- Formation of the propargyl radical in the reaction of 1CH2 and C2H2: Experiment and modelling
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The propargyl radical, C3H3, is thought to be an important precursor to the formation of aromatic compounds and of soot in combustion systems. These radicals are produced during combustion by the reaction of 1CH2 with acetylene, which proceeds via a three well mechanism. A master equation model of this system is constructed with the aim of determining the branching ratio for formation of the propargyl radical as a function of temperature and pressure. The rate limiting step is the initial formation of cyclopropene from the reactants and a knowledge of the rate of this reaction is necessary for accurate modelling. The rate coefficient for the overall reaction was measured, as a function of temperature, using laser flash photolysis of a ketene-acetylene mixture. The reaction was monitored by laser induced fluorescence of 1CH2. Experimental results are presented and used in the master equation model, which shows that the yield, γ, of dissociation products H + C3H3 decreases with increasing pressure and that the onset of the decrease shifts to higher pressures as the temperature increases. At higher pressures and temperatures, there is an overlap in the timescales of dissociation of thermalised C3H4 and of the nascent C3H*4 formed from 1CH2 + C2H2, so that a simple description through time independent rate coefficients is no longer possible.
- Blitz, Mark A.,Beasley, Martin S.,Pilling, Michael J.,Robertson, Struan H.
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- Propargyl from the Reaction of Singlet Methylene with Acetylene
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The technique of infrared kinetic spectroscopy has been used to study the production of propargyl radical from the reaction of singlet methylene with acetylene.The rate constant for this product channel was determined to be (3.5 +/- 0.7)E-10 cm3 molecule-1 s-1 at 295 K, measured relative to the known rate for 1CH2 with H2 or CH4.Methylene was produced in the singlet state by excimer laser photolysis of ketene at 308 nm in the presence of acetylene and either H2 or CH4.Reaction of 1CH2 with acetylene produces propargyl, and reaction of 1CH2 with either H2 or CH4 produces CH3.The intensity of a propargyl infrared absorption line was compared with that of a methyl infrared absorption line, and the rate of formation of propargyl was determined from the ratio of these two intensities and the known rate of reaction of singlet methylene with H2 (or CH4) to produce CH3.The relative peak infrared absorption cross sections of methyl and propargyl were calibrated under the conditions of the experiment by photolyzing crotyl bromide at 193 nm to produce methyl and propargyl in equal concentrations.
- Adamson, J. D.,Morter, C. L.,DeSain, J. D.,Glass, G. P.,Curl, R. F.
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- Decomposition of Picolyl Radicals at High Temperature: A Mass Selective Threshold Photoelectron Spectroscopy Study
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The reaction products of the picolyl radicals at high temperature were characterized by mass-selective threshold photoelectron spectroscopy in the gas phase. Aminomethylpyridines were pyrolyzed to initially produce picolyl radicals (m/z=92). At higher temperatures further thermal reaction products are generated in the pyrolysis reactor. All compounds were identified by mass-selected threshold photoelectron spectroscopy and several hitherto unexplored reactive molecules were characterized. The mechanism for several dissociation pathways was outlined in computations. The spectrum of m/z=91, resulting from hydrogen loss of picolyl, shows four isomers, two ethynyl pyrroles with adiabatic ionization energies (IEad) of 7.99 eV (2-ethynyl-1H-pyrrole) and 8.12 eV (3-ethynyl-1H-pyrrole), and two cyclopentadiene carbonitriles with IE′s of 9.14 eV (cyclopenta-1,3-diene-1-carbonitrile) and 9.25 eV (cyclopenta-1,4-diene-1-carbonitrile). A second consecutive hydrogen loss forms the cyanocyclopentadienyl radical with IE′s of 9.07 eV (T0) and 9.21 eV (S1). This compound dissociates further to acetylene and the cyanopropynyl radical (IE=9.35 eV). Furthermore, the cyclopentadienyl radical, penta-1,3-diyne, cyclopentadiene and propargyl were identified in the spectra. Computations indicate that dissociation of picolyl proceeds initially via a resonance-stabilized seven-membered ring.
- Reusch, Engelbert,Holzmeier, Fabian,Gerlach, Marius,Fischer, Ingo,Hemberger, Patrick
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supporting information
p. 16652 - 16659
(2019/12/24)
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- Formation of fulvene in the reaction of C2H with 1,3-butadiene
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Abstract Products formed in the reaction of C2H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C6H6. The photoionization spectrum of the C6H6 product indicates that fulvene is formed with a branching fraction of (57 ± 30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.
- Lockyear, Jessica F.,Fournier, Martin,Sims, Ian R.,Guillemin, Jean-Claude,Taatjes, Craig A.,Osborn, David L.,Leone, Stephen R.
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p. 232 - 245
(2015/04/14)
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- Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: An EPR spectroscopic study of chain propagation
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Hydrogen abstraction from 1-substituted cyclohexa-2,5-diene-1-carboxylic acids containing linear, branched and cyclic alkyl substituents, as well as allyl, propargyl (prop-2-ynyl), cyanomethyl and benzyl substituents, has been studied by EPR spectroscopy. For each carboxylic acid, EPR spectra of the corresponding cyclohexadienyl radicals were observed at lower temperatures, followed by spectra due to ejected carbon-centred radicals at higher temperatures. Rate constants, for release of the carbon-centred radicals from the cyclohexadienyl radicals, were determined from radical concentration measurements for the above range of substituents. The rate of cyclohexadienyl radical dissociation increased with branching in the 1-alkyl substituent and with electron delocalisation in the ejected carbon-centred radical; 3,5-and 2,6-dimethyl-substitution of the cyclohexadienyl ring led to reductions in the dissociation rate constants. Rate data for abstraction of bisallylic hydrogens from the cyclohexadienyl acids were also obtained for ethyl, n-propyl and isopropyl radicals. These results indicated a sharp drop in the rate of hydrogen abstraction as the degree of branching in the attacking radical increased. Small decreases in the hydrogen abstraction rate constants were observed for cyclohexadienes containing CO2R substituents.
- Jackson, Leon V.,Walton, John C.
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p. 1758 - 1764
(2007/10/03)
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- Photofragment translational spectroscopy of 1,2-butadiene at 193 nm
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The dissociation dynamics of 1,2-butadiene at 193 nm were analyzed by photofragment translational spectroscopy. Tunable vacuum ultraviolet (VUV) synchrotron radiation at the Advanced Light Source (ALS) was used for ionization of scattered photoproducts. I
- Robinson, Jason C.,Sun, Weizhong,Harris, Sean A.,Qi, Fei,Neumark, Daniel M.
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p. 8359 - 8365
(2007/10/03)
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- A combined crossed beam and ab initio investigation on the reaction of carbon species with C4H6 isomers. I. The 1,3-butadiene molecule, H2CCHCHCH2(X1A')
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The reaction between ground state carbon atoms, C(3Pj), and 1,3-butadiene, H2CCHCHCH2, was studied at three averaged collision energies between 19.3 and. 38.8 kJmol-1 using the crossed molecular beam technique. Our experimental data combined with electronic structure calculations show that the carbon atom adds barrierlessly to the ?-orbital of the butadiene molecule via a loose, reactantlike transition state located at the centrifugal barrier. This process forms vinylcyclopropylidene which rotates in a plane almost perpendicular to the total angular momentum vector J around its C-axis. The initial collision complex undergoes ring opening to a long-lived vinyl-substituted triplet allene molecule. This complex shows three reaction pathways. Two distinct H atom loss channels form 1- and 3-vinylpropargyl radicals, HCCCHC2H3(X2A ) and H2CCCC2H3(X2A ), through tight exit transition states located about 20 kJmol-1 above the products; the branching ratio of 1- versus 3-vinylpropargyl radical is about 8:1. A minor channel of less than 10 percent is the formation of a vinyl, C2H3(X2A'), and propargyl radical C3H3(X2B2). The unambiguous identification of two C5H5 chain isomers under single collision has important implications to combustion processes and interstellar chemistry. Here, in denser media such as fuel flames and in circumstellar shells of carbon stars, the linear structures can undergo a collision-induced ring closure followed by a hydrogen migration to cyclic C5H5 isomers such as the cyclopentadienyl radical-a postulated intermediate in the formation of polycyclic aromatic hydrocarbons (PAHs).
- Hahndorf, I.,Lee, H. Y.,Mebel, A. M.,Lin, S. H.,Lee, Y. T.,Kaiser, R. I.
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p. 9622 - 9636
(2007/10/03)
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- Photodissociation dynamics of the allyl radical
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The photochemistry and photodissociation dynamics of the allyl radical upon ultraviolet (UV) excitation is investigated in a molecular beam by using time- and frequency-resolved photoionization of hydrogen atoms with Lyman-a-radiation. The UV states of allyl decay by internal conversion to the ground state, forming vibrationally hot radicals that lose hydrogen atoms on a nanosecond time scale. Two channels are identified, formation of allene directly from allyl, and isomerization from allyl to 2-propenyl, with a subsequent hydrogen loss, resulting in both allene and propyne formation. The branching ratio is between 2:1 and 3:1, with direct formation of allene being the dominant reaction channel. This channel is associated with site-selective loss of hydrogen from the central carbon atom, as observed in experiments on isotopically labeled radicals. Ab initio calculations of the reaction pathways and Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the rates are in agreement with the mechanism and branching ratios. From the measured Doppler profiles a translational energy release of 14±1 kcal/mol is calculated. The calculated value of 66 kcal/mol for the barrier to the 1,2 hydrogen shift from allyl radical to 2-propenyl is confirmed by the experimental data.
- Deyerl, Hans-Juergen,Fischer, Ingo,Chen, Peter
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p. 1450 - 1462
(2007/10/03)
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- Detailed kinetics of cyclopentadiene decomposition studied in a shock tube
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Mixtures of cyclopentadiene diluted with argon were used to investigate its decomposition pattern in a single pulse shock tube. The temperatures ranged from 1080 to 1550 K and pressures behind the shock were between 1.7-9.6 atm. The cyclopentadiene concentrations ranged from 0.5 to 2%. Gas-chromatographic analysis was used to determine the product distribution The main products in order of abundance were acetylene, ethylene, methane, allene, propyne, butadiene, propylene, and benzene. The decomposition of cyclopentadiene was simulated with a kinetic scheme containing 44 species and 144 elementary reactions. This was later reduced to only 36 reactions The ring opening process of the cyclopentadienyl radical was found to be the crucial step in the mechanism. 1997 lohn Wiley and Sons, Inc.
- Burcat, Alexander,Dvinyaninov, Michael
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p. 505 - 514
(2007/10/03)
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- Study of Higher Hydrocarbon Production during Ethylacetylene Pyrolysis Using Laser-Generated Vacuum-Ultraviolet Photoionization Detection
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Higher hydrocarbon formation during the pyrolysis of ethylacetylene in a microjet reactor was studied by vacuum-ultraviolet photoionization time-of-flight mass spectrometry.At the wavelength employed, this ionization technique allows for the simultaneous detection of both stable and intermediate polyatomic species with ionization potentials below 10.49 eV, including most hydrocarbons with two or more carbon atoms.Minimal fragmentation simplifies the determination of parent species and allows identification of probable reaction pathways involving hydrocarbon radicals as well as stable species.The pyrolysis of ethylacetylene was carried out in the fast-flow microjet reactor (residence times 1-2 ms) at temperatures from 300 to 1600 K.At temperatures below 1500 K, products are predominantly linear conjugated compounds that are either primary pyrolysis products such as C3H3 or products of C1 and C2 addition and abstraction reactions.The first greater than four carbon hydrocarbons to be detected at 2-ms residence time were mass 78, at 1450 K, and mass 92, at 1280 K (likely predominantly benzene and toluene, respectively).At higher reactor temperatures, a progression of polymerization products was observed including likely aromatic species.Accompanying this increase, particularly above temperatures of 1600 K, are dramatic decreases in the concentration of species with fewer than 10 carbon atoms, due to their combination to form large polyaromatic hydrocarbon species.Analysis of the data shows that production rates of masses 78 and 92 amu are consistent with a low activation energy reaction of C4H5 with acetylene and methylacetylene (propyne).A mechanism involving the recombination of C3H3 radicals followed by isomerization can also account for mass 78 production, assuming it proceeded at high-pressure-limit recombination rate which has been estimated by previous investigators.
- Boyle, James,Pfefferle, Lisa
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p. 3336 - 3340
(2007/10/02)
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