RIces

Since my recruitment in 2009 in the ASTRO team of the laboratory Physique des Interactions Ioniques et Moléculaire, I take part in the development of the RIces project, whose goal is to identify potential chemical pathways for molecules detected in astrophysical environments. Based on analytical methods such as mid-infrared spectroscopy and low resolution mass spectrometry and isotope labelling, we identify the chemical reactivity that occurs in such environments as well as the physical parameters associated with it (activation energy, desorption energy, kinetic constant ...). In addition, the identification of these pathways allows proposing new molecules to be detected and to constrain the environments in which they should be searched for. Next to my collaborations within the pre-existing experiments related to the RIces project, I am particularly interested in the possibility of forming amino acid precursors, aminonitriles, in the bulk of astrophysical ices. The first aminonitrile that we studied was the aminoacetonitrile (NH₂CH₂CN) that was detected in the gas phase of Sagittarius B2. This molecule leads by hydrolysis to glycine (NH₂CH₂COOH), the simplest amino acid. We have shown that aminoacetonitrile can form with a low efficiency by the VUV irradiation at 20 K of an ice containing ammonia (NH₃) and acetonitrile (CH₃CN). Due to this low efficiency, we searched for an alternative pathway. We focused our investigations on the Strecker reaction. This reaction is one of the main pathways for the formation of amino acids detected in meteorites. However, for meteoritic amino acid, this reaction is supposed to occur in the aqueous phase inside the meteorite parent body. Our goal was to demonstrate that this Strecker reaction would also occur in a condensed phase in the bulk of interstellar and/or cometary icy grains (from 20 K to 300 K and 10^-8 mbar). We demonstrated that NH₂CH₂CN can indeed form into the icy grains following steps that depend on the temperature and on the life time of the object. We have also shown that NH₂CH₂CN formation competes with the hydroxyacetonitrile (HOCH₂CN) formation. However, the energy barrier for the glycine formation from aminoacetonitrile in a water ice is too high for this reaction to proceed. These results show that the precursors of amino acids detected in meteorites may have several origins, which could explain the particular isotope variations observed in meteoritic amino acids. With Aurélien Fresneau (PhD student CNES/Région PACA), I pursue our investigation on the Strecker synthesis by working on alanine and isobutyric acid (AIB) precursors. The latest results obtained with this project have shown the role of water as a catalyst for the HCN addition on CH₂O, which leads to HOCH₂CN formation. By studying the reactivity between acetone and NH₃ for the formation of H₂NC(CH₃)₂CN, we further demonstrated that water can trap reactants at higher temperature than their own desorption, allowing the reaction to occur. All our results suggest that a-hydrogenated amino acid precursors can form on the surface or in the bulk of icy grains, but a,a-dialkylated amino acid precursors cannot since the competitive formation of hydroxynitrile is more efficient than the aminoalcohol formation. In astrophysical ices, the Strecker synthesis may thus only occur for a-aminonitriles.

I am also interested in studying how molecules can serve as precursors to those detected in the gas phase of warmer regions, such as protoplanetary disks. For instance, we found that irradiation of HOCH₂CN leads to ketenimine (HN=C=CH₂), formylcyanide (CHOCN) and cyanogen (C₂N₂), the first two compounds being detected in the gas phase of various objects ¹¹. We were able to propose a scenario for the detection of these molecules, namely HOCH₂CN formation on or inside interstellar icy grains, following by its VUV irradiation around star formation regions leading to photoproduct formation, and their subsequent desorption in the gas phase of warmer regions.

We have to complete our investigations on aminonitrile formation precursors of alanine and AIB. We will also study the decomposition of these compounds under VUV irradiations and characterize the photoproducts formed.

All the reactivity described above is a step by step investigation. Therefore, for the aminoacetonitrile we have demonstrated that each step of the Strecker synthesis occurs. However, we do not show that the aminoacetonitrile can be formed if all reactants are present in the initial ice at 20 K (H2O, HCN, NH₃, CH₂O and HCOOH). This is not trivial since the first step of this synthesis is in competition with the HOCH₂CN formation. Because H₂NCH₂CN and HOCH₂CN have close masses and similar infrared features, it is impossible to determine if one or both of these compounds are formed using only FT-IR and low mass spectrometry analyses, even with isotopes. To obtain information on their formation, we will use the VAHIIA system that we specially developed for the analysis of volatile organic compounds. This system is described in the next section. This project will require time development for analytical protocols providing the characterization with GC-MS of H₂NCH₂CN and HOCH₂CN compounds. The VAHIIA system will be continuously developed for the characterization of molecules formed with the RIces project.