An experimental and theoretical study of toluene pyrolysis with tunable synchrotron VUV photoionization and molecular-beam mass spectrometry

Zhang, Taichang; Zhang, Youmin; Hong, Xin; Zhang, Kuiwen; Qi, Fei; Law, Chung K.; Ye, Taohong; Zhao, Ping; Chen, Yiliang

doi:10.1016/j.combustflame.2009.06.001

Cited by 118 publications

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References 64 publications

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“…A C 7 H 5 species was previously observed in a toluene flame, [4] but could not be unambiguously assigned to a specific isomer. Herein we report the direct observation of fulvenallenyl in a threshold photoelectron spectrum (TPES) obtained using synchrotron radiation.…”

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confidence: 93%

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Photoionization of C₇H₆ and C₇H₅: Observation of the Fulvenallenyl Radical

et al. 2010

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Species of the composition C 7 H n are known intermediates in combustion processes.[1] One out of five C 7 H 5 isomers, fulvenallenyl, was recently computed to be a resonance-stabilized radical [2] expected to be long-lived and to play an important role in the formation of soot and polycyclic aromatic hydrocarbons.[3] Thus, its structural and thermochemical characterization is relevant for the combustion community. A C 7 H 5 species was previously observed in a toluene flame, [4] but could not be unambiguously assigned to a specific isomer. Herein we report the direct observation of fulvenallenyl in a threshold photoelectron spectrum (TPES) obtained using synchrotron radiation. We also report a TPES of the C 7 H 6 isomer fulvenallene together with an accurate ionization energy (IE) of both species. Such data on reactive species are necessary for their isomer-selective detection in flames. [5] Fulvenallene as well as fulvenallenyl were produced by flash pyrolysis [6] of phthalide according to Scheme 1. This approach has been used before [7] to record conventional photoelectron spectra of fulvenallene, an unstable species despite being a closed-shell molecule. Figure 1 shows mass spectra recorded at a photon energy of 12.0 eV. Without pyrolysis (lower trace) a peak due to the phthalide precursor at m/z = 134 is visible as well as a smaller peak at m/z = 105, which is due to dissociative photoionization via the loss of HCO. When the pyrolysis is turned on (upper trace), the precursor signal decreases and a peak at m/z = 90 appears, corresponding to C 7 H 6 . In addition, a peak is revealed at m/z = 89, which grows in intensity with increasing pyrolysis temperature. The calculated bond dissociation energy of the allenic CÀH bond is 340 kJ mol À1 and is the by far lowest in fulvenallene.[2] The selective loss of this H-atom at high temperatures is thus not surprising. Small fragment peaks at lower masses are also present at high temperatures.We subsequently recorded TPE-spectra of both species using photoelectron-photoion coincidence spectroscopy. [8] This method permits recording a mass-selected photoelectron spectrum for each species, and is well-suited for the investigation of reactive intermediates.[9] Both spectra in Figure 2 are dominated by one large peak at 8.22 eV (C 7 H 6 ) and 8.19 eV (C 7 H 5 ). Additional scans below 8 eV did not reveal any further signal. The dissociative ionization of fulvenallene only sets in above 11 eV and does not perturb the mass-selected TPES of C 7 H 5 . Ionization energies of five different C 7 H 5 isomers were computed by density functional theory (DFT) using the B3LYP exchange correlation functional and the 6-311 + + G** basis. [10] The fulvenallenyl IE (Scheme 1) was computed to be 8.17 eV

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confidence: 93%

“…) with a resolving power of 10 4 . A rare gas filter operated with a Kr/Ar/Ne mixture at a pressure of 10 mbar suppresses the higher harmonic radiation.…”

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Photoionization of C₇H₆ and C₇H₅: Observation of the Fulvenallenyl Radical

et al. 2010

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“…Significant quantities of some unidentified C 11 H 7 isomer(s) have been detected however, along with the closed-shell C 11 H 8 parent ethynyl-1H-indene, in the pyrolysis of toluene. 19 Toluene pyrolysis produces large amounts of fulvenallene through thermal decomposition of the benzyl radical and the c-C 5 H 5 + C 2 H 2 reaction, providing a C 7 H 5 radical precursor. À9.380…”

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confidence: 99%

Chemically activated reactions on the C7H5 energy surface: propargyl + diacetylene, i-C5H3 + acetylene, and n-C5H3 + acetylene

Silva

Trevitt

2011

Phys. Chem. Chem. Phys.

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This study uses computational chemistry and statistical reaction rate theory to investigate the chemically activated reaction of diacetylene (butadiyne, C4H2) with the propargyl radical (C •H2CCH) and the reaction of acetylene (C 2H2) with the i-C5H3 (CH 2CCCC•H) and n-C5H3 (CHCC •HCCH) radicals. A detailed G3SX-level C7H 5 energy surface demonstrates that the C3H3 + C4H2 and C5H3 + C2H 2 addition reactions proceed with moderate barriers, on the order of 10 to 15 kcal mol-1, and form activated open-chain C 7H5 species that can isomerize to the fulvenallenyl radical with the highest barrier still significantly below the entrance channel energy. Higher-energy pathways are available leading to other C 7H5 isomers and to a number of C7H4 species + H. Rate constants in the large multiple-well (15) multiple-channel (30) chemically activated system are obtained from a stochastic solution of the one-dimensional master equation, with RRKM theory for microcanonical rate constants. The dominant products of the C4H2 + C 3H3 reaction at combustion-relevant temperatures and pressures are i-C5H3 + C2H2 and CH2CCHCCCCH + H, along with several quenched C7H 5 intermediate species below 1500 K. The major products in the n-C5H3 + C2H2 reaction are i-C 5H3 + C2H2 and a number of C 7H4 species + H, with C7H5 radical stabilization at lower temperatures. The i-C5H3 + C 2H2 reaction predominantly leads to C7H 4 + H and to stabilized C7H5 products. The title reactions may play an important role in polycyclic aromatic hydrocarbon (PAH) formation in combustion systems. The C7H5 potential energy surface developed here also provides insight into several other important reacting gas-phase systems relevant to combustion and astrochemistry, including C2H + the C3H4 isomers propyne and allene, benzyne + CH, benzene + C(3P), and C7H5 radical decomposition, for which some preliminary analysis is presented.

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“…1 product mass spectrum following corona discharge on toluene in rare gas is qualitatively similar to product mass spectra for toluene in rare gas following pyrolysis [35] or in a flames [36]. In each of these cases there is a clear pattern of formation of higher mass aromatic products; products at masses below toluene are seen as well.…”

Section: Resultsmentioning

confidence: 76%

Post Discharge Chemistry of Aromatic Molecules in Rare Gas

Sulkes

2011

Plasma Chem Plasma Process

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We have employed 800 nm 150 fs laser pulses to carry out photoionization (PI) time-of-flight mass spectrometric detection of intermediates following corona discharges on aromatic molecules (alkylbenzenes and pyridine) in He, stabilized by subsequent supersonic gas expansions. Observed product peaks appear to be at least roughly in proportion to actual number densities; PI induced fragmentation of parent ions appears not to be excessive. Consequently, 800 nm fs PI should be useful for general product analysis applications in plasma chemistry. For most alkylbenzenes subjected to corona discharges in rare gas, the overall trends in product chemistry are similar in many respects to observed flame and pyrolysis chemistry in rare gases for the same species. Following discharge, as in those other cases, H deficient carbon radical fragments initially produced react in turn to form larger aromatic species. However, compared with flame/pyrolysis, discharge produced a larger number of fragment species, which can lead to a wider and somewhat different range of higher mass aromatic products. Co-addition of even a small component of O 2 to the discharge mixes has a potent effect in inhibiting formation of higher mass aromatic products in alkylbenzenes.

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An experimental and theoretical study of toluene pyrolysis with tunable synchrotron VUV photoionization and molecular-beam mass spectrometry

Cited by 118 publications

References 64 publications

Photoionization of C₇H₆ and C₇H₅: Observation of the Fulvenallenyl Radical

Photoionization of C₇H₆ and C₇H₅: Observation of the Fulvenallenyl Radical

Chemically activated reactions on the C7H5 energy surface: propargyl + diacetylene, i-C5H3 + acetylene, and n-C5H3 + acetylene

Post Discharge Chemistry of Aromatic Molecules in Rare Gas

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An experimental and theoretical study of toluene pyrolysis with tunable synchrotron VUV photoionization and molecular-beam mass spectrometry

Cited by 118 publications

References 64 publications

Photoionization of C7H6 and C7H5: Observation of the Fulvenallenyl Radical

Photoionization of C7H6 and C7H5: Observation of the Fulvenallenyl Radical

Chemically activated reactions on the C7H5 energy surface: propargyl + diacetylene, i-C5H3 + acetylene, and n-C5H3 + acetylene

Post Discharge Chemistry of Aromatic Molecules in Rare Gas

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Photoionization of C₇H₆ and C₇H₅: Observation of the Fulvenallenyl Radical

Photoionization of C₇H₆ and C₇H₅: Observation of the Fulvenallenyl Radical