UV absorption spectrum,and kinetics and mechanism of the self reaction of CF3CF2O2 radicals in the gas phase at 295 K

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, σ = (2.74 ± 0.46) ×10^?18 cm² molecule^?1. This absorption cross section was used to derive the observed self reaction rate constant for reaction (1), defined as, ?d[CF₃CF₂O₂]/dt = 2k_1obs[CF₃CF₂O₂]²: k_1obs = (2.10 ± 0.38) ×10^?12 cm³ molecule^?1 s^?1 (2σ). The observed products following the self reaction of CF₃CF₂O₂ radicals were COF₂, CF₃O₃CF₃, 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 of (2.5 ± 0.6) ×10^?16 cm³ molecule^?s^?1 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). © 1993 John Wiley & Sons, Inc.     

Pulse radiolysis and fourier transform infrared study of neopentyl peroxy radicals in the gas phase at 297 K

The ultraviolet absorption spectrum of the neopentyl peroxy radical (CH₃)₃CCH₂O₂, and the kinetics and products of its self reaction have been studied in the gas phase at 298 K. Absorption cross sections were quantified over the wavelength range 230–290 nm. The measured cross section at 250 nm was; Errors represent statistical (2σ) together with our estimate of potential systematic errors(15%). The kinetics of the decay of the UV absorption following the generation of the neopentyl peroxy radicals was complicated by the rapid decomposition of the (CH₃)₃CCH₂O radicals formed in channel (4a). By measuring the yield of t-butyl peroxy radicals, the branching ratio k_4a/(k_4a + k_4b) was determined to be 0.39 ± 0.03. The rate constant for the self reaction of neopentyl peroxy radicals was k₄ = (1.07 ± 0.22) × 10^?12 cm³ molecule^?1 s^?1. Quoted errors represent 2σ. These results are discussed with respect to the available literature data. © John Wiley & Sons, Inc.     

UV absorption spectra and kinetics of the self reaction of CFCl2CH2O2 and CF2ClCH2O2 radicals in the gas phase at 298 K

The ultraviolet absorption spectra of the peroxy radicals derived from hydrochlorofluorocarbons 141b and 142b, (CFCl₂CH₂O₂ and CF₂ClCH₂O₂, respectively), and the kinetics of their self reactions have been studied in the gas phase at 298 K using a pulse radiolysis technique. Absorption cross sections were quantified over the wavelength range 220–300 nm. Measured absorption cross sections at 250 nm were indistinguishable within the experimental uncertainties (≈10%) and yield; Errors represent the sum of statistical uncertainty and our estimate of potential systematic errors. Our absorption cross section data were then used to derive the observed self reaction rate constants for reactions (1) and (2), defined as ?d[RO₂]/dt = 2k[RO₂]² (R = CFCl₂CH₂ or CF₂ClCH₂), of k_1obs = (4.36 ± 0.64) × 10^?12 and k_2obs = (4.13 ± 0.58) × 10^?12 cm³ molecule^?1 s^?1, quoted errors represent 2σ. These results are discussed with respect to previous studies of the absorption spectra and kinetics of peroxy radicals.     

FTIR study of the reaction of ethyl radicals with O2 and Cl2 in air at 295 K

Using Fourier transform infrared spectroscopy, the ethene yield from the reaction of C₂H₅ radicals with O₂ has been determined to be 1.50 ± 0.09%, 0.85 ± 0.11%, and <0.1% at total pressures of 25, 50, and 700 torr, respectively. Additionally, the rate constant of the reaction of C₂H₅ radicals with molecular chlorine was measured relative to that with molecular oxygen. (1) A ratio k₆/k₇ = 1.99 ± 0.14 was measured at 700 torr total pressure which, together with the literature value of k₇ = 4.4 × 10^?12 cm³ molecule^?1s^?1, yields k₆ = (8.8 ± 0.6) × 10^?12 cm³ molecule^?1s^?1. Quoted errors represent 2σ. These results are discussed with respect to previous kinetic and mechanistic studies of C₂H₅ radicals.     

Fourier transform infrared study of the self reaction of C2H5O2 radicals in air at 295 K

Fourier transform infrared spectroscopy was used to identify and quantify products of the self reaction of ethylperoxy radicals, C₂H₅O₂, formed in the photolysis of Cl₂/C₂H₆ mixtures in 700 torr total pressure of synthetic air at 295 K. From these measurements, branching ratios for the reaction channels (1) of k_1a/(k_1a + k_1b) = 0.68 and k_1c/(k_1a + k_1b + k_1c) ? 0.06 were established. Additionally, using the relative rate technique, the rate constant for the reaction of Cl atoms with C₂H₅OOH was determined to be (1.07 ± 0.07) × 10^?10 × cm³ molecule^?1 s^?1. Results are discussed with respect to the previous kinetic and mechanistic studies of C₂H₅O₂ radicals.     

The reaction of C2F5 radicals with H2S

The reaction of C₂F₅ radicals with H₂S was studied over the range 1°?123°C using C₂F₅ radicals generated by photolysis of perfluoropropionic anhydride. The rate constant k_H for reaction (2) is given by where θ = 2.303RT/cal mole^?1. The relevance of this result to conflicting published data on the analogous reaction between CF₃ radicals and H₂S is discussed. It is concluded that there is little difference in the Arrhenius parameters for reaction of CF₃ and C₂F₅ radicals with H₂S.     

Kinetic study of the reaction of chlorine atoms with CF3I and the reactions of CF3 radicals with O2, Cl2 and NO at 296 K

The kinetics of the gas-phase reaction of Cl atoms with CF₃I have been studied relative to the reaction of Cl atoms with CH₄ over the temperature range 271–363 K. Using k(Cl + CH₄) = 9.6 × 10^?12 exp(?2680/RT) cm³ molecule^?1 s^?1, we derive k(Cl + CF₃I) = 6.25 × 10^?11 exp(?2970/RT) in which E_a has units of cal mol^?1. CF₃ radicals are produced from the reaction of Cl with CF₃I in a yield which was indistinguishable from 100%. Other relative rate constant ratios measured at 296 K during these experiments were k(Cl + C₂F₅I)/k(Cl + CF₃I) = 11.0 ± 0.6 and k(Cl + C₂F₅I)/k(Cl + C₂H₅Cl) = 0.49 ± 0.02. The reaction of CF₃ radicals with Cl₂ was studied relative to that with O₂ at pressures from 4 to 700 torr of N₂ diluent. By using the published absolute rate constants for k(CF₃ + O₂) at 1–10 torr to calibrate the pressure dependence of these relative rate constants, values of the low- and high-pressure limiting rate constants have been determined at 296 K using a Troe expression: k₀(CF₃ + O₂) = (4.8 ± 1.2) × 10^?29 cm⁶ molecule^?2 s^?1; k_∞(CF₃ + O₂) = (3.95 ± 0.25) × 10^?12 cm³ molecule^?1 s^?1; F_c = 0.46. The value of the rate constant k(CF₃ + Cl₂) was determined to be (3.5 ± 0.4) × 10^?14 cm³ molecule^?1 s^?1 at 296 K. The reaction of Cl atoms with CF₃I is a convenient way to prepare CF₃ radicals for laboratory study. © 1995 John Wiley & Sons, Inc.     

Kinetics and mechanism of the reaction of OH with ClO

The kinetics and mechanism of the following reactions have been studied in the temperature range 230–360 K and at total pressure of 1 Torr of helium, using the discharge‐flow mass spectrometric method: 1a : (1a) 1b : (1b) The following Arrhenius expression for the total rate constant was obtained from the kinetics of OH consumption in excess of ClO radical, produced in the Cl + O₃ reaction either in excess of Cl atoms or ozone: k₁ = (6.7 ± 1.8) × 10^?12 exp {(360 ± 90)/T} cm³ molecule^?1 s^?1 (with k₁ = (2.2 ± 0.4) × 10^?11 cm³ molecule^?1 s^?1 at T = 298 K), where uncertainties represent 95% confidence limits and include estimated systematic errors. The value of k₁ is compared with those from previous studies and current recommendations. HCl was detected as a minor product of reaction (1) and the rate constant for the channel forming HCl (reaction (1b)) has been determined from the kinetics of HCl formation at T = 230–320 K: k_1b = (9.7 ± 4.1) × 10^?14 exp{(600 ± 120)/T} cm³ molecule^?1 s^?1 (with k_1b = (7.3 ± 2.2) × 10^?13 cm³ molecule^?1 s^?1 and k_1b/k₁ = 0.035 ± 0.010 at T = 298 K), where uncertainties represent 95% confidence limits. In addition, the measured kinetic data were used to derive the enthalpy of formation of HO₂ radicals: Δ H_f,298(HO₂) = 3.0 ± 0.4 kcal mol^?1. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 587–599, 2001     

Reaction of CF3 radicals with H2S

Hydrogen abstration from H₂S by CF₃ radicals, generated by the photolysis of both CF₃COCF₃ and CF₃I, has been studied in the temperature range 314–434 K. The rate constant, based on the value of 10^13.36 cm³/mol · s for the recombination of CF₃ radicals, is given by with CF₃COCF₃ as the radical source, and with CF₃I as the radical source, where k₂ is in cm³/mol · s and E is in J/mol. These results resolve a previously existing controversy concerning the values of the rate constants for this reaction. They show that CF₃ radicals are less reactive than CH₃ radicals in attacking H₂S, and this behavior indicates that polar effects play a significant role in the hydrogen transfer reactions of CF₃ radicals.     

A Kinetic study of the reaction of nitrogen dioxide with tetrafluoroethylene

The reaction of NO₂ with C₂F₄ was studied at 30°, 68°, 114°, and 157°C by in situ monitoring the infrared absorption bands of the products. The major primary products of the reaction are O₂NCF₂CFO and FNO. Smaller amounts of CF₂O (and presumably NO) are also produced. There was no evidence for other primary products, though they may have been produced in minor amounts. The rate laws for the production of both O₂NCF₂CFO and CF₂O are first order in both [NO₂] and [C₂F₄]. CF₂O production is at least partly heterogeneous as demonstrated by packing the quartz reaction vessel with Pyrex beads and by using a Monel cell. The homogeneous rate constant obtained from the high-temperature results gives a rate constant of 3.4 × 10⁸ exp (minus;17000/RT) M^?1sec^?1 for CF₂O production. Actually these Arrhenius parameters represent lower limits, since the heterogeneous reaction may still be playing a significant role. The production rate of O₂NCF₂CFO is not much affected by changing the nature of the surface or the surface to volume ratio. However the reaction may be heterogeneous, since the rate constant for its formation of 1.3 × 10⁴ e×p (?7500/RT) M^?1sec^?1 has an abnormally low pree×ponential factor. E×periments in the presence of NO indicate that the mechanism for O₂NCF₂CFO formatlon is The intermediate can also react with NO: with k₁₃/k₁₂ = 1.3.     

The gas phase reactions of hydroxyl radicals with a series of aliphatic ethers over the temperature range 240–440 K

Absolute rate constants were determined for the gas phase reactions of OH radicals with a series of linear aliphatic ethers using the flash photolysis resonance fluorescence technique. Experiments were performed over the temperature range 240–440 K at total pressures (using Ar diluent gas) between 25–50 Torr. The kinetic data for dimethylether (k₁), diethylether (k₂), and dipropylether (k₃) were used to derive the Arrhenius expressions and At 296 K, the measured rate constants (in units of 10^?13 cm³ molecule^?1 s^?1) were: k₁ = (24.9 ± 2.2), k₂ = (136 ± 9), and k₃ = (180 ± 22). Room temperature rate constants for the OH reactions with several other aliphatic ethers were also measured. These were (in the above units): di-n-butylether, (278 ± 36); di-n-pentylether, (347 ± 20); ethyleneoxide, (0.95 ± 0.05); propyleneoxide, (4.95 ± 0.52); and tetrahydrofuran, (178 ± 16). The results are discussed in terms of the mechanisms for these reactions and are compared to previous literature data.     

Kinetics of the gas phase reaction of OH radicals with pyrrole and thiophene

Absolute rate constants for the gas phase reaction of OH radicals with pyrrole (k₁) and thiophene (k₂) have been measured over the temperature ranges 298–440 and 274–382 K, respectively, using the flash photolysis-resonance fluorescence technique. The rate constants obtained were independent of the total pressure of argon diluent over the range 25–100 torr andwere fit by the Arrhenius expressions and with rate constants at 298 ± 2 K of k₁ = (1.03 ± 0.06) × 10^?10 cm³ molecule^?1 s^?1 and k₂ = (8.9 ± 0.7) × 10^?12 cm³ molecule^?1 s^?1. [These errors represent two standard deviations (systematic errors could constitute an additional ca. 10% uncertainty)]. These results are discussed with respect to the previous literature data and the atmospheric lifetimes of pyrrole and thiophene.     

Reaction of carbon monoxide with ozone and oxygen atoms

The reaction between ozone and carbon monoxide was reinvestigated in the range of 80–160°C. The previously reported rate law ?d[O₃]/dt = k_a[O₃][CO] + k_b[O₃]² was confirmed and simulated using a mechanism based on an impurity-initiated chain reaction. When the CO was sufficiently purified, k_b tended to zero and k_a reduced to the value expected for the thermal decomposition of O₃. Subsequent reactions of O atoms with CO produced chemiluminescence which was used to measure k₃ for as 10^?14.0±0.3 exp[?(1630 ± 325)/T] cm³ molecule^?1 s^?1. The implications of this are discussed.     

UV absorption spectrum of CF3CFClO2 and kinetics of the self reaction of CF3CFCl and CF3CFClO2 and the reactions of CF3CFClO2 with NO and NO2

The ultraviolet absorption spectrum of CF₃CFClO₂ and the kinetics of the self reactions of CF₃CFCl and CF₃CFClO₂ radicals and the reactions of CF₃CFClO₂ with NO and NO₂ have been studied in the gas phase at 295 K by pulse radiolysis/transient UV absorption spectroscopy. The UV absorption cross section of CF₃CFCl radicals was measured to be (1.78 ± 0.22) × 10^?18 cm² molecule^?1 at 220 nm. The UV spectrum of CF₃CFClO₂ radicals was quantified from 220 nm to 290 nm. The absorption cross section at 250 nm was determined to be (1.67 ± 0.21) × 10^?18 cm² molecule^?1. The rate constants for the self reactions of CF₃CFCl and CF₃CFClO₂ radicals were (2.6 ± 0.4) × 10^?12 cm³ molecule^?1 s^?1 and (2.6 ± 0.5) × 10^?12 cm³ molecule^?1 s^?1, respectively. The reactivity of CF₃CFClO₂ radicals towards NO and NO₂ was determined to (1.5 ± 0.6) × 10^?11 cm³ molecule^?1 s^?1 and (5.9 ± 0.5) × 10^?12 cm³ molecule^?1 s^?1, respectively. Finally, the rate constant for the reaction of F atoms with CF₃CFClH was determined to (8 ± 2) × 10^?13 cm³ molecule^?1 s^?1. Results are discussed in the context of the atmospheric chemistry of HCFC-124, CF₃CFClH. © 1994 John Wiley & Sons, Inc.     

Reactions of alkoxy radicals with O2. I. C2H5O radicals

C₂H₅ONO was photolyzed with 366 nm radiation at ?48, ?22, ?2.5, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yield of CH₃CHO, Φ{CH₃CHO}, was measured as a function of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?_1a = 0.29 ± 0.03 independent of temperature. The C₂H₅O radicals can react with NO by two routes The C₂H₅O radical can also react with O₂ via Values of k₆/k₂ were determined at each temperature. They fit the Arrhenius expression: Log(k₆/k₂) = ?2.17 ± 0.14 ? (924 ± 94)/2.303 T. For k₂ ? 4.4 × 10^?11 cm³/s, k₆ becomes (3.0 ± 1.0) × 10^?13 exp{?(924 ± 94)/T} cm³/s. The reaction scheme also provides k_8a/k₈ = 0.43 ± 0.13, where      

Pulse radiolysis study of CF3CCI2 and CF3CCI2O2 radicals in the gas phase at 295K

The ultraviolet absorption spectra and self reaction kinetics of CF₃CCI₂ and CF₃CCI₂O₂ radicals have been studied in the gas phase at 295K. Absorption cross sections were quantified over the wavelength range 220–300 nm. Measured cross sections near the absorption maxima were γ_CF3CCI2(230 nm) = (9.70 ± 1.47) x 10^-18 and γ_CF3CCI2O2(250 nm) = (1.70 ± 0.26) x 10^-18 cm² molecule^-1. Errors are statistical (2γ) together with our estimate of potential systematic errors. Rate constants for the self reaction of CF₃CCI₂ and CF₃CCI₂O₂ radicals were measured to be k₆ = (2.46 ± 0.43) x 10^-12 and k_7obs = (3.33 ± 0.53) x 10^-12 cm³ molecule^-1 s^-1, respectively. Results are discussed with respect to the existing database concerning halogenated peroxy radicals.     

Reaction of CF3 radicals with C2H6

The hydrogen transfer reaction between C₂H₆ and CF₃ radicals, generated by the photolysis of CF₃I, has been studied in the temperature range 298–617 K. The rate constant, based on the value of 10^13.36 cm³ mol^?1 s^?1 for the recombination of CF₃ radicals, is given by where k₂ is in cm³ mol^?1 s^?1 and E is in J mol^?1. These results are compared with those previously reported, and the following best value for k₂ is recommended:      

The reactions of alkoxy radicals with O2. II. n-C3H7O radicals

n-C₃H₇ONO was photolyzed with 366 nm radiation at ?26, ?3, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yields of C₂H₅CHO, C₂H₅ONO, and CH₃CHO were measured as a function of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?₁ = 0.38 ± 0.04 independent of temperature. The n-C₃H₇O radicals can react with NO by two routes The n-C₃H₇O radical can decompose via or react with O₂ via Values of k₄/k₂ ? k_4b/k₂ were determined to be (2.0 ± 0.2) × 10¹⁴, (3.1 ± 0.6) × 10¹⁴, and (1.4 ± 0.1) × 10¹⁵ molec/cm³ at 55, 88, and 120°C, respectively, at 150-torr total pressure of N₂. Values of k₆/k₂ were determined from ?26 to 88°C. They fit the Arrhenius expression: For k₂ ? 4.4 × 10^?11 cm³/s, k₆ becomes (2.9 ± 1.7) × 10^?13 exp{?(879 ± 117)/T} cm³/s. The reaction scheme also provides k_4b/k₆ = 1.58 × 10¹⁸ molec/cm³ at 120°C and k_8a/k₈ = 0.56 ± 0.24 independent of temperature, where      

The kinetics of the gas phase decomposition reactions of the hexafluoro-t-butoxy radical

Hexafluoro-t-butoxy radicals have been generated by reacting fluorine with hexafluoro-2-methyl isopropanol: Over the temperature range of 406–600 K the hexafluoro-t-butoxy radical decomposes exclusively by loss of a CF₃ radical [reaction (-2)] rather than by loss of a CH₃ radical [reaction (-1)]: (1) The limits of detectability of the product CF₃COCF₃, by gas-chromatographic analysis, place a lower limit on the ratio k_?2/k_-1 of ～80. The implications of this finding in relation to the reverse radical addition reactions to the carbonyl group are briefly discussed. A thermochemical kinetic calculation reveals a discrepancy in the kinetics and thermodynamics of the decomposition and formation reactions of the related t-butoxy radical:      

Temperature dependence of the reactions of HO2 with NO and NO2

Mixtures of N₂O, H₂, O₂, and trace amounts of NO and NO₂ were photolyzed at 213.9 nm, at 245°–328°K, and at about 1 atm total pressure (mostly H₂). HO₂ radicals are produced from the photolysis and they react as follows: Reaction (1b) is unimportant under all of our reaction conditions. Reaction (1a) was studied in competition with reaction (3) from which it was found that k_1a/k₃^1/2 = 6.4 × 10^?6 exp { z?(1400 ± 500)/RT} cm^3/2/sec^1/2. If k₃ is taken to be 3.3 × 10^?12 cm³/sec independent of temperature, k_1a = 1.2 × 10^?11 exp {?(1400 ± 500)/RT} cm³/sec. Reaction (2a) is negligible compared to reaction (2b) under all of our reaction conditions. The ratio k_2b/k₁ = 0.61 ± 0.15 at 245°K. Using the Arrhenius expression for k_1a given above leads to k_2b = 4.2 × 10^?13 cm³/sec, which is assumed to be independent of temperature. The intermediate HO₂NO₂ is unstable and induces the dark oxidation of NO through reaction (?2b), which was found to have a rate coefficient k_?2b = 6 × 10¹⁷ exp {?26,000/RT} sec^?1 based on the value of k_1a given above. The intermediate can also decompose via Reaction (10b) is at least partially heterogeneous.