1
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He C, Kaiser RI, Lu W, Ahmed M, Pivovarov PS, Kuznetsov OV, Zagidullin MV, Mebel AM. Unconventional Pathway in the Gas-Phase Synthesis of 9H-Fluorene (C 13 H 10 ) via the Radical-Radical Reaction of Benzyl (C 7 H 7 ) with Phenyl (C 6 H 5 ). Angew Chem Int Ed Engl 2023; 62:e202216972. [PMID: 36524679 DOI: 10.1002/anie.202216972] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2022] [Revised: 12/16/2022] [Accepted: 12/16/2022] [Indexed: 12/23/2022]
Abstract
The simplest polycyclic aromatic hydrocarbon (PAH) carrying a five-membered ring-9H-fluorene (C13 H10 )-is produced isomer-specifically in the gas phase by reacting benzyl (C7 H7 ⋅) with phenyl (C6 H5 ⋅) radicals in a pyrolytic reactor coupled with single photon ionization mass spectrometry. The unconventional mechanism of reaction is supported by theoretical calculations, which first produces diphenylmethane and unexpected 1-(6-methylenecyclohexa-2,4-dienyl)benzene intermediates (C13 H12 ) accessed via addition of the phenyl radical to the ortho position of the benzyl radical. These findings offer convincing evidence for molecular mass growth processes defying conventional wisdom that radical-radical reactions are initiated through recombination at their radical centers. The structure of 9H-fluorene acts as a molecular building block for complex curved nanostructures like fullerenes and nanobowls providing fundamental insights into the hydrocarbon evolution in high temperature settings.
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Affiliation(s)
- Chao He
- Department of Chemistry, University of Hawai'i at Mānoa, Honolulu, HI-96822, USA
| | - Ralf I Kaiser
- Department of Chemistry, University of Hawai'i at Mānoa, Honolulu, HI-96822, USA
| | - Wenchao Lu
- Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA-94720, USA
| | - Musahid Ahmed
- Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA-94720, USA
| | - Pavel S Pivovarov
- Samara National Research University, Samara, 443086, Russian Federation
| | - Oleg V Kuznetsov
- Samara National Research University, Samara, 443086, Russian Federation
| | | | - Alexander M Mebel
- Department of Chemistry and Biochemistry, Florida International University, Miami, FL-33199, USA
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2
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Li W, Zhao L, Kaiser RI. A unified reaction network on the formation of five-membered ringed polycyclic aromatic hydrocarbons (PAHs) and their role in ring expansion processes through radical-radical reactions. Phys Chem Chem Phys 2023; 25:4141-4150. [PMID: 36655590 DOI: 10.1039/d2cp05305e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Exploiting a chemical microreactor in combination with an isomer-selective product identification through fragment-free photoionization utilizing tunable vacuum ultraviolet (VUV) light in tandem with the detection of the ionized molecules by a high resolution reflection time-of-flight mass spectrometer (Re-TOF-MS), the present investigation reveals molecular mass growth processes to four distinct polycyclic aromatic hydrocarbons carrying two six- and one five-membered ring (C13H10): 3H-cyclopenta[a]naphthalene, 1H-cyclopenta[b]naphthalene, 1H-cyclopenta[a]naphthalene, and fluorene in the gas phase. Temperatures of 973 and 1023 K simulating conditions in combustion settings along with circumstellar envelopes of carbon-rich stars and planetary nebulae. These reactions highlight the importance of methyl-substituted aromatic reactants (biphenyl, naphthalene) which can be converted to the methylene (-CH2˙) motive by hydrogen abstraction or photolysis. Upon reaction with acetylene, methylene-substituted aromatics carrying a hydrogen atom at the ortho position of the ring can be then converted to cyclopentadiene-annulated aromatics thus providing a versatile pathway to five-membered ring aromatics at elevated temperatures.
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Affiliation(s)
- Wang Li
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China.
| | - Long Zhao
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China. .,School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, Anhui 230027, China
| | - Ralf I Kaiser
- Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822, USA.
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3
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Lemmens AK, Rap DB, Brünken S, Buma WJ, Rijs AM. Polycyclic aromatic hydrocarbon growth in a benzene discharge explored by IR-UV action spectroscopy. Phys Chem Chem Phys 2022; 24:14816-14824. [PMID: 35695165 PMCID: PMC9215700 DOI: 10.1039/d2cp01631a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Infrared signatures of polycyclic aromatic hydrocarbons (PAHs) are detected towards many phases of stellar evolution. PAHs are major players in the carbon chemistry of the interstellar medium, forming the connection between small hydrocarbons and large fullerenes. However, as details on the formation of PAHs in these environments are still unclear, modeling their abundance and chemistry has remained far from trivial. By combining molecular beam mass-selective IR spectroscopy and calculated IR spectra, we analyze the discharge of benzene and identify resulting products including larger PAHs, radicals and intermediates that serve as promising candidates for radio astronomical searches. The identification of various reaction products indicates that different gas-phase reaction mechanisms leading to PAH growth must occur under the same conditions to account for all observed PAH-related species, thereby revealing the complex and interconnected network of PAH formation pathways. The results of this study highlight key (exothermic) reactions that need to be included in astrochemical models describing the carbon chemistry in our universe.
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Affiliation(s)
- Alexander K. Lemmens
- Van't Hoff Institute for Molecular Sciences, University of AmsterdamScience Park 9041098 XHAmsterdamThe Netherlands,Institute for Molecules and Materials, FELIX Laboratory, Radboud UniversityToernooiveld 76525 EDNijmegenThe Netherlands
| | - Daniël B. Rap
- Institute for Molecules and Materials, FELIX Laboratory, Radboud UniversityToernooiveld 76525 EDNijmegenThe Netherlands
| | - Sandra Brünken
- Institute for Molecules and Materials, FELIX Laboratory, Radboud UniversityToernooiveld 76525 EDNijmegenThe Netherlands
| | - Wybren Jan Buma
- Van't Hoff Institute for Molecular Sciences, University of AmsterdamScience Park 9041098 XHAmsterdamThe Netherlands,Institute for Molecules and Materials, FELIX Laboratory, Radboud UniversityToernooiveld 76525 EDNijmegenThe Netherlands
| | - Anouk M. Rijs
- Division of BioAnalytical Chemistry, AIMMS Amsterdam Institute of Molecular and Life Sciences, Vrije Universiteit AmsterdamDe Boelelaan 11081081 HV AmsterdamThe Netherlands
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4
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Galimova GR, Medvedkov IA, Mebel AM. The Role of Methylaryl Radicals in the Growth of Polycyclic Aromatic Hydrocarbons: The Formation of Five-Membered Rings. J Phys Chem A 2022; 126:1233-1244. [PMID: 35138856 DOI: 10.1021/acs.jpca.2c00060] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The regions of the C13H11 potential energy surface (PES) related to the unimolecular isomerization and decomposition of the 1-methylbiphenylyl radical and accessed by the 1-/2-methylnaphthyl + C2H2 reactions have been explored by ab initio G3(MP2,CC)//B3LYP/6-311G(d,p) calculations. The kinetics of these reactions relevant to the growth of polycyclic aromatic hydrocarbons (PAH) under high-temperature conditions in circumstellar envelopes and in combustion flames has been studied employing the RRKM-Master Equation approach. The unimolecular reaction of 1-methylbiphenylyl proceeding via a five-membered ring closure followed by H elimination is predicted to be very fast, on a submicrosecond scale above 1000 K and to result in the formation of an embedded five-membered ring in the 9H-fluorene product. The 1-/2-methylnaphthyl + C2H2 reaction mechanism involves acetylene addition to the radical on the methylene group followed by a six- or five-membered ring closure and aromatization via an H atom loss. Despite of the complexity of the C13H11 PES, these straightforward pathways are dominant in the high-temperature regime (above ∼1000 K), with the prevailing products being phenalene, with a significant contribution of 1H-cyclopenta(a)naphthalene, for 1-methylnaphthyl + C2H2, and 1H-cyclopenta(b)naphthalene and 3H-cyclopenta(a)naphthalene, for 2-methylnaphthyl + C2H2. The methylnaphthyl reactions with acetylene represent a clean source of the three-ring PAHs, but they are relatively slow owing to the high entrance barriers of ∼10 kcal/mol, with the rate constants of about an order of magnitude lower as compared to those for naphthyl + allene and σ-aryl + C2H2. The 1-methylnaphthyl + C2H2 and 2-methylnaphthyl + C2H2 reactions represent prototypes for PAH growth by an extra six- and five-membered ring on a zigzag edge or a corner of PAH and the generated modified Arrhenius expressions are recommended for kinetic modeling of PAH expansion by the mechanism of acetylene addition to methylaryl radicals.
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Affiliation(s)
- Galiya R Galimova
- Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, United States.,Department of Physics, Samara National Research University, Samara 443086, Russian Federation
| | - Iakov A Medvedkov
- Department of Physics, Samara National Research University, Samara 443086, Russian Federation.,Lebedev Physical Institute, Samara 443011, Russian Federation
| | - Alexander M Mebel
- Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, United States
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5
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Zhao L, Prendergast M, Kaiser RI, Xu B, Lu W, Ahmed M, Hasan Howlader A, Wnuk SF, Korotchenko AS, Evseev MM, Bashkirov EK, Azyazov VN, Mebel AM. A molecular beam and computational study on the barrierless gas phase formation of (iso)quinoline in low temperature extraterrestrial environments. Phys Chem Chem Phys 2021; 23:18495-18505. [PMID: 34612388 DOI: 10.1039/d1cp02169a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Despite remarkable progress toward the understanding of the formation pathways leading to polycyclic aromatic hydrocarbons (PAHs) in combustion systems and in deep space, the complex reaction pathways leading to nitrogen-substituted PAHs (NPAHs) at low temperatures of molecular clouds and hydrocarbon-rich, nitrogen-containing atmospheres of planets and their moons like Titan have remained largely obscure. Here, we demonstrate through laboratory experiments and computations that the simplest prototype of NPAHs - quinoline and isoquinoline (C9H7N) - can be synthesized via rapid and de-facto barrier-less reactions involving o-, m- and p-pyridinyl radicals (C5H4N˙) with vinylacetylene (C4H4) under low-temperature conditions.
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Affiliation(s)
- Long Zhao
- Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822, USA.
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6
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Ghildina AR, Zavershinskiy IP, Mebel AM, Vinogradov KY, Bulanova AV, Zhu H. Theoretical Study of the Mechanism and Kinetics of the Oxidation of Cyclopenta[ a]Naphthalenyl Radical C 13H 9 with Molecular Oxygen. J Phys Chem A 2021; 125:6796-6804. [PMID: 34323493 DOI: 10.1021/acs.jpca.1c05421] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Electronic structure/Rice-Ramsperger-Kassel-Marcus Master equation calculations were applied to unravel the oxidation mechanism and kinetics of the cyclopenta[a]naphthalenyl radical with molecular oxygen. The reaction has been shown to proceed through the addition of O2 in the ortho-position in the five-membered ring of C13H9. At low temperatures, the reaction yields a collisionally stabilized C13H9O2 complex, which rapidly decomposes back to the reactants. In the high-temperature regime, above 800, 900, 1125, and 1375 K at pressures of 0.03, 1, 10, and 100 atm, respectively, the reaction forms bimolecular products including 3H-/1H-cyclopenta[a]naphthalen-3-one + OH as the prevailing product together with 1-ethanol-substituted 2-naphthyl radical + CO and 3H-benzo[f]chromen-3-one + H as minor ones, with the branching ratio of the OH elimination channel growing with temperature and the rate constants for the individual bimolecular channels being independent of pressure. The calculated rate constants and product branching for cyclopenta[a]naphthalenyl + O2 closely agree with those reported earlier for the indenyl + O2 reaction and are recommended for the combustion kinetic models for the oxidation reactions of five-membered rings on free edges of larger polycyclic aromatic hydrocarbon molecules. The results also confirm that the oxidation of a π radical located on a five-membered ring with molecular oxygen is very slow.
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Affiliation(s)
- A R Ghildina
- Lebedev Physical Institute, Samara 443011, Russia
| | | | - A M Mebel
- Samara University, Samara 443086, Russia.,Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, United States
| | | | | | - Hong Zhu
- Institute of Modern Catalysis, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China
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7
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Roy T, Ghosal S, Thimmakondu VS. Six Low-Lying Isomers of C 11H 8 Are Unidentified in the Laboratory-A Theoretical Study. J Phys Chem A 2021; 125:4352-4364. [PMID: 34003652 DOI: 10.1021/acs.jpca.1c02247] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Isomers of C11H8 have been theoretically examined using density functional theory and coupled-cluster methods. The current investigation reveals that 2aH-cyclopenta[cd]indene (2), 7-ethynyl-1H-indene (6), 4-ethynyl-1H-indene (7), 6-ethynyl-1H-indene (8), 5-ethynyl-1H-indene (9), and 7bH-cyclopenta[cd]indene (10) remain elusive till date in the laboratory. The puckered low-lying isomer 2 lies at 9 kJ mol-1 below the experimentally known molecule, cyclobuta[de]naphthalene (3), at the fc-CCSD(T)/cc-pVTZ//fc-CCSD(T)/cc-pVDZ level of theory. 2 lies at 36 kJ mol-1 above the thermodynamically most stable and experimentally known isomer, 1H-cyclopenta[cd]indene (1), at the same level. It is identified that 1,2-H transfer from 1 yields 2H-cyclopenta[cd]indene (14) and subsequent 1,2-H shift from 14 yields 2. Appropriate transition states have been identified, and intrinsic reaction coordinate calculations have been carried out at the B3LYP/6-311+G(d,p) level of theory. Recently, 1-ethynyl-1H-indene (11) has been detected using synchrotron-based vacuum ultraviolet ionization mass spectrometry. 2-Ethynyl-1H-indene (4) and 3-ethynyl-1H-indene (5) have been synthetically characterized in the past. While the derivatives of 7bH-cyclopenta[cd]indene (10) have been isolated elsewhere, the parent compound remains unidentified till date in the laboratory. Although C11H8 is a key elemental composition of astronomical interest for the formation of polycyclic aromatic hydrocarbons in the interstellar medium, none of its low-lying isomers have been characterized by rotational spectroscopy though they are having a permanent dipole moment (μ ≠ 0). Therefore, energetic and spectroscopic properties have been computed, and the present investigation necessitates new synthetic studies on C11H8, in particular 2, 6-10, and also rotational spectroscopic studies on all low-lying isomers.
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Affiliation(s)
- Tarun Roy
- Department of Chemistry, National Institute of Technology Durgapur, M G Avenue, Durgapur 713 209, India
| | - Subhas Ghosal
- Department of Chemistry, National Institute of Technology Durgapur, M G Avenue, Durgapur 713 209, India
| | - Venkatesan S Thimmakondu
- Department of Chemistry and Biochemistry, San Diego State University, San Diego 92182-1030, California, United States
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8
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Abstract
This Perspective presents recent advances in our knowledge of the fundamental elementary mechanisms involved in the low- and high-temperature molecular mass growth processes to polycyclic aromatic hydrocarbons in combustion systems and in extraterrestrial environments (hydrocarbon-rich atmospheres of planets and their moons, cold molecular clouds, circumstellar envelopes). Molecular beam studies combined with electronic structure calculations extracted five key elementary mechanisms: Hydrogen Abstraction-Acetylene Addition, Hydrogen Abstraction-Vinylacetylene Addition, Phenyl Addition-DehydroCyclization, Radical-Radical Reactions, and Methylidyne Addition-Cyclization-Aromatization. These studies, summarized here, provide compelling evidence that key classes of aromatic molecules can be synthesized in extreme environments covering low temperatures in molecular clouds (10 K) and hydrocarbon-rich atmospheres of planets and their moons (35-150 K) to high-temperature environments like circumstellar envelopes of carbon-rich Asymptotic Giant Branch Stars stars and combustion systems at temperatures above 1400 K thus shedding light on the aromatic universe we live in.
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Affiliation(s)
- Ralf I Kaiser
- Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States
| | - Nils Hansen
- Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551, United States
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9
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Zhao L, Doddipatla S, Kaiser RI, Lu W, Kostko O, Ahmed M, Tuli LB, Morozov AN, Howlader AH, Wnuk SF, Mebel AM, Azyazov VN, Mohamed RK, Fischer FR. Gas-phase synthesis of corannulene – a molecular building block of fullerenes. Phys Chem Chem Phys 2021; 23:5740-5749. [DOI: 10.1039/d0cp06537d] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Corannulene can be formed through molecular mass growth processes in circumstellar envelopes.
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10
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Howlader AH, Diaz K, Mebel AM, Kaiser RI, Wnuk SF. Iodoindenes: Synthesis and application to cross-coupling. Tetrahedron Lett 2020. [DOI: 10.1016/j.tetlet.2020.152427] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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11
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Zhao L, Kaiser RI, Lu W, Kostko O, Ahmed M, Evseev MM, Bashkirov EK, Oleinikov AD, Azyazov VN, Mebel AM, Howlader AH, Wnuk SF. Gas phase formation of cyclopentanaphthalene (benzindene) isomers via reactions of 5- and 6-indenyl radicals with vinylacetylene. Phys Chem Chem Phys 2020; 22:22493-22500. [DOI: 10.1039/d0cp03846f] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The reaction of indenyl radicals with vinylacetylene leads to cyclopentanaphthalene at low temperature.
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Affiliation(s)
- Long Zhao
- Department of Chemistry
- University of Hawaii at Manoa
- Honolulu
- USA
| | - Ralf I. Kaiser
- Department of Chemistry
- University of Hawaii at Manoa
- Honolulu
- USA
| | - Wenchao Lu
- Chemical Sciences Division
- Lawrence Berkeley National Laboratory
- Berkeley
- USA
| | - Oleg Kostko
- Chemical Sciences Division
- Lawrence Berkeley National Laboratory
- Berkeley
- USA
| | - Musahid Ahmed
- Chemical Sciences Division
- Lawrence Berkeley National Laboratory
- Berkeley
- USA
| | | | | | - Artem D. Oleinikov
- Samara National Research University
- Samara 443086
- Russian Federation
- Lebedev Physical Institute
- Samara 443011
| | - Valeriy N. Azyazov
- Samara National Research University
- Samara 443086
- Russian Federation
- Lebedev Physical Institute
- Samara 443011
| | - Alexander M. Mebel
- Samara National Research University
- Samara 443086
- Russian Federation
- Department of Chemistry and Biochemistry
- Florida International University
| | - A. Hasan Howlader
- Department of Chemistry and Biochemistry
- Florida International University
- Miami
- USA
| | - Stanislaw F. Wnuk
- Department of Chemistry and Biochemistry
- Florida International University
- Miami
- USA
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12
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Zhao L, Kaiser RI, Lu W, Ahmed M, Oleinikov AD, Azyazov VN, Mebel AM, Howlader AH, Wnuk SF. Gas phase formation of phenalene via 10π-aromatic, resonantly stabilized free radical intermediates. Phys Chem Chem Phys 2020; 22:15381-15388. [DOI: 10.1039/d0cp02216k] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
1H-Phenalene can be synthesized via the reaction of the 1-naphthyl radical with methylacetylene and allene under high temperature conditions prevalent in carbon-rich circumstellar environments and combustion systems.
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Affiliation(s)
- Long Zhao
- Department of Chemistry
- University of Hawaii at Manoa
- Honolulu
- USA
| | - Ralf I. Kaiser
- Department of Chemistry
- University of Hawaii at Manoa
- Honolulu
- USA
| | - Wenchao Lu
- Chemical Sciences Division
- Lawrence Berkeley National Laboratory
- Berkeley
- USA
| | - Musahid Ahmed
- Chemical Sciences Division
- Lawrence Berkeley National Laboratory
- Berkeley
- USA
| | - Artem D. Oleinikov
- Department of Physics
- Samara National Research University
- Samara 443086
- Russian Federation
- Lebedev Physical Institute
| | - Valeriy N. Azyazov
- Department of Physics
- Samara National Research University
- Samara 443086
- Russian Federation
- Lebedev Physical Institute
| | - Alexander M. Mebel
- Department of Physics
- Samara National Research University
- Samara 443086
- Russian Federation
- Department of Chemistry and Biochemistry
| | - A. Hasan Howlader
- Department of Chemistry and Biochemistry
- Florida International University
- Miami
- USA
| | - Stanislaw F. Wnuk
- Department of Chemistry and Biochemistry
- Florida International University
- Miami
- USA
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13
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Kozliak E, Sulkes M, Alhroub I, Kubátová A, Andrianova A, Seames W. Influence of early stages of triglyceride pyrolysis on the formation of PAHs as coke precursors. Phys Chem Chem Phys 2019; 21:20189-20203. [PMID: 31486462 DOI: 10.1039/c9cp02025j] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Molecular beam (MB) time-of-flight mass spectrometry has been used to investigate thermal decomposition of triolein, to reveal the mechanisms of low temperature soot/coke formation characteristic for triglycerides (TGs). Mass detected pyrolysis products were observed at incremented temperatures using both VUV single photon ionization (general product detection) and REMPI based selective detection of aromatic products. To augment the simple mass characterizations, we have employed stoichiometric considerations; we have supplemented the analysis further by using the detailed information available from product analysis of batch reactor TG cracking. Both the VUV photoionization and batch reactor studies indicated that formation of C7-sized stable products is a marker of significant triolein decomposition that is coupled with PAH formation. A significant fraction of the C7 species observed likely formed as a result of a C-C bond scission at the allylic position to the ω-9 double bond of oleic acid. REMPI detection indicated a high specificity for PAH formation at three distinct molecular weight values, 276, 352 and 444 amu (the latter being a fullerene precursor). The stoichiometric analysis has shown that these PAHs likely arise from condensation reactions of either C7- or C8-sized fragments (three, four and five, respectively). The C8-sized intermediate would become essential whenever the PAH product of C7 fragment condensation contained an odd number of carbon atoms, resulting in a less stable aromatic structure with an incomplete double bond conjugation. MB experiments involving either addition or in situ generation of hydrogen resulted in an enhancement of lower molecular weight PAH formation, i.e., a decrease in the effective number of condensing fragments. In contrast, an increase in temperature yielded the opposite effect.
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