This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes. Article Cite This: ACS Earth Space Chem. 2019, 3, 986−999 http://pubs.acs.org/journal/aesccq Alcohols on the Rocks: Solid-State Formation in a H3CC CH + OH Cocktail under Dark Cloud Conditions † ‡ § † # † ∥ Danna Qasim,*, Gleb Fedoseev, Thanja Lamberts, Ko-Ju Chuang, , Jiao He, Sergio Ioppolo, ⊥ † Johannes Kastner,̈ and Harold Linnartz † Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, P. O. Box 9513, NL-2300 RA Leiden, The Netherlands ‡ INAF−Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy § Leiden Institute of Chemistry, Leiden University, P. O. Box 9502, NL-2300 RA Leiden, The Netherlands ∥ School of Electronic Engineering and Computer Science, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom ⊥ Institute for Theoretical Chemistry, University of Stuttgart, 70569 Stuttgart, Germany *S Supporting Information ABSTRACT: A number of recent experimental studies have shown that solid-state complex organic molecules (COMs) can form under conditions that are relevant to the CO freeze- out stage in dense clouds. In this work, we show that alcohols can be formed well before the CO freeze-out stage (i.e., during the very early stage of the H2O-rich ice phase). This joint experimental and computational investigation shows that isomers n-propanol and isopropanol (H3CCH2CH2OH and H3CCHOHCH3)andn-propenol and isopropenol (H3CCH CHOH and H3CCOH CH2) can be formed in radical-addition reactions starting from propyne (H3CC CH) + OH at the low temperature of 10 K, where H3CC CH is one of the simplest representatives of stable carbon chains already identified in the interstellar medium (ISM). The resulting average abundance ratio of 1:1 for n- propanol:isopropanol is aligned with the conclusions from the computational work that the geometric orientation of strongly interacting species is influential to the extent of which “mechanism” is participating and that an assortment of Downloaded via LEIDEN UNIV on September 30, 2019 at 20:56:11 (UTC). geometries leads to an averaged-out effect. Three isomers of fi propanediol are also tentatively identi ed in the experiments. It is also shown that propene and propane (H3CCH CH2 and fi H3CCH2CH3) are formed from the hydrogenation of H3CC CH. This experimental nding falls in line with the lower See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. activation barrier of hydrogenation of a C C bond in comparison to a C C bond. Reactants and products are probed by temperature-programmed desorption−quadrupole mass spectrometry (TPD-QMS) and reflection absorption infrared spectroscopy (RAIRS). Product relative abundances are determined from TPD-QMS data. Computationally derived activation barriers give additional insight into what types of reactions and mechanisms are more likely to occur in the laboratory and in the ISM. Our findings not only suggest that the alcohols studied here share common chemical pathways and therefore can show up − simultaneously in astronomical surveys but also that their extended counterparts that derive from polyynes containing H3C − (C C)n H structures may exist in the ISM. Such larger species, such as fatty alcohols, are the possible constituents of simple lipids that primitive cell membranes on the early Earth are thought to be partially composed of. continued... Special Issue: Complex Organic Molecules (COMs) in Star- Forming Regions Received: March 18, 2019 Revised: April 28, 2019 Accepted: April 30, 2019 Published: April 30, 2019 © 2019 American Chemical Society 986 DOI: 10.1021/acsearthspacechem.9b00062 ACS Earth Space Chem. 2019, 3, 986−999 ACS Earth and Space Chemistry Article KEYWORDS: astrochemistry, astrobiology, solid-state, interstellar, ice, complex organic molecules, infrared, mass spectrometry 1. INTRODUCTION The mechanism proposed in this work results in the formation The origin of cosmic carbon lies in the outflows of carbon-rich of alcohols already in H O-rich ices, meaning, the alcohols can stars.1,2 How carbon evolves into hydrocarbon species, from 2 be synthesized before the CO freeze-out stage, well below small molecules such as methane (CH4) and acetylene (C2H2) 32 extinctions (AV) of 9 in dense atomic/molecular cores. In the to polycyclic aromatic hydrocarbons (PAHs), to carbon literature, most solid-state laboratory experiments report the nanoparticles, or other carbon-containing species such as formation of simple alcohols (e.g., methanol (CH OH) and alcohols, is far from understood. In addition to “bottom-up” 3 ethanol (H3CCH2OH)) and polyalcohols (e.g., ethylene glycol approaches, which merge smaller precursors into larger species (HOCH CH OH), glycerol (HOCH CHOHCH OH), and “ ” 2 2 2 2 (in the gas-phase or solid-state), also top-down approaches methoxymethanol (H COCH OH)) in the context where much 3 2 − have been proposed. Both scenarios are considered as likely road CO has already been frozen out.33 40 These experiments can be maps toward molecular complexity in space, but as mentioned, divided into two subgroups: “energetic” and “non-energetic” 3,4 many details are lacking. processing, where “non-energetic” refers to a radical-induced From a bottom-up perspective, simple species such as C2H2 process without the involvement of UV, cosmic rays, and/or 5,6 41 can polymerize to form polyacetylene ([C2H2]n). The other “energetic” particles. In the “energetic”-induced studies, complexity can increase by addition of a methyl (-CH3) group CH3OH- and/or CO-containing ices are irradiated to form to form methylpolyacetylene. Such species can eventually alcohols, and CH OH is either mixed with CO or explicitly 3 − accrete onto carbonaceous dust grains that were formed from stated to be a product of CO hydrogenation.33 36 In nucleation of PAHs.2,7 An alternative is that these species form experiments that focus on “non-energetic” processes, alcohols through surface reactions in the ice layers that are on top of dust are formed by reactions that involve the hydrogenation of a CO- − grains. In this article, we focus on the simplest representative of rich ice.37 40 the latter row of species: methylacetylene, also known as Many of the icy alcohols that have been formed in the propyne (H CCCH). H CCCH has been detected in laboratory have also been detected as gas-phase species in the − 3 3 space.8 16 Not only is it reported to be observed toward carbon- interstellar medium (ISM). This includes the detection of 13 rich stars, as expected, but it is also reported to be detected CH3OH, H3CCH2OH, vinyl alcohol (H2CCHOH), 9,12 fi toward cold, dark, and dense clouds, where it has a column HOCH2CH2OH, and H3COCH2OH, which were rst reported − × 13 −2 9 by Ball et al.,42 Zuckerman et al.,43 Turner and Apponi,44 Hollis density of (4 8) 10 cm in TMC-1. Assuming an H2 − 3 45 46 abundance of ∼1021 cm 2, this leads to <1% with respect to et al., and McGuire et al., respectively. CH3OH also has been detected in the solid-state.47 Much effort has been recently put H2O ice following the calculation from Herbst and van Dishoeck.17 into explaining the transition from frozen to gas-phase CH3OH − H O ice is formed on dust grains.18 23 Laboratory experi- to explain, for example, CH3OH abundances observed in 2 protoplanetary disks.48 Other alcohols are still elusive, such as n- ments show that every formed H2O molecule, in the solid-state and gas-phase, has an OH radical and/or ion as an propanol (H3CCH2CH2OH) (Qasim et al., submitted for 24 publication). intermediate. In the ice, OH radicals have the chance to react with other species that are in direct proximity prior to This work overviews the reaction of solid-state H3CC CH with H and OH under conditions relevant to the dense cloud hydrogenation to yield H2O, as the accretion of an H atom onto − stage. Section 2 provides details on the experimental and a dust grain of radius 10 5 cm happens once a day.25 An example computational parameters used for this study. Section 3 presents of this is the formation of CO2 from CO and OH in H2O-rich fi 26 the ndings from the laboratory experiments. Section 4 reports ices. Thus, the interaction between OH radicals and H3CC the computationally derived energies that are applicable to the CH is a valid topic to be addressed in astrochemical laboratories. reactions taking place in the experiments. These results are This interaction may result in the formation of simple alcohols combined with the laboratory work to uncover the products and polyalcohols. formed and their formation pathways. Section 5 discusses how The formation of alcohols is particularly intriguing as they the formation of such icy alcohols can take place at the interface may have a role in astrobiology, assuming that alcohols can be 27 of astrochemical and astrobiological environments. Finally, the delivered to planetary bodies such as the early Earth. Simple conclusions of this study are summarized in section 6. alcohols are amphiphilic molecules, i.e., both sides of the Given the many different species that will be discussed in the ff ffi 28 molecule have di erent a nities from each other. Because next sections, Table 1 is added and summarizes the names and they are composed of a polar head (-OH group) and a chemical structure formulas of all relevant species. A majority of hydrophobic tale (aliphatic group), they can take part in the the listed chemicals are the expected products of solid-state formation of micelles in primordial oceans. More complex H3CC CH hydrogenation or hydroxylation.