What Are the Intermediates That Could React in the Interstellar Ices?

Special Issue Review

Received: 3 August 2014, Revised: 5 September 2014, Accepted: 8 September 2014, Published online in Wiley Online Library: 28 November 2014

(wileyonlinelibrary.com) DOI: 10.1002/poc.3380 What are the intermediates that could react in the interstellar ices?‡ Fabien Borgeta*, Fabrice Duvernaya, Grégoire Dangera, Patrice Theuléa, Jean-Baptiste Bossaa, Vassilissa Vinogradoffa†, Florent Mispelaera, Sandra Müllerb, Dirk Groteb and Thierry Chiavassaa

Interstellar chemical reactivity forms complex organic molecules implying different intermediates. In this mini review, reactions arising in/on interstellar ices are presented. These ices are a place where molecules are formed and where they can evolve under thermal and nonthermal effects. We present in this paper different intermediates that have been detected in experimental simulations of interstellar ices. Ionic, neutral, and radical intermediates have been characterized by different means allowing the building of an ice chemical network explaining their formation. Copyright © 2014 John Wiley & Sons, Ltd.

Keywords: intermediates; interstellar chemistry; ISRIUM; water ice reactivity

INTRODUCTION to finish with the more recent studies using radicals. We can propose to astronomers different target molecules that could be detected in Actually, around 180 molecules have been detected so far in the the ISM and original processes to understand the production and interstellar medium (ISM) (see, for instance, the Cologne data- the synthesis in the ISM of different molecules already detected. base).[1] Most of those molecules can be considered as complex organic molecules (COMs) following Herbst’sdefinition, which [2] define a COM as a molecule with more than six atoms. To ex- EXPERIMENTAL plain the diversity and the complexity of these molecules, the [3,4] solid phase of the ISM is thought to play a crucial role. This The composition of the interstellar ices is known from infrared (IR) obser- solid phase represents less than 1% of the global mass of a dark vations that can be realized on Earth by telescope or by satellites such as cloud. This solid phase is mainly composed of dust grains, which IRAS, ISO, and SPITZER. Using a star as a source of light, it is possible to are chemically very important because it is the place where mol- analyse the IR absorption and then obtain information about the ice [8] ecules can be formed, diffuse, react, and be released in the gas compositions. From these observations, we know that ices are mainly phase, where they can be detected by radioastronomy. at 10 K, and depending of the proximity and on the evolution stage of This dust is composed of a silicate core embedded in an ice layer the protostar, the ices can be warmed up. essentially composed of water with traces of other molecules such With these initial conditions, we can simulate the ice thermal processing, [5] depositing at low temperature a gas mixture in the cosmic ratio under a low as CH OH, CO, CO ,NH,andHCO. Depending on the proximity 7 3 2 3 2 pressure, typically 10 mbar that will directly condense as a molecular solid. of a star, the grains can chemically evolve because of thermal Then, by increasing the temperature, we can simulate thermal effects and or photochemical effects. Indeed, the range of temperature is obtain information on the reactivity occurring in the formed ice. We simulate between few kelvins (K) till a few hundred K. Radiations are mainly also the VUV light from the protostar using a H2 flow lamp. The typical flux of composed of a vacuum ultraviolet (VUV) field including the in- tense Lyman-α band of H at 121.6 nm. Interstellar radiation field photolyzes all the molecules producing reactive radicals; then, an * Correspondence to: F. Borget, Laboratoire de Physique des Interactions Ioniques et fi [6] Moléculaires, Aix-Marseille Université et CNRS, UMR 7345, Centre de Saint- ef cient chemistry begins producing more complex molecules. Jérôme, case 252, Avenue Escadrille Normandie-Niemen, 13397 Marseille, France. In order to understand the different basic processes that lead E-mail: [email protected] to the increase of molecular complexity in the interstellar ices, it † is important to disentangle the role of the thermal activation Current address: Institut de Mineralogie, Physique des Materiaux et Cosmochimie Museum National d'Histoire Naturelle, UMR CNRS 759, 75231, Paris, France from the role of the photochemical activation.[7] To investigate these types of chemistry in the laboratory, we ‡ This article is published in Journal of Physical Organic Chemistry as a special experimentally simulate the interstellar ices using model chemi- issue on the International Symposium on Reactive Intermediates and Unusual cal system that take into account few compounds allowing to Molecules 2014 on Physical Organic Chemistry by Robert Moss (Rutgers study thermal reactions and/or photochemistry. University USA) and Anna Gudmundsdottir (University of Cinncinnati, USA). After a description of the experimental simulations used in the a F. Borget, F. Duvernay, G. Danger, P. Theulé,J.-B.Bossa,V.Vinogradoff, laboratory to study the ice evolution, we propose a view of the differ- F. Mispelaer ent reactions by type of intermediates that have been observed and Laboratoire de Physique des Interactions Ioniques et Moléculaires, Aix- studied directly in this interstellar framework. In consequence, in a Marseille Université et CNRS, 13397, Marseille, France fi 163 rst part, different reactions implying ionic intermediates will be in- b S.Müller,D.Grote troduced followed by reactions implying neutral intermediates and Lehrstuhl für Organische Chemie II, Ruhr-Universität, 44801, Bochum, Germany

Biography Biography

Fabien Borget studied Chemistry in Vassilissa Vinogradoff studied organic Cergy-Pontoise and Marseille, France. chemistry at the University of Lyon. He defended his PhD at the University She received her PhD in 2013 at the of Provence in 2000 under the supervi- University of Aix-Marseille, under the sion of Prof. Jean-Pierre Aycard. Then supervision of Prof. Thierry Chiavassa he joined Prof. Curt Wentrup in Australia and Dr Fabrice Duvernay on the forma- at the University of Queensland, where tion of complex organic molecules in he worked on iminopropadienone reac- the interstellar ices, focusing on the tivity. In 2002, he became an Assistant impact of thermal effect leading to com- Professor at the University of Provence, plex reactions. She is now doing a post- which became Aix-Marseille University doc at the Musuém National d’Histoire in 2012. His research ﬁeld concerns the reactivity of interstellar Naturelle with Laurent Remusat. Her research interests are ices studied experimentally by laboratory simulation implying now in the meteoritic context, aiming to understand the diver- thermal and photochemical effects. sity of organic matter observed in carboneous chondrites.

Biography Biography

Fabrice Duvernay studied chemistry in Sandra Müller studied chemistry at the Marseille, France. He defended his PhD University of Cologne, Germany. She at the University of Provence in 2005 received her PhD degree in 2014 at the under the supervision of Prof. Thierry Ruhr-University Bochum under the super- Chiavassa. Then he joined Prof. David vision of Wolfram Sander. Birney at Texas Tech University, where he worked on infrared multiphoton Biography chemistry. In 2007, he became an Assistant Professor at the University of Dirk Grote studied Chemistry at the Provence, which became Aix-Marseille University of Bonn, Germany. After he had University in 2012. His research field received his PhD degree in 2007 at the Ruhr concerns the formation of complex organic molecules in University in Bochum, Germany, under the interstellar ices. supervision of Wolfram Sander, he obtained a permanent research position. His research fi Biography interests are in the eld of physical organic chemistry: matrix isolation and spectro- Grégoire Danger obtained a PhD in scopic characterization of organic high-spin ‘Chemistry of biomolecules: synthesis, systems, specializing in matrix electron structure and reactivity’ at Montpellier paramagnetic resonance spectroscopy. 2 University, France, in 2006. Thereafter, he undertook three post-doctorate Biography positions: the first at the ‘NIST’ at – Gaithersburg, MD, USA (2006 2007), Thierry Chiavassa studied chemistry at the followed with a second one from the University of Provence, in Marseille, France, ‘ ’ ’ Institut d Astrophysique Spatiale at and received his PhD degree in 1992. He – Orsay, France (2007 2008), and a third obtained a permanent research position in – one in Montpellier, France (2008 2009). 1993 at the same university and then be- Since 2009, he has had a lecturer position at the Aix-Marseille came a Professor in 2006, in the PIIM labora- fi University. His main elds of interest comprise chemistry in tory (Aix Marseille University). His research prebiotic environments and astrochemistry. interests are in the field of astrochemistry with studies related to the chemical reactivity of molecules in interstellar and cometary ices. Biography

Patrice Theulé studied Physics in Greno- Biography ble, France. He obtained his PhD in 2003 attheEcolePolytechniqueFédéralede Jean-Baptiste Bossa studied Chemistry at Lausanne (Switzerland) under the supervi- Aix-Marseille University in France. He received sion of Prof. T. Rizzo. Then, he joined Prof. his PhD in Chemistry from Aix-Marseille Uni- P. Thaddeus at the Harvard-Smithsonian versity in 2010 under the supervision of Prof Center for Astrophysics, Boston, MA, USA, Thierry Chiavassa. He is currently the principal wherehestudiedcarbonchainsand investigator of a research project founded by carbon rings in an astrophysical context. the Marie Skłodowska-Curie actions and In2005,hebecameanAssistantProfessor conducted at both Leiden University (the at Aix-Marseille University where he works Netherlands) and the University of Cologne on the solid-state astrochemistry topics investigating the reac- 164 (Germany). His research interests are in the tivity and the dynamics of interstellar ice analogues. ﬁeld of laboratory astrophysics.

these carbamates in these conditions.[17] From the methylammonium + methycarbamate CH3NH3 CH3NHCOO irradiation, glycinate has been detected by direct compari- son with experimental deposition of + CH3NH3 NH2CH2COO . This com- pound is the result of an isomeriza- tion of the formed carbamate through a dissociation recombination Figure 1. Evolution of the grain in interstellar medium, concepts for the laboratory simulations process (Fig. 2). The global scheme of this reaction 14 2 this type of lamp is 2.10 photons/cm /s, compared with the average ISM’s implies that carbamate is an intermediate in this original pathway fl 3 2 ux in dense molecular cloud of 10 photon/cm /s. Then, 1 h of laboratory to form an amino acid salt. irradiation corresponds to 106 yearsofirradiationintheISM(Fig.1). Our experimental setup is made of a cryostat maintained at 10 7 mbar, where a gold-platted copper surface is in contact with a cold Neutral intermediates head of a cryogenerator (ARS cryo DE-204 SB Advanced Research System Macungie PA, USA) reaching a temperature as cold as 7 K. With a resis- The description of global pathways implies also that neutral mol- tance heater, and using a Lakeshore temperature regulator (model ecules can be also considered as intermediates. In this para- 331), temperatures between 10 and 300 K can be reached, and we can graph, we present different neutral molecules that could act as also simulate an annealing using temperature ramps of few K/min. intermediates in different pathways. Because our principal diagnosis is the IR spectroscopy, with this setup, we can compare directly our experimental results with the satellites’ observations.[9] Furthermore, a quadrupole mass spectrometer is also Hydroxyacetonitrile connected to our cryostat allowing a mass analysis of compounds that + Mixing HCN and NH3 forms the salt NH4 CN ;wedemonstratedthat desorb during the ice warming and can help to the identification of hydroxyacetonitrile (HOCH2CN) could be formed when this mixture the new-formed species. [18] is deposited with formaldehyde (CH2O) andthenwarmedup. This reaction competes with the formation of aminomethanol, DIFFERENT TYPES OF INTERMEDIATES whichisanaminoacidprecursorthrough the Strecker synthesis.[19] We irradiated with VUV light the hydroxyacetonitrile and ob- Amongst the 180 molecules already detected or strongly suspected [20] served different photoprocesses. Three photodegradation in ISM, most of them are organic molecules. Actually, the most pathways were observed (Fig. 3). The first one corresponds to a recent models of the ISM chemistry[6] show that the influence of photolysis of the molecule giving back the starting molecules, the grain surface is fundamental to understand the diversity and formaldehyde (CH O) and HCN. The second one corresponds to the complexity of the detected molecules. Then, it is necessary to 2 a dehydrogenation yielding formylcyanide (CHOCN) as a first use simple system to determine different chemical reaction product. If the irradiation is prolonged, CO and HCN are formed constants such as kinetic constants. A mechanistic approach allows as secondary products. And the third pathway corresponds to a precise description of this ISM reactivity including the observa- the formation of a ketenimine (H C¼C¼NH). tion of different types of intermediates. 2 Hydroxyacetonitrile is then a central molecule in the ISM conditions playing an intermediate role. Ionic intermediates + [10] Ions are detected in ISM as stable species as H2NCO , + [11,12] + [13] [14] Intermediates generated during hydrogenation process NH4, or OH , for instance, for cations or CN for an- ions. In solid phase, ions can be also observed, and depending One of the most important classes of reactions happening in on the conditions, some of those ions can be also considered the ISM is the hydrogenation process. H atom bombardment pro- as intermediates in global pathways studied in a laboratory. vokes the hydrogenation of unsaturated molecules. CO hydrogena- [21–23] Amines are well known to react with carbon dioxide (CO2) tion formed H CO and CH OH. Hydrogenation of CH CHO 2 3 3 – – [24] to formed carbamates (R NH COO ). We demonstrated that yields saturated alcohol C2H5OH, CH3OH and CH4 and H2CO. this reaction could happen in ISM simulations at low tempera- The same experiments were also made on O2 and O3, and water [25–29] tures. NH3 or CH3NH2 have been deposited at 10 K with CO2 was produced. Recently, we investigated the H atom bom- in different experiments; no reaction was observed at this bardment on HCN molecules. We showed that methylamine temperature. As soon as 40 K is reached, new IR bands are (CH NH ) is produced.[30] 3 2 growing on the spectra. Carbamates NH2COO and CH3NHCOO The natural intuitive intermediate for this last reaction is obviously fi [15,16] were clearly identi ed as the products and the correspond- the corresponding imine CH2NH. But from HCN hydrogenation, we + + ing ammonium counter ions NH4 and CH3NH3, respectively. were not able to observe, by mass spectrometry, this intermediate In ISM, ices are irradiated; we studied the VUV irradiation of between HCN and CH3NH2. We synthesized CH2NHandusedthe 165

[7,15–17] Figure 2. Scheme showing the formation from methylamine and CO2 reaction of carbamate as a photochemical intermediate

important implying the presence of polymers of high molecular weight.[32] When the imine is the minor reactant with the salt, we observed the thermal formation of the aminoacetonitrile (NH2CH2CN) from 135 K. Aminoacetonitrile was detected in 2008[33] and was characterized in ISM conditions.[34] Aminoacetonitrile is a glycine precursor through the Strecker’s synthesis. We demonstrated Figure 3. Scheme showing the formation of hydroxyacetonitrile as a photochemical intermediate[18] that this step of this synthesis could be carried out in ISM conditions. Neverthe- less, the further step that leads to the formation of glycine by aminoacetonitrile hydration cannot be achieved in the ISM conditions because

[30] of the necessary temperature that can- Figure 4. Scheme showing methanimine as intermediate in the HCN hydrogenation process not be reached before the aminoacetonitrile sublimation.

same protocol to hydrogenate this imine. We detected, as a final Imines observed under vacuum ultraviolet processes product, CH3NH2 following the reaction described in Fig. 4. The imine can be considered here as a neutral intermediate. In The stardust mission results, which confirmed the secure pres- ISM condition, the imine hydrogenation kinetic is faster than ence of methylamine (CH3NH2) and glycine and the suspected [35] the HCN hydrogenation and that is the reason why it could presence of ethylamine (C2H5NH2) in the comet 81P/Wild 2, not be observed directly from the HCN reaction in our experi- give an observational framework to our studies in the ices. This mental conditions. mission collected cometary grains in aerogels and sent them back on Earth for high accuracy analyses. The cometary ices are considered as a memory of the initial nebula compositions Imines reactivity in interstellar medium ices because these grains are the results of the accretion of the orig- We investigated different reactions implying imines in ISM inal solid phase of our solar system, when this latter was in simulations. Methanimine has been detected in gas phase of formation. the ISM in 1973.[31] We irradiated successively methylamine[36] and We focused on the reactivity of the imine with a mixture of ethylamine[37] deposited as ices. In both cases, the corre- [19] fi fi HCN and NH3 deposited as a neat solid (Fig. 5). In this case, sponding imine was detected as a rst product identi ed by + HCN is in a basic environment and forms the salt CN NH4. IR spectroscopy and mass spectrometry (Fig. 6). Indeed, When the imine is deposited in excess compared with the salt, CH3CH¼NH was characterized in IR by the νC¼N mode at 1 1 we observed the polymerization of the imine forming 1652 cm ,andCH2¼NH at 1635 cm . This observation polymethylenimine that can be kept on the surface as a residue shows that one of the irradiation products is a H2 loss. Con- at high temperature (260 K), when all the volatile compounds tinuing the irradiation, nitriles were formed and clearly have already desorbed. This refractory layer’s formation is of observed with the elimination of a second H2 molecule. great significance for the ISM because the COMs included in this Under VUV, ethanimine yields acetonitrile and ketenimine. residue can survive at high temperature and be incorporated in Concerning the methanimine, HCN was observed. new objects such as asteroids. The analyses of the ISM ice’s Amines photolysis produced imines that can be again consid- simulation residue show that the molecular diversity is really ered as intermediate for photochemical reactions towards nitriles through successive H2 loss.

Radicals Characterization using scavengers During the photoreaction of dehydra- tion, it is clear that H radicals are produced by a homolytic breakage of a bond. The two radicals that are formed have a very short lifetime, and it is almost impossible to observe them as intermediate. Nevertheless, we develop a strategy in order to be able to observe indi- 166 rectly radicals using scavengers. In Figure 5. Methanimine as an intermediate of thermal reaction the methylamine photolysis study[36]

producing H2O in ISM conditions. These experiments show that radicals are important and need to be characterized in ISM conditions. These intermediates are reactive enough to react with H2 to form hydrogenated com- Figure 6. Ethanimine as a photochemical intermediate[19] pounds or can react with other radicals or neutral molecules and going towards the molecular complexity. This sort of reaction shows also a way to form HCN in ISM. This approach is really important because the different pathways to explain the formation of simple molecules can be implemented in different chemical model of ISM on the basis of a large network of reactions.

CONCLUSIONS These few examples of possible reactions happening in the ices of the ISM showed that a mechanistic approach could help to understand the chemical pathways occurring in this medium. In consequence, it is possible to propose target molecules to the astronomers for detection, and it is

[36] possible to obtain precise data to imple- Figure 7. CO and CO2 as H radical scavenger ment the chemical models of the ISM. The intermediates involved in the ISM previously described, we used CO and CO2 as H radical trap- chemistry are very diverse and show that despite of the harsh con- ping agent forming HCO and HOCO radicals that are observ- ditions of this medium a rich chemistry occurs. The molecular evo- 1 able in ices with an IR band at 1836 and 1850 cm , lution goes towards more complex molecules showing that most respectively. We studied the full photoreactivity of methyl- of the elementary molecules, such as the formation of Glycine, aminefragmentswiththesescavengersandwereabletopro- could be achieved in the solid phase of ISM, asteroids, or comets. pose a precise pathway including the reaction with HCO and But this rich chemistry is still not very well understood. And the HOCO radicals (Fig. 7). characterization of intermediates in ISM conditions seems very This experiment shows how the radicals can be important in the necessary. Actually, we are able to propose a mechanistic approach ice’s photolytic processes. Moreover, just using a scavenger, we to explain the formation of different ISM molecules, but we believe can see that the molecular diversity increases forming molecules that other intermediates are probably present in ISM allowing a that have been detected in ISM. That is the case of acetaldehyde largely richest chemistry. As we have seen, radicals are really detected in 1974,[38] the acetic acid in 1997,[39] and the formamide important, but much work is still necessary to understand their in 1971.[40] According to the fact that the scavengers we used behaviour under the ISM conditions. To achieve that, more corresponds to abundant molecules in ISM in gaseous and in experiments allowing trapping and characterization of such inter- solid phase, this sort of pathways are totally reliable to explain mediates are necessary using techniques that have been not yet the formation of such molecules. used in such studies. By example, ESR study could help to understand the ISM reactivity characterizing precisely radicals. Moreover, an opening towards new intermediates seems also necessary. Reaction with H 2 Indeed, the global diversity of the detected molecules and the Recently, we irradiated cyanogen (C2N2) at different wave- diversity of the conditions raises the possibility that carbenes or length in the VUV and UV.[41] The corresponding isonitrile was nitrenescouldbealsopossibleintermediates of ISM reactions. detected and the CN radical characterized by a band at Carbenes and nitrenes have been already observed in the gaseous 2050 cm1 in Ar matrix. We realized the same experiment in phase of ISM[47–49] but needs to be characterized in interstellar H2 matrix.InISM,H2 is the most abundant molecule, and it chemical pathways showing probably that those intermediates supposed to exist in solid state as ﬂakes[42] or contaminated could go towards more molecular complexity. [43] [44,45] H2 ice macroparticles or as sublayers on the ISM grains. We observed the direct reaction of the CN radical with H2 forming HCN as monomer and oligomers. We also observed Acknowledgements the presence of the radical H2CN. Towards the radicals, which are intermediates, H2 is then a This work funded by a Hubert Curien Program from Campus France 167 hydrogenation reactant. This sort of reaction have been and DAAD agencies for the travelling cost between France and already observed with OH radical by Watanabe et al.,[46] Germany. The authors thank Dr Jean-Claude Guillemin for indicating

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