Glycolaldehyde (CHOCH2OH) and ethylene glycol (HOCH2CH2OH) are among many complex organic molecules detected in the interstellar medium (ISM). Astrophysical models proposed very often that the formation of these compounds would be directly linked to the hydrogenation of glyoxal (CHOCHO), a potential precursor which is not yet detected in the ISM. We have performed, in this work, surface and bulk hydrogenations of solid CHOCHO under ISM conditions in order to confirm or invalidate the astrophysical modelling of glyoxal transformation. Our results show that the hydrogenation of glyoxal does not lead to the formation of detectable amounts of heavier organic molecules such as glycolaldehyde and ethylene glycol but rather to lighter CO-bearing species such as CO, H2CO, and CO–H2CO, a reaction intermediate resulting from an H-addition–elimination process on CHOCHO and where CO is linked to H2CO. The solid phase formation of such a reaction intermediate has been confirmed through the neon matrix isolation of CO–H2CO species. Additionally, the CHOCHO + H solid-state reaction might also lead to the production of CH3OH formed under our experimental conditions as a secondary product resulting from the hydrogenation of formaldehyde.
Glycolaldehyde, the simplest sugar, is a complex organic molecule detected in many regions of the interstellar medium (ISM). Although its synthetic routes are fairly well known and consistent with many laboratory studies, queries still arise about its reactivity and its role in the complex chemistry of the ISM. The present study shows the surface and bulk hydrogenation of glycolaldehyde at 10 K in order to confirm or invalidate the astrophysical models which suggest that CHOCH2OH would be a precursor of ethylene glycol through hydrogenation processes occurring on the surface of interstellar dust grains. By coupling IR spectroscopy and mass spectrometry, we show that the formation of HOCH2CH2OH from CHOCH2OH + H solid state reaction occurs, supporting the existence of a chemical link between these two organics in the ISM. The present work suggests that while CHO + CH2OH and CH2OH + CH2OH radical recombination would lead to CHOCH2OH and HOCH2CH2OH, respectively, the presence of H-atoms in the ISM would be a secondary source to favor ethylene glycol over glycolaldehyde. These results are in good agreement with different astronomical observations which show simultaneous detections of glycolaldehyde and ethylene glycol with an abundance ratio HOCH2CH2OH/CHOCH2OH ranged between 1 and 15.
Methanol, one of the most abundant organic molecules in the interstellar medium, plays an important role in the complex grain surface chemistry which is believed to be a source of many organic compounds. Under energetic processing such as UV photons or cosmic rays, methanol may decompose into CH4, CO2, CO, HCO, H2CO, CH3O, CH2OH which in turn lead to complex organic molecules such as CH3OCHO, CHOCH2OH, HOCH2CH2OH through radical recombination reactions. However, although molecular oxygen has been the subject of many debates about its detection, abundance and role in the interstellar medium, a few experiments on the oxidation of organic compounds have been carried out under interstellar conditions. The present study shows the behavior of solid methanol when treated by UV light and thermal processing in oxygen-rich environments. Methanol has been irradiated in absence and presence of O2 at different concentrations in order to study how oxidized complex organic molecules may form and also to investigate the O-insertion reaction in the C-H bound to form methanediol HOCH2OH through CH3OH + O(1D) solid state reaction. The adding of O2 in the thermal and photochemical reaction of solid methanol leads to the formation of O3, H2O and HO2, in addition to three main organics, HCOOH, CHOCHO and HOCH2OH. We show that in an O2 rich environment, species such as CO, CH4, HCO, CH3OH and CHOCH2OH are oxidized into CO2, CH3OH, HC(O)OO, HOCH2OH and CHOCHO, respectively, while HCOOH might be formed through H2CO + O(3P) → (OH + HCO)cage → HCOOH H-abstraction reaction.
Organic residues are considered as part of the chemical composition of the interstellar dust grains. They are formed under the extreme conditions of the interstellar medium and play an important role in exobiology. They may contain prebiotic organic species such as amino acids, constituents of proteins and building blocks of DNA and RNA, key elements of life. By investigating the formation of organic residues in an astrophysical context, many groups have been focusing in the UV irradiation and subsequent warm-up of astrophysical ice analogs. This aims to suggest that organic residues are mainly formed in regions of molecular clouds exposed to UV light or cosmic rays. The present study shows an organic residues formation involving glyoxal ice and H-atoms. While the hydrogenation of glyoxal at 10 K leads mainly to small molecules such as CO and H2CO and CH3OH, we show that the heating of the hydrogenated ice in the 10–300 K temperature range leads to solid residues which structure is similar to that of glycolaldehyde but they remain stable in solid phase at 300 K and atmospheric pressure. The analysis of the IR data shows that the organic residues formed through the thermal processing of CHOCHO + H reaction would be a mixture of hydroxypyruvaldehyde and methyl glyoxylate, two solid organics which formation starts with an H-abstraction from glyoxal to form CHOCO• radical which recombines to •CH2OH and •OCH3 radicals. These latter may be formed and trapped in glyoxal ice as secondary products from H2CO + H secondary reaction.
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