Temperature-induced transformations of peroxide radicals formed in polytetrafluoroethylene with γ-ray irradiation

At 200°C, temperature-induced transformations of peroxide radicals from ? CF₂CF(OO·)CF₂? (radical I) to ? CF₂CFCF₂? (radical 1) and ? CF₂CF₂OO· (radical II) to ? CF₂CF₂ (radical 2) in vacuo, and from I to II in oxygen atmosphere were observed. In these thermal transformations, the evolution of CO₂ with a small amount of CO and CF₂O was observed. Further, measurements of the heat of crystallization of the treated polytetrafluoroethylene suggest that a reduction of polymer molecular weight occurs during the transformations from I to 1 and I to II.     

CF3CH(ONO)CF3: Synthesis,IR spectrum,and use as OH radical source for kinetic and mechanistic studies

The synthesis, IR spectrum, and first‐principles characterization of CF₃CH(ONO)CF₃ as well as its use as an OH radical source in kinetic and mechanistic studies are reported. CF₃CH(ONO)CF₃ exists in two conformers corresponding to rotation about the RCO? NO bond. The more prevalent trans conformer accounts for the prominent IR absorption features at frequencies (cm^?1) of 1766 (N?O stretch), 1302, 1210, and 1119 (C? F stretches), and 761 (O? N? O bend); the cis conformer contributes a number of distinct weaker features. CF₃CH(ONO)CF₃ was readily photolyzed using fluorescent blacklamps to generate CF₃C(O)CF₃ and, by implication, OH radicals in 100% yield. CF₃CH(ONO)CF₃ photolysis is a convenient source of OH radicals in the studies of the yields of CO, CO₂, HCHO, and HC(O)OH products which can be difficult to measure using more conventional OH radical sources (e.g., CH₃ONO photolysis). CF₃CH(ONO)CF₃ photolysis was used to measure k(OH + C₂H₄)/k(OH + C₃H₆) = 0.29 ± 0.01 and to establish upper limits of 16 and 6% for the molar yields of CO and HC(O)OH from the reaction of OH radicals with benzene in 700 Torr of air at 296 K. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 159–165, 2003     

Formation and decay of trifluoromethyl radicals during photodecomposition of the long-lived perfluoro-2,4-dimethyl-3-ethylpent-3-yl radical

UV irradiation of the long-lived radical [(CF₃)₂CF]₂C^·C₂F₅ (1) in a hexafluoropropylene trimer (HFPT) glassy matrix at 77 K and in a HFPT solution at 300 K leads to its decomposition to the ^·CF₃ radical and perfluoroolefin molecule. About 90% of the ^·CF₃ radicals formed recombine at 300 K. The remaining radicals add to the HFPT molecules generating the long-lived radicals [(CF₃)₂CF]₃C^·. Unlike the ^·CF₃ radicals produced by the photodecomposition of radicals 1, the ^·CF₃ radicals formed during radiolysis of HFPT are not stabilized in the glassy HFPT matrix at 77 K.     

Activation of molecular oxygen in trifluoroacetic acid

The interaction of molecular oxygen with aqueous trifluoroacetic acid (TFA) led to an increase in pH. This effect was explained by a decrease in the concentration of the protonated CF₃CO₂H₂⁺ and H₃O⁺ species after oxygen was fed in the reactor. Quantum-chemical calculations show that a radical pair can be formed in an activation-free exothermal reaction involving the radical residue of the acid, the CF₃CO₂H₂⁺...³O₂...CF₃CO₂⁻ peroxide radical, and the acid molecule in the CF₃C₂^·...HOO^· collision complex. It was assumed that the activation of molecular oxygen in aqueous TFA solutions, providing the activity of the system in oxidations of various organic and inorganic substrates, is related to the formation of peroxide radicals in them.     

The ESR spectra of the spin-adducts of branched perfluorinated σ-radicals with fullerene-60

The ESR spectra of the radical adducts of the^.CF=CFC(CF₃)₃ and^.C(O)CF(CF₃)₂ radicals with C₆₀ are characterized by hyperfine interaction with the nucleus of the δ-fluorine atom located above the five-membered cycle in the fullerenyl radical. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1222–1224, June, 1997.     

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.     

ESR study of radicals formed by the reduction of α-diketones C(O)C(O)CF3 (R=(CF3)2CF, C6F5, (CF3)3C) with several group I–III metals

The ESR spectra of radical anions formed by reduction of α-diketones RC(O)C(O)CF₃ (R=(CF₃)₂CF, C₆F₅, (CF₃)₃C) with metals (Li, Na, K, Mg, Cd, Zn, Hg, In, and TI) in THF were studied. For R=(CF₃)₂CF and C₆F₅, the radical anions are formed ascis-isomers, whereas for R=(CF₃)₃C,trans-isomers are obtained. Line broadening due to solvation and desolvation of the cation is observed in the latter case. The reduction of α-diketone (CF₃)₂CFC(O)C(O)CF₃ with Group II metals (Mg, Cd, Zn) results in the formation of radical pairs. Translated fromIzvestiya Akadmii Nauk. Seriya Khimicheskaya, No. 11, pp. 2228–2231, November, 1998.     

The nascent translational energy of difluorocarbene produced in IR MPD of the pentafluoroethyl radical

We report upon the direct detection of difluorocarbene following infrared multiphoton photolysis of pentafluoroethyl iodide using well-defined (SLM, TEM₀₀, 80 ns pulse width) TEA CO₂-laser pulses. The rate of appearance of CF₂ at 1 mTorr pressure and RRKM modelling of the unimolecular dissociation of C₂F₃I and C₂F₅ using reasonable input parameters are presented. These support a mechanism whereby CF₂ is produced by secondary photolysis of pentafluoroethyl radicals. Measurements of the velocity of CF₂ by the transient diffusion technique lead to an estimate of 2.6 kcal/mol for its average translational energy acquired from the homolytic cleavage of the CI and CC bonds. This value is higher than that predicted from the models using reasonable spontaneous dissociation rates ( = 10⁹ s^?1). An inherent assumption of the models is that the excess energy of dissociation is distributed statistically among the vibrational modes of the reaction complex and that there are no small barriers in the exit channel.     

Temperature Dependence Kinetic Studies of the Reaction of O(3P) with CHI3 and C2H5I and the 298 K Reaction of OH(X2Π) with CHI3

A flash photolysis–resonance fluorescence technique was used to investigate the kinetics of the OH(X²Π) radical and O(³P) atom‐initiated reactions with CHI₃ and the kinetics of the O(³P) atom‐initiated reaction with C₂H₅I. The reactions of the O(³P) atom with CHI₃ and C₂H₅I were studied over the temperature range of 296 to 373 K in 14 Torr of helium, and the reaction of the OH (X²Π) radical with CHI₃ was studied at T = 298 K in 186 Torr of helium. The experiments involved time‐resolved resonance fluorescence detection of OH (A²Σ⁺ → X²Π transition at λ = 308 nm) and of O(³P) (λ = 130.2, 130.5, and 130.6 nm) following flash photolysis of the H₂O/He, H₂O/CHI₃/He, O₃/He, and O₃/C₂H₅I/He mixtures. A xenon vacuum UV (VUV) flash lamp (λ > 120 nm) served as a photolysis light source. The OH radicals were produced by the VUV flash photolysis of water, and the O(³P) atoms were produced by the VUV flash photolysis of ozone. Decays of OH radicals and O(³P) atoms in the presence of CHI₃ and C₂H₅I were observed to be exponential, and the decay rates were found to be linearly dependent on the CHI₃ and C₂H₅I concentrations. Measured rate coefficients for the reaction of O(³P) atoms with CHI₃ and C₂H₅I are described by the following Arrhenius expressions (units are cm³ s^?1): k_O+C2H5I(T) = (17.2 ± 7.4) × 10^?12 exp[?(190 ± 140)K/T] and k_O+CHI3(T) = (1.80 ± 2.70) × 10^?12 exp[?(440 ± 500)K/T]; the 298 K rate coefficient for the reaction of the OH radical with CHI₃ is k_OH+CHI3(298 K) = (1.65 ± 0.06) × 10^?11 cm³ s^?1. The listed uncertainty values of the Arrhenius parameters are 2σ‐standard errors of the calculated slopes by linear regression.     

Long path-FTIR studies of some atmospheric reactions involving CF3OO and CF3O radicals

In attempts to obtain kinetic and mechanistic data required for an assessment of atmospheric fate of alternative halocarbons containing a CF₃ group, reactions of the key free radical intermediates CF₃OO and CF₃O with several atmospheric compounds (i.e., NO, NO₂, alkanes and alkenes) have been studied at 297 ± 2 K in 700 torr of air. Experiments employed the long path-FTIR spectroscopic method for product analysis and the visible (400 nm) photolysis of CF₃NO → CF₃ + NO as a source for the precursor radical CF₃. Numerous labile and stable F-containing molecular products have been characterized based on kinetic and spectroscopic data obtained at sufficiently short photolysis time (≤1 min) to minimize heterogeneous decay on the reactor walls. Major new findings have been made for the reactions involving CF₃O radicals. The behavior of CF₃O radicals has been shown to be markedly different from that of CH₃O radicals, i.e., (1) O₂-reaction: no evidence for the F-atom transfer reaction CF₃O + O₂ → CF₂ O + FOO; (2) NO-reaction: addition reaction CH₃O + NO (+M) → CH₃ONO (+M), but F-transfer reaction CF₃O + NO → CF₂O + FNO; (3) NO₂-reaction: addition reaction for both radicals, but F-transfer reaction CF₃ + NO₂ → CF₂O + FNO₂ to a minor extent; (4) alkane-reaction: much faster H-abstraction by CF₃O, comparable to HO; (5) alkene-reaction: much faster addition reaction of CF₃O, comparable to HO. These results are summarized in this paper.     

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.     

Photostabilizing activity of sterically hindered piperidines—II: Radical trapping ability

The abilities of both 2,2,6,6-tetramethylpiperidine (I) and its nitroxyl (II) to trap radicals involved in hydrocarbon photo-oxidations have been studied in cumene and 1,3,5-trimethylcyclohexane at 27° using AIBN, hydroperoxide and dialkylperoxide as initiators: the light was either the band 300–400 nm or 366 nm. Under conditions of photolysis of ROOH (degenerate branching), I is oxidized to II. II is capable of trapping R' radicals, the rate constant being ～50 times lower than that for RO^.₂ formation. RO^.₂ radicals react with neither I nor II. Under the condition of degenerate branching, II is capable of intercepting the radical fragments from decomposing hydroperoxide. The rate constant of this process is ～500 times higher than that for hydrogen abstraction by these fragments. A reaction mechanism is suggested: hydrogen bonded associates formed between an N-containing stabilizer and ROOH play a dominant role. The principal intermediates in this mechanism are represented by >NO^., >NOH and >NOR species.     

On the mechanism of C2F5I photolysis in the presence of trifluoroacetic acid

The photolysis of C₂F₅I with CF₃COOH in the temperature range 473–533 K was studied in the gas phase. Evidence is presented that supports a complex mechanism for the formation of C₂F₅H through the H-atom abstraction reaction from CF₃COOH by C₂F₅ radicals as well as the participation of I(²P_½).     

Photochemistry of Long-Lived [(CF3)2CF]2C·C2F5 Radicals in a Hexafluoropropylene Trimer Matrix

The kinetics of photodecomposition of long-lived [(CF₃)₂CF]₂C^·C₂F₅ radicals (I) in glassy and liquid hexafluoropropylene trimer was studied at 77 and 300 K, respectively. It was found that the phase state of the hexafluoropropylene trimer matrix did not affect the photodecomposition mechanism. In both cases, I eliminates ^·CF₃ radical from the perfluoroethyl fragment. The molar absorption coefficient of I was determined: ₂₅₀ = 49 m² mol^–1. It was shown that the photodecomposition of I in a liquid matrix at 300 K led to the formation of other long-lived radicals. One of these species is [(CF₃)₂CF]₃C^· radical, which results from addition of ^·CF₃ radical to the double bond of a hexafluoropropylene trimer molecule.     

The IR multiphoton decomposition of CF3I. The effect of inert and reactive gases

The multiphoton decomposition of CF₃I with a pulsed CO₂ laser has been studied at incident fluences of 0.6 and 1.2 J/cm². The effect of pressure on the reaction probability for dissociation of CF₃I was measured in the presence of added isobutane, Ar and CO₂. In the experiments with isobutane, the CF₃ radicals generated by the decomposition of excited CF₃I react to yield CF₃H in competition with the recombination to C₂F₆. The laser absorption cross section was also measured as a function of fluence at a pressure of 0.1 torr of CF₃I and with 0.5–2.0 torr of added isobutane. The experimental results were modeled with a master equation in order to obtain information on the energy transferred by collisions of excited CF₃I with the bath molecules. An energy dependent value of 〈ΔE〉_d produces the best fit to the experimental data. Integration of the rate equations to account for the fractional product yield, [CF₃I]/[C₂F₆], allowed for the calculation of the specific rate constant for hydrogen abstraction from isobutane by CF₃ radicals. The value obtained is dependent on the total pressure and higher than expected at room temperature. From these results, an effective temperature for the reaction mixture was calculated. © 1994 John Wiley & Sons, Inc.     

ESR spectra of trifluoromethyl radical formed during the solid-phase photodecomposition of long-lived perfluoro-2,4-dimethyl-3-ethylpentyl-3 radicals

The products of photolysis of the long-lived radical [(CF₃)₂CF]₂C^·C₂F₅ at 77 K were studied by ESR. The mechanism of photodecomposition to form ^·CF₃ radicals was proposed. The ESR spectra of the trifluoromethyl radicals stabilized at 77 K in a glassy hexafluoropropylene trimer matrix were simulated, and the parameters À = 25.15 mT, À = 9.1 mT, and g = l.9996, g = 2.0056 were determined.     

Chemical effects of radical vibrational energy content on the abstraction reaction: CF3 + Br2 → CF3Br + Br

We have studied the effect of vibrational mode activation in the CF₃ radical on the bromine abstraction reaction; CF₃ + Br₂ → CF₃Br + Br. Excess vibrational energy resides in the symmetric modes of the radical after 248 nm photolysis of the parent molecule, CF₃I. Our data indicate that the hot radicals react no faster than thermalized CF₃, and may actually have a lower cross-section for reaction. Dynamical factors that result in poor coupling of the vibrational energy to the reaction coordinate, as well as other similar considerations, could be responsible for the experimental observations. In addition, we have made an independent determination of the rate for the bromine abstraction reaction of (1.08 ± .13) × 10¹² s^?1 cm³ mol^?1.     

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.     

Kinetics and mechanism of the thermal gas‐phase oxidation of perfluorobutene‐2 in presence of trifluoromethyl hypofluorite

The oxidation of perfluorobutene‐2 (C₄F₈) initiated by trifluoromethyl hypofluorite (CF₃OF) in presence of O₂ has been studied at 323.1, 332.6, 342.5, and 352.0 K, using a conventional static system. The initial pressure of CF₃OF was varied between 4.8 and 23.6 Torr, that of C₄F₈ between 48.7 and 302.4 Torr, and that of O₂ between 51.5 and 270.4 Torr. Several runs were made in presence of 325.5–451.2 Torr of N₂. The main products were COF₂, CF₃C(O)F, and CF₃OC(O)F. Small amounts of compound containing ? CF(CF₃)? O? C(O)CF₃ group were also formed, as detected by ¹³C NMR spectroscopy. The oxidation is a homogeneous short‐chain reaction, attaining, at the pressure of O₂ used, the pseudo‐zero‐order condition with respect to O₂ as reactant. The reaction is independent of the total pressure. Its basic steps are as follows: the thermal generation of CF₃O^? radicals by the abstraction of fluorine atom of CF₃OF by C₄F₈, the addition of CF₃O^? to the alkene, the formation of perfluoroalkoxy radicals RO^? in presence of O₂, and the decomposition of these radicals via the C? C bond scission, giving products containing ? C(O)F end group and reforming RO^? and CF₃O^? radicals. The mechanism consistent with experimental results is postulated. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 532–541, 2003