Computational Insights into the Formation of Methyl Formate and Glycolaldehyde via Amorphous ISM Ice

Abstract

In the realm of Complex Organic Molecules (COMs) detected within the Interstellar Medium (ISM), glycolaldehyde stands out as a pivotal player due to its dual significance as the simplest sugar and a precursor of life-sustaining molecules such as ribose. Coexisting in substantial concentrations with glycolaldehyde is its isomer, methyl formate, raising questions about a potential shared formation pathway. Recent research hints at the essential role of amorphous water ice in COM formation within dense ISM nebulae. This study revisits a promising two-step pathway for glycolaldehyde synthesis, initiated by the reaction of formaldehyde with the formyl radical, followed by hydrogenation of the resulting intermediate. In addition to employing a more rigorous level of calculation, the Langmuir–Hinshelwood mechanism is explored, while also predicting the possibility of methyl formate formation from the same starting materials. Simulation efforts, using water clusters comprising 18 and 25 molecules, leverage both Density Functional Theory (DFT) and Coupled Cluster (CC) methods at distinct sites of the surface. The findings reveal that, at the DFT level, the proposed reaction leads to glycolaldehyde formation with no energy barriers, exhibiting a significant relative energy reduction of up to 129% relative to the gas-phase reaction. Conversely, at the CC level, a modest 19 kJ mol^–1 barrier is encountered at 10 K, representing a 45% reduction. We show that formation of methyl formate via this pathway is not feasible, with higher energy barriers than the primary glycolaldehyde production. Analysis of the adsorption energies suggests distinct behaviors, with glycolaldehyde intermediates remaining adsorbed until the formation of the final molecule, while methyl formate exhibits sufficient energy for desorption before the final hydrogenation step.