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Synthesis of the First Nitrogen-Heterocycles in Interstellar

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Synthesis of the First Nitrogen-Heterocycles in Interstellar MNRAS 000,1–16 (2021) Preprint 11 June 2021 Compiled using MNRAS LATEX style file v3.0 Synthesis of the first nitrogen-heterocycles in interstellar ice analogs containing methylamine (CH3NH2) exposed to UV radiation: Formation of trimethylentriamine (TMT, c-(-CH2-NH)3) and hexamethylentetramine (HMT, (CH2)6N4). H. Carrascosa,1¢ C. González Díaz, 1 G. M. Muñoz Caro,1y P. C. Gómez,2 M. L. Sanz, 3 1Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain 2Dep. Química Física, Fac. Química, Univ. Complutense, 28040 Madrid, Spain 3Department of instrumental analysis and environmental chemistry, IQOG-CSIC, Juan de la Cierva 3, 28006 Madrid, Spain Accepted XXX. Received YYY; in original form ZZZ ABSTRACT Hexamethylentetramine has drawn a lot of attention due to its potential to produce prebiotic species. This work aims to gain a better understanding in the chemical processes concerning methylamine under astrophysically relevant conditions. In particular, this work deeps into the formation of N-heterocycles in interstellar ice analogs exposed to UV radiation, which may lead to the formation of prebiotic species. Experimental simulations of interstellar ice analogs were carried out in ISAC. ISAC is an ultra-high vacuum chamber equipped with a cryostat, where gas and vapour species are frozen forming ice samples. Infrared and ultraviolet spectroscopy were used to monitor the solid phase, and quadrupole mass spectrometry served to measure the composition of the gas phase. The variety of species detected after UV irradiation of ices containing methylamine revealed the presence of 12 species which have been already detected in the ISM, being 4 of them typically classified as complex organic molecules: formamide (HCONH2), methyl cyanide (CH3CN), CH3NH and CH3CHNH. Warming up of the irradiated CH3NH2-bearing ice samples lead to the formation of trimethylentriamine (TMT), a N-heterocycle precursor of HMT, and the subsequent synthesis of HMT at temperatures above 230 K. Key words: Astrochemistry – Methods: laboratory: molecular – techniques: spectroscopy – software: simulation – ultraviolet: ISM – ISM: molecules 1 INTRODUCTION temperature increases and thermal energy is enough to overcome the activacion barriers to form new species. Up to now, more than Molecules with six or more atoms including at least one C atom 200 different molecules have been detected in the gas phase toward are known as complex organic molecules (COMs) (Herbst & van interstellar and circumstellar environments, including around 50 Dishoeck 2009). COMs have been detected in dense interstellar COMs. An important fraction of these species is thought to be arXiv:2106.05622v1 [astro-ph.IM] 10 Jun 2021 clouds and circumstellar regions. Deep inside dense clouds, produced in ice mantles. the interstellar UV field cannot penetrate and dust temperature decreases to around 10 K. At these low temperatures, H2O and Among them, methylamine (CH NH ) has drawn consider- other molecules including CO, CO2, CH3OH, CH4, or NH3 3 2 accrete onto dust grains forming ice mantles. These ice mantles able attention due to its potential to produce prebiotic species. It are exposed to secondary-UV photons generated by the interaction was first detected in Sagitarius B2 and Orion A molecular clouds 15 −2 15 between cosmic rays and hydrogen molecules, as well as direct with column densities ranging from 1×10 cm to 4×10 −2 cosmic ray impact, producing radicals and ions which can lead cm (Kaifu et al. 1974; Fourikis et al. 1974; Belloche et al. to the formation of new species. Some intermediate species can 2013). Goesmann et al.(2015) and Altwegg et al.(2017) reported react at low temperature, while others remain in the ice until the CH3NH2 abundances from 0.6% to 1.2% relative to H2O on comet 67P/Churyumov-Gerasimenko. Finally, CH3NH2 has been also detected in hot cores. Ohishi et al.(2019) reported the presence of ¢ E-mail: [email protected] CH3NH2 in G10.47+0.03 while Bøgelund et al.(2019) observed y E-mail: [email protected] column densities between 3.0×1015 and 2.7×1017 in three dif- © 2021 The Authors 2 H. Carrascosa et al. ferent hot cores from the high-mass star forming region NGC 6334I. genated by UV radiation to produce formaldehyde, or formaldehyde is formed, albeit less efficiently, from hydrogenation of CO in Different mechanisms have been proposed for methylamine the ice (Briggs et al. 1992; Muñoz Caro & Schutte 2003; Muñoz formation under astrophysical conditions. In the gas phase, Herbst Caro et al. 2004). The reaction between a carbonyl group and a ¸ primary amine typically leads to the formation of imine groups (1985) modelled the possible association between CH3 ions and NH3 molecules to produce CH3NH2, concluding that it (R=N-R’), releasing a water molecule. Thus, from formaldehyde was an efficient route to produce methylamine under interstellar and ammonia, methylenimine (CH2NH) is obtained. This highly conditions, despite the existence of exothermic reaction channels. reactive methylenimine polymerises, leading to a 6-member stable Gardner & McNesby(1980) and Ogura et al.(1989) reported ring, known as trimethylentriazine (TMT). Methylen groups are the formation of methylamine from photolysis of gas mixtures electron-deficient units which incorporate hydroxymethyl radicals containing CH4 and NH3. In their experiments, UV-irradiation to form 1,3,5-trihydroxymethyltrimethylentriazine (TMT-triol). produced CH3· and NH2· radicals able to recombine to produce The latter leads to HMT in presence of ammonia. The last step, CH3NH2. This pathway was also explored in ice samples by Kim however, only takes place at high temperatures, when ammonia is & Kaiser(2011) and Förstel et al.(2017), who carried out electron already thermally desorbed to the gas phase in the ice irradiation bombardment and photon irradiation of CH4:NH3 ice mixtures, and warm-up experiments. Indeed, Muñoz Caro & Schutte(2003) respectively, reporting the formation of methylamine in both works. observed the formation of HMT at room temperature monitoring An alternative route is the successive hydrogenation of gas phase the growth of its main absorption bands in the IR spectra at −1 HCN molecules, analogous to the formation of H2CO and CH3OH 1007 and 1234 cm . These authors proposed the presence of from CO molecules, as pointed out by Dickens et al.(1997) and carboxylate ammonium salts, which would remain in the ice at high Theule et al.(2011). temperature, able to provide the amino groups. Several experiments reveal the potential of CH3NH2 to form Vinogradoff et al.(2012, 2013) and references therein prebiotic molecules. Holtom et al.(2005) prepared CH 3NH2:CO2 deepened in the formation mechanism of HMT. Those works binary ice mixtures which resulted in glycine (NH2CH2COOH) reported measurements of complex ice mixtures, confirming that formation under electron bombardment. Bossa et al.(2009a) repro- formaldehyde reacts with ammonia to produce methylenimine duced the same ice mixture in a water dominated ice. They found (CH2NH). However, the stability of organic rings favours the evidence on the formation of methylammonium methylcarbamate, cyclation of the 6-member species CH2NH-CH2NH-CH2NH, ¸ − [CH3NH3 ][CH3NHCOO ], a glycine salt precursor in astrophysi- leading to the production of TMT (Vinogradoff et al. 2012). TMT cal environments dominated by thermal and UV processing. In the is then converted into HMT near room temperature. same year, Lee et al.(2009) reported the formation of several COMs from UV irradiation of CH3NH2:CO2 mixtures on top of a H2O ice. Within this work, we explore an alternative mechanism in the production of HMT. We have focused on the primary steps and The main motivation to study the UV-photoprocessing the intermediate species. We have calculated the theoretical IR of methylamine-bearing ices in this work is to gain a better spectrum of TMT since we found no spectrum for this species in the understanding on the formation of N-heterocycles in interstellar literature. The validity of this calculation was checked comparing ice analogs. In particular, we targeted the synthesis of hexamethy- the calculated HMT spectrum to the one measured in the laboratory. lentetramine (HMT), a molecule commonly identified among the Gardner & McNesby(1982) reported the preferential formation of refractory products in ice irradiation experiments, and its precursor, CH2NH from UV irradiation of CH3NH2 molecules, supported by trimethylentriamine (TMT). the efficient formation of H2 molecules. By using methylamine, we have been able to study in more depth the chemical pathways of Hexamethylentetraamine (C6N3H12, HMT) is a molecule of C-N bearing molecules in different environments. Noble gas matrix prime interest in astrochemistry. Firstly, it is a stable molecule isolation experiments were also carried out to isolate CH2NH efficiently made by UV or ion processing of interstellar ice molecules, thus allowing the study of its spectroscopic features analogs, which remains in the generated organic residue at room and radical formation. This will be explained in more detail in the temperature (Briggs et al. 1992; Cottin et al. 2001). Second, it may paragraphs below. catalyze organic reactions leading to the formation of other COMs (Vinogradoff et al. 2012). Third, its acid hydrolysis leads to the In addition to the primary processes leading to HMT, we also formation of several aminoacids (Fox & Windsor 1970). HMT has studied the formation of other photoproducts arising upon UV been recently detected in Murchison (846±37 ppb), Murray (29±9 irradiation of methylamine. When H2O is added to CH3NH2 in the ppb) and Tagish Lake (671±9 ppb) carbonaceous chondrites, as ice, in a likely more realistic astrophysical scenario, oxygenated well as the methyl, amino, hidroxy and hidroxymethyl derivatives species are formed. Some of them, such as HCN, HNCO or of HMT (see Oba et al. 2020). The synthesis of HMT derivatives in HCONH2 were found in comet 67P/Churyumov-Gerasimenko ice irradiation experiments was previously reported (Muñoz Caro surface during the Rossetta mission (Goesmann et al.
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