Non-Energetic Formation of Ethanol via CCH Reaction with Interstellar
  H2O Ices. A Computational Chemistry Study
  Ethanol (CH$_3$CH$_2$OH) is a relatively common molecule, often found in star
forming regions. Recent studies suggest that it could be a parent molecule of
several so-called interstellar complex organic molecules (iCOMs). Yet, the
formation route of this species remains debated. In the present work, we study
the formation of ethanol through the reaction of CCH with one H$_2$O molecule
belonging to the ice, as a test case to investigate the viability of chemical
reactions based on a "radical + ice component" scheme as an alternative
mechanism for the synthesis of iCOMs, beyond the usual radical-radical
coupling. This has been done by means of DFT calculations adopting two clusters
of 18 and 33 water molecules as ice models. Results indicate that
CH$_3$CH$_2$OH can potentially be formed by this proposed reaction mechanism.
The reaction of CCH with H$_2$O on the water ice clusters can be barrierless
(thanks to the help of boundary icy water molecules acting as proton transfer
assistants) leading to the formation of vinyl alcohol precursors (H$_2$CCOH and
CHCHOH). Subsequent hydrogenation of vinyl alcohol yielding ethanol is the only
step presenting a low activation energy barrier. We finally discuss the
astrophysical implications of these findings.
