  A number of recent experimental studies have shown that solid-state complex
organic molecules (COMs) can form under conditions that are relevant to the CO
freeze-out stage in dense clouds. In this work, we show that alcohols can be
formed well before the CO freeze-out stage (i.e., in the H2O-rich ice phase).
This joint experimental and computational investigation shows that the isomers,
n- and i-propanol (H3CCH2CH2OH and H3CCHOHCH3) and n- and i-propenol (H3CCHCHOH
and H3CCOHCH2), can be formed in radical-addition reactions starting from
propyne (H3CCCH) + OH at the low temperature of 10 K, where H3CCCH is one of
the simplest representatives of stable carbon chains already identified in the
interstellar medium (ISM). The resulting average abundance ratio of 1:1 for
n-propanol:i-propanol is aligned with the conclusions from the computational
work that the geometric orientation of strongly interacting species is
influential to the extent of which 'mechanism' is participating, and that an
assortment of geometries leads to an averaged-out effect. Three isomers of
propanediol are also tentatively identified in the experiments. It is also
shown that propene and propane (H3CCHCH2 and H3CCH2CH3) are formed from the
hydrogenation of H3CCCH. Computationally-derived activation barriers give
additional insight into what types of reactions and mechanisms are more likely
to occur in the laboratory and in the ISM. Our findings not only suggest that
the alcohols studied here share common chemical pathways and therefore can show
up simultaneously in astronomical surveys, but also that their extended
counterparts that derive from polyynes containing H3C(CC)nH structures may
exist in the ISM. Such larger species, like fatty alcohols, are the possible
constituents of simple lipids that primitive cell membranes on the early Earth
are thought to be partially composed of.
