Oxidation of S and SO by O2 in high-temperature pyrolysis and photolysis reaction systems

The reaction of S atoms with O₂ was studied behind reflected shock waves applying atomic resonance absorption spectroscopy (ARAS) for concentration measurements of S and O atoms. S atoms were generated either by laser-flash photolysis (LFP) of CS₂ or by the high-temperature pyrolysis of COS, respectively. The concentrations of O₂ in the mixtures ranged between 50 ppm and 400 ppm, and those of the S precursors, CS₂ and COS, between 5 and 25 ppm. The rate coefficient of the reaction was determined from the observed decay of the S absorption signals for temperatures 1220 K ? T ? 3460 K. The measured O-atom concentration profiles in COS/O₂/Ar reaction systems were evaluated, using simplified kinetic mechanism, to verify the given rate coefficient k₅. In experiments with the highest value of the [O₂]/[COS] ratio the measured O-atom concentrations were found to be sensitive to the reaction: The fitting of the calculated O-atom profiles to the measured ones results in mean value of: which is to be valid for the temperature range 2570 K ? T ? 2980 K. A first-order analysis of the observed S absorption decay in LFP shock wave experiments on CS₂/Ar gas mixtures resulted in a rate coefficient of the background reaction (R4): for temperatures 1260 K ? T ? 1820 K. © 1995 John Wiley & Sons, Inc.     

The rate constant for t-C4H9 → H + i-C4H8 and the thermodynamic parameters of t-C4H9

The rate of the reverse reaction of the system has been measured in the range of 584–604 K from a study of the azomethane sensitized pyrolysis of isobutane. Assuming the published value for the rate constant of recombination of t-butyl we obtain Combination with our published data for k₁ permits the evaluation We have modified a previously published structural model of t-butyl by the inclusion of a barrier to free rotation of the methyl groups in order to calculate values of the entropy and enthalpy of t-butyl as a function of temperature. Using standard data for H and for i-C₄H₈ we obtain We have obtained other, independent values of this quantity by a reworking of published data using our new calculations of the entropy and enthalpy of t-butyl. There is substantial agreement between the different values with one exception, namely, that derived from published data on the equilibrium which is significantly lower than the other values. We conclude that the value obtained from the present work and a reworking of published data which involves the use of experimental data on t-butyl recombination is incompatible with the result based on iodination data.     

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.     

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 and mechanism of the reaction H + CH3ONO

The reaction of hydrogen atoms with methyl nitrite was studied in a fast-flow system using photoionization mass spectrometry and excess atomic hydrogen. The associated bimolecular rate coefficient can be expressed by in the temperature range of 223-398°K. NO, CH₃OH, CH₄, C₂H₆, CH₂O, and H₂O are the main products; OH and CH₃ radicals were detectable intermediates. The mechanism was deduced from the observed product yields using normal and deuterated reactants. The primary reaction steps were identified as followed by a rapid unimolecular decomposition of CH₂ONO into CH₂O and NO. Since the extent of reaction channel (1b) could not be determined independently, only extreme limits could be obtained for the individual contributions of the two channels of reaction (3) which follows the generation of CH₃O radicals: The most probable values, k_3a/k₃ = 0.31 ± 0.30 and k_3b/k₃ = 0.69 ± 0.30, support the previous results on this reaction, although the range of uncertainties is much greater here.     

Absolute rate constants for the reactions of O(3P) atoms and OH radicals with SiH4 over the temperature range of 297–438°K

Absolute rate constants for the reaction of SiH₄ with O(³P) atoms and OH radicals have been determined over the temperature range 297°–438°K using flash photolysis–NO₂ chemiluminescence and flash photolysis–resonance fluorescence techniques, respectively. The Arrhenius expressions obtained are where the error limits in the Arrhenius activation energies are the estimated overall error limits. Rate data for the reactions of SiH₄, CH₄, and H₂S with O(³P), H, and F atoms and with OH, CH₃, and CF₃ radicals are compared, showing that H₂S and SiH₄, which have similar bond energies, have reasonably similar reactivities toward these atoms and radicals.     

Chlorine-photosensitized reactions in the Cl2 + O2 + 1,1,1,2-C2H2Cl4 system

The gas-phase photochlorination (λ = 436 nm) of the 1,1,1,2-C₂H₂Cl₄ has been studied in the absence and the presence of oxygen at temperatures between 360 and 420°K. Activation energies have been estimated for the following reaction steps: The dissociation energy D(CCl₃CHCl? O₂) ± (24.8 ± 1.5) kcal/mole has also been estimated from the difference in activation energy of the direct and reverse reactions The mechanism is discussed and the rate parameters are compared to those obtained for a series of other chlorinated ethanes.     

A shock tube study of H + HNCO → NH2 + CO

The reaction of atomic hydrogen with isocyanic acid (HNCO) to produce the amidogen radical (NH₂) and carbon monoxide, has been studied in shock-heated mixtures of HNCO dilute in argon. Time-histories of the ground-state NH₂ radical were measured behind reflected shock waves using cw, narrowlinewidth laser absorption at 597 nm, and HNCO time-histories were measured using infrared emission from the fundamental v₂-band of HNCO near 5 μm. The second-order rate coefficient of reaction (2(a)) was determined to be: cm³ mol^?1 s^?1, where f and F define the lower and upper uncertainty limits, respectively. An upper limit on the rate coefficient of was determined to be:      

H2S-promoted thermal isomerization of butene-2 CIS to butene-1 or butene-2 trans around 500°C

H₂S increases the thermal isomerization of butene-2 cis (B_c) to butene-1 (B₁) and butene-2 trans (B_t) around 500°C. This effect is interpreted on the basis of a free radical mechanism in which buten-2-yl and thiyl free radicals are the main chain carriers. B₁ formation is essentially explainedby the metathetical steps: whereas the free radical part of B_t formation results from the addition–elimination processes: . It is shown that the initiation step of pure B_c thermal reaction is essentially unimolecular: and that a new initiation step occurs in the presence of H₂S: . The rate constant ratio has been evaluated: and the best values of k₁ and k_1', consistent with this work and with thermochemical data, are . From thermochemical data of the literature and an “intrinsic value” of E_?3 ? 2 kcal/mol given by Benson, further values of rate constants may be proposed: is shown to be E₄ ? 3.5 ± 2 kcal/mol, of the same order as the activation energy of the corresponding metathetical step.     

Time-resolved vibrational chemiluminescence: Rate constants for the reactions of F atoms with H2O and HCN,and for the relaxation of HF (v = 1) by H2O and HCN

Rate constants have been determined at (298 ± 4) K for the reactions: and the relaxation processes: Time-resolved HF(1,0) emission was observed following the photolysis of F₂ with pulses from an excimer laser operating on XeCl (λ = 308 nm). Analysis of the emission traces gave first-order constants for reaction and relaxation, and their dependence on [H₂O] and [HCN] yielded:      

The determination of the arrhenius parameters for the reaction Cl + C2F5I → ICl + C2F5 using a competitive method

The reactions have been studied competitively over the range of 28–182°C by photolysis of mixtures of Cl₂ + C₂F₅I+ CH₄. We obtain where θ = 2.303RT J/mol. The use of published data on reaction (2) leads to log (k₁cm³/mol sec) = (13.96 ± 0.2) ? (11,500 ± 2000)/θ.     

Temperature dependence of the rate constant for the reaction OH + H2S

Absolute rate constants for the reaction of OH with H₂S have been measured over the temperature range of 239–425 K using the flash photolysis–resonance fluorescence technique. The results showed that the rate constants deviate slightly from Arrhenius behavior but can still be represented adequately by the following Arrhenius equation: Comparisons with recent literature values are presented.     

Rate data for O + OCS → SO + CO and SO + O3 → SO2 + O2 by a new time-resolved technique

Pulsed laser photolysis of O₃ in a large excess of N₂ has been used to generate O(³P) atoms in the presence of OCS. By observing chemiluminescence from the small fraction of electronically excited SO₂ formed in the reaction of SO with O₃, rate constants of (1.7 ± 0.2) × 10^?14 and (8.7 ± 1.6) × 10^?14 cm³/molecule sec have been determined at 296 ± 4 K for the reactions and In addition, it has been shown that any reaction between SO and OCS has a rate constant 10^?14 cm³/molecule sec.     

High-temperature determination of the rate coefficient for the reaction H2 + CN → H + HCN

The rate coefficient of the reaction has been determined in the temperature range of 2700–3500 K using a shock tube technique. C₂N₂? H₂? Ar mixtures were heated behind incident shock waves and the early-time CN history was monitored using broad-band absorption spectroscopy. The rate coefficient providing the best fit to the data was in good agreement with extrapolations of previously published low-temperature results.     

The pyrolysis of dimethyl sulfide,kinetics and mechanism

The kinetics of the gas phase pyrolysis of dimethyl sulfide (DMS) was studied in a static system at 681–723 K by monitoring total pressure-time behavior. Analysis showed the pressure increase to follow DMS loss. The reaction follows two concurrent paths: with a slow, minor, secondary reaction: In a seasoned reactor the reaction follows a 3/2 order rate law with rate coefficient given by with θ = 2.303 RT in kcal/mol. A free radical mechanism is proposed to account for the data and a theoretical rate coefficient is derived from independent data: which agrees well with the experimental one over the range studied. The reaction is initiated by Me₂S → Me + MeS? and propagated by metathetical radical attack on Me₂S. C₂H₄ is formed by an isomerization reaction which may in part be due to a hot radical: Thermochemical data are listed, many from estimations, for both molecular and radical species of interest in the present system.     

Flash photolysis of biacetyl in gas phase. Rate constants of reactions between methyl and acetyl radicals

The flash photolysis of biacetyl produces CO, C₂H₆, and CH₃COCH₃ as main products, and in small amounts CO₂, C₂H₄, and CH₃CHO. The rate constants of reactions (2) and (3) of thermally equilibrated radicals were calculated from the amounts of products: .     

Über Chalkogenolate. 121. Untersuchungen über N-Cyanomonothiocarbimidsäure 1. Darstellung und Eigenschaften von Alkalimetall-N-cyanomonothiocarbimaten

On Chalcogenolates. 121. Studies on N-Cyanomonothiocarbimic Acid. 1. Synthesis and Properties of Alkali Metal N-Cyanomonothiocarbimates The hitherto unknown N-cyanomonothiocarbimates M₂[SOC?N? CN] · H₂O, where M = Na, K, Rb, Cs, have been prepared by reaction of the corresponding alkali metal salt of cyanamide with COS. N-Cyanomonothiocarbimates react with sulfur to form the ion, which gives with an acid and with CH₃I the methyl compound . The reaction of the latter compound with H₂O₂ yields . All compounds have been characterized by means of diverse methods.     

The gas phase reaction of CF3 radicals with C6F5H in the temperature range 373–658 K

CF₃ radicals were generated by the photolysis of perfluoroacetic anhydride. In the presence of pentafluorobenzene, the CF₃ radicals react according to the following mechanism: It was found that the addition reaction (3) becomes reversible above ca. 453 K. The addition rate parameters have been revised and they satisfactorily agree with those reported previously. At temperatures higher than 593 K, only true H-abstraction occurs. The rate constant k_H for reaction (5) is given by: where θ = 2.303 RT kJmol^?1 and k_c is the rate constant for combination of CF₃ radicals. The reactions of CF₃ with benzene and pentafluorobenzene are compared.     

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.     

Photolysis of alkyl iodides at 147.0 nm. The reaction H + CnH2n + 1I → HI + CnH2n + 1

The 147 nm (8.4 eV) photolysis of gaseous C₂H₅I, n-C₃H₇I, and sec-C₃H₇I was investigated in the presence of and absence of HI. The main overall processes are: These dissociative processes occur mainly as a result of initial cleavage of the weak C? I bond, followed by decomposition of the internally excited alkyl radicals. In all cases, approximately 5-10% of the alkyl radicals thus formed do not undergo dissociation at pressures around 3-7 torr. There is also evidence for the elimination of HI as well as C? C cleavage in the primary dissociation. The former is indicated by deuterium labeling experiments and the formation of cyclopropane (Φ = 0.04) as a product in the photolysis of n-C₃H₇I. Because the processes listed above provide a constant source of H atoms whose quantum yield can be exactly determined, it was feasible to obtain accurate values for k_a/k_b: For thermally equilibrated H atoms (300 K), k_a/k_b is 0.44 ± 0.04, 0.57 ± 0.06, 0.95 ± 0.1, and 0.024 ± 0.01 for C₂H₅I, n-C₃H₇I, sec-C₃H₇I, and C₂H₅Br, respectively.