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A Photoionization Reflectron Time‐Of‐Flight Mass Spectrometric Articles ChemPhysChem doi.org/10.1002/cphc.202100064 A Photoionization Reflectron Time-of-Flight Mass Spectrometric Study on the Detection of Ethynamine (HCCNH2) and 2H-Azirine (c-H2CCHN) Andrew M. Turner,[a, b] Sankhabrata Chandra,[a, b] Ryan C. Fortenberry,*[c] and Ralf I. Kaiser*[a, b] Ices of acetylene (C2H2) and ammonia (NH3) were irradiated with 9.80 eV, and 10.49 eV were utilized to discriminate isomers energetic electrons to simulate interstellar ices processed by based on their known ionization energies. Results indicate the galactic cosmic rays in order to investigate the formation of formation of ethynamine (HCCNH2) and 2H-azirine (c-H2CCHN) C2H3N isomers. Supported by quantum chemical calculations, in the irradiated C2H2:NH3 ices, and the energetics of their experiments detected product molecules as they sublime from formation mechanisms are discussed. These findings suggest the ices using photoionization reflectron time-of-flight mass that these two isomers can form in interstellar ices and, upon spectrometry (PI-ReTOF-MS). Isotopically-labeled ices confirmed sublimation during the hot core phase, could be detected using the C2H3N assignments while photon energies of 8.81 eV, radio astronomy. 1. Introduction acetonitrile (CH3CN; 1) and methyl isocyanide (CH3NC; 2) ‘isomer doublet’ (Figure 2) – the methyl-substituted counterparts of For the last decade, the elucidation of the fundamental reaction hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) – has pathways leading to structural isomers – molecules with the been detected toward the star-forming region SgrB2.[4,8–9] same molecular formula, but distinct connectivities of atoms – However, none of their higher energy isomers has ever been of complex organic molecules (COMs) in the interstellar identified in any astronomical environment: 2H-azirine (c- [10–14] [15–19] medium (ISM) has received considerable interest from the NCHCH2; 3), ethynamine (HCCNH2; 4), ketenimine [1–3] [20] [21] astrochemistry and physical chemistry communities. This is (HNCCH2; 5), or 1H-azirine (c-HNCHCH; 6). because structural isomers have been recognized as tracers of In terrestrial laboratories, 2H-azirine (c-NCHCH2; 3) may act physical and chemical conditions of, e.g., cold molecular clouds both as a nucleophile as well as an electrophile;[22–25] therefore, like TMC-1 and of star-forming regions like SgrB2, which are its derivatives have been exploited in preparative organic used to test chemical models through astrochemical modeling chemistry[23,26–27] toward the synthesis of heterocyclic rings,[24] combined with astronomical observations.[4] Whereas consider- Diels-Alder reactions,[28] and photochemically-activated able progress has been made toward a better understanding of reactions.[9,29] The gas phase preparation of 3 was accomplished the formation routes of COMs within carbon-, hydrogen-, and via pyrolysis of vinyl azide[30] and 1H-1,2,3-triazole;[31] in addition, oxygen-bearing systems such as C3H6O [acetone (CH3COCH3), 3 was matrix-isolated and tentatively identified via infrared [5] [32] propanal (HCOC2H5), propylene oxide (c-C3H6O)], C2H6O2 spectroscopy by exposing 1 to laser-ablated transition metals. [ethylene glycol (HOCH2CH2OH), methoxymethanol Crossed molecular beam experiments along with statistical [6] (CH3OCH2OH)], and C3H6O2 [hydroxyacetone (CH3COCH2OH), calculations on the possible identification of 3 yielded conflict- [33–34] ethyl formate (HCOOC2H5), methyl acetate (CH3COOCH3)] (Fig- ing results. The preparation of 4, which is an amino (-NH2) ure 1)[7] within the ices deposited on interstellar nanoparticles, substituted acetylene molecule, was first reported in 1988 few advances have been achieved with respect to structural through vacuum pyrolysis of the keto and amine derivative of [35] isomers of carbon-, hydrogen-, and nitrogen-bearing systems. isoxazole (c-H3C3NO), whereas 5 was observed through Structural isomers of C2H3N represent a peculiar problem. The photolysis of hydrazoic acid (HN3) in the presence of acetylene (C2H2). This isomer has been exploited in preparative organic synthesis involving nucleophilic additions, radical additions, [a] Dr. A. M. Turner, Dr. S. Chandra, Prof. Dr. R. I. Kaiser cycloaddition reactions, electrocyclic ring closure reactions, and Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy [36] Mall, Honolulu, HI 96822 (USA) σ rearrangements. The thermodynamically least stable isomer E-mail: [email protected] 6 has not been prepared to date. [b] Dr. A. M. Turner, Dr. S. Chandra, Prof. Dr. R. I. Kaiser In this work, the preparation and detection of 2H-azirine (c- W. M. Keck Laboratory in Astrochemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, HI 96822 (USA) NCHCH2; 3) along with ethynamine (HCCNH2; 4) by exposing [c] Prof. R. C. Fortenberry low-temperature ices of acetylene (C2H2) and ammonia (NH3) Department of Chemistry & Biochemistry, University of Mississippi, along with their isotopically substituted counterparts Mississippi 38677-1848 (USA) 13 15 13 15 E-mail: [email protected] ( C2H2 :NH3, C2H2 : NH3, C2H2 : NH3, C2D2 :NH3) to energetic Supporting information for this article is available on the WWW under electrons is reported. The neutral molecules formed are then https://doi.org/10.1002/cphc.202100064 sublimed into the gas phase, ionized according to their distinct ChemPhysChem 2021, 22, 985–994 985 © 2021 Wiley-VCH GmbH Wiley VCH Donnerstag, 06.05.2021 2110 / 201099 [S. 985/994] 1 Articles ChemPhysChem doi.org/10.1002/cphc.202100064 Figure 1. Selected key classes of complex organic molecules (COMs) within the carbon, hydrogen, and oxygen system. Figure 2. Isomers of C2H3N are shown in the sequence of decreasing adiabatic ionization energies. CCSD(T)/CBS+ZPVE adiabatic ionization energies are corrected by incorporating the error analysis (Table S9). Relative energies and point groups of each isomer are also shown. ChemPhysChem 2021, 22, 985–994 www.chemphyschem.org 986 © 2021 Wiley-VCH GmbH Wiley VCH Donnerstag, 06.05.2021 2110 / 201099 [S. 986/994] 1 Articles ChemPhysChem doi.org/10.1002/cphc.202100064 adiabatic ionization energies by single photon ionization (PI) the ices were monitored with a Fourier-transform infrared spec- À 1 À 1 using vacuum ultraviolet (VUV) photons, and subsequently trometer (FTIR, Nicolet 6700) in the range of 4000 cm to 700 cm À 1 mass resolved and detected within a reflectron time-of-flight at 4 cm resolution averaging spectra over 2 minutes, i.e. up to 30 accumulated spectra in total during the irradiation. After the mass spectrometer (ReTOF-MS).[37–38] Discrete photon energies electron irradiation, temperature programmed desorption (TPD) of 10.49 eV, 9.80 eV, and 8.81 eV along with isotopic substitu- was performed to sublime the ices along with the newly formed tion experiments were exploited to determine the molecular products. Each exposed sample was heated from 5 K to 300 K at a formula of the products, to discriminate between structural rate of 1 KminÀ 1. During TPD, a reflectron time-of-flight mass isomers, and to infer in combination with electronic structure spectrometer (ReTOF-MS, Jordan TOF Products, Inc.) coupled with calculations possible reaction pathways to their formation. photoionization (PI) at photon energies of 10.49 eV, 9.80 eV and 8.81 eV was utilized to ionize the subliming molecules. To generate Although acetylene has not been identified on interstellar ices, these photon energies, pulsed (30 Hz) coherent vacuum ultraviolet laboratory studies provide explicit evidence that acetylene (VUV) light was generated via four-wave mixing using krypton represents a major reaction product of the radiolysis and (Specialty Gases, 99.999%) or xenon (Specialty Gases, 99.999%) as a [39–41] photolysis of methane (CH4) ices, which has been detected nonlinear medium (Table S3). The VUV light was separated from the [59] on interstellar grains at levels of up to 6%.[42] Along with fundamentals using a lithium fluoride (LiF) biconvex lens (ISP Optics) based on distinct refractive indices of the lens material for ammonia, present at levels of up to 10%, in interstellar ices[42–44] different wavelengths.[60] The photoionized molecules were ex- and also observed, e.g., on moons of Saturn (Enceladus), Uranus tracted, and mass-to-charge ratios were determined on the basis of [45–49] (Miranda), and Pluto (Charon) as well as the Kuiper Belt the arrival time of the ions at a multichannel plate. The signal was Object Quaoar,[50] both 3 and 4 are expected to be formed in amplified with a fast preamplifier (Ortec 9305) and recorded using a interstellar ices. In the hot-core stage, sublimation drives the bin width of 4 ns that was triggered at 30 Hz (Quantum Composers, newly formed molecules into the gas phase, where they might 9518). Infrared and PI-ReTOF-MS assignments were cross checked and verified with ices containing isotopically substituted counter- be detected with radio telescopes such as the Atacama Large 13 13 13 parts such as C2H2:NH3 ( C2H2; >99 C%, Cambridge Isotope), Millimeter/submillimeter Array (ALMA) thus providing a better 15 15 15 13 15 C2H2: NH3 ( NH3; > 98 N%, Sigma Aldrich), C2H2: NH3, and C2D2: understanding on the intrinsic formation pathways of structural NH3 (C2D2; >99 D%, C/D/N Isotope) isomers within the carbon-, hydrogen-, and nitrogen-bearing system. Computational Details The quantum chemical computations rely upon the CCSD(T) Experimental Methods method[61] and the aug-cc-pVTZ basis set[62–63] in large part. The minimum energy
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