- Atmospheric Chemistry of CF2BrH: Kinetics and Mechanism of Reaction with F and Cl Atoms and Fate of CF2BrO Radicals
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A pulse radiolysis technique was used to investigate the kinetics and products of the reaction of CF2BrH with fluorine atoms at 296 K.This reaction forms an adduct which is in dynamic equilibrium with CF2BrH and fluorine atoms.The UV absorption spectrum of the adduct was measured relative to the UV spectrum of the CH3O2 radical over the range 230-380 nm.At 280 nm, an absorption cross section of (1.3 +/- 0.3)E-17 cm2 molecule-1 was determined.From the absorption at 280 nm the equilibrium constant K5 = /() was measured to be (1.59 +/- 0.13)E-17 cm3 molecule-1.In 1 atm of SF6, the forward rate constant k5 = (1.4 +/- 0.5)E-11 cm3 molecule-1 s-1 and the backward rate constant k-5 = (8.8 +/- 3.0)E5 s-1 were determined by monitoring the rate of formation and loss of the adduct.As part of the present work a relative rate technique was used to measure k(Cl + CF2BrH) = (5.8 +/- 1.0)E-15 cm3 molecule-1 s-1 at 296 K and 700 Torr of N2.The fate of the oxy radical, CF2BrO, in the atmosphere is bromine atom elimination and formation of COF2.
- Bilde, Merete,Sehested, Jens,Mogelberg, Trine E.,Wallington, Timothy J.,Nielsen, Ole J.
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- Fluorine-atom initiated oxidation of CF3CF2H (HFC-125) studied by FTIR spectroscopy: Product yields and kinetic modelling
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A comprehensive study has been carried out of the oxidation of CF3CF2H (HFC-125). Reaction was initiated by continuous photolysis of F2 in the near-UV. The F atoms produced abstracted a hydrogen atom from CF3CF2H initiating oxidation in gas mixtures containing variable amounts of O2 and made up to a total pressure of 700 Torr with N2. Product yields were measured as a function of time using FTIR spectroscopy. Experiments were performed at room temperature and in mixtures containing different ratios of [F2] to [CF3CF2H]. The major products were COF2, CF3O3CF3 and CF3O3C2F5, consistent with C-C bond scission being the dominant loss process for CF3CF2O radicals and with previous studies which used chlorine atoms to initiate oxidation. Attempts to match the experimental results with predictions using the FACSIMILE chemical modelling program were moderately successful and confirmed recent results regarding the equilibrium constant for: F + O2(+M) = FO2(+M).
- Hasson, Alam S.,Moore, Christopher M.,Smith, Ian W. M.
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- Kinetic and mechanistic studies for reactions of CF3CH2CHF2 (HFC-245fa) initiated by H-atom abstraction using atomic chlorine
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Kinetics and mechanisms of Cl atom initiated oxidation reactions of CF3CH2CHF2 (HFC-245fa) were investigated by long-path FTIR spectroscopic methods at 297 ± 2 K. The Cl atoms initiated the reaction mainly via H-atom abstraction from the terminal carbon (≥95%) to produce a CF3CH2CF2 radical. Subsequent reactions in 700 Torr of air produced CF3CHO and CF2O as the primary products. Secondary reactions of CF3CHO with Cl atoms led to the formation of CF3C(O)OH, CF3OH, CO2, and CF3OOOCF3 in the experimental system. However, under atmospheric conditions, CF3CHO would undergo a series of rapid oxidation and decomposition reactions ultimately leading to HF and CO2, and no long-lived organic decomposition products would be produced.
- Chen, Junyi,Young, Valerie,Niki, Hiromi,Magid, Hillel
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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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- A kinetic and mechanistic study of the gas-phase reactions of OH radicals and Cl atoms with some halogenated acetones and their atmospheric implications
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Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with a series of halogenated acetones of the type CX3COCH 3 (X = H, Cl, F) have been determined using a photolytic relative-rate technique at T = 298 K and at 760 Torr total pressure. The reactions studied and the rate coefficients obtained are shown in the table. Reaction Reaction number Rate coefficient/cm3 molecule-1 s-1 OH + CH3COCH3 → products (1) (2.2 ± 0.5) × 10-13 OH + CH2ClCOCH3 → products (2) (4.2 ± 0.8) × 10-13 OH + CHCl 2COCH3 → products (3) (3.8 ± 0.8) × 10-13 OH + CCl3COCH3 → products (4) (1.5 ± 0.3) × 10-14 OH + CH2FCOCH3 → products (5) (2.1 ± 0.4) × 10-13 OH + CF 3COCH3 → products (6) (6.9 ± 1.3) × 10-15 Cl + CH3COCH3 → products (7) (2.2 ± 0.4) × 10-12 Cl + CH2ClCOCH3 → products (8) (2.0 ± 0.2) × 10-12 Cl + CHCl 2COCH3 → products (9) (1.7 ± 0.3) × 10-13 Cl + CCl3COCH3 → products (10) ( 1. 7 ± 0.3) × 10-14 Cl + CH2FCOCH3 → products (11) (8.2 ± 1.6) × 10-13 Cl + CF 3COCH3 → products (12) (8.0 ± 1.6) × 10-15 The errors quoted reflect an estimate of the absolute uncertainty in the measured rate coefficients of ±20%. For reactions (7)-(12), Fourier transform infrared spectroscopy was used to identify products. Qualitative ultra-violet absorption spectra were also recorded for most of the halogenated species investigated in this study, and have been used together with the kinetic data to derive atmospheric lifetimes for these species.
- Carr, Sinead,Shallcross, Dudley E.,Canosa-Mas, Carlos E.,Wenger, John C.,Sidebottom, Howard W.,Treacy, Jack J.,Wayne, Richard P.
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p. 3874 - 3883
(2007/10/03)
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- The Open-Chain Trioxide CF3OC(O)OOOC(O)OCF3
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The open-chain trioxide CF3OC(O)OOOC(O)OCF3 is synthesised by a photochemical reaction of CF3C(O)OC(O)CF 3, CO and O2 under a low-pressure mercury lamp at -40°C. The isolated trioxide is a colourless solid at -40°C and is characterised by IR, Raman, UV and NMR spectroscopy. The compound is thermally stable up to -30°C and decomposes with a half-life of 1 min at room temperature. Between -15 and +14°C the activation energy for the dissociation is 86.5 kJ mol-1 (20.7 kcal mol-1). Quantum chemical calculations have been performed to support the vibrational assignment and to discuss the existence of rotamers.
- Von Ahsen, Stefan,Garcia, Placido,Willner, Helge,Paci, Maximiliano Burgos,Argueello, Gustavo A.
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p. 5135 - 5141
(2007/10/03)
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- 1,1,1,3,3,-pentafluorobutane (HFC-365mfc): Atmospheric degradation and contribution to radiative forcing
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The rate constant for the reaction of the hydroxyl radical with 1,1,1,3,3-pentafluorobutane (HFC-365mfc) has been determined over the temperature range 278-323 K using a relative rate technique. The results provide a value of k(OH + CF3CH2CF2CH3) = 2.0 × 10-12 exp(- 1750 ± 400/T) cm3 molecule-1 s-1 based on k(OH + CH3CCl3) = 1.8 × 10-12 exp (- 1550 ± 150/T) cm3 molecule-1 s-1 for the rate constant of the reference reaction. Assuming the major atmospheric removal process is via reaction with OH in the troposphere, the rate constant data from this work gives an estimate of 10.8 years for the tropospheric lifetime of HFC-365mfc. The overall atmospheric lifetime obtained by taking into account a minor contribution from degradation in the stratosphere, is estimated to be 10.2 years. The rate constant for the reaction of Cl atoms with 1,1,1,3,3-pentafluorobutane was also determined at 298 ± 2 K using the relative rate method, k(Cl + CF3CH2CF2CH3) = (1.1 ± 0.3) × 10-15 cm3 molecule-1 s-1. The chlorine initiated photooxidation of CF3CH2CF2CH3 was investigated from 273-330 K and as a function of O2 pressure at 1 atmosphere total pressure using Fourier transform infrared spectroscopy. Under all conditions the major carbon-containing products were CF2O and CO2, with smaller amounts of CF3O3CF3. In order to ascertain the relative importance of hydrogen abstraction from the - CH2 - and - CH3 groups in CF3CH2CF2CH3, rate constants for the reaction of OH radicals and Cl atoms with the structurally similar compounds CF3CH2CCl2F and CF3CH2CF3 were also determined at 298 K k(OH + CF3CH2CCl2F) = (8 ± 3) × 10-16 cm3 molecule-1 s-1; k(OH + CF3CH2CF3) = (3.5 ± 1.5) × 10-16 cm3 molecule-1 s-1; k(Cl + CF3CH2CCl2F) = (3.5 ± 1.5) × 10-17 cm3 molecule-1 s-1; k(C; + CF3CH2F3) -17 cm3 molecule-1 s-1. The results indicate that the most probable site for H-atom abstraction from CF3CH2CF2CH3 is the methyl group and that the formation of carbonyl compounds containing more than a single carbon atom will be negligible under atmospheric conditions, carbonyl difluoride and carbon dioxide being the main degradation products. Finally, accurate infrared absorption cross-sections have been measured for CF3CH2CF2CH3, and jointly used with the calculated overall atmospheric lifetime of 10.2 years, in the NCAR chemical-radiative model, to determine the radiative forcing of climate by this CFC alternative. The steady-state Halocarbon Global Warming Potential, relative to CFC-11, is 0.17. The Global Warming Potentials relative to CO2 are found to be 2210, 790, and 250, for integration time-horizons of 20, 100, and 500 years, respectively.
- Barry, John,Locke, Garrett,Scollard, Donncha,Sidebottom, Howard,Treacy, Jack,Clerbaux, Cathy,Colin, Reginald,Franklin, James
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p. 607 - 617
(2007/10/03)
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- Rate Mechanism of the Atmospheric Degradation of 1,1,1,2-Tetrafluoroethane (HFC-134a)
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Ihe atmospheric chemical behaviour of 1,1,1,2-tetrafluoroethane (CF3CFH2, HFC-134a) with respect to its rate and mechanism of degradation in the troposphere has been investigated. The rate coefficient for the reaction of (i a) CF3CFH2 +OH→CF3CFH + H2O has been determined in direct time-resolved experiments using iaser-pulse initiation and laser long-path absorption. A value of kia = (4.6±0.5)×10-15 cm3 A at T = 295K has been found. The ratio of the rate coefficients for the reactions of the CF3CFHO-radical with O2, (4) CF3CFHO + O2→CF3CFO + HO2, and C-C bond fission, (5) CF3CFHO + M→CFHO + CF3 + M, for T =295 K and ptotal = 50 mbar (O2) has been obtained to be k4/(k5 [M]) = 1.5×10-19 cm3, with the individual values being k4 = 2.7×10-15 cm3/s and k5 [M] = 1.8×104 s-1. The mechanism of the atmospheric degradation of HFC-134a has been investigated in the temperature range 244-295 K and ptotal = 1000 mbar using UV photolysis/FTIR product studies. From these experiments the Arrhenius expression k4/(k5 [M]) = 8.7 ×10-25 exp ((3240-610+990) K/T) cm3 was determined. Moreover, it is concluded that the major carbonyl products are CF3CFO, CFHO and CF2O which result from the two possible reactions of the oxy radical CF3CFHO. As a consequence the yield of these carbonyl products is dependent on temperature, O2 partial pressure and total pressure. For the reaction of CFHO with Cl atoms the Arrhenius expression k18 = 5.7×10-14 exp (-(1130 ±160)K/T)cm3/s was determined in the temperature range 244-298 K. VCH Verlagsgeseuschaft mbH, 1996.
- Bednarek,Ereil,Hoffmann,Kohlmann,Moers,Zellner
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p. 528 - 539
(2007/10/03)
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- The reaction of CF3OOCF2OF with fluoroolefins
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The reaction of fluoroxydifluoromethyl trifluoromethyl peroxide, CF3OOCF2OF (1), with fluoroolefins leads to the formation of novel fluorinated peroxides as major products.When 1 was added to 1,2-dichloro-1,2-difluoroethylene, CFCl=CFCl, under controlled conditions, the formation of 4,5-dichloro-2,2,4,5-tetrafluoro-1,3-dioxolane was also observed.A mechanism is proposed to rationalize these results. - Keywords: Fluoroolefins; Fluoroxydifluoromethyl trifluoromethyl peroxide; Synthesis; IR spectroscopy; NMR spectroscopy; Mass spectrometry
- Russo, A.,Montanari, V.,Navarrini, W.,DesMarteau, Darryl D.
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- Kinetics and mechanisms for the reaction of hydroxyl radicals with trifluoroacetic acid under atmospheric conditions
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The rate constant for the gas-phase reaction of the hydroxyl radical with trifluoroacetic acid has been determined over the temperature range 283-323 K using a relative rate technique.The results provide a value k(OH+CF3C(O)OH)=2.0*10-13exp(-146+/-500/T) cm3molecule-1s-1 based on k(OH+C2H6)=7.8*10-12exp(-1020+/-100/T) cm3molecule-1s-1 for the rate constant of the reference reaction.The OH-radical initiated oxidation of CF3C(O)OH was investigated at 298 K in oxygen at 1 atm total pressure using Fourier-transform infrared spectroscopy.The major carbon-containing products were C(O)F2, CF3O3CF3 and CO2 suggesting that the dominant reaction channel for loss of the CF3C(O)O radical, formed by hydrogen atom abstraction, is carbon-carbon bond cleavage.The results indicate that the gas-phase reaction of OH radicals with trifluoroacetic acid may provide a sink for this compound in the troposphere that cannot be neglected.
- Carr, Sinead,Treacy, Jack J.,Sidebottom, Howard W.,Connell, Richenda K.,Canosa-Mas, Carlos E.,et al.
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- Atmospheric Chemistry of HFC-134a: Kinetic and Mechanistic Study of the CF3CFHO2 + HO2 Reaction
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Rate constant measurements for the title reaction and for F + CF3CFH2 are reported over the 210-363 K temperature range.Reacting mixtures of CF3CFHO2 and HO2 radicals are created by the flash photolysis of F2 in the presence of HFC-134a (CF3CFH2), H2, and O2 and are probed by time-resolved UV absorption spectroscopy.The deconvolution of spectra taken at various delay times provides concentration versus time profiles for the respective radical species.A comparison of the initial CF3CFHO2 and HO2 concentrations yields a relative rate determination of k1 = (9.8+9-5) * 10-11e(-1130+/-190)/T cm3s-1 for the F + CF3CFH2 rate constant.A simultaneous fit of the radical decay curves to a mechanism containing the known HO2 self-reaction kinetics, the previously determined CF3CFHO2 self-reaction mechanism, and a cross reaction between these species yields a rate constant of k7 = (1.8+2.4-1.0) * 10-13e(910+/-220)/T cm3s-1 for the HO2 + CF3CFHO2 reaction.A product stucy at 296 K by Fourier transform infrared spectroscopy reveals that less than 5percent of the product appears as CF3C(O)F.By inference, >95percent of the reaction gives the hydroperoxide CF3CFHOOH.Implications of these results are discussed with respect to the atmospheric degradation of HFC-134a (CF3CFH2).
- Maricq, M. Matti,Szente, Joseph J.,Hurley, Michael D.,Wallington, Timothy J.
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p. 8962 - 8970
(2007/10/02)
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- Atmospheric Chemistry of CF3O Radicals: Reaction with H2O
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Evidence is presented that CF3O radicals react with H2O in the gas phase at 296 K to give CF3OH and OH radicals.This reaction is calculated to be exothermic by 1.7 kcal mol-1 implying a surprisingly strong CF3O-H bond energy of 120 +/- 3 kcal mol-1.Results from a relative rate experimental study suggest that the rate constant for the reaction of CF3O radicals with H2O lies in the range (0.2-40) x 10-17 cm3 molecule-1 s-1.Implications for the atmospheric chemistry of CF3O radicals are discussed.
- Wallington, Timothy J.,Hurley, Michael D.,Schneider, William F.,Sehested, Jens,Nielsen, Ole John
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p. 7606 - 7611
(2007/10/02)
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- UV absorption spectrum, and kinetics and mechanism of the self reaction of CF3CF2O2 radicals in the gas phase at 295 K
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The ultraviolet absorption spectrum, kinetics and mechanism of the self reaction of CF3CF2O2 radicals have been studied in the gas phase at 295 K. Two techniques were used; pulse radiolysis UV absorption to measure the spectrum and kinetics, and long-path length FTIR spectroscopy to identify and quantify the reaction products. Absorption cross sections were quantified over the wavelength range 220-270 nm. At 230 nm, ΣCF3CF2O2 =(2.74±0.46)×10-18 cm2 molecule-1. The observed products following the self reaction of CF3CF2O2 radicals were COF2CF3O3CF3, CF3O3C2F5, and CF3OH. CF3O2CF3 was tentatively identified as a product. The carbon balance was 90-100%. The self reaction of CF3CF2O2 radicals was found to proceed via one channel to produce CF3CF2O radicals which then decompose to give CF3 radicals and COF2. In the presence of O2, CF3 radicals are converted into CF3O radicals. CF3O radicals have several fates; self reaction to give CF3O2CF3; reaction with CF3O2 radicals to give CF3O3CF3; reaction with C2F5O2 radicals to give CF3O3C2F5; or reaction with CF3CF2H to give CF3OH. As part of this work a rate constant (2.5±0.6)×10-16 cm3 molecule-s- was measured for the reaction of Cl atoms with CF3CHF2 using a relative rate technique. Results are discussed with respect to the atmospheric chemistry of CF3CF2H(HFC-125).
- Sehested,Ellermann,Nielsen
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p. 701 - 717
(2007/10/03)
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