Formation of Complex Organic Molecules on dust grains surfaces: a Quantum Mechanical approach

The presence of molecules in the extreme physical conditions of the interstellar medium (ISM) was considered impossible, until in 1937 the first diatomic species were observed. Since then, about 300 gaseous species were identified and, in the coldest environments, the presence of dust grains made of silicates and carbonaceous material covered in water-dominated ice mantles was discovered^1,2. Ethanol (CH₃CH₂OH) is a relatively common molecule, often found in star-forming regions. Recent studies suggest that it could be a parent molecule of several so-called interstellar complex organic molecules (iCOMs), that are the building blocks of the molecules responsible for the origin of life, and are thought to be inherited through the different stages of the evolution of a planetary system³. Similarly, urea (NH₂CONH₂) is thought to be the precursor of purines and pirimidines. It has been detected in few sources of the ISM and in Murchison meteorite, with the unique characteristic of possessing two C–N bonds. Being stable against ultraviolet radiation and high-energy electron bombardment, urea is expected to be present in interstellar ices⁴.

However, the formation route of these species remains under debate. In the present work, we investigated the formation of ethanol and urea on the surface of the icy mantles coating dust grains in the ISM with quantum chemical simulations. Two clusters of 18 and 33 water molecules were adopted as ice models and DFT calculations were run with Gaussian and ORCA codes.

The “radical + ice component” scheme was tested as an alternative mechanism for the synthesis of ethanol, beyond the usual radical−radical coupling^5,6. Results indicate that CH₃CH₂OH can potentially be formed by this proposed reaction mechanism. The reaction of CCH with an H₂O belonging to the water ice clusters can be barrierless, leading to the formation of vinyl alcohol precursors (H₂CCOH and CHCHOH). Subsequent hydrogenation of vinyl alcohol yielding ethanol is the only step presenting a low activation energy barrier. In this case, the positive outcome of the “radical + ice component” scheme is due to the establishment of a hemibond interaction between the two reactants.⁷

Theoretical and experimental studies suggest that isocyanic acid (HNCO) and formamide (NH₂CHO) are possible precursors of urea.⁸ Here, the application of the same “radical + ice component” scheme to the synthesis of urea was not straightforward, as HNCO and NH₂CHO (or, alternatively, NH₃) are less abundant then water on the icy mantles.⁹ Alternatively, different mechanisms involving both closed-shell and open-shell species were investigated, and the radical–radical NH₂CO + NH₂ coupling was found to be the most favourable pathway due to being almost barrierless and more favoured than the competitive H-abstraction reaction returning NH₃ +HNCO. In this path, the presence of the icy surfaces is crucial for acting as reactant concentrators/suppliers, as well as third bodies able to dissipate the energy liberated during the urea formation.¹⁰

 

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