The photolysis of pentafluoroacetone

The photolysis of pentafluoroacetone has been investigated in the 3130 Å region, from room temperature to 360°C. The Φ_CO varies from 0.7 to 0.9 over this range, and the decomposition is represented by CF₂HCOCF₃ → CF₂H + CO + CF₃. The disproportionation/combination ratio for CF₃ and CF₂H (→ CF₃H + CF₂) radicals is found to be 0.09. Arrhenius parameters for hydrogen atom abstraction from the ketone are log₁₀A = 12.7 (units are mole^?1 cc sec^?1) and E = 14.3 kcal mole^?1 for CF₂H, and log₁₀A = 12.1 and E = 11.8, for CF₃ radicals. At low pressures HF elimination reactions are observed from the vibrationally excited fluoroethanes, C₂F₅H* and C₂F₄H₂*, formed in the system. A rough estimate of the activation energy for the process C₂F₅H → C₂F₄ + HF of 60–65 kcal mole^?1 is made.     

Gas phase radiation chemistry of hexafluoroethane

The Co-60 gamma radiolysis of gaseous C₂F₆ was investigated at 50 Torr pressure, both pure and with 10% oxygen added. For the pure system, the radiolytic products and their respective C values were CF₄, 2.27; C₃F₈, 0.23; C₄F₁₀, 0.09; C₅F₁₂, 0.015; and C₆F₁₄, 0.009. All radiolysis products except for CF₄ (C = 0.61) were eliminated when 10% O₂ was added as scavenger. The results are discussed mainly in terms of the decomposition of excited C₂F₆ into free radicals, which can then combine. The unscavenged CF₄ is accounted for by the ion-molecule reaction CF₃⁺ + C₂F₆ → CF₄ + C₂F₅⁺.     

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.     

Broadening Of The CF3I UV Absorption Spectrum BY CO2-Laser-Induced Vibrational Excitation

Absorption of CO₂ laser photons by CF₃I leads to a broadening of the CF₃I UV absorption band near 270 nm. The fwhm is increased by a factor of ≈2.3 for CO₂ laser fluence of 0.4 J/cm². Significant increase in the bandwidth is accompanied by partial decomposition of the CF₃I.     

Mechanism of thermal decomposition of peroxide radicals formed in polytetrafluoroethylene by γ-ray irradiation

Thermal decomposition of polymer peroxide radicals formed in γ-irradiated polytetrafluoroethylene, ～CF₂CF(OO·)CF₂～ (radical I) and ～CF₂CF₂(OO～) (radical II), was studied by mass spectral analysis of the gas evolved in comparison with their photolysis with ultraviolet light. In the thermal decomposition of radicals I and II, CO₂ was the most abundant component, with smaller amount of CO, CF₂O, and gases present. In the photolysis, CO instead of CO₂ was the most abundant in the case of radical I, while in the case of radical II, CO₂ was again the main product. When a labeled polymer peroxide radical, ～CF₂CF(¹⁸O-¹⁸O·)CF₂～, was treated with heat or ultraviolet light, C¹⁸O¹⁶O was detected as a main component. In the treatment with ultraviolet light, a large amount of C¹⁸O comparable to that of C¹⁸O¹⁶O was also obtained. The mechanism of main-chain scission of radicals I and II is discussed.     

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.     

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     

Multiphoton dissociation of molecules with low power cw infrared lasers: collisional enhancement of dissociation probabilities

Multiphoton dissociation of C₃F⁺₆ is observed using low intensity cw CO₂ laser radiation. Ion cyclotron resonance (ICR) techniques are used to store ions for irradiation. Ion storage times up to 2 s are used. Multiphoton dissociation is observed at laser intensities below 100 W cm^?2 and at pressures below 10^?5 Torr. Only the lowest energy decomposition of C₃F⁺₆, to give C₂⁺₄ and CF₂, is observed. Multiphoton dissociation probabilities show a sharp wavelength dependence in contrast to typical pulsed laser multiphoton dissociation experiments. The photodissociation spectrum of C₃F⁺₆ is similar to the infrared absorption spectrum of neutral C₃F₆ at both low and high resolution. Collisions between C₃F⁺₆ and unreactive buffer gases (Ar, N₂, SF₆) are seen to enhance multiphoton dissociation, while collisions with C₃F₆ deactive the laser excited species. The results are discussed in terms of mechanisms for slow multiphoton dissociation.     

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.     

Thermal decomposition of perfluoro-di-t-butyl peroxide

The thermal decomposition of perfluoro-di-t-butyl peroxide has been studied for the first time. The reaction was carried out in the gas phase between 5 and 600 torr in the 108-149°C temperature region. The products consisted solely of C₂F₆ and CF₃COCF₃. The decomposition was found to be first order and homogeneous. The rate constant is given by log k_decomp(sec^?1) = (16.2 ± 1.2) - (148.7 4.4)/2.3RT where R is 0.008314kJ/mol · °K. These Arrhenius parameters are consistent with those determined for the decomposition of di-t-butyl peroxide.     

The reaction of photochemically generated CF3O radicals with CO

CF₃O₂CF₃ was photolyzed at 254 nm in the presence of CO in 760 torr N₂ or air at 296 K in a static reactor. In N₂, the products CF₃OC(O)C(O)OCF₃ and CF₃OC(O)O₂C(O)OCF₃ were detected by FTIR spectroscopy. In air, the only observed products were CF₂O and CO₂ and a chain process, initiated by CF₃O, was invoked for the conversion of CO to CO₂. From both product studies, a mechanism for the CF₃O initiated oxidation of CO was derived, involving the addition reaction CF₃O₂ + CO → CF₃OC(O). The rate constant for the reaction CF₃O + CO at 296 K at a total pressure of 760 torr air was determined to be k(CF₃O + CO) = (5.0 ± 0.9) × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹. © 1997 John Wiley & Sons, Inc.     

Decomposition Mechanism of Fluorinated Compounds in Water Plasmas Generated Under Atmospheric Pressure

Decomposition mechanism of HFC-134a, HFC-32, and CF₄ in water plasmas at atmospheric pressure has been investigated. The decomposition efficiency of 99.9% can be obtained up to 3.17 mol kWh⁻¹ of the ratio of hydrofluorocarbon (HFC) feed rate to the arc power and 1.89 mol kWh⁻¹ of the ratio of perfluorocarbon (PFC) feed rate to the arc power. The species such as H₂, CO, CO₂, CH₄, and CF₄ were detected from the effluent gas of both PFC and HFC decomposition. However, CH₂F₂ and CHF₃ were observed only in the case of HFC decomposition. The HFC and PFC decomposition generate CH₂F, CHF_{x (x:1–2)}, and CF_{y (y:1–3)} radicals, then those radicals were subsequently oxidized by oxygen, leading to CO and CO₂ generation in the excess oxygen condition. However, when there is insufficient oxygen available, those radicals were easily recombined with fluorine to form by-product such as CH₂F₂, CHF₃, and CF₄.     

The kinetics and the mechanism of the thermal gas-phase reaction between bis(trifluoromethyl)trioxide,CF3O3CF3 and 1,1-dichlorodifluoroethylene,CF2CCI2

The thermal addition of CF₃O₃CF₃(T) to CF₂CCl₂(E) has been investigated between 49.6 and 69.5°C. The initial pressure of CF₃O₃CF₃ was varied between 7 and 240 torr and that of CF₂CCl₂ between 4 and 600 torr. Four products of formula CF₃O(E)_j OOCF₃, where j = 1 → 4 are formed. The sum of the products Σ CF₃O(E)_jOOCF₃ is equal to the amount of trioxide decomposed. The reaction is homogeneous. Its rate is not affected by the total pressure and the presence of inert gas. It is a free radical telomerization with four basic steps: thermal decomposition of CF₃O₃CF₃ into CF₃O^. and CF₃O₂^., chain initiation by addition of CF₃O^. to olefin incorporated in, and telomeric radicals termination. The consumption of alkene is well represented by the equation: where (d[E]/d[T]) = is the mean chain length of telomerization. varies from 1.45 at 1.5 torr of E to 3.3 at 400 torr of E. Above this pressure E has no influence on . The estimated value of the constant for the addition of telomeric radicals to alkene is:      

Comparative performance of CF3I,CD3I and CH3I in an atomic iodine photodissociation laser

We have compared the performance of CF₃I, CD₃I, and CH₃I in an atomic iodine photodissociation laser over the pressure range 1–200 torr. At pressures below 5 torr, CD₃I produces larger energy outputs, while above 5 torr CF₃I gives superior performance. The crossing of the laser energy output versus pressure curves is explained on the basis of collisional quenching of I(²P_{$\text{1}\text{2}$})(≡I^*) by undissociated alkyl iodide.     

Photolysis of 1,1,1-trifluoromethylazomethane as a source of methyl and trifluoromethyl radicals

The photochemistry of 1,1,1-trifluoromethylazomethane has been partially characterized. The quantum yield for N₂ formation from photolysis at 366 nm and room temperature was unity at low pressure and decreased to 0.5 at 630 torr. At room temperature the principal products were C₂H₆, C₂F₆, CH₃CF₃ (or CH₂CF₂ + HF at reduced pressures), plus substituted hydrazines, which mainly arise from addition of CF₃ to the parent followed by combination of these radicals with CH₃ or CF_3. These fluorinated methyl hydrazine products detract from the general utility of CF₃-N₂-R compounds as sources for simultaneous study of the chemistry of CF₃ and R radicals. At room temperature the hydrazine products accounted for more than 50% of the total yield; however, these products can be reduced by lowering the temperature and at 195°K their yields are negligible. The quantum yield for intramolecular (direct) formation of CH₃CF₃ + N₂ was shown to be ≤0.002.     

TEA CO2 laser-induced reaction of CH3NO2 with CF2HCl: A mechanistic study

Dissociation of nitromethane has been observed when a mixture of CF₂HCl and CH₃NO₂ is irradiated using pulsed TEA CO₂ laser at 9R (24) line (1081 cm^-1), which is strongly absorbed by CF₂HCl but not by CH₃NO₂. Under low laser fluence conditions, only nitromethane dissociates, whereas at high fluence CF₂HCl also undergoes dissociation, showing that dissociation occurs via the vibrational energy transfer processes from the TEA CO₂ laser-excited CF₂HCl to CH₃NO₂. Time-resolved infrared fluorescence from vibrationally excited CF₂HCl and CH₃NO₂ molecules as well as UV absorption of CF₂ radicals are carried out to elucidate the dynamics of excitation/dissociation and the chemical reactions of the dissociation products.     

High intensity infrared laser irradiation calorimetry: direct-determination of heat input to chlorodifluoromethane and ethyl acetate

The heat absorbed when chlorodifluoromethane (CF₂HCl) and ethyl acetate were irradiated with a pulsed infrared CO₂-TEA laser was measured using a laser calorimeter. The measurements show that CF₂HCl at high pressure (> 10 torr) was completely thermally equilibrated during the laser pulse, while at low pressure (<2 torr) this system deviated considerably from thermal equilibrium. Ethyl acetate, however, was found to be non-thermal over the entire pressure range studied (0.5 to 20 torr). Implications of these observations to infrared induced unimolecular decompositions and molecular energy transfer are discussed.     

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.     

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).     

The dark reactions F2 + CF3I,C2F5I,and n-C3F7I

The reactions between F₂ and the lowest members of the homologous series of perfluoroalkyl iodides (CF₃I, C₂F₅I, and n-C₃F₇I) have been studied. For these compounds, an exponential decrease in the alkyl iodide concentration over time following an induction period is observed for certain experimental conditions. Other conditions lead to chaotic-like kinetic behavior where the rate of alkyl iodide consumption continually changes. Kinetic rate data with CF₃I show that the disappearance rate depends upon both the type of surface and surface preparation. For all three compounds, Arrhenius plots reveal activation energies on the order of 10 kcal/mol, consistent with effective initiation steps of F₂ + RI → RIF + F, where R represents the CF₃, C₂F₅, or n-C₃F₇ radical respectively. The end products of the F₂ + RI reactions are RF, R₂, and IF₅, suggesting that the R radicals play an important kinetic role. Introducing O₂ into the F₂ + RI reaction systems results in successive oxidation of R by O₂, leading to the formation of CF₂O as an additional end product. IF(B → X) emission is observed from the RI-rich F₂ + RI reactions, confirming the existence of IF as an intermediate. © 1996 John Wiley & Sons, Inc.