- Kinetics and mechanism of the reaction of CF3 radicals with NO2
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The reaction of CF3 with NO2 was studied at 296 ± 2K using two different absolute techniques. Absolute rate constants of (1.6 ± 0.3) × 10-11 and (2.1-0.3+0.7) × 10-11 cm3 molecule-1 s-1 were derived by IR fluorescence and UV absorption spectroscopy, respectively. The reaction proceeds via two reaction channels: CF3 + NO2 → CF2O + FNO, (70 ± 12)% and CF3 + NO2 → CF3O + NO, (30 ± 12)%. An upper limit of 11% for formation of other reaction products was determined. The overall rate constant was within the uncertainty independent of total pressure between 0.4 to 760 torr.
- Sehested, Jens,Nielsen, Ole John,Rinaldi, Carlos A.,Lane, Silvia I.,Ferrero, Juan Carlos
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- Kinetics and mechanisms of the thermal gas-phase reactions of CF3OF and CF3OOCF3 with NO2
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The kinetics of the reactions of CF3OF and CF3OOCF3 with NO2 have been investigated using a conventional static system. The reaction between CF3OF and NO2 has been studied in a quartz reactor in the temperature range of 313.2-334.2 K, varying the initial pressure of CF3OF between 19.4 and 165.2 Torr and that of NO2 + N2O4 between 18.2 and 179.2 Torr. Some experiments were made in presence of 506.5-600.8 Torr of N2. The total pressure had no influence on the reaction rate. COF2 and FNO2 were identified as reaction products. The expression obtained for the rate constant for the abstraction of fluorine atom from CF3OF by NO2 was: k1 = (1.1±0.2) × 109 exp(-16.4±1 kcal mol-1/ RT) dm3 mol-1 s-1. The reaction of CF3OOCF3 with NO2 has been studied in an aluminum reactor in the temperature range of 474.0-512.5 K, varying the initial pressure of CF3OOCF3 between 24.1 and 202.5 Torr and that of NO2 between 24.7 and 202.7 Torr. Several experiments were made in presence of 399.8-490.5 Torr of N2. The reaction rate was proportional to [CF3OOCF3]1/2. The reaction approached the first order with respect to NO2 at low pressure of NO2. Increasing the pressure of NO2, the ratio of the reaction rates increased more rapidly than the ratio of the corresponding concentrations of NO2. Three products were formed: COF2, FNO and O2. The expression obtained for the rate constant for the abstraction of the fluorine atom from the radical CF3O by NO2 was: k8 = (1.72±0.4) × 109 exp(-10.8±1 kcal mol-1/RT) dm3 mol-1 s-1. The mechanisms for both reactions were postulated. by Oldenbourg Wissenschaftsverlag, Muenchen.
- Czarnowski
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- Kinetic study of the reactions of CF3O2 radicals with Cl and NO
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Kinetic studies of the reactions CF3O2 + Cl and CF3O2 + NO were performed at room temperature in the gas phase using the discharge flow mass spectrometric technique (DFMS). The reactions were investigated under pseudo- first-order conditions with Cl or NO in large excess with respect to the CF3O2 radicals. The rate constant for the reaction CF3O2 + NO was measured at 298 K and the value of (1.6 ± 0.3) x 10-11 cm3 molecule-1 s-1 is in very good agreement with all previous values. For the reaction CF3O2 + Cl, we obtain a rate constant at 298 K of (4.2 ± 0.8) x 10-11 cm3 molecule-1 s-1 in excellent agreement with the only published value. Product analysis shows that this reaction occurs via the major reaction pathway CF3O2 + Cl → CF3O + ClO at room temperature. In addition, an ab initio theoretical study was performed to gain insights on the different postulated reaction pathways. There is a significant disagreement between experimental and ab initio values recommended for the formation enthalpies of CF2O, CF3O and related molecules produced in this system. Consequently, we provide self-consistent values of enthalpies based on isodesmic reactions for the CF3O2 + Cl reaction system using the G2, G2(MP2) and CBS-Q methods. These values are also compared with BAC-MP4 heats of formation calculated in this work.
- Louis, Florent,Burgess Jr., Donald R.,Rayez, Marie-Therese,Sawerysyn, Jean-Pierre
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p. 5087 - 5096
(2007/10/03)
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- Kinetic and mechanistic studies of the reactions of CF3O radicals with NO and NO2
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The reactions of CF3O radicals with (1) NO and (2) NO2 were-studied using two different experimental techniques. A laser photolysis/LIF detection method was applied for measuring the rate constants as a function of temperature (T=222-302 K) and total pressure (ptot = 7-107 mbar). Whereas the reaction with (1) NO was found to be independent of temperature and pressure with k1 = (4.5±1.2)×10-11 cm3 s-1, the reaction with (2) NO2 was found to be dependent on both of these variables. The temperature dependence of k2 in the high pressure limit can be given by the expression k2, ∞ (T)=(8±5)×10-13 exp ((863±194) K/T) cm3 s-1. The product distributions of the two reactions were determined in separate experiments using steady-state photolysis combined with FTIR spectroscopy. For reaction (1) only CF2O was found as a reaction product with a yield of 0.93±0.10, independent of temperature. For reaction (2) several products (CF3ONO2, CF2O, FNO2) were identified, the overall yield, however, is dominated (≥90%) by the recombination product CF3ONO2. A theoretical analysis of the detailed mechanisms of both reactions was made by performing ab initio energy and geometry predictions in combination with RRKM calculations. Both reactions were found to proceed via an initial addition mechanism involving the CF3ONOx (x=1, 2) intermediate and a four-center transition state. A direct abstraction of an F atom by NO or NO2 can be excluded. WILEY-VCH Verlag GmbH, 1997.
- Fockenberg,Somnitz,Bednarek,Zellner
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p. 1411 - 1420
(2007/10/03)
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- Kinetics of the Reaction of CF3O with NO
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The reaction between CF3O and NO has been studied by laser flash photolysis/transient diode laser absorption over the range 213 K a small extent (Φ=0.15).A significant fraction of the CF3O (ca. 30percent) is born with sufficient internal excitation to dissociate into F2CO and F atoms.The presence of NO leads to additional F2CO formation.The analysis of this formation rate, as well as the NO loss rate, yields a temperature dependent rate constant of (4.4 +/- 1.5) x 10-11e(+100+/-88)/T cm3 s-1 for the CF3O + NO reaction.The measured rate constants agree well with previous results based on observations of CF3O disappearance.They confirm expectations that the reaction yields exclusively the products FNO and F2CO and that it serves as an efficient stratospheric sink for the CF3O radical.The reaction between CF3O and HCl has also been examined, and an upper limit of 2 x 10-13 cm3 s-1 has been determined for its rate constant at 295 K.The small rate constant for this reaction relative to the rapid removal of CF3O by NO severely limits the possibility that hydrofluorocarbons contribute to stratospheric ozone depletion by their CF3O degradation product liberating chlorine atoms from HCl.
- Dibble, Theodore S.,Maricq, M. Matti,Szente, Joseph J.,Francisco, Joseph S.
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p. 17394 - 17402
(2007/10/02)
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- Temperature Dependence and Product Distribution for the Reaction of CF3O Radicals with Methane
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The rate coefficient for the reaction (1) CF3O + CH4 --> CF3OH + CH3 has been determined using a combined laser photolysis/LIF technique.From measurements in the temperature range 235-401 K the Arrhenius expression k1 = (3.3 +/- 0.8)E-12 exp. cm3/s, corresponding to k1 = 2.5E-14 cm3/s at 298 K is obtained.The products of reaction (1) have been identified by independent experiments using a steady-state/FTIR technique.It is concluded that the main primary product of reaction (1) is CF3OH which decomposes under our experimental conditions to yield CF2O and HF.An upper limit of the decomposition rate coefficient for (7) CF3OH --> CF2O + HF at 298 K of k7 -1 is determined. - Keywords: CF3O radicals / Laser photolysis / LIF technique / H abstraction from CH4 / Decomposition of CF3OH
- Bednarek, G.,Kohlmann, J. P.,Saathoff, H.,Zellner, R.
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- Chemical Ionization Mass Spectrometric Studies of the Gas-Phase Reactions CF3O2 + NO, CF3O + NO, and CF3O + RH
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We have used a flow tube reactor coupled to a chemical ionization mass spectrometric (CIMS) detector to study the reactions of trifluoromethylperoxy (CF3O2) and trifluoromethoxy (CF3O) radicals with NO and the reaction of CF3O with isobutane.We have determined the rate coefficients at 297 K for the reaction CF3O2 + NO to be (1.53 +/- 0.20) x 10-11 cm3 molecule-1 s-1 (all uncertainties are for 95percent confidence limits), in excellent agreement with two previous measurements.The use of CIMS detection technique has allowed us to observe both CF3O and NO2 as the products of this reaction.Modeling of a secondary reaction between CF3O and NO observed in these studies has yielded an estimate of k = (2 +/- 1) x 10-11 cm3 molecule-1 s-1 for this reaction, in which FNO was observed as a product.A relatively rapid reaction was also observed between CF3O and isobutane, for which a rate coefficient of (5 +/- 3) x 10-12 cm3 molecule-1 s-1 is estimated.This reaction was seen to proceed by hydrogen abstraction, yielding trifluoromethanol, CF3OH.A much slower H-abstraction reaction was observed between CF3O and methane.The significance of these reactions for the atmospheric fate of the trifluoromethoxy radical is discussed.
- Bevilacqua, Thomas J.,Hanson, David R.,Howard, Carleton J.
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p. 3750 - 3757
(2007/10/02)
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- Flash Photolysis-Time-Resolved UV Spectroscopy of the CF3CFHO2 Self-Reaction
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The self-reaction of CF3CFHO2 has been studied via time-resolved ultraviolet spectroscopy over the temperature range 211-372 K.The absorption spectrum of CF3CFHO2 extends from 190 to 275 nm with a maximum cross section of (5.2 +/- 0.3) * 10-18 cm2 molecule-1 at 213 nm.The UV absorbance of the reaction mixture decreases and shifts to the blue as the reaction progresses.This is consistent with the CF3CFHO2 self-reaction producing CF3CFHO, the alkoxy radical then decomposing to yield CF3, which adds molecular oxygen to form CF3O2.The CF3CFHO2 self-reaction has a negative temperature dependence with rate constant given by k1 = (7.8 +/- 1.3) * 10-13 e(605+/-40)/T cm3 s-1.The rate of alkoxy radical dissociation, at 230 Torr of total pressure, is k2a = (3.7 +/- 0.7) * 107 e-(2200+/-150)/T s-1.The rate constants for CF3O2 reaction with CF3CFHO2 and itself are determined to be k8 = (8 +/- 3) * 10-12 and k9 = (1.8 +/- 0.5) * 10-12 cm3 s-1, respectively, at 297 K.
- Maricq, M. Matti,Szente, Joseph J.
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p. 10862 - 10868
(2007/10/02)
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