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

J. Enrique-Romero , A. Rimola , C. Ceccarelli (1) , N. Balucani (1)

1 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

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.

Mots clés: astrochemistry; molecular processes; ISM: clouds; ISM: molecules; Astrophysics - Earth and Planetary Astrophysics

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

J. Enrique-Romero, A. Rimola, C. Ceccarelli, N. Balucani. The (impossible?) formation of acetaldehyde on the grain surfaces: insights from quantum chemical calculations. Monthly Notices of the Royal Astronomical Society: Letters, 2016, 459, pp.L6-L10. ⟨10.1093/mnrasl/slw031⟩. ⟨insu-03691536⟩