Thierry Chiavassa

NH2CH2OH THERMAL FORMATION IN INTERSTELLAR ICES CONTRIBUTION TO THE 5-8 μm REGION TOWARD EMBEDDED PROTOSTARS

Aminomethanol (NH2CH2OH) is formed at low temperature from the purely thermal reaction of NH3 and H2CO in laboratory interstellar ice analogs. We report for the first time its infrared and mass spectra. We study its reaction and desorption kinetics using Fourier transform infrared spectroscopy and mass spectrometry. Its reaction rate is estimated to be k(T) = 0.05 × exp(–4.5(kJ mol-1)/RT) and its desorption energy to be E des = 58 ± 2 kJ mol–1. NH2CH2OH can also contribute to the 5-8 μm region of thermally processed ices encountered in many young stellar objects. Gas phase NH2CH2OH may be present in hot core regions, when the frozen material is desorbed.

Hydrogenation of solid hydrogen cyanide HCN and methanimine CH2NH at low temperature

Context. Hydrogenation reactions dominate grain surface chemistry in dense molecular clouds and lead to the formation of complex saturated molecules in the interstellar medium. Aims. We investigate in the laboratory the hydrogenation reaction network of hydrogen cyanide HCN. Methods. Pure hydrogen cyanide HCN and methanimine CH2NH ices are bombarded at room temperature by H-atoms in an ultra-high vacuum experiment. Warm H-atoms are generated in an H2 plasma source. The ices are monitored with Fourier-transform infrared spectroscopy in reflection absorption mode. The hydrogenation products are detected in the gas phase by mass spectroscopy during temperature-programmed desorption experiments. Results. HCN hydrogenation leads to the formation of methylamine CH3NH2 ,a nd CH 2NH hydrogenation leads to the formation of methylamine CH3NH2, suggesting that CH2NH can be a hydrogenation-intermediate species between HCN and CH3NH2. Conclusions. In cold environments the HCN hydrogenation reaction can produce CH3NH2, which is known to be a glycine precursor, and to destroy solid-state HCN, preventing its observation in molecular clouds ices.

Experimental investigation of aminoacetonitrile formation through the Strecker synthesis in astrophysical-like conditions: reactivity of methanimine (CH2NH), ammonia (NH3), and hydrogen cyanide (HCN)

Astronomy & Astrophysics Experimental investigation of aminoacetonitrile formation through the Strecker synthesis in astrophysical-like conditions: reactivity of methanimine (CH 2 NH), ammonia (NH 3), and hydrogen cyanide (HCN) ABSTRACT Context. 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. Aims. We present the first experimental investigation of the formation of aminoacetonitrile NH 2 CH 2 CN from the thermal processing of ices including methanimine (CH 2 NH), ammonia (NH 3), and hydrogen cyanide (HCN) in interstellar-like conditions without VUV photons or particules. Methods. We use Fourier Transform InfraRed (FTIR) spectroscopy to monitor the ice evolution during its warming. Infrared spec-troscopy and mass spectroscopy are then used to identify the aminoacetonitrile formation. Results. 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.

Methylammonium methylcarbamate thermal formation in interstellar ice analogs: a glycine salt precursor in protostellar environments

Context. Analyses of dust cometary grains collected by the Stardust spacecraft have shown the presence of amines and amino acids molecules, and among them glycine (NH2CH2COOH). We show how the glycine molecule could be produced in the protostellar environments before its introduction into comets. Aims. We study the evolution of the interstellar ice analogues affected by both thermal heating and vacuum ultraviolet (VUV) photons, in addition to the nature of the formed molecules and the confrontation of our experimental results with astronomical observations. Methods. Infrared spectroscopy and mass spectrometry are used to monitor the evolution of the H2O:CO2:CH3NH2 and CO2:CH3NH2 ice mixtures during both warming processes and VUV photolysis. Results. We first show how carbon dioxide (CO2) and methylamine (CH3NH2) thermally react in water-dominated ice to form methylammonium methylcarbamate [CH3NH + ][CH3NHCOO − ] noted C. We then determine the reaction rate and activation energy. We show that C thermal formation can occurs in the 50–70 K temperature range of a protostellar environment. Secondly, we report that a VUV photolysis of a pure C sample produces a glycine salt, methylammonium glycinate [CH3NH + ][NH2CH2COO − ] noted G. We propose a scenario explaining how C and subsequently G can be synthesized in interstellar ices and precometary grains. Conclusions. [CH3NH + ][CH3NHCOO − ] could be readily formed and would act as a glycine salt precursor in protostellar environments dominated by thermal and UV processing. We propose a new pathway leading to a glycine salt, which is consistent with the detection of glycine and methylamine within the returned samples of comet 81P/Wild 2 from the Stardust mission.

Carbamic acid and carbamate formation in NH

Context. We study carbamic acid [ NH 2 COOH] and ammonium carbamate [ NH 2 COO - ] [ NH 4 + ] formation in interstellar ice analogs. Aims. We demonstrate how carbamic acid [ NH 2 COOH] and ammonium carbamate [ NH 2 COO - ] [ NH 4 + ] can be formed from both thermal reactions and energetic photons in an NH 3 :CO 2 ice mixture. Methods. Infrared and mass spectroscopy are used to monitor NH 3 :CO 2 ice mixture evolution during both warming and VUV photon irradiation. Results. Carbamic acid and ammonium carbamate can be produced thermally in a 1:1 ratio from NH 3 and CO 2 above 80 K. They can be also formed in a 28:1 ratio by less efficient processes such as energetic photons. Our study and its results provide fresh insight into carbamic acid formation in interstellar ices. Conclusions. We demonstrate that care is required to separate irradiation-induced reactivity from purely thermal reactivity in ices in which ammonia and carbon dioxide are both present. From an interstellar chemistry point of view, carbamic acid and ammonium carbamate are readily produced from the ice mantle of a typical interstellar grain and should therefore be a detectable species in molecular clouds.

_{\sf 3}

Context. Much of the formaldehyde (H2CO) is formed from the hydrogenation of CO on interstellar dust grains, and is released in the gas phase in hot core regions. Radio-astronomical observations in these regions are directly related to its desorption from grains. Aims. We study experimentally the thermal desorption of H2CO from bare silicate surfaces, from water ice surfaces and from bulk water ice in order to model its desorption from interstellar grains. Methods. Temperature-programmed desorption experiments, monitored by mass spectrometry, and Fourier transform infrared spectroscopy are performed in the laboratory to determine the thermal desorption energies in: (i.) the multilayer regime where H2CO is bound to other H2CO molecules; (ii.) the submonolayer regime where H2CO is bound on top of a water ice surface; (iii.) the mixed submonolayer regime where H2CO is bound to a silicate surface; and (iv.) the multilayer regime in water ice, where H2CO is embedded within a H2O matrix. Results. In the submonolayer regime, we find the zeroth-order desorption kinetic parameters nu(0) = 10(28) mol cm(-2) s(-1) and E = 31.0 +/- 0.9 kJmol(-1) for desorption from an olivine surface. The zeroth-order desorption kinetic parameters are nu(0) = 10(28) mol cm(-2) s(-1) and E = 27.1 +/- 0.5 kJmol(-1) for desorption from a water ice surface in the submonolayer regime. In a H2CO:H2O mixture, the desorption is in competition with the H2CO + H2O reaction, which produces polyoxymethylene, the polymer of H2CO. This polymerization reaction prevents the volcano desorption and co-desorption from happening. Conclusions. H2CO is only desorbed from interstellar ices via a dominant sub-monolayer desorption process (E = 27.1 +/- 0.5 kJmol-1). The H2CO which has not desorbed during this sub-monolayer desorption polymerises upon reaction with H2O, and does not desorb as H2CO at higher temperature.

:CO

Context. Water is the major component of the interstellar ice mantle. In interstellar ice, chemical reactivity is limited by the diffusion of the reacting molecules, which are usually present at abundances of a few percent with respect to water. Aims. We want to study the thermal diffusion of H2CO, NH3, HNCO, and CO in amorphous water ice experimentally to account for the mobility of these molecules in the interstellar grain ice mantle. Methods. In laboratory experiments performed at fixed temperatures, the diffusion of molecules in ice analogues was monitored by Fourier transform infrared spectroscopy. Diffusion coefficients were extracted from isothermal experiments using Ficks second law of diffusion. Results. We measured the surface diffusion coefficients and their dependence with the temperature in porous amorphous ice for HNCO, H2CO, NH3, and CO. They range from 10(-15) to 10(-11) cm(2) s(-1) for HNCO, H2CO, and NH3 between 110 K and 140 K, and between 5-8 x 10(-13) cm(2) s(-1) for CO between 35 K and 40 K. The bulk diffusion coefficients in compact amorphous ice are too low to be measured by our technique and a 10(-15) cm(2) s(-1) upper limit can be estimated. The amorphous ice framework reorganization at low temperature is also put in evidence. Conclusions. Surface diffusion of molecular species in amorphous ice can be experimentally measured, while their bulk diffusion may be slower than the ice mantle desorption kinetics.

_{\sf 2}

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 in molecular 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.

ices – UV irradiation versus thermal processes

Context. 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. Aims. The purpose of this work is to present the first experimental approach for the formation of acetonitrile and amino acetonitrile in interstellar-like conditions. Methods. We use Fourier Transform InfraRed (FTIR) spectroscopy and mass spectrometry to study the formation at 20 K of ace-tonitrile CH 3 CN from VUV irradiation of ethylamine and of amino acetonitrile NH 2 CH 2 CN from VUV irradiation of ammonia: acetonitrile mixture. Isotopic substitutions are used to confirm identifications. Results. 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 for-mation. These results suggest that acetonitrile and amino acetonitrile can be formed in astrophysical environments that are submitted to VUV irradiations.

The desorption of H2CO from interstellar grains analogues

Abstract The structure and energy of the 1:1 complexes between isocyanic acid (HNCO) and H 2 O are investigated using FTIR matrix isolation spectroscopy and quantum calculations at the MP2/6-31G(d,p) level. Calculations yield two stable complexes. The first and most stable one (Δ E =23.3 kJ/mol) corresponds a form which involves a hydrogen bond between the acid hydrogen of HNCO and the oxygen of water. The second form involves a hydrogen bond between the terminal oxygen of HNCO and hydrogen of water. In an argon matrix at 10 K, only the first form is observed. Adsorption on amorphous ice water at 10 K shows the formation of only one adsorption site between HNCO and ice. It is comparable to the complex observed in matrix and involves an interaction with the dangling oxygen site of ice. Modeling using computer code indicates the formation of polymeric structure on ice surface. Warming of HNCO, adsorbed on H 2 O ice film or co-deposited with H 2 O samples above 110 K, induces the formation of isocyanate ion (OCN − ) characterized by its ν as NCO infrared absorption band near 2170 cm −1 . OCN − can be produced by purely solvation-induced HNCO dissociative ionization. The transition state of this process is calculated 42 kJ/mol above the initial state, using the ONIOM model in B3LYP/6-31g(d,p).