- The vinyl radical investigated by infrared diode laser kinetic spectroscopy
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A c-type band was observed at around 895 cm-1 by infrared diode laser kinetic spectroscopy combined with the excimer laser photolysis of vinyl halides at 193 nm and was assigned to the CH2 wagging mode of the vinyl radical.The band was found to consist of two component bands separated by 0.0541(11) cm-1.Both component bands showed clearly the statistical weight in an alternative way, that is, of one shows the weight 1:3 for even:odd Ka levels, the other exhibits 3:1, indicating that the radical is of C2υ effective symmetry, executing a double-minimum motion probably associated with the C-H in-plane rocking vibration.The upper states of the two bands were found to be perturbed weakly, possibly by a Coriolis interaction with the first overtone state of the C-H rocking mode.
- Kanamori, Hideto,Endo, Yasuki,Hirota, Eizi
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Read Online
- Competitive hydrogen-atom abstraction versus oxygen-atom and electron transfers in gas-phase reactions of [X4O10]+ (X = P, V) with C2H4
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(Figure Presented) Why so different? The comparison of the reaction of "bare" [P4O10],+ and [V4O 10]"+with ethene by mass-spectrometric and computational studies permits insight into mechanistic aspects of the c
- Dietl, Nicolas,Engeser, Marianne,Schwarz, Helmut
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Read Online
- 193 nm photolysis of vinyl bromide: Nascent product distribution of the C2H3Br→C2H2 (vinylidene)+HBr channel
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The internal energy content of fragment HBr and C2H2 following photolysis of the precursor, vinyl bromide, at 193 nm was determined using time-resolved Fourier transform spectroscopy (TR FTS). Data taken 1 μs after the laser photolys
- Liu,Letendre,Dai
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p. 1734 - 1741
(2007/10/03)
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- Site-specific dissociation dynamics of ethylene at 157 nm: atomic and molecular hydrogen elimination
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Photodissociation of five ethylene isotopomers at 157 nm was studied using a molecular beam apparatus. From this study, complete and interesting information on the site and isotope effects on the molecular hydrogen elimination processes from ethylene were also detected. Using the new improved experimental technique, dynamical differences between different microchannels of molecular hydrogen elimination processes from ethylene were also detected. Site and isotopic effects on the molecular hydrogen elimination processes were also clearly observed.
- Lin, Jim J.,Wang, Chia C.,Lee, Yuan T.,Yang, Xueming
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p. 9668 - 9677
(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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- Products of the ultraviolet photodissociation of trifluoroacetic acid and acrylic acid
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The photodissociation of trifluoroacetic and acrylic acids by the ultraviolet light from a flashlamp has been investigated by measuring the relative yields of some of the major products by time-resolved infrared absorption using tunable, narrow band diode lasers. Yields of CO2 were measured both in the absence and presence of added O2. The former experiments measure the CO2 produced directly by decarboxylation of the acid, channel (2) below, the latter the sum of the yields from channels (1) and (2) since HOCO is rapidly converted to CO2. The yields of CO from the decarbonylation channel (3) have also been measured. For trifluoroacetic acid, the relative yields are found to be [HOCO]: [CO2]: [CO] = (0.28 ± 0.07): (0.61 ± 0.09): (0.11 ± 0.06) and, for acrylic acid, [HOCO]: [CO2]: [CO] = (0.32 ± 0.08): (0.37 ± 0.08) :(0.31 ± 0.09). The results are discussed in relation to the other, limited, measurements on the photodissociation of these acids and whether these three processes are likely to occur independently of one another.
- Osborne, Michael C.,Li, Qiang,Smith, Ian W. M.
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p. 1447 - 1454
(2007/10/03)
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- Absolute Rate Constant and Product Branching Fractions for the Reaction between F and C2H4 at T = 202-298 K
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The discharge-flow kinetic technique coupled to mass-spectrometric detection has been used to determine the variable-temperature dependence of the rate constant and product branching fractions for the reaction between F(2P) and C2H4 at P = 1 Torr nominal pressure (He). The reaction was studied at T = 202 and 236 K by monitoring the decay of C2H4 in the presence of a large excess of F(2P). The overall rate coefficients were determined to be k1(202 K) = (1.7 ± 0.4) x 10-10 cm3 molecule-1 s-1 and k1(236 K) = (2.1 ± 0.5) x 10-10 cm3 molecule-1 s-1 with the quoted uncertainty representing total errors. Further, the branching fractions for the two observed reaction channels F + C2H4 → C2H3 + HF (1a) and F + C2H4 → C2H3F + H (1b) were determined by quantitatively measuring the yield of C2H3F under conditions of excess C2H4. The stabilized adduct, C2H4F, was not detected at T = 202 K. The derived branching fractions were Γ1a(202 K) = 0.25 ± 0.09, Γ1b (202 K) = 0.75 ± 0.16, and Γ1a(236 K) = 0.27 ± 0.13, and Γ1b (236 K) = 0.73 ± 0.20, where the quoted uncertainty represents total errors. By inclusion of k1(298 K) = (3.0 ± 0.8) x 10-10 cm3 molecule-1 s-1, a revised value that used data from our previous study and Γ1a(298 K) = 0.35 ± 0.04 and Γ1b (298 K) = 0.65 ± 0.04 from a laser photolysis/photoionization mass spectrometry study, we obtain the Arrhenius expressions k1a(T) = (7.5 ± 4.0) x 10-10 exp[(-1.2 ± 0.3)/(RT)] and k1b(T) = (5.2 ± 1.0) x 10-10 exp[(-0.6 ± 0.1)/(RT)] in units of cm3 molecule-1 s-1 for k and in units of kcal mol-1 for activation energy. The quoted uncertainty represents total errors at 1σ precision errors plus 15% systematic errors. RRKM calculations have shown that the critical energy for H addition to C2H3F is less than 6 kcal mol-1 larger than that for the addition of F to C2H4 and that the competitive decomposition of chemically activated C2H4F radicals favor C-H bond rupture by a factor greater than 1000 over that for C-F bond rupture.
- Nesbitt, Fred L.,Thorn Jr., R. Peyton,Payne Jr., Walter A.,Tardy
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p. 4470 - 4479
(2007/10/03)
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- Photodissociation of acrylonitrile at 193 nm: A photofragment translational spectroscopy study using synchrotron radiation for product photoionization
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We have investigated the photodissociation of acrylonitrile (H2CCHCN) at 193 nm using the technique of photofragment translational spectroscopy. The experiments were performed at the Chemical Dynamics Beamline at the Advanced Light Source and used tunable vacuum ultraviolet synchrotron radiation for product photoionization. We have identified four primary dissociation channels including atomic and molecular hydrogen elimination. HCN elimination, and CN elimination. There is significant evidence that all of the dissociation channels occur on the ground electronic surface following internal conversion from the initially optically prepared state. The product translational energy distributions reflect near statistical simple bond rupture for the radical dissociation channels, while substantial recombination barriers mediate the translational energy release for the two molecular elimination channels. Photoionization onsets have provided additional insight into the chemical identities of the products and their internal energy content.
- Blank, David A.,Suits, Arthur G.,Lee, Yuan T.,North, Simon W.,Hall, Gregory E.
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p. 5784 - 5794
(2007/10/03)
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- Crossed molecular beam study of the reaction O(3P) + allene
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The reaction between ground state (3P) oxygen atoms and allene was studied under single collision conditions using the crossed molecular beams method.Product angular distributions and the translational energy distribution were determined for ea
- Schmoltner, A. M.,Huang, S. Y.,Brudzynski, R. J.,Chu, P. M.,Lee, Y. T.
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p. 1644 - 1653
(2007/10/02)
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- Long Path FTIR Spectroscopic Study of the Reactions of CF3O Radicals with Alkenes
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The reactions of CF3O radicals with alkenes have been studied using the long path FTIR method in the visible (λ >/= 400 nm) photolysis of mixtures containing CF3NO, NO, and alkene (in milliTorr range) in 700 Torr of O2/N2 at 297 +/- 2 K.Kinetic and spectroscopic evidence has been obtained for the occurrence of the addition reaction (a) CF3O* + >C=C adduct, and the subsequent reactions involving the adducts, for several alkenes.Using the reaction (b) CF3O* + NO -> CF2O + FNO as the reference reaction, values for the relative rate constants, ka/kb have been determined to be 0.7 +/- 0.2(?), 0.9 +/- 0.2(?), 1.1 +/- 0.3(?), and 1.7 +/- 0.4(?) for CH2=CH2, CH3CH=CH2, trans-CH3CH=CHCH3, and (CH3)2C=C(CH3)2, respectively.Using the value of the rate constant for reaction b recently reported by Bavilacqua et al. (kb = (2 +/- 1) x 10-11 cm3 molecule-1 s-1), the values of the rate constant ka have been calculated to be (1.4 +/- 0.8) x 10-11, (1.8 +/- 1.0) x 10-11, (2.2 +/- 1.3) x 10-11, and (3.4 +/- 1.9) x 19-11 cm3 molecule-1 s-1 for CH2=CH2, CH3CH=CH2, trans-CH3CH=CHCH3, and (CH3)2C=C(CH3)2, respectively.
- Chen, J.,Zhu, T.,Young, V.,Niki, H.
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p. 7174 - 7177
(2007/10/02)
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- Radical Analysis in the Pyrolysis of Hydrocarbons by Scavenging with Dimethyl Disulfide
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Radicals formed in the low pressure pyrolysis of hydrocarbons can be quantitatively analyzed by scavenging them with dimethyl disulfide (DMDS) in the condensed phase the pyrolyses of several hydrocarbons (ethyne, ethene, 1.3-butadiene, and benzene) have been studied in a flow reactor at 1300 K and 10.7 mbar.Samples of the hot gas mixture leaving the reactor were taken continuously by a quartz probe, the resulting supersonic nozzle beam was frozen together with an excess of DMDS on the inner surface of two hollow hemispheres cooled by liquid nitrogen.While melting the scavenging reaction takes place.The mixture of DMDS, stable pyrolysis products, and methylthio compounds was separated and identified with a GC-MS-system.CH3-radical, C2H3-radical, C6H5-radical radicals and H-atoms have been detected.The presence of acetone in ethyne pyrolysis has an important influence on the radical concentrations and the product yields.
- Guthier, K.,Hebgen, P.,Loock, H. P.,Homann, K. H.,Zimmermann, G.,Hofmann, J.
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p. 140 - 142
(2007/10/02)
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- Photodissociation of ethylene at 193 nm
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The photodissociation of ethylene at 193 nm was studied by measuring the product translational energy distributions for the H + C2H3 and H2 + C2H2 channels.In agreement with previous workers, it was determined that atomic and molecular elimination occur in relatively equal amounts.Using 1,1 D2CCH2 and 1,2 cis HDCCDH, it was shown that both acetylene and vinylidene are formed and that the acetylene/vinylidene ratio is approximately 2/3 in the molecular elimination.This H2 elimination channel has a translational energy distribution peaked at around 20 kcal/mol, indicating that it is a concerted process with a substantial exit barrier.It was found that the H atom elimination channel is best described as a simple bond rupture occurring after internal conversion of the electronically excited molecule to the vibrationally excited ground state ethylene.Some of the primary C2H3 product has sufficient internal energy to spontaneously decompose to H + HCCH.At higher laser intensity a large fraction of the C2H3, however, absorbs another photon and fragments to H + H2C=C: (1A1 and 3B2).
- Balko, B. A.,Zhang, J.,Lee, Y. T.
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p. 935 - 942
(2007/10/02)
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- Photodissociation of Thiirane in a Molecular Beam at 193 nm
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A comprehensive study on the 193-nm photodissociation of thiirane has been carried out using the time-of-flight crossed laser-molecular beam technique.Following excitation of the transition with 4p S + C2H3, C2H3 -> H + C2H2 and HS -> H + S
- Felder, P.,Wannenmacher, E. A. J.,Wiedmer, I.,Huber, J. Robert
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p. 4470 - 4477
(2007/10/02)
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- Is the Homogeneous Thermal Dimerization of Acetylene a Free-Radical Chain Reaction? Kinetic and Thermochemical Analysis
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Basic kinetic and thermochemical arguments incorporated into calculations modeling the initial stages of the homogeneous pyrolysis of acetylene reveal that the usual assumption of a free-radical mechanism is flawed.The key findings are that updated thermochemistry for ethynyl, vinyl, and propargyl radicals and the inclusion of falloff corrections in radical reactions lead to (I) exceedingly slow decomposition rates below 1300 K, (2) kinetic inert gas effects, and (3) the formation of benzene rather than vinylacetylene or diacetylene, all predictions at variance with experimental observations.On this basis the involvement of vinylidene is proposed.
- Duran, R. P.,Amorebieta, V. T.,Colussi, A. J.
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p. 636 - 640
(2007/10/02)
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- Selected-ion flow tube studies of reactions of the radical cation (HC3N)+. in the interstellar chemical synthesis of cyanoacetylene
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The radical cation (HC3N)+. was produced in a Selcted-Ion Flow Tube (SIFT) apparatus from cyanoacetylene by electron impact and reacted at room temperature in helium buffer gas with a selection of molecules including H2, CO, HCN, CH4, H2O, O2, HC3N, C2H2, OCS, C2H4, and C4H2.The observed reactions exhibited a wide range of reactivity and a variety of pathways including charge transfer, hydrogen atom transfer, proton transfer, and association.Association reactions were observed with CO, O2, HCH and HC3N.With the latter two molecules association was observed to proceed close to the collision limit, which is suggestive of covalent bond formation perhaps involving azine-like N-N bonds.For HC3N an equally rapid association has been observed by Buckley et al. with ICR (Ion Cyclotron Resonance) measurements at low pressures and this is suggestive of radiative association.The hydrogen atom transfer reaction of ionized cyanoacetylene with H2 is slow while similar reactions with CH4 and H2O are fast.The reaction with CO fails to transfer a proton.These results have implications for synthetic schemes for cyanoacetylene as proposed in recent models of the chemistry of interstellar gas clouds.Proton transfer was also observed to be curiously unfavourable with all other molecules having a proton affinity higher than (C3N)..Also, hydrogen-atom transfer was inefficient with the polar molecules HCN and HC3N.These results suggest that informations at close separations may lead to preferential alignment of the reacting ion and molecule which is not suited for proton transfer or hydrogen atom transfer.
- Fox, A.,Raksit, A. B.,Dheandhanoo, S.,Bohme, D. K.
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p. 399 - 403
(2007/10/02)
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- A Direct Study of the Reaction CH2(3B1) + C2H4 in the Temperature Range 296 K = T = 728 K
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The reaction between CH2-radicals in the triplet electronic ground state 3B1(3CH2) and C2H4 3CH2 + C2H4 --> products was studied in the gas phase at temperatures between 296 K 3CH2 in the presence of a large excess of C2H4 was found to be k1exp = 1E(12.69+/-0.12)exp(-(2810+/-140)K/T)cm3/mols.Correcting for the small contribution to the depletion of 3CH2 due to collisional excitation to the singlet first excited state 1A1(1CH2) followed by consecutive reactions of 1CH2, the rate constant for the direct reaction of 3CH2 with C2H4 was obtained to be k1 = 1E(12.50+/-0.10)exp(-(22.1+/-1.0)kj mol-1/RT)cm3/mols.A reaction mechanism describing both the present data as well as the results of earlier related studies in the CH2 + C2H4 system is proposed.The mechanism and product yields are discussed in terms of unimolecular rate theory.- Chemical Kinetics / Elementary Reactions / Laser Magnetic Resonance / Radicals
- Boehland, T.,Temps, F.,Wagner, H. Gg.
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p. 468 - 475
(2007/10/02)
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- Abschaetzung kinetischer und thermodynamischer Daten fuer Wasserstoffabspaltungsreaktionen von niederen Olefinen
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Mit Hilfe der BEBO-Methode sind fuer 500 ... 1200 K die Aktivierungsparameter der H-Uebertragungsreaktionen .R1 + HR2 --> R1H + R2.(R1: H, CH3, C2H5, n-C3H7, i-C3H7, CH3CO, CH2COCH3; R2H: Ethen, Propen, But-1-en) berchnet worden.Unter Verwendung entsprechender Daten der Rueckreaktion wurden daraus ΔRGθ, ΔRHθ und ΔRSθ bestimmt und mit den aus thermochemischen Inkrementen zugaenglichen Werten verglichen.Die Beweiskraft der guten Uebereinstimmung beider Datensaetze fuer die Anwendbarkeit des BEBO-Verfahrens auf H-Abstraktionen von Olefinen wird diskutiert.Ausserdem wird gezeigt, dass bei der Pyrolyse von Olefinen die H-Abstraktion aus der Vinylgruppe bis ueber 1500 K gegenueber der aus den Alkylgruppen zu vernachlaessigen ist.
- Scherzer, K.,Lohse, K.,Loeser, U.
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p. 196 - 204
(2007/10/02)
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- Dissociation of 3-Methylpenta-1,4-diene and the Resonance Energy of the Pentadienyl Radical
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From measurements of the initial rate of formation of methane in the pyrolysis of 3-methylpenta-1,4-diene at temperatures over the range 653-716 K and pressures between 15 and 200 Torr, methane has been shown to be formed by an homogeneous, first-order process.Examination of the other products of pyrolisis shows the most reasonable mode of formation of primary methane to be followed by .The rate expression for reaction (i) is where θ=2.303 RT/cal mol-1 (1 cal=4.18 J) and this leads to D=76.4 kcal mol-1, which by comparison with the value of 95.0 for the secondary C-H bond dissociation energy in alkanes leads to a figure of 18.6+/-1.5 kcal mol-1 for the resonance stabilisation energy of the pentadienyl radical.
- Trenwith, Antony B.
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p. 3131 - 3136
(2007/10/02)
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- Vacuum-ultraviolet (147.0 nm and 123.6 nm) photolysis of 1,1-dimethylcyclopropane
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The gas-phase photolysis of 1,1-dimethylcyclopropane has been investigated using xenon (147.0 nm) and krypton (123.6 nm) resonance radiation.Major products observed in order of decreasing importance were isobutene, ethylene, hydrogen, 1,3-butadiene, 2-methyl-1,3-butadiene, propylene, allene, methylacetylene, and acetylene.Radical scavengers, NO and O2, and radical interceptors, H2S/D2S and HI, were used to determine the relative importance of radical and molecular processes.CH3, C2H3, C3H5, and C4H7 radical species were identified and quantified.Ten primary reaction channels were postulated, of which the elimination of methylene was the most predominant, accounting for 34percent of the photo-decomposition at 147.0 nm and 39percent at 123.6 nm.Although ionization was established at 123.6 nm (η=0.10) the nature of a charge transfer of other ion-molecule reaction channel leading to the formation of 2-methyl-1-butene and 2-methyl-2-butene could not be determined.
- Binkewicz, Joseph B.,Kaplan, Michael,Doepker, Richard D.
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p. 537 - 542
(2007/10/02)
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