Results

Role of Water in the reactivity occurring in astrophysical ices

Water is the most abundant compound in interstellar and cometary ices. Laboratory experiments on ice analogues have shown that water has a great influence on the chemical activity of these ices. In this study, we investigated the reactivity of acetone–ammonia ices, showingthat water is an essential component in chemical reactions with high activation energies. In a water-free binary ice, acetone and ammonia do not react due to high activation energy, as the reactants desorb before reacting (at 120 and 140 K, respectively). By contrast, our study shows that under experimental conditions of ∼150 K, this reaction does occur in the presence of water. Here, water traps reactants in the solid phase above their desorption temperatures, allowing them to gather thermal energy as the reaction proceeds. Using infrared spectroscopy and mass spectrometry associated with isotopic labelling, as well as quantum chemical calculations, 2-aminopropan-2-ol (2HN-C(CH3)2-OH) was identified as the acetone–ammonia reaction product. The formation of this product may represent the first step towards formation of 2-aminoisobutyric acid (AIB) in the Strecker synthesis. The activation energy for the formation of 2-aminopropan-2-ol was measured to be 42 ± 3 kJ mol−1, while its desorption energy equalled 61.3 ± 0.1 kJ mol−1. Our work demonstrates that astrophysical water, rather than being a non-thermally reactive species, is crucial for the evolution of complex chemistry occurring in the Universe.

Strecker synthesis in astrophysical ices

Studing chemical reactivity in astrophysical environments is an important means for improving our understanding of the origin of the organic matter in molecular clouds, in protoplanetary disks, and possibly, as a final destination, in our solar system. Laboratory simulations of the reactivity of ice analogs provide important insight into the reactivity in these environments. Here, we use these experimental simulations to investigate the Strecker synthesis leading to the formation of aminoacetonitrile in astrophysical like conditions. The aminoacetonitrile is an interesting compound because it was detected in SgrB2, hence could be a precursor of the smallest amino acid molecule, glycine, in astrophysical environments. We present the first experimental investigation of the formation of aminoacetonitrile NH2CH2CN from the thermal processing of ices including methanimine (CH2NH), ammonia (NH3), and hydrogen cyanide (HCN) in interstellar-like conditions without VUV photons or particules. We use Fourier Transform InfraRed (FTIR) spectroscopy to monitor the ice evolution during its warming. Infrared spectroscopy and mass spectroscopy are then used to identify the aminoacetonitrile formation. We demonstrate that methanimine can react with −CN during the warming of ice analogs containing at 20 K methanimine, ammonia, and [NH+4−CN] salt. During the ice warming, this reaction leads to the formation of poly(methylene-imine) polymers. The polymer length depend on the initial ratio of mass contained in methanimine to that in the [NH+4−CN] salt. In a methanimine excess, long polymers are formed. As the methanimine is progressively diluted in the [NH+4−CN] salt, the polymer length decreases until the aminoacetonitrile formation at 135 K. Therefore, these results demonstrate that aminoacetonitrile can be formed through the second step of the Strecker synthesis in astrophysical-like conditions.

Aminoacetonitrile (AAN) has been detected in 2008 in the hot core SgrB2. This molecule is of particular interest because it is a central molecule in the Strecker synthesis of amino acids. This molecule can be formed from methanimine (CH2NH), ammonia (NH3) and hydrogen cyanide (HCN) in astrophysical icy conditions. Nevertheless, few studies exist about its infrared (IR) identification or its astrophysical characterization. Aims. We present in this study a characterization of the pure solid AAN and when it is diluted in water to study the influence of H2O on the main IR features of AAN. The reactivity with CO2 and its photoreactivity are also studied and the main products were characterized. Methods. Fourier transformed infrared (FTIR) spectroscopy of AAN molecular ice was performed in the 10–300 K temperature range. We used temperature-programmed desorption coupled with mass spectrometry detection techniques to evaluate the desorption energy value. The influence of water was studied by quantitative FTIR spectroscopy and the main reaction and photochemical products were identified by FTIR spectroscopy. We determined that in our experimental conditions, the IR limit of AAN detection in the water ice is about 1 × 1016 molecule cm−2, which means that the AAN detection is almost impossible within the icy mantle of interstellar grains. The desorption energy of pure solid AAN is of 63.7 kJmol−1 with ν0 to 1028 molecule cm−2 s−1, which implies that the presence of this molecule in the gas phase is only possible in hot cores. The glycine (Gly) formation from the AAN through the last step of the Strecker synthesis seems to be impossible in astrophysical-like conditions. Furthermore, AAN is photoresistant to vacuum ultra-violet radiation, which emphasizes the fact that AAN can be considered as a Gly reservoir molecule.

This contribution is focused on the concurrent pathway to the Strecker synthesis of amino acids in an astrophysicallike environment. We indeed use experimental and modeling simulations to investigate the possibility to form the aminomethanol (HOCH2NH2) in concurrence with the hydroxyacetonitrile (HOCH2CN) from ices containing at 40 K formaldehyde (CH2O), ammonia (NH3), and cyanide ion (CN−).We demonstrate using infrared spectroscopy and mass spectrometry that the formation of the aminomethanol (Ea = 4.5 kJ mol−1) is competing with the hydroxyacetonitrile formation (Ea = 3.9 kJ mol−1). The ratio between aminomethanol and hydroxyacetonitrile depends on the initial ratio in the ice between ammonia and cyanide. An increase of cyanide ion provides a decrease in aminomethanol formation. Since the aminomethanol is the first step through the formation of glycine in astrophysical environments, these data are important for understanding the possibility of forming glycine in such environments. Furthermore, using a reduced kinetic model, we evaluate the astrophysical environments in which the aminomethanol and hydroxyacetonitrile can be formed and evolved without degradation. The results suggest that these two molecules could be formed inmolecular clouds or protostellar disks, and subsequently incorporated inside comets or asteroids. Therefore, hydroxyacetonitrile and aminomethanol could be formed before the formation of the solar system, which suggests that hydroxyacids and amino acids, such as those detected inside meteorites, have been formed in various astrophysical environments.

The reactivity in astrophysical environments can be investigated in the laboratory through experimental simulations, which provide understanding of the formation of specific molecules detected in the solid phase or in the gas phase of these environments. In this context, the most complex molecules are generally suggested to form at the surface of interstellar grains and to be released into the gas phase through thermal or non-thermal desorption, where they can be detected through rotational spectroscopy. Here, we focus our experiments on the photochemistry of hydroxyacetonitrile (HOCH2CN), whose formation has been shown to compete with aminomethanol (NH2CH2OH), a glycine precursor, through the Strecker synthesis. We present the first experimental investigation of the ultraviolet (UV) photochemistry of hydroxyacetonitrile (HOCH2CN) as a pure solid or diluted in water ice. We used Fourier transform infrared (FT-IR) spectroscopy to characterize photoproducts of hydroxyacetonitrile (HOCH2CN) and to determine the di
erent photodegradation pathways of this compound. To improve the photoproduct identifications, irradiations of hydroxyacetonitrile 14N and 15N isotopologues were performed, coupled with theoretical calculations. We demonstrate that the photochemistry of pure hydroxyacetonitrile (HOCH2CN) under the influence of UV photons, or diluted in water ice, leads to the formation of formylcyanide (CHOCN), ketenimine (CH2CNH), formaldehyde (CH2O), hydrogen cyanide (HCN), carbon monoxyde (CO), and carbon dioxyde (CO2); the presence of water increases its photodegradation rate. Furthermore, because hydroxyacetonitrile is more highly refractory than water, our results suggest that in astrophysical environments, hydroxyacetonitrile can be formed on icy grains from formaldehyde and hydrogen cyanide, and can be subsequently photodegradated in the water ice, or irradiated as a pure solid at the surface of dry grains after water desorption. As some of the hydroxyacetonitrile photochemistry products are detected in protostellar cores (e.g. formylcyanide or ketenimine), this compound maybe considered as one of the possible sources of these molecules at the grain surface in fairly cold regions. These photoproducts can then be released in the gas phase in a warmer region.

The understanding of compound formation in laboratory simulated astrophysical environments is an important challenge in obtaining information on the chemistry occurring in these environments. We here investigate by means of both laboratory experiments and quantum chemical calculations the ice-based reactivity of acetaldehyde (CH3CHO) with ammonia (NH3) and hydrogen cyanide (HCN) in excess of water (H2O) promoted by temperature. A priori, this study should give information on alanine (2HN–CH(CH3)–COOH) formation (the simplest chiral amino acid detected in meteorites), since these reactions concern the first steps of its formation through the Strecker synthesis. However, infrared spectroscopy, mass spectrometry with HC14N or HC15N isotopologues and B3LYP-D3 results converge to indicate that an H2O-dominated ice containing CH3CHO, NH3 and HCN not only leads to the formation of α-aminoethanol (2HN–CH(CH3)–OH, the product compound of the first step of the Strecker mechanism) and its related polymers (2HN–(CH(CH3)–O)n–H) due to reaction between CH3CHO and NH3, but also to the 2-hydroxypropionitrile (HO–CH(CH3)–CN) and its related polymers (H–(O–CH(CH3))n–CN) from direct reaction between CH3CHO and HCN. The ratio between these two species depends on the initial NH3/HCN ratio in the ice. Formation of α-aminoethanol is favoured when the NH3 concentration is larger than HCN. Wealso show that the presence of water is essential for the formation of HO–CH(CH3)–CN, contrarily to 2HN–CH(CH3)–OH whose formation also takes place in absence of H2O ice. As in astrophysical ices NH3 is more abundant than HCN, formation of α-aminoethanol should consequently be favoured compared to 2-hydroxypropionitrile, thus pointing out α-aminoethanol as a plausible intermediate species for alanine synthesis through the Strecker mechanism in astrophysical ices.

Reactivity in astrophysical environments is still poorly understood. In this contribution, we investigate the thermal reactivity of interstellar ice analogs containing acetone ((CH3)2CO), ammonia (NH3), hydrogen cyanide (HCN) and water (H2O) by means of infrared spectroscopy and mass spectrometry techniques, complemented by quantum chemical calculations. We show that no reaction occurs in H2O:HCN:(CH3)2CO ices. Nevertheless, HCN does indeed react with acetone once activated by NH3 into CN− to form 2-hydroxy-2-methylpropanenitrile (HO–C(CH3)2–CN), with a calculated activation energy associated with the rate determining step of about 51 kJ mol−1. This reaction inhibits the formation of 2-aminopropan-2-ol (HO–C(CH3)2–NH2) fromacetone and NH3, even in the presence ofwater, which is the first step of the Strecker synthesis to form 2-aminoisobutyric acid (NH2C(CH3)2COOH). However, HO–C(CH3)2–CN formation could be part of an alternative chemical pathway leading to 2-hydroxy-2-methyl-propanoic acid (HOC(CH3)2COOH), which could explain the presence of hydroxy acids in some meteorites.

Polyoxymethylene formation in astrophysical ices

Studying chemical reactivity is an important way to improve our understanding of the origin of organic matter in astrophysical environments such as molecular clouds, protoplanetary disks, and possibly, as a final destination, in our solar system bodies such as in comets. Laboratory simulations on the reactivity of ice analogs can provide important insights into this complex reactivity. Here, the role of water as a catalytic agent is investigated under the conditions of simulated interstellar and cometary grains in the formation of complex organic molecules: the hydroxyacetonitrile (HOCH2CN) and formaldehyde polymers (polyoxymethylene POM). Using infrared spectroscopy and mass spectrometry, we show that HCN reacts with CH2O only in the presence of H2O, whereas in the absence of H2O, HCN is not sufficiently reactive to promote this reaction. Furthermore, depending on the dilution of CH2O and HCN in the water matrix, 1-cyanopolyoxymethylene polymers can also be formed (H–(O–CH2)n–CN, POM–CN), as confirmed by mass spectrometry using the HC15N isotopologue. Moreover, quantum chemical calculations allowed us to suggest mechanistic proposals for these reactions, the first step being the activation of HCN by water forming H3O+ and CN, which subsequently condense on a neighbouring CH2O promoting the formation of OCH2CN. Once OCH2CN is formed, it can either recover a proton by reacting with H3O+ or condense on CH2O molecules leading to POM–CN structures. Implications of this work for the forthcoming Rosetta mission are also addressed.

Aminoacetonitrile formation from the VUV irradiation of acetonitrile and NH3 ices

The study of the chemical reactivity in interstellar ices in astrophysical environments is an important tool for understanding the origin of the organic matter in molecular clouds, in protoplanetary disks, and possibly, as a final destination, in our solar system. The laboratory simulations of the reactivity in ice analogs provide important information for understanding the reactivity in these environments. Here, we used these experimental simulations to trace some formation pathways of two nitriles, acetonitrile and amino acetonitrile, which are two potential precursors of amino acids in astrophysical environments. The purpose of this work is to present the first experimental approach for the formation of acetonitrile and amino acetonitrile in interstellar-like conditions. We use Fourier Transform InfraRed (FTIR) spectroscopy and mass spectrometry to study the formation at 20 K of acetonitrile CH3CN from VUV irradiation of ethylamine and of amino acetonitrile NH2CH2CN from VUV irradiation of ammonia: acetonitrile mixture. Isotopic substitutions are used to confirm identifications. We demonstrate that acetonitrile can be formed at 20 K from the VUV irradiation of ethylamine with a yield of 4%. Furthermore, in presence of ammonia, at 20 K and under VUV irradiation, the acetonitrile can lead to the amino acetonitrile formation. These results suggest that acetonitrile and amino acetonitrile can be formed in astrophysical environments that are submitted to VUV irradiations.