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.     

The kinetics of the addition of alkyl radicals to carbonyl groups. I. The kinetics of the gas phase decomposition reactions of the trifluoro-t-butoxy radical

Trifluoro-t-butoxy radicals have been generated by reacting fluorine with 2-trifluoromethyl propan-2-ol: Over the temperature range 361-600 K the trifluoro-t-butoxy radical decomposes exclusively by loss of the ? CF₃ group [reaction (?2)] rather than by loss of ? CH₃ group [reaction (?1)]: The limits of detectability of the product CF₃COCH₃, by gas-chromatographic analysis, place a lower limit on the ratio k_?2/k_?1 of ca. 75. The implications of these results in relation to the reverse radical addition reactions to the carbonyl group are discussed along with the thermochemistry of the reactions.     

The methyl radical combination rate constant as determined by kinetic spectroscopy

The rate constant for the reaction has been determined by means of vacuum ultraviolet flash photolysis and time-resolved kinetic spectroscopic observations of the 1504-Å absorption band of CH₃. The measurements made using three different sources of methyl radicals (azomethane, dimethylmercury, and ketene-hydrogen) were in accord and yielded a value for the rate constant of k₁ = (9.53 ± 1.17) × 10^?11 cc molec^?1 sec^?1. A detailed error analysis is presented. The f-value for the 1504-Å band of CH₃ is determined to be (2.5 ± 0.7) × 10^?2.     

An evaluation of the kinetic parameters for the reaction of trifluoromethyl radicals with CH3Cl in the gas phase in the temperature range from 416 to 636 K

The hydrogen and chlorine atom abstraction reactions from CH₃Cl by CF₃ radicals produced by the photolysis of hexafluoroacetone (HFA) and CF₃I were studied relative to the recombination of CF3 radicals (I) The Arrhenius parameters obtained in the temperature range 416 to 578 K are: where Θ = 2,303.RT cal mol^?1 and k₂ is the recombination rate constant for the CF₃ radicals. The factors that influence the transfer processes of chlorine and hydrogen are analyzed in a series of reactions of halomethanes with CF₃ and CH₃ radicals. © 1993 John Wiley & Sons, Inc.     

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:      

Iodine catalyzed pyrolysis of dimethyl sulfide. Heats of formation of CH3SCH2I,the CH3SCH2 radical,and the pibond energy in CH2S

A kinetic study has been made of the gas phase, I₂-catalyzed decomposition of (CH₃)₂S at 630–650 K. Some I₂ is consumed initially, reaching a steady-state concentration. The initial major products are CH₄ and CH₂S together with small amounts of CH₃SCH₂I, CH₃I, HI, and CS₂. The initial reaction corresponds to a pseudo-equilibrium: accompanied by: and which brings (I₂) into steady state and a final complex reaction: From the initial rate of I₂ loss it is possible to obtain Arrhenius parameters for the iodination: We measure k₁, (644 K) = 150 L/mol s and from both the Arrhenius plot and independent estimates A₁ (644 K) = 10^{11.2 ± 0.3} L/mol s. Thus, E₁ = 26.7 ± 1 kcal/mol. From the steady-state I₂ concentration, an assumed mechanism and the known rate parameters for the CH₃I/HI system. It is possible to deduce K_A (644) = 3.8 × 10^?2 with an uncertainty of a factor of 2. Using an estimated ΔS (644) = 4.2 ± 1.0 e.u. we find ΔH_A (644) = 7.0 ± 1.1 kcal. With 〈ΔC_PA〉644 = 1.2 this becomes: ΔH_A(298) = 6.6 ± 1.1 kcal/mol. Then ΔH (CH₃SCH₂I) = 6.3 ± 1 kcal/mol. Making the assumption that E_?1 = 1.0 ± 0.5 kcal/mol we find ΔH (644) = 25.7 ± 0.7 kcal/mol and with 〈ΔC_PI〉 = 1.2; ΔH = 25.3 ± 0.8 kcal/mol. This gives ΔH (CH₃S?H₂) = 35.6 ± 1.0 kcal/mol and DH (CH₃SCH₂? H) = 96.6 ± 1.0 kcal/mol. This then yields E_π(CH₂S) = 52 ± 3 kcal. From the observed rate of pressure increase in the system and the preceding data k₃, is calculated for the step CH₃SCH₂ → CH₃ + CH₂S. From an estimated A-factor E₃ is deduced and from the overall thermochemistry values for k_?3 and E_?3. A detailed mechanism is proposed for the I-atom catalyzed conversion of CH₂S to CS₂ + CH₄.     

Kinetic study of gas-phase reactions with CF2Cl2 and CFCl3. Analysis using the transition state theory (TST) of the chlorine atom transfer reactions by CF3 and CH3 radicals from halomethanes

The following gas-phase reactions: were studied by the competitive method with CF₃I as the source of radicals. The kinetic parameters obtained in the temperature range 533–613 K and 503–613 K respectively for chlorine atom transfer reactions are given by: where θ = 2.303 RT (cal mol^?1). The Arrhenius A values were calculated for seven chlorine atom transfer reactions (CF₂Cl₂, CFCl₃, CCl₄ with CF₃ radicals; CF₃Cl, CF₂Cl₂, CFCl₃ and CCl₄ with CH₃ radicals) by using the thermochemical kinetic version of the Transition State Theory (TST).     

UV absorption spectra,kinetics, and mechanisms of the self reaction of CF3O2 radicals in the gas phase at 295 K

The ultraviolet absorption spectrum and the self reaction kinetics of CF₃O₂ radicals have been studied in the gas phase at 298 K using the pulse radiolysis technique. Long pathlength Fourier transform infrared (FTIR) spectroscopy was used to identify and quantify reaction products. Absorption cross sections were quantified over the wavelength range 215–270 nm. The measured cross section at 230 nm was; Errors represent statistical (2σ) together with our estimate of potential systematic errors. The absorption cross section data were then used to derive the observed self reaction rate constant for reaction (1), defined as ?d[CF₃O₂]/dt = 2k _obs[CF₃O₂]² k_lobs = (3.6 ± 0.9) × 10^?12 cm³ molecule^?1 s^?1. The only carbon containing product observed by FTIR spectroscopy was CF₃OOOCF₃. Consideration of the loss of CF₃O₂ radicals to form the trioxide CF₃OOOCF₃ allows derivation of the true bimolecular rate constant for reaction (1); k₁ = (1.8 ± 0.5) × 10^?12 cm³ molecule^?1 s^?1. These results are discussed with respect to previous studies of the absorption spectra of peroxy radicals, the kinetics, and mechanisms of their self reaction. © John Wiley & Sons, Inc.     

The kinetics and equilibrium of the gas-phase reaction CH3CF2Br + I2 = CH3CF2I + IBr the C-Br bond dissociation energy in 1,1-difluorobromoethane

The kinetics and equilibrium of the gas-phase reaction of CH₃CF₂Br with I₂ were studied spectrophotometrically from 581 to 662°K and determined to be consistent with the following mechanism: A least squares analysis of the kinetic data taken in the initial stages of reaction resulted in log k₁ (M^?1 · sec^?1) = (11.0 ± 0.3) - (27.7 ± 0.8)/θ where θ = 2.303 RT kcal/mol. The error represents one standard deviation. The equilibrium data were subjected to a “third-law” analysis using entropies and heat capacities estimated from group additivity to derive ΔH_r° (623°K) = 10.3 ± 0.2 kcal/mol and ΔH_rr (298°K) = 10.2 ± 0.2 kcal/mol. The enthalpy change at 298°K was combined with relevant bond dissociation energies to yield DH°(CH₃CF₂ - Br) = 68.6 ± 1 kcal/mol which is in excellent agreement with the kinetic data assuming that E₂ = 0 ± 1 kcal/mol, namely; DH°(CH₃CF₂ - Br) = 68.6 ± 1.3 kcal/mol. These data also lead to ΔH_f°(CH₃CF₂Br, g, 298°K) = -119.7 ± 1.5 kcal/mol.     

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.     

The gas-phase kinetics of the reaction of 1,1,1-trifluoroethyl iodine with HI: The C?I bond dissocation energy in 2,2,2-trifluoroethyl iodide

The kinetics of the gas-phase reaction of 2,2,2-trifluoroethyl iodide with hydrogen iodide has been studied over the temperature range of 525°K to 602°K and a tenfold variation in the ratio of CF₃CH₂I/HI. The experimental results are in good agreement with the expected free radical-mechanism: An analysis of the kinetic data yield: where θ =2.303RT in kcal/mol. If these results are combined with the assumption that E₂ = 0 ± 1 kcal/mol, then one obtains DH (CF₃CH₂? I) = 56.3 kcal/mol. This result may be compared with DH(CH₃CH₂? I) = 52.9 kcal/mol and suggests that substitution of three fluorines for hydrogen in the beta position strengthens the C? I bond slightly.     

Kinetic studies of the reactions CH2I2 + HI⇄CH3I + I2 and 2 CH3I⇄CH4 + CH2I2 the heat of formation of the iodomethyl radical

The rate of the reaction CH₂I₂ + HI ? CH₃I + I₂ has been followed spectrophotometrically from 201.0 to 311.2°. The rate constant for the reaction fits the equation, log (k₁/M^?1 sec^?1) = 11.45 ± 0.18 - (15.11 ± 0.44)/θ. This value, combined with the assumption that E₂ = 0 ± 1 kcal/mole, leads to ΔH (CH₂I, g) = 55.0 ± 1.6 kcal/mole and DH (H? CH₂I) = 103.8 ± 1.6 kcal/mole. The kinetics of the disproportionation, 2 CH₃I ? CH₄ + CH₂I₂ were studied at 331° and are compatible with the above values.     

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:      

The gas-phase kinetics of the reaction of 1,1-difluoroiodoethane with hydrogen iodide: The C–I bond dissociation energy in 1,1-difluoroiodoethane

The kinetics of gas-phase reaction of CH₃CF₂I with HI were studied from 496 to 549K and have been shown to be consistent with the following mechanism: A least squares treatment of the data gave where θ = 2.303 RT kcal/mole. The observed activation energy E₁ was combined with E₂ = 0 ± 1 kcal/mole to yield The result, combined with data for several C? I bond dissociation energies, leads us to conclude that the C(sp³)? I bond is relatively insensitive to F for H substitution and that the C(sp²)–I bond has considerable double-bond character.     

Very low-pressure pyrolysis (VLPP) of alkyl cyanides. II. n-propyl cyanide and isobutyl cyanide. The heat of formation and stabilization energy of the cyanomethyl radical

The very low-pressure pyrolysis (VLPP) technique has been used to study the pyrolysis of n-propyl cyanide over the temperature range of 1090–1250°K. Decomposition proceeds via two pathways, C₂? C₃ bond fission and C₃? C₄ bond fission, with the former accounting for >90% of the overall decomposition. Application of unimolecular reaction rate theory shows that the experimental unimolecular rate constants for C₂? C₃ fission are consistent with the high-pressure Arrhenius parameters given by where θ=2.303RT kcal/mole. The activation energy leads to DH₂₉₈⁰[C₂H₅? CH₂CN]=76.9±1.7 kcal/mole and ΔH(?H₂CN, g)=58.5±2.2 kcal/mole. The stabilization energy of the cyanomethyl radical has been found to be 5.1±2.6 kcal/mole, which is the same as the value for the α-cyanoethyl radical. This result suggests that DH[CH₂(CN)? H] ～ 93 kcal/mole, which is considerably higher than previously reported. The value obtained for ΔH_?⁰(?H₂CN) should be usable for prediction of the activation energy for C₂? C₃ fission in primary alkyl cyanides, and this has been confirmed by a study of the VLPP of isobutyl cyanide over the temperature range of 1011–1123°K. The decomposition reactions parallel those for n-propyl cyanide, and the experimental data for C₂? C₃ fission are compatible with the Arrhenius expression A significant finding of this work is that HCN elimination from either compound is practically nonexistent under the experimental conditions. Decomposition of the radical, CH₃CHCH₂CN, generated by C₃? C₄ fission in isobutyl cyanide, yields vinyl cyanide and not the expected product, crotonitrile. This may be explained by a radical isomerization involving either a 1,2-CN shift or a 1,2-H shift.     

The kinetics of the reactions of O(3P) atoms with dimethyl ether and methanol

The kinetics of the reaction of O + CH₃OCH₃ were investigated using fast-flow apparatus equipped with ESR and mass-spectrometric detection. The concentration of O(³P) atoms to CH₃OCH₃ was varied over an unusually large range. The rate constant for reaction was found to be k = (5.0 ± 1.0) × 10¹² exp [(?2850 ± 200/RT)] cm³ mole^?1 sec^?1. The reaction O + CH₃OH was studied using ESR detection. Based on an assumed stoichiometry of two oxygen atoms consumed per molecule of CH₃OH which reacts, we obtain a value of k = (1.70 ± 0.66) × 10¹² exp [(?2,280 ± 200/RT)] cm³ mole^?1 sec^?1 for the reaction The results obtained in this study are compared with the results from other workers on these reactions. The observation of essentially equal activation energies in these two reactions is indicative of approximately equal C? H bond strengths in CH₃OCH₃ and CH₃OH. This is in agreement with recent measurements of these bond energies.     

The rate constants for the reaction of the hydroxyl radical with methyl,n-propyl,and n-butyl nitrites

The kinetics of the reaction of OH radicals with methyl, n-propyl, and n-butyl nitrite have been studied in a discharge flow system under pseudo first-order conditions. The OH radicals were generated by the reaction of H atoms with NO₂ and the concentration of OH; monitored by resonance fluorescence, was followed as a function of time in an excess of each nitrite. Values of k(CH₃ONO) = (0.6 ± 0.09) × 10⁹ dm³ mol^?1 s^?1 k(n – C₃H₇ONO) = (1.39 ± 0.20) × 10⁹ dm³ mol^?1 s^?1, and k(n – C₄H₉ONO) = (2.89 ± 0.43) × 10⁹ dm³ mol^?1 s^?1 at 295 K were obtained. These results agree with previous relative rate measurements from this laboratory but the value for k (CH₃ONO) is a factor of 7 greater than the value obtained by relative rate measurements elsewhere using a different OH source.     

Kinetics studies of reactions of OH radicals with four haloethanes part I. Experiment and BEBO calculation

The rate constants for reactions of OH with ClCH₂CH₂Cl, Cl₂CHCHCl₂, BrCH₂CH₂Br, CH₃CH₂Br have been measured by using a discharge flow-resonance fluorescence (DF-RF) technique over the temperature range 293–418 K. The rate constants as a function of temperature were fitted to the Arrhenius expression as follows: (k's units are 10^?12 cm³ molecule^?1 s^?1, and E_a's units are cal/mol) The experimental data have been compared with theoretical calculations of the semi-empirical BEBO-transition state method. Life-times of the four haloethanes in the troposphere have been estimated.     

Kinetics of the reaction between methane and iodine from 830 to 1150 K in the presence and absence of oxygen

The formation of methyl iodide was determined by radiochemical methods and by massspectroscopic analyses in mixtures of Ar–CH₄–I₂ and Ar? CH₄? I₂? O₂, heated by a reflected shock wave to temperatures of 830–1150 K. The rate of formation of CH₃I was consistent with the chain mechanism where the indicated rate constant for reaction between I and CH₄ is given by k₂(cm³/mol · s) = 10^14.17 exp(?32.9 ± 0.8 kcal/mol/RT). No effect on the reaction rate by the presence of O₂ was detected. However, in one experiment at 1097 K with 3.86 mol % O₂ the formation of CH₂O was indicated by the mass-spectroscopic analysis, presumably from the reaction of O₂ with CH₃.     

The kinetics of the addition of alkyl radicals to carbonyl groups. II. The addition of methyl radicals to trifluoroacetone in the gas phase. The formation reaction of the trifluoro-t-butoxy radical

By photolyzing azomethane over the temperature range 331–491 K in the presence of trifluoroacetone the kinetics of the addition reaction (1), ?H₃ + CF₃COCH₃ → CF₃C(?)(CH₃)₂ have been studied. Detailed analyses have shown that the principal product of the adduct radical, CF₃C(?)(CH₃)₂, is CH₃COCH₃ from reaction (?2), CF₃C(?)(CH₃)₂ → CH₃COCH₃ + ?F₃. The rate constant of the addition reaction has been determined to be k₁(dm³/mol s) = (4.5 ± 1.4) × 10⁷ exp(-(3370 ± 120)/T) over the temperature range 331–491 K, based on the value k₃ = 2.2 × 10¹⁰ dm³/mol s for the reaction (3), 2?H₃ → C₂H₆. The results are discussed in relation to existing data for radical additions to groups.