- A Shock-Tube, Laser-Schlieren Study of the Dissociation of 1,1,1-Trifluoroethane: An Intrinsic Non-RRKM Process
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We report a shock-tube, laser-schlieren investigation of the molecular dissociation of the title trifluoroethane, CF3CH3 → CH2CF2 + HF, over very high temperatures, 1600-2400 K, and a wide range of sub-atmospheric pressures, 15-550 Torr. The density gradients are well fit by a simple two-reaction mechanism and accurate dissociation rates obtained. The results are compared with a k∞ calculated from a G3 TS for this molecular elimination, which is a superb fit to the available lower-T data and a reliable extrapolation of k∞ to high temperatures. The derived rate constants show a very deep falloff from, this extrapolation but surprisingly little variation with pressure. This peculiarity is so severe that RRKM calculations dramatically fail to account for the behavior. The dissociation seems to be a clear example of an intrinsic non-RRKM process (nonstatistical dissociation). This conclusion is strongly supported by the observation of double vibrational relaxation at both dissociating and nondissociating temperatures, an unambiguous demonstration of slow IVR. Using a simple model with division into two groups of states, the deep falloff is found to be consistent with a rate-controlling slow IVR, not with low collision efficiency. The model suggests an IVR rate of ~108 s-1 for dissociation energies.
- Kiefer,Katopodis,Santhanam,Srinivasan,Tranter
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- Competing Pathways in the Infrared Laser Photolysis of 1-Chloro-1-fluoroethylene
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The multiphoton-induced decomposition of 1-chloro-1-fluoroethylene (CH2CClF) and the effect of laser fluence upon the decomposition pathways has been investigated by using a pulsed TEA CO2 laser operating at 10.632 μm.CH2CClF has been found to decompose via two competing pathways, yielding either HF and HCCCl or HCl and HCCf.The decomposition reactions have been followedin real time by monitoring the IR fluorescence from the vibrationally excited HF and HCl.Variation of the laser fluence causes the HF/HCl product ratio to change.This change is explained in terms of a competition between the reaction pathways and the optical pumping rate.
- Jalenak, W. A.,Nogar, N. S.
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- Reactions of photogenerated fluorine atoms with molecules trapped in solid argon 4.* spectroscopic characteristics of β-c2h2f. radicals generated in reactions of mobile f atoms with c2h2 molecules tra
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Reactions of mobile fluorine atoms with C2H2, C2D2, and C2HD molecules in solid argon were studied by ESR and IR spectroscopic techniques. Highly resolved ESR spectra of the stabilized radicals CHF=s
- Misochko,Goldschleger,Akimov,Wight
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p. 989 - 995
(2007/10/03)
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- Photodissociation dynamics of 1,1-difluoroethylene at 157 nm excitation
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Photodissociation of 1,1-difluoroethylene (F2CCH2) at 157 nm has been investigated using photofragment translational spectroscopy. Five dissociation channels have been experimentally observed; molecular HF elimination, H atom elimination, molecular hydrogen (H2) elimination, F atom elimination, and double bond breaking. Molecular HF elimination and H atom elimination channels are found to be the two major dissociation pathways in photodissociation of F2CCH2 at 157 nm excitation. Molecular hydrogen (H2) elimination and double bond cleavage are also significant, while F atom elimination is a minor process. Product translational energy distributions for all dissociation channels have also been measured. All translational energy releases are peaked at energies away from zero, indicating that the dissociation of F2CCH2 at 157 nm excitation most likely occurs with exit barriers on the ground electronic potential surface through internal conversion from the initially excited electronic state. Branching ratios and averaged energy partitions for different channels have also been estimated.
- Lin,Wu,Hwang,Lee,Yang
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p. 10838 - 10846
(2007/10/03)
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- Chlorofluoroacetylene and bromofluoroacetylene by gas phase dehalogenation of 1,1-difluoroethylenes
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Chlorofluoroacetylene and bromofluoroacetylene have been prepared by dehalogenation of dichlorodifluoroathylene and dibromodifluoroetnylene, respectively. The products of the direct dehalogenation with alkali metals as well as of the pyrolysis of trimethylstannyl compounds as "stabilized carbenoids" have been isolated in Ar matrices.
- Runge, Andreas,Sander, Wolfram W.
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p. 5453 - 5456
(2007/10/02)
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- Laser-Induced Reactions of Hexafluorobenzene and Selected Hydride Compounds
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Infrared-laser-induced reactions between C6F6 and general hydrides R-H (R = H, D, CH3, HCC, H2C=CH, and Cl) were studied by irradiating C6F6 at 1027 cm-1 in C6F6/R-H mixtures.In general, two competitive pathways involving C-F bond cleavage in C6F6 were observed as follows: (1) C6F6 + R-H C6F5H + R-F and (2) C6F6 + R-H C6F5R + HF.C6F6 decomposition also took place to a minor extent depending on the mole fraction of C6F6 and gave rise to C2F4 and C2H2.From infrared and GC/MS analysis of the product mixtures after 20-200 pulses, C6F5H was observed in all reactions except that involving D2.When D2 was used C6F5D was the major product.C6F5H was the major product in the reactions involving H2 and C2H2.In the reaction with C2H4, C6F5H was the major product derived from C6F6 though C2H2 was the major product of the reaction.The large amount of C2H2 seems to be derived from an additional sensitized decomposition of C2H4.C6F5H was present in minor amounts in the reaction with CH4 and HCl.Besides C6F5H, other monosubstituted products derived from C6F6 were also formed, generally within 20-100 pulses.Thus, C6F5CH3, C6F5CH=CH2, C6F5CCH, and C6F5Cl were produced, respectively, in the reaction of C6F6 with CH4, C2H4, C2H2, and HCl.In the first and last cases these products were the major ones observed.The results are discussed mechanistically in terms of the initial formation of the C6F5. radical and synthetically in terms of the utility of obtaining selective-laser-induced reduction of C6F6.
- Koga, Yoshinori,Chen, Ruth,Keehn, Philip M.
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p. 306 - 311
(2007/10/02)
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- Pulse-Duration Effects on Competitive Reactions in Infrared Multiple-Photon Decomposition of CH2ClCHClF and CHClFCHClF
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Vibrationally excited 1,2-dichlorofluoroethane and 1,2-dichloro-1,2-difluoroethane have been observed to dissociate competitively via two channels to form vibrationally excited HCl and HF.The fluence dependences of the branching ratio have been measured for both "short"-pulse (80-ns fwhm) and "long"-pulse (80-ns fwhm with 1-μs-fwhm tail) irradiations.The branching ratio shows not only fluence dependence but also pulse-duration dependence, that is, intensity dependence.When the reactant pressure is 1.0 Torr, collisional deactivation is expected to occur to a considerable extent under long-pulse irradiation while it can be ignored under short-pulse irradiation.The experimental results are interpreted by using the exact stochastic method based on the energy-grained master equations, which take into account collisional deactivation.
- Ishikawa, Yo-ichi,Sugita, Kyoko,Arai, Shigeyoshi
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p. 5067 - 5071
(2007/10/02)
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- Infrared Multiphoton Decomposition of 1-Chloro-1-fluoroethene
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Unimolecular dissociation and subsequent reactions of CH2=CFCl subjected to the infrared multiphoton excitation were studied with a focused geometry using the P(10) line of the 10.6 μm CO2 band at 952.9 cm-1.The final products of the photolysis observed were CH=CCl, CH=CF, CH=CH, CH2CHF, and CH2=CF2.A series of diagnostic experiments shows that the primary processes of photolysis involve the molecular elimation of HF and HCl, to minor extend C-Cl bond rupture.The relative importance of the primary steps is tentatively estimated to be approximately 100:65:25, respectively.Primarily formed CH=CCl and CH=CF suffer from secondary photolysis absorbing additional photons.More fraction of CH=CF primary yield is decomposed than that of CH=CCl, due to the accidental resonance with the laser excitation line.CH=CH and CH2=CHF are concluded to be formed by the H atom abstraction reaction of CH2=C. and CH=CF. radicals generated during the photolysis respectively, CH2=CF2 beging formed by the recombination reaction between CH2=CF. radical and F atom.In the shock tube pyrolysis the formation of CH=CF predominates over that of CH=CCl.A mechanistic change to form CH=CF in the shock tube pyrolysis is suggested.
- Ogura, Hiroo
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p. 1358 - 1366
(2007/10/02)
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- CO2 Laser-induced Decomposition of 1,2-Dichloro-1-fluoroethane
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CH2ClCHFCl was photolyzed with a TEA CO2 laser at 1033.5 cm-1.The infrared multiphoton dissociation mechanism of CH2ClCHFCl was investigated under various conditions: Sample gas pressure, additive gas presure, pulse number, pulse energy, and pulse duration.It is concluded that primary process of the IRMPD is direct eleimination of molecular HCl and HF, HCl elimination being predominant channel.Primary HCl elimination products cis-and trans-CHF=CHCl, and CH2=CFCl are formed at high vibrational levels, from which additional photon absorption occures in the secondary photolysis to give rise to CH=CCl, CH=CF, and CH2=CHF.All of the secondary products are concluded to be derived from mainly CH2=CFCl among the chlorofluoroethene isomers.CH2=CFCl decomposes via HF and HCl elimination channels together with the C-Cl bond repture channel. appears to be generated by the H atom abstraction reaction of C2H. radical, which may result from further decomposition of and/or .The neat IRMPD at higher pressures gives quite similar primary product distribution, but markedly different secondary product distribution from those in shock tube pyrolysis.
- Ogura, Hiroo,Yano, Takayuki
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p. 1239 - 1250
(2007/10/02)
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- Vibrationally excited populations from IR-multiphoton absorption. II. Infrared fluorescence measurements
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Infrared emission spectra were obtained for 1,1,2-trifluoroethane (TFE) excited by infrared multiphoton absorption (1079.85 cm-1).The emission features show that the HF reaction product is formed in vibrational states up to about v = 3.Furthermore, emission attributed to F-CC-H was observed near 3320 cm-1, indicating that the difluoroethylene primary products of TFE decomposition undergo secondary photolysis; since the difluoroethylene products at room temperature do not absorb laser light, they must be formed vibrationally excited.The emission from the C-H stretch modes of TFE was readily identified near 2980 cm-1 and the emission intensity was obtained as a function of laser fluence.These data are in excellent agreement with predictions based on the theoretical expression for fluorescence intensity and the reconstructed populations determined by the Master Equation calculations described in the preceding paper.These results provide additional support for the accuracy of the reconstructed population distributions and for the theory relating infrared fluorescence intensity to total vibrational energy in polyatomic molecules.
- Zellweger, Jean-Michel,Brown, Trevor C.,Barker, John R.
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p. 6261 - 6267
(2007/10/02)
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- Infrared Laser Multiphoton Dissociation of CF2ClCH2Cl
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The infrared multiphoton decomposition (IRMPD) of CF2ClCH2Cl was studied with focusing geometry using the P(34) line of the 9.6-μm CO2 band (1033.6 cm-1).The principal reaction product is CF2CHCl.Other products of significance include CFClCHCl, CF2CH2, and CFCH.It is concluded that the primary processes of photodecomposition involve the molecular elimination of HCl and HF and, to a very minor extent, C-C bond rupture.The relative importance of the primary steps is approximately 1000:30:1, respectively.From a series of diagnostic experiments in the presence of hydrogen donors and D2, it is shown that CF2CH2 derives from the secondary photolysis of CF2CHCl.The decomposition yield, the HF/HCl ratio, and the CF2CH2/CF2CHCl ratio were investigated as a function of reactant and argon pressure, the latter serving as a buffer gas.From the dependence of the decomposition yield on pulse number at different pulse energies, E0, the specific rate of decomposition, b, was found to be proportional to b E01.8, the power dependence being somewhat higher than the standard 3/2 power law for focusing geometry.These phenomena are interpreted in terms of a simple geometric fluence model which includes contributions from collisionally induced reactions in the outermost (lower fluence) irradiated region.The nonresonant photodecomposition of C2H6 and C2H4 at 1033.6 cm-1 observed in auxiliary, diagnostic experiments is interpreted in terms of photosensitization processes.
- Yano, T.,Ozaki, S.,Ogura, H.,Tschuikow-Roux, E.
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p. 1108 - 1116
(2007/10/02)
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- A Photoelectron Spectroscopic Study of the Ground States of CH2F+ and CD2F+
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The fluorine atom/methyl fluoride reaction has been studied by photoelectron spectroscopy.A new product band with vibrational components at 9.04 +/- 0.01 eV adiabatic and 9.22 +/- 0.01 eV vertical ionization energies is assigned to the CH2F free radical.The ν'= 0-1 vibronic separation measured as 1450 +/- 30 cm-1 is due to the C-F stretching fundamental of the ground state of CH2F+; this vibronic interval was 1530 +/- 30 cm-1 for CD2F+.The positive deuterium shift for CH2F+ is due to interaction with the H-C-H bending mode, which shifts below the C-F stretching mode on deuteration.The substantial increase in the C-F stretching modes for CH2F+ and CD2F+, as compared to 1163- and 1193-cm-1 values for the CH2F and CD2F free radicals in solid argon, respectively, is due to increased net C-F bonding in the cations.
- Andrews, Lester,Dyke, John M.,Jonathan, Neville,Keddar, Noureddine,Morris, Alan,Ridha, Abed
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p. 2364 - 2368
(2007/10/02)
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- Vacuum Ultraviolet Photochemistry of Fluoroethene and 1,1-Difluoroethene
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Products from the broad-band vacuum ultraviolet photolysis of CH2CHF and CH2CF2 were collected by using a novel gas collection technique and analyzed by using gas chromatography.The primary route of decay for both parents is through α-β elimination of HF.Primary branching ratios for HF elimination, F atom ejection, and HH elimination from CH2CHF were determined: 0.82, 0.13, and 0.05, respectively.The technique does not permit detection of single H atom ejection.The ratio of (C2F2H3) stabilization by He vs. decomposition, formed by the addition of F to CH2CHF, is 0.029 +/- 0.004 torr-1.The lifetime of the excited complex is approximately a factor of 5 longer relative to other related systems.A less detailed study of excited-CH2CF2 decay indicates similar trends.
- Sirkin, Eric R.,Pimentel, George C.
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p. 1833 - 1840
(2007/10/02)
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