Vassilissa Vinogradoff

Importance of thermal reactivity for hexamethylenetetramine formation from simulated interstellar ices

Recent observations of high ionization rates of molecular hydrogen in diffuse interstellar clouds point to a distinct low-energy cosmic-ray component. Supposing that this component is made of nuclei, two models for the origin of such particles are explored and low-energy cosmic-ray spectra are calculated which, added to the standard cosmic ray spectra, produce the observed ionization rates. The clearest evidence of the presence of such low-energy nuclei between a few MeV per nucleon and several hundred MeV per nucleon in the interstellar medium would be a detection of nuclear \gamma-ray line emission in the range E_ 0.1 - 10 MeV, which is strongly produced in their collisions with the interstellar gas and dust. Using a recent \gamma-ray cross section compilation for nuclear collisions, \gamma-ray line emission spectra are calculated alongside with the high-energy \gamma-ray emission due to {\pi} 0 decay, the latter providing normalization of the absolute fluxes by comparison with Fermi-LAT observations of the diffuse emission above E \gamma = 0.1 GeV. Our predicted fluxes of strong nuclear \gamma-ray lines from the inner Galaxy are well below the detection sensitivies of INTEGRAL, but a detection, especially of the 4.4-MeV line, seems possible with new-generation \gamma-ray telescopes based on available technology. We predict also strong \gamma-ray continuum emission in the 1-8 MeV range, which in a large part of our model space for low-energy cosmic rays exceeds considerably estimated instrument sensitivities of future telescopes.Context. Complex organic molecules are observed in a broad variety of astrophysical objects, but little is known about their formation mechanism. Laboratory simulations on interstellar ice analogues are therefore crucial for understanding the origin of these complex organic molecules. In this context, we focus on the thermal reactivity for the formation of the organic residue obtained after photolysis at 25 K of the interstellar ice analogue (H2O:CH3OH:NH3) warmed to 300 K. Aims: We determine the formation mechanism of one major product detected in the organic residue: hexamethylenetetramine (HMT). We compare the warming of the photolysed interstellar ice analogue with the warming of the two non-photolysed specific ice mixtures H2CO:NH3:HCOOH and CH2NH:HCOOH, which are used as references. Using both general and specific approaches, we show the precise role of the UV photons and the thermal processing in the HMT formation. Methods: We used Fourier transform infrared spectroscopy (FTIR) to monitor the chemical changes induced by the heating of the photolysed ice analogue and characterize some important species that will subsequently evolve in the formation of HMT in the residue. Results: We show that the thermal processes play a key role in the HMT formation in photolysed ice analogues heated at 300 K. We identify the stable intermediates in the HMT formation that are formed during the warming: the aminomethanol (NH2CH2OH) and the protonated ion trimethyletriamine (TMTH+, C3H10N3+). We also identify for the first time a new product in the organic residue, the polymethylenimine PMI (-(CH2 -NH)n). Results from this study will be interesting for the analysis of the forthcoming Rosetta mission.

Acetaldehyde Solid State Reactivity at Low Temperature: Formation of the Acetaldehyde Ammonia Trimer

We focus on low temperature reactivity from 25 to 300 K, in ice containing acetaldehyde, ammonia, and formic acid. We show that the warming of this ice mixture forms the acetaldehyde ammonia trimer (2,4,6-trimethyl-1,3,5-hexahydrotriazine, C(6)H(15)N(3)) after five steps. The reaction is monitored by FTIR spectroscopy and mass spectrometry. We propose a mechanism for its formation that differs from the one proposed in the liquid phase. The reaction intermediates, α-aminoethanol (from 80 K) and ethanimine (formed at 180 K), have been identified by a mechanistic approach: each step of the reaction has been treated separately. The chemical implications and the astrophysical relevance of the study are also discussed.

Aminoacetonitrile characterization in astrophysical-like conditions

Context. 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. Results. We determined that in our experimental conditions, the IR limit of AAN detection in the water ice is about 1 × 10 16 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 kJ mol −1 with ν0 to 10 28 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.

Formaldehyde and methylamine reactivity in interstellar ice analogues as a source of molecular complexity at low temperature

Context. Laboratory simulations on interstellar or cometary ice analogues are crucial to understand the formation of complex organic molecules that are detected in the interstellar medium (ISM). Results from this work give hints on physical and chemical processes occurring in space and can serve as a benchmark for dedicated space missions. Aims. The aim of this work is to consolidate the knowledge of ice evolution during the star formation process by investigating the influence of thermal reactions as a source of molecular complexity in the ISM. In this study, we are interested in the thermal reactivity between two interstellar molecules, formaldehyde (H2CO) and methylamine (CH3NH2) in water ice analogues. Methods. We used Fourier transform infrared spectroscopy, mass spectrometry, and B3LYP calculations to investigate the thermal reaction between formaldehyde and methylamine (14N and 15N) at low temperature in water ice analogues. Results. We demonstrate that methylamine and formaldehyde quickly react in water ice analogues for astronomically relevant temperatures and form N-methylaminomethanol CH3NHCH2OH. The measured activation energy of this reaction, 1.1 ± 0.05 kJ mol−1 (1.8 ± 0.08 kJ mol−1 with methylamine 15N), allows the reaction to proceed in interstellar ices, when the ices are gently warmed, as it occurs in young stellar objects, in photo-dissociation regions, or in comets. Therefore, CH3NHCH2OH is likely to be found in these objects. This hypothesis is confirmed by numerical simulations that clearly show that the formation of N-methylaminomethanol is likely at low temperature. Isotopic experiments as well as photochemical studies have also been performed to better characterise the ice evolution induced by heat and ultraviolet radiation during star formation.

CN radical hydrogenation from solid H2 reactions, an alternative way of HCN formation in the interstellar medium

Context. Molecular hydrogen (H 2) is the most abundant molecule of the interstellar medium (ISM) in gas phase and it has been assumed to exist in solid state or as coating on grains. Aims. Our goal is to show that solid H 2 can act as a hydrogenation agent, reacting with CN radicals to form HCN. Methods. In a H 2 matrix, we studied the hydrogenation of the CN radical generated from the vacuum ultraviolet photolysis (VUV-photolysis) of C 2 N 2 at 3.8 K. We modified the wavelengths and the host gas in order to be sure that CN radicals can abstract H from H 2 molecules. Results. HCN monomers, dimers, and oligomers have been characterised by Fourier transform infrared spectroscopy (FTIR). H 2 CN as well as CN radicals have also been clearly observed during the photolysis performed at 3.8 K. Conclusions. H 2 is a hydrogenation reagent towards CN radicals producing HCN. This type of reaction should be taken into account for the reactivity at low temperature in contaminated H 2 ice macro-particles (CHIMPs), H 2 flakes or in the first sublayers of grains where solid H 2 has accumulated.

Formation of Hexamethylenetetramine - Comment.

In their recent paper BFormation of Hexamethylenetetramine (HMT) from HCHO and NH3 Relevance to Prebiotic Chemistry and B3LYP consideration^ Zeffiro et al. (Orig Life Evol Biosph DOI 10.1007/s11084-015-9479-5) stated: B... a complete theoretical study of the formation of HMT has not received due considerations in the literature with regard to the thermodynamic feasibility of many of the mechanistically proposed intermediates in its formation.^ We wish to point out that in our publication BThe mechanism of hexamethylenetetramine (HMT) formation in the solid state at low temperature^ (Phys Chem Chem Phys, 2012, 14, 12309–12320), experimental and theoretical results were fruitfully combined to elucidate the mechanism of HMT formation, providing both thermodynamic and kinetic energy values. Orig Life Evol Biosph (2017) 47:215 DOI 10.1007/s11084-016-9481-6