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Formation Pathways of Complex Organic Molecules: OH• Projectile Colliding with Methanol Ice Mantle (CH3OH)10 Natalia Inostroza-Pino1, Diego Mardones2, Jixing J

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Formation Pathways of Complex Organic Molecules: OH• Projectile Colliding with Methanol Ice Mantle (CH3OH)10 Natalia Inostroza-Pino1, Diego Mardones2, Jixing J A&A 641, A14 (2020) Astronomy https://doi.org/10.1051/0004-6361/202037904 & c ESO 2020 Astrophysics Formation pathways of complex organic molecules: OH• projectile colliding with methanol ice mantle (CH3OH)10 Natalia Inostroza-Pino1, Diego Mardones2, Jixing J. X. Ge2, and Desmond MacLeod-Carey1 1 Universidad Autónoma de Chile, Facultad de Ingeniería, Núcleo de Astroquímica & Astrofísica, Av. Pedro de Valdivia 425, Providencia, Santiago, Chile e-mail: [email protected] 2 Universidad de Chile, Facultad de Ciencias Físicas y Matemáticas, Departamento de Astronomía, Camino el observatorio 1515, Las condes, Santiago, Chile Received 6 March 2020 / Accepted 12 June 2020 ABSTRACT In this article, we simulated the collisions of an OH• projectile impacting on a methanol cluster formed by ten units of methanol to mimic an ice mantle (CH3OH)10. The chemical processes occurring after the impact were studied through Born-Oppenheimer (ab-initio) molecular dynamics. We focus on collisions with initial kinetic impact energy of 10–22 eV, where the richest chemistry happens. We report the formation mechanisms of stable complex organic molecules (COMs) such as methoxymethanol CH3OCH2OH, • formic acid HCOOH, formyl radical HCO, formaldehyde H2CO and its elusive HCOH isomer. We show that CH2(OH)2, CH2OH + • − or CH2OH are key intermediates to generate H2CO and other COMs. We compare the outcomes using OH with those using OH projectiles. These processes are likely relevant to the production of COMs in astrophysical environments. We discuss its formation mechanism and the astrophysical implications of these chemical pathways in star-forming regions. Key words. astrochemistry – molecular processes – ISM: molecules – ISM: atoms – dust, extinction 1. Introduction ISM. Methanol can be released from the grain mantles onto the gas phase at low temperatures via non-thermal desorption by The study of the chemistry in the universe has developed strongly energetic photons or particles or by exothermic chemical reac- since the discovery of CO in 1970, leading to the detection of tions (Öberg et al. 2011; Charnley et al. 1992). Thus, methanol more than 200 molecules in the interstellar medium (ISM) to is thought to be one of the main sources of complex organic date. The start of operations of the ALMA observatory opened molecules in the ISM (Hama & Watanabe 2013; Garrod et al. the door to study the creation processes of complex organic 2007; Qasim et al. 2018). Dust grains are believed to be cov- molecules (COMs) with 6 or more atoms each in the universe, ered by molecular icy mantles. Thus, the composition of these essential to understanding from prebiotic chemistry to the ori- mantles are an essential ingredient to understand theoretically gin of life (Tielens 2013). COMs have been detected in many and observationally the chemistry on grain surfaces. Dust grain different astronomical sources (Herbst & Van Dishoeck 2009) ice mantle composition has been measured using infrared (IR) including, for example, urea (CO(NH2)2)(Belloche et al. 2019; absorption observations from the ground and space (Öberg et al. Inostroza & Senent 2012) and methoxymethanol CH3OCH2OH 2011). Water ice is found to be the dominant component fol- (McGuire et al. 2017). Even though the formation of COMs lowed by simple carbon compounds mostly in CO, CO2, and has been modeled since 1980s (Tielens & Hagen 1982), these CH3OH (Hama & Watanabe 2013), with smaller amounts of models fail to reproduce the observed abundances in the cold NH3, XCN, (Gibb et al. 2004) and other molecules. interstellar environments. Laboratory experiments (Öberg et al. Photo-dissociation experiments on CH3OH ices have shown 2009) and models (Garrod & Herbst 2006) show that COMs that radicals are synthesized in situ on the ice surfaces can be formed in the solid state on icy grains, typically fol- under hot ISM environmental conditions (T of 100–600 K) lowing atom-addition or UV-photon absorption processes. For (Garrod & Herbst 2006; Garrod 2008). In simulations “heavy” • • example successive hydrogen addition has been proposed as a radicals like CH3 and CH2OH diffuse within the ice man- route to obtain reduced alcohols from aldehydes starting from a tles to finally recombine to form complex molecules. Oberg variety of known interstellar molecules: formaldehyde H2CO to (Öberg et al. 2009) also investigated CH3OH-rich ices in lab- methanol (CH3OH); ethenone CH2O to vinyl alcohol CH2CHOH; oratory photochemistry experiments and confirmed the diffu- ethanal CH3CHO to ethanol CH3CH2OH; or glycolaldehyde sion and recombination of the radicals into more complex HOCH2CHO to ethylene glycol HOCH2CH2OH. CH3OH can species. Chemical reactions that eject products from the sur- also be formed via CO hydrogenation (Garrod et al. 2007) or face and energetic processes such as UV Photolysis which pro- by the reaction of CH4 + OH under cold dense ISM conditions duce radicals that quickly react with other molecules play a key (Qasim et al. 2018). However, in order to reproduce the observed role in the synthesis of COMs in the ISM (Öberg et al. 2009). abundances, methanol must be formed on icy dust grains. Since Last year we reported a new route to obtain H2CO and HCO − CH3OH has a similar volatility to water ice (Brown & Bolina starting from CH3OH-ice-mantles impacted by OH projectile • 2007); it is expected to be abundant on grains in the cold (Inostroza et al. 2019). We showed how formation of CH3 and Article published by EDP Sciences A14, page 1 of7 A&A 641, A14 (2020) • OCH3 undergo radical-radical reactions to form CH3OCH3. Our 2. Computational methods main results were in agreement with experiments on CH3OH + OH reactions by Oberg (Öberg et al. 2009). Thus, methanol We used molecular dynamics (BOMD) simulations to study the effect of an OH• projectile impacting with an energy of 10 to CH3OH, one of the dominant sources of reactive interstellar organic species, can suffer photo-dissociation (Laas et al. 2011) 22 eV. As target material we used a methanol cluster formed by • ten units of methanol to mimic an ice mantle (CH3OH)10. The to produce reactive radicals such as methyl CH3, hydroxymethyl • • details are presented below. CH2OH and methoxy CH3O . Cernicharo et al.(2012) proposed that the gas-phase reaction • • • CH3OH + OH is an efficient way to form CH3O and CH2OH 2.1. Simulations radicals (see Eqs. (1) and (2)). This helps to understand the The simulations were performed using the density functional lack of detection of CH2OH under ISM conditions, which is the more stable isomer formed on icy grain mantles (Jheeta et al. theory (DFT) formalism, under the micro-canonical ensemble 2013). More recently, Ocaña et al.(2019) show the reaction of approach (also called NEV ensemble), as explained in Paper I • • Inostroza et al.(2019). We used the long range-corrected hybrid CH3OH + OH to be a fast and effective source of CH2OH and • functional of Head-Gordon !B97X-D (Helgaker et al. 1990; CH3O at the low pressures and temperatures prevalent in the interstellar medium, emphasizing the role of dust-grain mantle Uggerud & Helgaker 1992; Bolton et al. 1998). This !B97X-D reactions are needed to understand gas phase molecular abun- functional is known to be flexible enough to describe reactive dances. collisions (McBride et al. 2013). We used BOMD at constant energy (i.e., in the NEV ensemble). The NEV ensemble repro- • • duces the conditions where the low concentration of particles CH3OH + OH ! CH3O + H2O (1) hinders the dissipation of thermal energy on timescales of that • • CH3OH + OH ! CH2OH + H2O: (2) of the collision (less than 400 fs). The two most relevant elec- tronic states for OH are the ground state X2Π and the first • excited state A2Σ+, which is approximately 4 eV higher. The col- From the above isomers, the methoxy radical CH3O has been detected in space (Cernicharo et al. 2012), although the lisions are assumed to happen in the ground state of the species, • which is a good approximation because the lowest excitation hydroxymethyl radical, CH2OH, which is thermodynamically more stable (Bermudez et al. 2017; Nguyen et al. 2019), has not energy of methanol, 6 eV (Varela et al. 2015) and hydroxyl, yet been accurately measured in the laboratory, which com- 4 eV (Schofield & Kjaergaard 2004) is a large fraction of the plicates its detection in the interstellar medium. The direct kinetic energy of the projectile used during the simulations, reaction of CH O• and •CH OH can form methoxymethanol which implies a small probability of non-adiabatic dynamics 3 2 (Inostroza et al. 2019). Moreover, the collision times in our cal- CH3OCH2OH (Reaction (3)), which has been observed in the high-mass star-forming region NGC6334 (McGuire et al. 2017). culations are much shorter than the radiative de-excitation times • for both methanol and hydroxyl. Thus, the species remains in This may explain the non-detection of CH2OH, since it is an intermediate in the CH OCH OH formation process. Thus, its ground state. We follow the simulations until the molecular 3 2 fragments and reaction products do not change further in time. CH3OCH2OH would be a molecular tracer for this radical- radical reaction in star-forming regions: All calculations were done using the electronic structure package Gaussian 09 (Frisch et al. 2009). • • CH3O + CH2OH ! CH3OCH2OH (3) 2.2. Chemical model It is known that COMs can be formed in icy grain mantles, We use a cluster formed by ten units of methanol to mimic an ice which can enhance their gas-phase molecular abundances upon mantle (CH3OH)10. This cluster corresponds to the most stable grain surface ejection of COMs. Thus, it is important to study the isomer (Boyd & Boyd 2007). Its size is a good representation possible gas or solid phase pathways that are likely to produce between the size of the dust ice-mantle and the computational one or more of these radicals (McGuire et al.
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