  Glycolaldehyde (HOCH2CHO) is the most straightforward sugar detected in the
Interstellar Medium (ISM) and participates in the formation pathways of
molecules fundamental to life, red such as ribose and derivatives. Although
detected in several regions of the ISM, its formation route is still debated
and its abundance cannot be explained only by reactions in the gas phase. This
work explores a new gas-phase formation mechanism for glycolaldehyde and
compares the energy barrier reduction when the same route happens on the
surface of amorphous ices. The first step of the mechanism involves the
formation of a carbon-carbon bond between formaldehyde (H2CO) and the formyl
radical (HCO), with an energy barrier of 27 kJ mol-1 (gas-phase). The second
step consists of barrierless hydrogen addition. Density functional calculations
under periodic boundary conditions were applied to study this reaction path on
10 different amorphous ice surfaces through an Eley-Rideal type mechanism. It
was found that the energy barrier is reduced on average by 49 per cent, leading
in some cases to a 100 per cent reduction. The calculated adsorption energy of
glycolaldehyde suggests that it can be promptly desorbed to the gas phase after
its formation. This work, thus contributes to explaining the detected relative
abundances of glycolaldehyde and opens a new methodological framework for
studying the formation routes for Complex Organic Molecules (COMs) in
interstellar icy grains.
