Kinetics of the reaction: 2C2H4 → C2H5 + C2H3· heat of formation of the vinyl radical

The initial rates of formation of the major products in the thermal reactions of ethylene at temperatures in the neighborhood of 800 K have been measured in the presence and absence of the additives neopentane and ethane. It has been shown that in the absence of the additive the main initiation process is (1) while in the presence of neopentane and ethane the following additional initiation processes occur: (2) From the ratios of the rates of formation of the major products in the presence and absence of the additive the ratios k_N/k₁ and k_E/k₁ were measured over the temperature range of 750–820 K. Taking values from the literature for k_N and k_E, the following value was obtained for k₁: Previous results using butene-1 as additive were rexamined and shown to be consistent with this measurement. From this measurement the following values were derived: ΔH_f(C₂H₃) = 63.4 ± 2 kcal/mol and D(C₂H₃? H) = 103 kcal/mol.     

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:      

Kinetics of the reactions Br2 + HCN,BrCN + I−, S(CN)2 + I− in aqueous acid solution

Spectrophotometric methods have been used to obtain rate laws and rate parameters for the following reactions: with k_a, k_b, E_a, E_b having the values 85±5 l./mole · s, 5.7±0.2 s^?1 (both at 298.2°K), and 56±4 and 66±2 kJ/mole, respectively. with k_c=0.106±0.004 l./mole ·s at 298.2°K and E_c=67±2 kJ/mole. with k_d=(3.06 ±; 0.15) × 10^?3 l./mole ·s at 298.2°K and E_d=66±2 kJ/mole. Mechanisms for these reactions are discussed and compared with previous work.     

Formation of HCOH + H2 through the reaction CH3 + OH. Experimental evidence for a hitherto undetected product channel

In an extension of our earlier studies at lower temperatures [4,5] the title reaction was measured directly in a flow reactor at temperatures of 600 and 700 K. The pressure of 0.65 mb was chosen that low in order to reduce the contribution of the stabilization channel. OH was used in an excess over CH₃. Both reactants along with the reaction products were monitored by mass spectrometry. CH₃ profiles served as the major observable quantity for the extraction of rate data. This had to be done by using computer simulation since it was impossible to work under pseudo-first-order conditions. The obtained total rate coefficients were divided into channel rate coefficients by means of branching ratios as determined by the mass spectrometric measurement of the reaction products. For CH₃ + OH, this led to a rate coefficient, k_1a into the stabilization channel, and another one, k_{1e + f} referring to the sum of two H₂-eliminating channels yielding the biradical HCOH and to a minor extent H₂CO. These latter channels have not been measured before. In order to distinguish between them we switched over from OH to OD to get so that the biradical and/or aldehyde channels could be determined by their by-products H₂ and HD, respectively. The use of OD makes it also possible to measure the channel through its by-product, HDO. A comparison of the rate coefficients of both systems, i.e., CH₃ + OH and CH₃ + OD, indicates that within our error limits no significant isotope effect takes place. For the rate coefficient into the HCOH channel, we arrive at a preliminary Arrhenius expression in units of cm, molec, and s: . The H₂CO channel could not be detected at our lower temperature rendering us with a rate coefficient at 700 K: . Since simulation is needed for the deduction of the total rate coefficients as well as of the branching ratios, an uncertainty factor of 1.5 has to be attributed to these numbers. © 1995 John Wiley & Sons, Inc.     

Addition of trichloromethyl radicals to tetrachloroethylene

The gamma-radiation-induced free-radical chain reactions in liquid CCl₄? C₂Cl₄? c? C₆H₁₂ mixtures were studied in the temperature range of 363–448°K. The main products in this system are chloroform, hexachloropropene and chlorocyclohexane. These products are formed via reactions (1)–(5): with G values (molec/100 eV) of the order of magnitude of 10² and 10³ at the lowest and highest temperatures, respectively. Values of k₂/k₁ were determined from the product distribution. In turn, these values gave the following Arrhenius expression for k₂/k₁ (θ = 2.303RT, in kcal/mol): From this result and the previously determined Arrhenius parameters of reaction (1), k₂ is found to be given by .     

Determination of the mechanism of the oxidation of the trichloromethyl radical by the photolysis of chloral in the presence of oxygen

The photooxidation of chloral was studied by infrared spectroscopy under steady-state conditions with irradiation of a blackblue fluorescent lamp (300 nm < λ < 400 nm, λ_max = 360 nm) at 296 ± 2 K. The products were hydrogen chloride, carbon monoxide, carbon dioxide, and phosgen. The kinetic results reveal that the reaction proceeds via chain reaction of the Cl atom: The results lead to the conclusion that mechanism (B) is confirmed to be more likely than mechanism (A), which was favored at one time by Heicklen for the mechanism of the oxidation of trichloromethyl radicals by oxygen molecules: The ratio of the initial rates of CO and CO₂ formation gave k₇/k₆ = 4.23M^?1, and the lower limit of reaction (5) was found to be 3.7 × 10⁸M^?1 sec^?1.     

Shock tube study of cyanogen oxidation kinetics

Mixtures of cyanogen and nitrous oxide diluted in argon were shock-heated to measure the rate constants of A broad-band mercury lamp was used to measure CN in absorption at 388 nm [B²Σ⁺(v = 0) ← X²Σ⁺(v = 0)], and the spectral coincidence of a CO infrared absorption line [v(2 ← 1), J(37 ← 38)] with a CO laser line [v(6 → 5), J(15 → 16)] was exploited to monitor CO in absorption. The CO measurement established that reaction (3) produces CO in excited vibrational states. A computer fit of the experiments near 2000 K led to An additional measurement of NO via infrared absorption led to an estimate of the ratio k₅/k₆: with k₅/k₆ ? 10^3.36±0.27 at 2150 K. Mixtures of cyanogen and oxygen diluted in argon were shock heated to measure the rate constant of and the ratio k₅/k₆ by monitoring CN in absorption. We found near 2400 K: and The combined measurements of k₅/k₆ lead to k₅/k₆ ? 10^?3.07 exp(+31,800/T) (±60%) for 2150 ≤ T ≤ 2400 K.     

A shock tube study of the reaction S + H2 ⇌ SH + H in pyrolysis and photolysis systems

Atomic resonance absorption spectroscopy (ARAS) was applied to measure S atoms, behind shock waves in COS/H₂ pyrolysis or CS₂/H₂ photolysis systems. Both the pyrolysis of COS and the photolysis CS₂ was used to generate the S atoms, which subsequently reacts with H₂ via the reaction: The photolysis experiments were designed to provide clear first-order conditions for reaction (R3); i.e., the H₂ concentration exceeds that of S by at least a factor of 100. The S atom profiles obtained during pyrolysis of highly diluted COS/H₂/Ar mixtures were analyzed by computer simulations based on a simplified reaction mechanism using the rate coefficient k₃ as a fitting parameter. Both groups of experiments covered the temperature range of 1257 K ? T ? 3137 K and lead to a rate coefficient of: . © 1995 John Wiley & Sons, Inc.     

Reaction of methoxy radicals with oxygen. I. Using dimethyl peroxide as a thermal source of methoxy radicals

Using dimethyl peroxide as a thermal source of methoxy radicals overthe temperature range of 110–160°C, and the combination of methoxy radicals and nitrogen dioxide as a reference reaction: a value was determined of the rate constant for the reaction of methoxy radicals with oxygen: is independent of nitrogen dioxide or oxygen concentration and added inert gas (carbon tetrafluoride). No heterogeneous effects were detected. The value of k₄ is given by the expression In terms of atmospheric chemistry, this corresponds to a value of 10^5.6 M^?1·sec^?1 at 298 K. Extrapolation to temperatures where the combustion of organic compounds has been studied (813 K) produces a value of 10^7.7 M^?1·sec^?1 for k₄. Under these conditions, reaction (4) competes with hydrogen abstraction or disproportionation reactions of the methoxy radical and its decomposition (3): In particular k₃ is in the falloff region under these conditions. It is concluded that reaction (4) takes place as the result of a bimolecular collision process rather than via the formation of a cyclic complex.     

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:      

Influence of butadiene-1,3 on ethanal pyrolysis at 495°C

At 495°C and a low extent of reaction, ethanal pyrolysis is slightly inhibited by the addition of small quantities of butadiene-1,3, whereas it is accelerated by more important quantities. The inhibiting effect is interpreted in terms of a free-radical chain mechanism in which the main chain carriers of ethanal pyrolysis (CH₃.free radicals) reversibly add to butadiene-1,3 and yield penten-2-yl (R.) free radicals. These free radicals either react in a metathetical step: or in terminating steps. But butadiene-1,3 also gives rise to new initiation steps: which account for the accelerating effect. Process (i₃) seems to be more important than process (i₂) in the experimental conditions, but its nature could not be identified. The results are consistent with literature data and the following value of k₆: (4.57T in cal/mol).     

The kinetics and the mechanism of the thermal decomposition of bis-pentafluorosulfurtrioxide (SF5OOOSF5)

The thermal decomposition of SF₅O₃SF₅ has been investigated between 5 and 25°C. In the presence of sufficient high pressures of O₂ the only products formed are SF₅O₂SF₅ and O₂: The reaction is homogeneous. Its rate is strictly first order with respect to the trioxide pressure and independent of the total pressure of the reaction products and of oxygen above a certain limiting pressure: The experimental results can be explained with the following mechanism: In the presence of O₂ > 100 Torr the concentration of SF₅ is insignificantly small. Therefore reactions (5) and (6) do not have to be considered any more, and steps (2) and (2′) will be of no importance. From reactions (1)–(4) it follows: The numerical value of the factor [1 + (k′₁²/2k₃k₄)^1/2] is small. It can be estimated that E₃ ? 2 ± 1 kcal; therefore, E – E₁ ≤ 1 kcal, and D = (26 – 1) ± 1.0 kcal.     

Kinetics of the gas-phase reaction CH3F + I2 ⇆ CH2FI + HI: The C?H bond dissociation energy in methyl and methylene fluorides

The kinetics of the gas-phase reaction of CH₃F with I₂ have been studied spectrophotometrically from 629 to 710 K, and were determined to be consistent with the following mechanism: (1) A least-squares analysis of the kinetic data taken in the initial stages of reaction resulted in where θ = 4.575T/1000 kcal/mol. The errors represent one standard deviation. The experimental activation energy E₄ = 30.8 ± 0.2 kcal/mol was combined with the assumption E₃ = 1 ± 1 kcal/mol and estimated heat capacities to obtain The enthalpy change at 298 K was combined with selected thermochemical data to derive The kinetic studies of ?HF₂ and CH₂F₂ have been reevaluated to yield These results are combined with literature data to yield the C? H, C? F, and C? Cl bond dissociation energies in their respective fluoromethanes, and the effect of α-fluorine substitution is discussed.     

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.     

Reactions of alkoxy radicals with O2. I. C2H5O radicals

C₂H₅ONO was photolyzed with 366 nm radiation at ?48, ?22, ?2.5, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yield of CH₃CHO, Φ{CH₃CHO}, was measured as a function of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?_1a = 0.29 ± 0.03 independent of temperature. The C₂H₅O radicals can react with NO by two routes The C₂H₅O radical can also react with O₂ via Values of k₆/k₂ were determined at each temperature. They fit the Arrhenius expression: Log(k₆/k₂) = ?2.17 ± 0.14 ? (924 ± 94)/2.303 T. For k₂ ? 4.4 × 10^?11 cm³/s, k₆ becomes (3.0 ± 1.0) × 10^?13 exp{?(924 ± 94)/T} cm³/s. The reaction scheme also provides k_8a/k₈ = 0.43 ± 0.13, where      

Direct determination of the rate constant of the reaction NO + HO2 → NO2 + OH with the LMR

The rate of the reaction was determined in an isothermal discharge flow reactor with a combined ESR–LMR detection under pseudo-first-order conditions in HO₂. The rate constant was identical in experiments with two different HO₂ sources: F + H₂O₂ and H + O₂ + M. The absolute rate constant at T = 293 K was measured as In the range 2 ≤ p mbar ≤ 17 no pressure dependence for k₁ was found.     

Induced heterogeneity,a novel technique for the study of gas-phase reactions. Part I. Determination of the arrhenius parameters for C?C bond scission in propane

It is shown that, by deliberate activation of the reaction vessel, heterogeneous reaction at the wall can be made to dominate chain termination in a complex gas-phase reaction. For a homogeneous process, characterized, as is often the case, by multiple terminations, this has the effect of simplifying the mechanism and allowing explicit solution of the relevant steady-state equations so that the rate constants of some individual steps can be evaluated without assumption as to the values of those of others. The pyrolysis of propane, in the vicinity of 500°C, has been used as an example of this approach. Enhancement of the wall activity leads to the reaction (1) providing, almost exclusively, chain termination. As a result, rate constants for the initiation step (2) can be directly determined. The results of this study provide the Arrhenius equation In combination with current thermochemical values this result gives k_?1 = 10^13.40 cm³/mol·s which, in turn, implies, via the geometric mean rule, k_Et-Et = 10^12.9 cm³/mol·s for ethyl–ethyl recombination, in good accord with the most recent determinations and compatible with the newly proposed value of the enthalpy of formation of ethyl. The first-order wall constant k₈ has been evaluated as k₈<10^4.2 s^?1. This appears to be the first occasion on which a wall constant has been evaluated from data for a high-temperature complex gas reaction.     

Some Reactions of OH(v = 1)

Reactions of OH(v = 1) with HBr, O, and CO have been studied at 295°K using a fast discharge flow apparatus: The reaction O + HBr → OH(v = 1) + Br was used as a source of OH(v = 1), and subsequent chemical reactions of the excited radical were followed using EPR spectroscopy. Rate constants for reactions (2b), (3b), and (6b) were measured as (4.5 ± 1.3) × 10^?11, (10.5 ± 5.3) × 10^?11, and <5 × 10^?12 cm³/molec·sec, respectively. The rate constant for physical deactivation of OH(v = 1) by CO was determined as <4 × 10^?13 cm³/molec·sec.     

The reactions of alkoxy radicals with O2. II. n-C3H7O radicals

n-C₃H₇ONO was photolyzed with 366 nm radiation at ?26, ?3, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yields of C₂H₅CHO, C₂H₅ONO, and CH₃CHO were measured as a function of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?₁ = 0.38 ± 0.04 independent of temperature. The n-C₃H₇O radicals can react with NO by two routes The n-C₃H₇O radical can decompose via or react with O₂ via Values of k₄/k₂ ? k_4b/k₂ were determined to be (2.0 ± 0.2) × 10¹⁴, (3.1 ± 0.6) × 10¹⁴, and (1.4 ± 0.1) × 10¹⁵ molec/cm³ at 55, 88, and 120°C, respectively, at 150-torr total pressure of N₂. Values of k₆/k₂ were determined from ?26 to 88°C. They fit the Arrhenius expression: For k₂ ? 4.4 × 10^?11 cm³/s, k₆ becomes (2.9 ± 1.7) × 10^?13 exp{?(879 ± 117)/T} cm³/s. The reaction scheme also provides k_4b/k₆ = 1.58 × 10¹⁸ molec/cm³ at 120°C and k_8a/k₈ = 0.56 ± 0.24 independent of temperature, where