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Alcohols on the Rocks: Solid-State Formation in a H3CCCH + OH Cocktail under Dark Cloud Conditions

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journal contribution
posted on 30.04.2019, 00:00 by Danna Qasim, Gleb Fedoseev, Thanja Lamberts, Ko-Ju Chuang, Jiao He, Sergio Ioppolo, Johannes Kästner, Harold Linnartz
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) and n-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 geometries leads to an averaged-out effect. Three isomers of propanediol are also tentatively identified in the experiments. It is also shown that propene and propane (H3CCHCH2 and H3CCH2CH3) are formed from the hydrogenation of H3CCCH. This experimental finding falls in line with the lower 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.