Induced heterogeneity,a novel technique for the study of gas-phase reactions. 2. Direct study of C?C bond scission in ethane

Deliberate activation of the reaction vessel surface leads to the domination of chain termination in ethane pyrolysis by the reaction As a result, chains are dramatically reduced in length, methane yields are entirely primary and larger in proportion to other products, and values of k₁ can be directly determined from methane yield data without ambiguity. Experiments carried out in the temperature range of 841–913K at initial ethane pressures of 1–20 torr, without and with added nitrogen, yield the infinite pressure Arrhenius equation It is shown that most previously published data can be combined with those of this study to yield Fall-off curves for k₁ as a function of pressure are in good agreement with those from other laboratories. From these the relevant data for k_?1 can be extracted for use in other kinetic studies.     

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.     

Kinetics of some reactions of vinyl and isopropyl radicals in the gas phase

Vinyl and isopropyl radicals were generated by the pyrolysis of azoisopropane in the presence of acrolein at 473–563 K. Reaction products were analyzed by gas chromatography. Rate constant ratios k₂/k₁ = 0.02 ± 0.01 and k₄/k₃ = 0.01 ± 0.005 are suggested for the following reactions: The rate constant ratio of reactions (7) and (c) obeys the Arrhenius equation The Arrhenius equation was derived for (k₈ + k₉).     

The thermal unimolecular decompositions of C2H5Cl,i-C3H7Cl,and t-C4H9Cl

The thermal decompositions of ethyl chloride, iso propyl chloride, and tertiary butyl chloride were studied in a static system in the pressure range of 0.1–300 torr. The following Arrhenius equations for the high-pressure limit were obtained: The pressure dependence of the first-order rate constant (falloff) for these three unimolecular dehydrochlorination reactions, starting with approximately equal k_ω values, by proper choice of temperature, is shifted to lower pressures with increasing molecular size:      

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:      

Reaction of CF3 radicals with H2O and D2O

The reaction of CF₃ radicals with H₂O (D₂O) has been studied over the range of 533–723 K using the photolysis and the pyrolysis of CF₃I as the free radical source. Arrhenius parameters for the reactions where X = H or D, relative to CF₃ radical recombination are given by where k/k is in cm^3/2/mol^1/2·s^1/2 and θ = 2.303RT/cal/mol. The activation energy and the primary kinetic isotope effect have been compared with those derived from the BEBO method.     

Kinetics,mechanism, and endo selectivity of Diels–Alder reactions of alkylmonosubstituted ethenes with cyclohexa-1,3-diene in the gas phase

The reactions where Y = CH₃ (M), C₂H₅ (E), i? C₃H₇ (I), and t? C₄H₉ (T) have been studied between 488 and 606 K. The pressures of CHD ranged from 16 to 124 torr and those of YE from 57 to 625 torr. These reactions are homogeneous and first order with respect to each reagent. The rate constants (in L/mol·s) are given by The Arrhenius parameters are used as a test for a biradical mechanism and to discuss the endo selectivity of the reactions.     

Kinetics of the reaction of OH with pernitric and nitric acids

The absolute rate constants for the reactions of OH + HO₂NO₂ (1) and OH + HNO₃ (2) have been measured with the technique of flash photolysis resonance fluorescence over the temperature ranges of 240–330 K at 760 torr He for reaction (1) and of 240–370 K at 50 and 760 torr He for reaction (2). Reactant concentrations were monitored continuously by ultraviolet and infrared spectrophotometry. The data can be fitted to the following Arrhenius expressions: These results are in very good agreement with recent studies of reaction (2), and also of reaction (1) at 295 K.     

Reaction of pentafluoroethyl radicals with hydrogen cyanide

The reaction of C₂F₅ radicals with HCN has been studied over the range of 533–673 K using the pyrolysis of pentafluoroethyl iodide as the free-radical source. Arrhenius parameters for the reaction relative to C₄F₁₀ recombination are given by where θ = 2.303RT kJ/mol and k_H/k is in cm^3/2/mol^1/2·s^1/2.     

The addition of methyl radicals to hexafluoroacetone

The reaction of methyl radicals (Me) with hexafluoroacetone (HFA), generated from ditertiary butyl peroxide (dtBP), was studied over the temperature range of 402–433 K and the pressure range of 38–111 torr. The reaction resulted in the following displacement process taking place: where TFA refers to trifluoroacetone. The trifluoromethyl radicals that were generated abstract a hydrogen atom from the peroxide: such that k_6a is given by: where θ = 2.303RT kcal/mol. The interaction of methyl and trifluoromethyl radicals results in the following steps: Product analysis shows that k₁₇/kk = 2.0 ± 0.2 such that k₁₇ = 10^10.4±0.5M^?1 · s^?1. The rate constant k₅ is given by: It is concluded that the preexponential factor for the addition of methyl radicals to ketones is lower than that for the addition of methyl radicals to olefins.     

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.     

Rate constants and kinetic isotope effects for hydrogen/deuterium abstraction by chlorine atoms from the chloromethyl group in CH2ClCH2Cl,CD2ClCD2Cl,CH2ClCHCl2, and CH2ClCDCl2

The abstraction of hydrogen and deuterium from 1,2-dichloroethane, 1,1,2-trichloroethane, and two of their deuterated analogs by photochemically generated ground state chlorine atoms has been investigatedin the temperature range 0–95°C using methane as a competitor. Rate constants and their temperature coefficients are reported for the following reactions Over the temperature range of this investigation an Arrhenius law temperature dependence was observed in all cases. Based on the adopted rate coefficient for the chlorination of methane [L.F. Keyser, J. Chem. Phys., 69 , 214 (1978)] which is commensurate with the present temperature range, the following rate constant values (cm³ s^?1) are obtained: The observed pure primary, and mixed primary plus α- and β3-secondary kinetic isotope effects at 298 K are k₃/k₆ = 2.73 ± 0.08, and k₁/k₂ = 4.26 ± 0.12, respectively. Both show a normal temperature dependence decreasing to k₃/k₆ = 2.39 ± 0.06 and k₁/k₂ = 3.56 ± 0.09 at 370 K. Contrary to some simple theoretical expectations, the kinetic isotope effect for H/D abstraction decreases with increasing number of chlorine substituents in the geminal group in a parallel manner to the trend established previously for C1-substitution in the adjacent group. The occurrence of a β-secondary isotope effect, k₄/k₅, is established; this effect suggests a slight inverse temperature dependence.     

Kinetics and mechanism of the thermal gas-phase reaction between NO2 and trichloroethene at 303–362.2 K

The kinetics of the gas-phase reaction between NO₂ and trichloroethene has been investigated in the temperature range 303–362.2 K. The pressure of NO₂ was varied betwen 5.1 and 48.7 torr and that of trichloroethene between 7.3 and 69.5 torr. The reaction was homogeneous. Two products were formed: nitrosyl chloride, ClNO, and glyoxyloxyl chloride, HC[O]C[O]Cl, which was identified by its infrared spectrum and its molecular weight determined by chromatography. The rate of consumption of the reactants was independent of the total pressure and can be represented by a second-order reaction: The following mechanism was proposed to explain the experimental results: The following expression was obtained for k: . © John Wiley & Sons, Inc.     

Effect of olefins on the thermal decomposition of propane part I. The model reaction

Study of the thermal decomposition of propane at very low conversions in the temperature range 760–830 K led to refinement of the mechanism of the reaction. The quotient V/V characterizing the two decomposition routes connected with the 1- and 2-propyl radicals proved to depend linearly on the initial propane concentration. This suggested the occurrence of intermolecular radical isomerization: in competition with decomposition of the 2-propyl radical: The linearity led to the conclusion that the selectivity of H-abstraction from the methyl and methylene groups by the methyl radical is practically the same as that by the H atom. The temperature-dependence of this selectivity ( μ = kCH₃/kCH₂) was given by Further evaluation of the dependence gave the Arrhenius representation for the ratio of the rate coefficients of the above isomerization and decomposition reactions. Steady-state treatment resulted in the rate equation of the process, comparison of which with measurements gave further Arrhenius dependences.     

Determination of the gas-phase reactivity of hydroxyl with chlorinated methanes at high temperature: Effects of laser/thermal photochemistry

Atmospheric pressure absolute rate coefficients have been determined for the gas phase reaction of OH radicals with methyl chloride (k₁), methylene chloride (k₂), and chloroform (k₃) over an extended temperature range using a laser photolysis/laser-induced fluorescence technique. The rate coefficients are best described by the following modified Arrhenius equations: Measurements were obtained as a function of excimer photolysis intensity and are compared with previous results and extended to higher temperatures. Photolysis intensities in excess of 12 mJ-cm^?2 were found to measurably increase (up to a factor of 2) the rate coefficients for k₃ between 400–775 K, with the effect increasing with increasing temperature. A similar, yet much smaller (ca. 20–35%) increase was observed for k₂ between 675–955 K. No effect was observed for k₁ at any temperature. Relative absorption coefficient measurements at 193.3 nm indicated that chlorinated methane photolysis increases with both increasing temperature and increasing chlorine substitution. These measurements suggest that reactant photolysis may be responsible for the observed dependence of k₂ and k₃ on photolysis intensity at elevated temperatures. The puzzling and disconcerting discrepancy between previously published high temperature measurements of k₃ and transition state model predictions is reconciled with these latest measurements. © 1993 John Wiley & Sons, Inc.     

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

Absolute rate constants were determined for the gas phase reactions of OH radicals with a series of aliphatic alcohols 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 methanol (k₁), ethanol (k₂), and 2-propanol (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 = (8.61 ± 0.47), k₂ = (33.3 ± 2.3), and k₃ = (58.1 ± 3.4). Room temperature rate constants for the OH reactions with several other aliphatic alcohols were also measured. These were (in the above units): 1-propanol, (53.4 ± 2.9); 1-butanol, (83.1 ± 6.3) and 1-pentanol, (108 ± 11). The results are discussed in terms of the mechanisms for these reactions and are compared to previous literature data.     

Intramolecular hydrogen isotope effect in the pyrolysis of diethyl carbonate-1,1,1,2,2-d5

The thermal unimolecular decomposition of diethyl carbonate-1,1,1,2,2-d₅ has been examined in the high-pressure-limiting region. The observed chemistry is consistent with a simple, competitive two-channel model: The intramolecular isotope effect k_H/k_D has been determined, and the relative Arrhenius parameters for the two channels are given by over the temperature range of 540–620 K. These Arrhenius parameters predict an isotope effect k_H/k_D = 5.4 at 300 K.     

Decomposition of the t-butoxy radical: II. Studies over the temperature range 303—393 K

The near U-V photolysis of t-butyl nitrite has been studied over the temperature range 303–393 K. Under these conditions t-butyl nitrite was shown to be a very clean photochemical source of t-butoxy radicals. This allows a study of the decomposition of the t-butoxy radical to be made over this temperature range (3). (1) Extrapolation of the rate constants k₃ to high pressure and combination with our previous thermal data give the results:      

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

The temperature dependencies of the rate constants for the gas phase reactions of OH radicals with a series of carboxylic acids were measured in a flash photolysis resonance fluorescence apparatus over the temperature range 240–440 K. The data at total pressures (using Ar diluent gas) between 25–50 torr for acetic acid (k₁), propionic acid (k₂), and i-butyric acid (k₃) were used to derive the Arrhenius expressions and At 298 K, the measured rate constants (in units of 10^?12 cm³ molecule^?1 s^?1) were: k₁ = (0.74 ± 0.06), k₂ = (1.22 ± 0.12), and k₃ = (2.00 ± 0.20). In addition a rate constant of (0.37 ± 0.04), in the above units, was determined for the reaction of OH with formic acid. The error limits cited above are 2σ from the linear least squares analyses. These results are discussed in terms of the mechanisms for these reactions and are compared to literature data.     

Intramolecular isotopic effect in the pyrolysis of ethyl-2d1 chloride

The thermal decomposition of deuterated ethyl chloride CH₂DCH₂Cl was studied in a static system in the pressure range of 0.1–26 torr, and the Arrhenius expression for the overall decomposition at the high-pressure limit and in the temperature range of 670–1100 K was found to be The intramolecular isotopic effects were first examined in the pressure range of 0.1–26 torr at 837 K, and the branching ratio k_H/k_D was found to decrease with increasing pressure. The RRKM-theory calculations describe the experimental data well. The intramolecular isotopic effect was also examined in the temperature range of 728–926 K, and the branching ratio at the high pressure limit was given by the expression when k_H and k_D are the rate constants for the HCl and DCl channels of elimination. The Arrhenius A factors obtained at the high-pressure limit together with the temperature-dependent expression of the branching ratio provided additional experimental data for an assignment (fine-tuned) of the vibrational frequencies of both activated complexes involved in the thermal decomposition of CH₂DCH₂Cl. The evaluated vibrational frequencies were then used in the RRKM calculations describing the pressure dependence of the intramolecular isotopic effect. The RRKM calculations and the experimental data were in good agreement, supporting the choice of vibrational frequencies for both the activated complexes as well as the transition-state model.