The (impossible?) formation of acetaldehyde on the grain surfaces: insights from quantum chemical calculations

TL;DR

This study uses quantum chemical calculations to investigate whether acetaldehyde can form on interstellar grain surfaces, finding that surface constraints likely prevent its formation via radical coupling on icy grains.

Contribution

The paper provides the first quantum chemical analysis of acetaldehyde formation on water ice surfaces, revealing surface constraints inhibit its synthesis from radicals.

Findings

Acetaldehyde formation via radical coupling is unlikely on icy grain surfaces.

Gas-phase pathways may produce acetaldehyde, but surface reactions are constrained.

Implications for the formation mechanisms of other complex organic molecules in space.

Abstract

Complex Organic Molecules (COMs) have been detected in the interstellar medium (ISM). However, it is not clear whether their synthesis occurs on the icy surfaces of interstellar grains or via a series of gas-phase reactions. As a test case of the COMs synthesis in the ISM, we present new quantum chemical calculations on the formation of acetaldehyde (CH3CHO) from the coupling of the HCO and CH3 radicals, both in gas phase and on water ice surfaces. The binding energies of HCO and CH3 on the amorphous water ice were also computed (2333 and 734 K, respectively). Results indicate that, in gas phase, the products could be either CH3CHO, CH4 + CO, or CH3OCH, depending on the relative orientation of the two radicals. However, on the amorphous water ice, only the CH4 + CO product is possible due to the geometrical constraints imposed by the water ice surface. Therefore, acetaldehyde cannot be synthesized by the CH3 + HCO coupling on the icy grains. We discuss the implications of these results and other cases, such as ethylene glycol and dimethyl ether, in which similar situations can occur, suggesting that formation of these molecules on the grain surfaces might be unlikely.