic
molecules). The results are used to compare with ALMA observations. The
resulting 1-propanol:propanal ratio concludes an upper limit of < 0:35-0:55,
which is complemented by computationally-derived activation barriers in
addition to the experimental results.
