Atomistic Modeling of Methyl Formate and Glycolaldehyde Formation on Interstellar Dirty Ice Mantles via a "Radical + Ice" Mechanism.

Methylformate (MF) and glycolaldehyde (GA) are two primogenital organic molecules detected in both cold and warm regions of the interstellar medium (ISM). Both gas-phase and grain-surface pathways have been proposed to explain their abundances, yet uncertainties remain, since prevailing grain-surface mechanisms favor the formation of GA over MF, which mismatch observations in different ISM regions. In this work, MF and GA synthetic reactions are atomistically modeled on surfaces containing variable H ${}_2$ O and CO percentages (interstellar dirty ices), in which one of the reactants coming from the gas phase reacts with an icy CO, thus adopting the following two-step "radical + ice" mechanism: for MF, OCH ${}_3$ + ${\mathrm{CO}}_{\left(\mathrm{ice}\right)}$ $\to$ COOCH ${}_3$ + H $\to$ HCOOCH ${}_3$ ; for GA, CH ${}_2$ OH +  ${\mathrm{CO}}_{\left(\mathrm{ice}\right)}$ $\to$ COCH ${}_2$ OH + H $\to$ HCOCH ${}_2$ OH. Calculations show that the first step presents an energy barrier (32-38 kJ mol ${}^{-1}$ for MF and 17-20 kJ mol ${}^{-1}$ for GA), while the second step is nearly barrierless. Although the energetics favor GA formation, the observed abundances are better explained by desorption phenomena rather than reaction barriers are argued. Specifically, the weaker binding energies of MF (16.8-46.1 kJ mol ${}^{-1}$ ) than GA (28.4-90.2 kJ mol ${}^{-1}$ ) support its higher abundance in the ISM.