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  1. Oxirene was prepared and stabilized in matrices through resonant energy transfer prior to identification in the gas phase. 
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    Free, publicly-accessible full text available March 10, 2024
  2. Although methanediamine (CH 2 (NH 2 ) 2 ) has historically been the subject of theoretical scrutiny, it has never been isolated to date. Here, we report the preparation of methanediamine (CH 2 (NH 2 ) 2 )—the simplest diamine. Low-temperature interstellar analog ices composed of ammonia and methylamine were exposed to energetic electrons which act as proxies for secondary electrons produced in the track of galactic cosmic rays. These experimental conditions, which simulate the conditions within cold molecular clouds, result in radical formation and initiate aminomethyl (ĊH 2 NH 2 ) and amino ( N . H 2 ) radical chemistry. Exploiting tunable photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) to make isomer-specific assignments, methanediamine was identified in the gas phase upon sublimation, while its isomer methylhydrazine (CH 3 NHNH 2 ) was not observed. The molecular formula was confirmed to be CH 6 N 2 through the use of isotopically labeled reactants. Methanediamine is the simplest molecule to contain the NCN moiety and could be a vital intermediate in the abiotic formation of heterocyclic and aromatic systems such as nucleobases, which all contain the NCN moiety. 
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  3. Free, publicly-accessible full text available March 2, 2024
  4. Abstract

    Carbonyl-bearing complex organic molecules (COMs) in the interstellar medium (ISM) are of significant importance due to their role as potential precursors to biomolecules. Simple aldehydes and ketones like acetaldehyde, acetone, and propanal have been recognized as fundamental molecular building blocks and tracers of chemical processes involved in the formation of distinct COMs in molecular clouds and star-forming regions. Although previous laboratory simulation experiments and modeling established the potential formation pathways of interstellar acetaldehyde and propanal, the underlying formation routes to the simplest ketone—acetone—in the ISM are still elusive. Herein, we performed a systematic study to unravel the synthesis of acetone, its propanal and propylene oxide isomers, as well as the propenol tautomers in interstellar analog ices composed of methane and acetaldehyde along with isotopic-substitution studies to trace the reaction pathways of the reactive intermediates. Chemical processes in the ices were triggered at 5.0 K upon exposure to proxies of Galactic cosmic rays in the form of energetic electrons. The products were detected isomer-selectively via vacuum ultraviolet (VUV) photoionization reflectron time-of-flight mass spectrometry. In our experiments, the branching ratio of acetone (CH3COCH3):propylene oxide (c-CH3CHOCH2):propanal (CH3CH2CHO) was determined to be (4.82 ± 0.05):(2.86 ± 0.13):1. The radical–radical recombination reaction leading to acetone emerged as the dominant channel. The propenols appeared only at a higher radiation dose via keto–enol tautomerization. The current study provides mechanistic information on the fundamental nonequilibrium pathways that may be responsible for the formation of acetone and its (enol) isomers inside the interstellar icy grains.

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  5. We unravel, for the very first time, the formation pathways of hydroxyacetone (CH 3 COCH 2 OH), methyl acetate (CH 3 COOCH 3 ), and 3-hydroxypropanal (HCOCH 2 CH 2 OH), as well as their enol tautomers within mixed ices of methanol (CH 3 OH) and acetaldehyde (CH 3 CHO) analogous to interstellar ices in the ISM exposed to ionizing radiation at ultralow temperatures of 5 K. Exploiting photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) and isotopically labeled ices, the reaction products were selectively photoionized allowing for isomer discrimination during the temperature-programmed desorption phase. Based on the distinct mass-to-charge ratios and ionization energies of the identified species, we reveal the formation pathways of hydroxyacetone (CH 3 COCH 2 OH), methyl acetate (CH 3 COOCH 3 ), and 3-hydroxypropanal (HCOCH 2 CH 2 OH) via radical–radical recombination reactions and of their enol tautomers (prop-1-ene-1,2-diol (CH 3 C(OH)CHOH), prop-2-ene-1,2-diol (CH 2 C(OH)CH 2 OH), 1-methoxyethen-1-ol (CH 3 OC(OH)CH 2 ) and prop-1-ene-1,3-diol (HOCH 2 CHCHOH)) via keto-enol tautomerization. To the best of our knowledge, 1-methoxyethen-1-ol (CH 3 OC(OH)CH 2 ) and prop-1-ene-1,3-diol (HOCH 2 CHCHOH) are experimentally identified for the first time. Our findings help to constrain the formation mechanism of hydroxyacetone and methyl acetate detected within star-forming regions and suggest that the hitherto astronomically unobserved isomer 3-hydroxypropanal and its enol tautomers represent promising candidates for future astronomical searches. These enol tautomers may contribute to the molecular synthesis of biologically relevant molecules in deep space due to their nucleophilic character and high reactivity. 
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  6. Abstract

    The formation of complex organic molecules by simulated secondary electrons generated in the track of galactic cosmic rays was investigated in interstellar ice analogs composed of methanol and carbon dioxide. The processed ices were subjected to temperature-programmed desorption to mimic the transition of a cold molecular cloud to a warmer star-forming region. Reaction products were detected as they sublime using photoionization reflectron time-of-flight mass spectrometry. By employing isotopic labeling, tunable photoionization and computed adiabatic ionization energies isomers of C2H4O3were investigated. Product molecules carbonic acid monomethyl ester (CH3OCOOH) and glycolic acid (HOCH2COOH) were identified. The abundance of the reactants detected in analog interstellar ices and the low irradiation dose necessary to form these products indicates that these molecules are exemplary candidates for interstellar detection. Molecules sharing a tautomeric relationship with glycolic acid, dihydroxyacetaldehyde ((OH)2CCHO), and the enol ethenetriol (HOCHC(OH)2), were not found to form despite ices being subjected to conditions that have successfully produced tautomerization in other ice analog systems.

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  7. Silicon monoxide (SiO) is classified as a key precursor and fundamental molecular building block to interstellar silicate nanoparticles, which play an essential role in the synthesis of molecular building blocks connected to the Origins of Life. In the cold interstellar medium, silicon monoxide is of critical importance in initiating a series of elementary chemical reactions leading to larger silicon oxides and eventually to silicates. To date, the fundamental formation mechanisms and chemical dynamics leading to gas phase silicon monoxide have remained largely elusive. Here, through a concerted effort between crossed molecular beam experiments and electronic structure calculations, it is revealed that instead of forming highly-stable silicon dioxide (SiO 2 ), silicon monoxide can be formed via a barrierless, exoergic, single-collision event between ground state molecular oxygen and atomic silicon involving non-adiabatic reaction dynamics through various intersystem crossings. Our research affords persuasive evidence for a likely source of highly rovibrationally excited silicon monoxide in cold molecular clouds thus initiating the complex chain of exoergic reactions leading ultimately to a population of silicates at low temperatures in our Galaxy. 
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