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1. A Mechanistic Study on the Formation of Acetone (CH 3 COCH 3 ), Propanal (CH 3 CH 2 CHO), Propylene Oxide (c-CH 3 CHOCH 2 ) along with Their Propenol Enols (CH 3 CHCHOH/CH 3 C(OH)CH 2 ) in Interstellar Analog Ices

   https://doi.org/10.3847/1538-4357/ac8c92  

   Singh, Santosh K. ; Fabian Kleimeier, N. ; Eckhardt, André K. ; Kaiser, Ralf I. ( December 2022 , The Astrophysical Journal)

   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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2. Synthesis of methanediol [CH 2 (OH) 2 ]: The simplest geminal diol

   https://doi.org/10.1073/pnas.2111938119  

   Zhu, Cheng ; Kleimeier, N. Fabian ; Turner, Andrew M. ; Singh, Santosh K. ; Fortenberry, Ryan C. ; Kaiser, Ralf I. ( January 2022 , Proceedings of the National Academy of Sciences)

   Geminal diols—organic molecules carrying two hydroxyl groups at the same carbon atom—have been recognized as key reactive intermediates by the physical (organic) chemistry and atmospheric science communities as fundamental transients in the aerosol cycle and in the atmospheric ozonolysis reaction sequence. Anticipating short lifetimes and their tendency to fragment to water plus the aldehyde or ketone, free geminal diols represent one of the most elusive classes of organic reactive intermediates. Here, we afford an exceptional glance into the preparation of the previously elusive methanediol [CH 2 (OH) 2 ] transient—the simplest geminal diol—via energetic processing of low-temperature methanol–oxygen ices. Methanediol was identified in the gas phase upon sublimation via isomer-selective photoionization reflectron time-of-flight mass spectrometry combined with isotopic substitution studies. Electronic structure calculations reveal that methanediol is formed via excited state dynamics through insertion of electronically excited atomic oxygen into a carbon–hydrogen bond of the methyl group of methanol followed by stabilization in the icy matrix. The first preparation and detection of methanediol demonstrates its gas-phase stability as supported by a significant barrier hindering unimolecular decomposition to formaldehyde and water. These findings advance our perception of the fundamental chemistry and chemical bonding of geminal diols and signify their role as an efficient sink of aldehydes and ketones in atmospheric environments eventually coupling the atmospheric chemistry of geminal diols and Criegee intermediates. 

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3. A vacuum ultraviolet photoionization study on the formation of methanimine (CH 2 NH) and ethylenediamine (NH 2 CH 2 CH 2 NH 2 ) in low temperature interstellar model ices exposed to ionizing radiation

   https://doi.org/10.1039/c8cp06002a  

   Zhu, Cheng ; Frigge, Robert ; Turner, Andrew M. ; Abplanalp, Matthew J. ; Sun, Bing-Jian ; Chen, Yue-Lin ; Chang, Agnes H. ; Kaiser, Ralf I. ( January 2019 , Physical Chemistry Chemical Physics)

   Methylamine (CH 3 NH 2 ) and methanimine (CH 2 NH) represent essential building blocks in the formation of amino acids in interstellar and cometary ices. In our study, by exploiting isomer selective detection of the reaction products via photoionization coupled with reflectron time of flight mass spectrometry (Re-TOF-MS), we elucidate the formation of methanimine and ethylenediamine (NH 2 CH 2 CH 2 NH 2 ) in methylamine ices exposed to energetic electrons as a proxy for secondary electrons generated by energetic cosmic rays penetrating interstellar and cometary ices. Interestingly, the two products methanimine and ethylenediamine are isoelectronic to formaldehyde (H 2 CO) and ethylene glycol (HOCH 2 CH 2 OH), respectively. Their formation has been confirmed in interstellar ice analogs consisting of methanol (CH 3 OH) which is ioselectronic to methylamine. Both oxygen-bearing species formed in methanol have been detected in the interstellar medium (ISM), while for methanimine and ethylenediamine only methanimine has been identified so far. In comparison with the methanol ice products and our experimental findings, we predict that ethylenediamine should be detectable in these astronomical sources, where methylamine and methanimine are present. 

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4. Complex Reactive Acids from Methanol and Carbon Dioxide Ice: Glycolic Acid (HOCH 2 COOH) and Carbonic Acid Monomethyl Ester (CH 3 OCOOH)

   https://doi.org/10.3847/1538-4357/ac97e3  

   Marks, Joshua H. ; Wang, Jia ; Evseev, Mikhail M. ; Kuznetsov, Oleg V. ; Antonov, Ivan O. ; Kaiser, Ralf I. ( January 2023 , The Astrophysical Journal)

   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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5. Synthesis of interstellar propen-2-ol (CH 3 C(OH)CH 2 ) – the simplest enol tautomer of a ketone

   https://doi.org/10.1039/D3CP02307A  

   Wang, Jia ; Nikolayev, Anatoliy A ; Zhang, Chaojiang ; Marks, Joshua H ; Azyazov, Valeriy N ; Eckhardt, André K ; Mebel, Alexander M ; Kaiser, Ralf I ( July 2023 , Physical Chemistry Chemical Physics)

   Astronomically elusive propen-2-ol and methyl vinyl ether were prepared in irradiated low-temperature acetone ices and detected in the gas phase via photoionization.

    

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