Kinetic study of the gas-phase reaction c-C5H10 + I2 ⇄ c-C5H8 + 2HI the heat of formation of cyclopentyl radical

The kinetics of the gas-phase dehydrogenation of cyclopentane to cyclopentene is found to be consistent with a slow attack by an I atom (step 4, text) on cyclopentane in the range 282–382°C. The measured rate constants fit the Arrhenius equation, log k₄ = 11.95 ± 0.08 – (24.9 ± 0.23)/θ 1 mole^?1 sec^?1, where θ = 2.303 R T in kcal/mole. This leads to a value of ΔH = 24.3 ± 1 kcal/mole and a bond dissociation energy DH = 94.9 ± 1 kcal/mole. The latter value is identical with DH⁰(i-Pr-H) = 95 ± 1 kcal/mole and signifies that cyclopentane and the cyclopentyl radical have the same strain energy. Arrhenius parameters are deduced for all six steps in the reaction mechanism. Surface reactions are shown to be unimportant. Cyclopentyl iodide is an unstable intermediate in the reaction and the rate constant for its bimolecular formation from HI + cyclopentene is found to be log k₆ = 8.40 ± 0.29 - (26.9 ± 0.8)/θ 1 mole^?1 sec^?1. Together with the equilibrium constant, this yields for the unimolecular elimination of HI from cyclopentyl iodide, the rate constant, log k₅ = 13.3 ± 0.3 – (42.8 ± 1.2)/θ sec^?1.     

Laser-induced kinetics: Arrhenius parameters for t-C4H9 + XI → i-C4H9X + I (X = H,D) and the Heat of Formation of the t-butyl radical

The metathesis reaction of DI with t-C₄H₉ generated by 351-nm photolysis of 2,2′-azoisopropane was studied in a low-pressure reactor (VLP? Knudsen cell) in the temperature range of 302–411 K. The data obeyed the following Arrhenius relation when combined with recent data by Rossi and Golden gathered by the same technique (t-C₄H₉ by thermal decomposition of 2,2′-azoisobutane): log k₂^D(M^?1s^?1) = 9.60 – 1.90/θ, where θ = 2.303RT kcal/mol for 302 K < T > 722 K. The metathesis reaction of HI with t-C₄H₉ was studied at 301 K and resulted in k₂^H(M^?1·s^?1) = (3.20 ± 0.62) × 10⁸. An analogous Arrhenius relation was calculated for the protiated system if the small primary isotope effect k₂^H/k₂^D was assumed to be √2 at 700 K. It was of the following form: log k₂^H(M^?1·s^?1) = 9.73 – 1.68/θ. Preliminary data of Bracey and Walsh indicate that earlier Arrhenius parameters determined for the reverse reaction are somewhat in error. Their value of log k₁(M^?1·s^?1) = 11.5 – 23.8/θ yields 7delta;H_f,300⁰(t-butyl) = 9.2 kcal/mol and S₃₀₀⁰(t-butyl) = 74.2 cal/mol7°K when taken in conjuction with this study.     

The decomposition of dimethyl peroxide and the rate constant for CH3O + O2 → CH2O + HO2

The decomposition of dimethyl peroxide (DMP) was studied in the presence and absence of added NO₂ to determine rate constants k₁ and k₂ in the temperature range of 391–432°K: The results reconcile the studies by Takezaki and Takeuchi, Hanst and Calvert, and Batt and McCulloch, giving log k₁(sec^?1) = (15.7 ± 0.5) - (37.1 ± 0.9)/2.3 RT and k₂ ≈ 5 × 10⁴M^?1· sec^?1. The disproportionation/recombination ratio k_7b/k_7a = 0.30 ± 0.05 was also determined: When O₂ was added to DMP mixtures containing NO₂, relative rate constants k₁₂/k_7a were obtained over the temperature range of 396–442°K: A review of literature data produced k_7a = 10^9.8±0.5M^?1·sec^?1, giving log k₁₂(M^?1·sec^?1) = (8.5 ± 1.5) - (4.0 ± 2.8)/2.3 RT, where most of the uncertainty is due to the limited temperature range of the experiments.     

Thermal decomposition of di-t-butyl peroxide in the presence of (CH3)2CCH2: Reactions of CH3, (CH3)2ĊCH2CH3, and (CH3)2ĊCH2C(CH3)2CH2CH3 radicals

A study of the reaction initiated by the thermal decomposition of di-t-butyl peroxide (DTBP) in the presence of (CH₃)₂C?CH₂ (B) at 391–444 K has yielded kinetic data on a number of reactions involving CH₃ (M·), (CH₃)₂CCH₂CH₃ (MB·) and (CH₃)₂?CH₂C(CH₃)₂CH₂CH₃ (MBB·) radicals. The cross-combination ratio for M· and MB· radicals, rate constants for the addition to B of M· and MB· radicals relative to those for their recombination reactions, and rate constants for the decomposition of DTBP, have been determined. The values are, respectively, where θ = RT ln 10 and the units are dm^3/2 mol^?1/2 s^?1/2 for k₂/k and k₉/k, s^?1 for k₀, and kJ mol^?1 for E. Various disproportionation-combination ratios involving M·, MB·, and MBB· radicals have been evaluated. The values obtained are: Δ₁(M·, MB·) = 0.79 ± 0.35, Δ₁(MB·, MB·) = 3.0 ± 1.0, Δ₁(MBB·, MB·) = 0.7 ± 0.4, Δ₁(M·, MBB·) = 4.1 ± 1.0, Δ₁(MB·, MBB·) = 6.2 ± 1.4, and Δ₁(MBB·, MBB·) = 3.9 ± 2.3, where Δ₁ refers to H-abstraction from the CH₃ group adjacent to the center of the second radical, yielding a 1-olefin. © 1994 John Wiley & Sons, Inc.     

A flash photolysis study of the rate of the reaction OH + CH4 → CH3 + H2O over an extended temperature range

A flash photolysis system has been used to study the rate of reaction (1), OH + CH₄ → CH₃ + H₂O, using time-resolved resonance absorption to monitor OH. The temperature was varied between 300 and 900°K. It is found that the Arrhenius plot of k₁ is strongly curved and k₁ (T) can best be represented by the expression The apparent Arrhenius activation energy changes from 15±1 kJ/mole at 300°K to 32±2 kJ/mole at 1000°K. On either side of our temperature range, both absolute rates and their temperature dependence are in good agreement with the results from most previous investigations.     

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.     

Kinetic study of the gas-phase reaction c-C5H8 + I2 ⇄ c-C5H6 + 2HI the heat of formation and the stabilization energy of the cyclopentenyl radical

The gas-phase dehydrogenation of cyclopentene to cyclopentadiene catalyzed by iodine in the range 178–283°C has been found to obey a rate law consistent with the slow rate-determining step, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm I} + {\rm c} - {\rm C}_5 {\rm H}_8 \stackrel{4}{\rightarrow}{\rm HI} + {\rm c} - {\rm C}_5 {\rm H}_7 $\end{document}, log [k₄/(1 mole^?1 sec^?1)] = 10.25 ± 0.08 - (12.26 ± 0.18)/θ, where θ = 2.303 R T in kcal/mole. Surface effects are not important. This value of E₄ leads to a value of DH = 82.3 ± 1 kcal/mole and ΔH_f298 = 38.4 ± 1 kcal/mole. From difference in bond strengths in the alkane and the alkene, the allylic resonance stabilization in the cyclopentenyl radical is 12.6 ± 1.0 kcal/mole, in excellent agreement with the value for the butenyl radical. Arrhenius parameters for the other steps in the mechanism are evaluated. The low value of A₄ (compared with A₄ for cyclopentane) suggests a “tighter” transition state for H-atom abstraction from alkenes than from alkanes.     

The elimination kinetics and mechanisms of ethyl piperidine‐3‐carboxylate,ethyl 1‐methylpiperidine‐3‐carboxylate,and ethyl 3‐(piperidin‐1‐yl)propionate in the gas phase

The gas‐phase elimination kinetics of the above‐mentioned compounds were determined in a static reaction system over the temperature range of 369–450.3°C and pressure range of 29–103.5 Torr. The reactions are homogeneous, unimolecular, and obey a first‐order rate law. The rate coefficients are given by the following Arrhenius expressions: ethyl 3‐(piperidin‐1‐yl) propionate, log k₁(s^?1) = (12.79 ± 0.16) ? (199.7 ± 2.0) kJ mol^?1 (2.303 RT)^?1; ethyl 1‐methylpiperidine‐3‐carboxylate, log k₁(s^?1) = (13.07 ± 0.12)–(212.8 ± 1.6) kJ mol^?1 (2.303 RT)^?1; ethyl piperidine‐3‐carboxylate, log k₁(s^?1) = (13.12 ± 0.13) ? (210.4 ± 1.7) kJ mol^?1 (2.303 RT)^?1; and 3‐piperidine carboxylic acid, log k₁(s^?1) = (14.24 ± 0.17) ? (234.4 ± 2.2) kJ mol^?1 (2.303 RT)^?1. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene through a concerted six‐membered cyclic transition state type of mechanism. The intermediate β‐amino acids decarboxylate as the α‐amino acids but in terms of a semipolar six‐membered cyclic transition state mechanism. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 106–114, 2006     

The kinetics of the reaction F + H2 → HF + H. A critical review of literature data

Published experimental studies concerning the determination of rate constants for the reaction F + H₂ → HF + H are reviewed critically and conclusions are presented as to the most accurate results available. Based on these results, the recommended Arrhenius expression for the temperature range 190–376 K is k = (1.1 ± 0.1) × 10⁻¹⁰ exp |-(450 ± 50)/T| cm³ molecule⁻¹ s⁻¹, and the recommended value for the rate constant at 298 K is k = (2.43 ± 0.15) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. The recommended Arrhenius expression for the reaction F + D₂ → DF + D, for the same temperature range, based on the recommended expression for k and accurate results for the kinetic isotope effect k/k is k = (1.06 ± 0.12) × 10^×10 exp |-(635 ± 55)/T|cm³ molecule⁻¹ s⁻¹, and the recommended value for 298 K is k = (1.25 ± 0.10) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 67–71, 1997.     

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.     

Rates and equilibria in the reaction system Br + i-C4H10 ⇌ HBr + t-C4H9. The heat of formation of the t-butyl radical

The very low pressure reactor (VLPR) technique has been used to measure the bimolecular rate constant of the title reaction at 300 K. The rate constant is given by log k₁ (1/mol s) = (11.6 ± 0.4) ? (5.9 ± 0.6)/θ the equilibrium constant has also been measured at the same temperature and is given by K₁ = (5.6 ± 1) × 10^?3 and hence log k_?1 (1/mol s) = 9.5 ± 0.1. The results show that the reaction Br + t? C₄H₉ → HBr + i? C₄H₈ is unimportant under the present experimental conditions. Assigning the entropy of t-butyl radical to be 74 ± 2 eu which is in the possible range, the value of K₁ gives ΔH (t-butyl) = 9.1 ± 0.6 kcal/mol^?1. This yields for the bond dissociation, DH° (t-butyl-H) = 93.4 ± 0.6 kcal/mol. Both of these values are found to be in good agreement with recent VLPP studies.     

The absolute rate constant for the metathesis t-C4H9• + DI → C4H9D + I• and the heat of formation of the t-butyl radical

The Arrhenius parameters for the title reaction have been measured in a very-low-pressure pyrolysis apparatus in the temperature range 644–722 K and are given by log k₂ (M^?1 · sec^?1) = 9.68 - 2.12/θ, where θ = 2.303RT in kcal/mol. Together with the published Arrhenius parameters for the reverse reaction from iodination studies, they result in a standard heat of formation of the t-butyl radical of 8.4 kcal/mol, accepting S⁰(C₄H₉·) = 72.2 e.u. at 300 K from other kinetic data, and thus confirm the accepted value for ΔH_f⁰(t-C₄H₉·), at variance with recent investigations which yielded significantly higher values. This value for ΔH_f⁰(t-C₄H₉·) results in a bond-dissociation energy (BDE) for isobutane of 92.7 kcal/mol.     

The gas‐phase elimination kinetics of ethyl 2‐furoate and ethyl 2‐thiophenecarboxylate

The gas‐phase elimination kinetics of ethyl 2‐furoate and 2‐ethyl 2‐thiophenecarboxylate was carried out in a static reaction system over the temperature range of 623.15–683.15 K (350–410°C) and pressure range of 30–113 Torr. The reactions proved to be homogeneous, unimolecular, and obey a first‐order rate law. The rate coefficients are expressed by the following Arrhenius equations: ethyl 2‐furoate, log k₁ (s^?1) = (11.51 ± 0.17)–(185.6 ± 2.2) kJ mol^?1 (2.303 RT)^?1; ethyl 2‐thiophenecarboxylate, log k₁ (s^?1) = (11.59 ± 0.19)–(183.8 ± 2.4) kJ mol^?1 (2.303 RT)^?1. The elimination products are ethylene and the corresponding heteroaromatic 2‐carboxylic acid. However, as the reaction temperature increases, the intermediate heteroaromatic carboxylic acid products slowly decarboxylate to give the corresponding heteroaromatic furan and thiophene, respectively. The mechanisms of these reactions are suggested and described. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 145–152, 2009     

Rate constants for the gas‐phase β‐myrcene + OH and isoprene + OH reactions as a function of temperature

Rate constants for the gas‐phase reactions of the hydroxyl radical with the biogenic hydrocarbons, β‐myrcene and isoprene, were measured using the relative rate technique over the temperature range 313–413 K and at ～1 atm total pressure. OH was produced by the photolysis of H₂O₂, and helium was the diluent gas. The reactants were detected by online mass spectrometry, which resulted in high time resolution allowing for large amounts of data to be collected and used in the determination of the Arrhenius parameters. Many experiments were performed over the temperature range of interest, leading to more accurate parameters than previous investigations. The following Arrhenius expression has been determined for these reactions (in units of cm³ molecule^?1 s^?1): for isoprene k = (3.14) × 10^?11 exp [(338 ± 19)/T] and for β‐myrcene k = (9.19) × 10^?12 exp[(1071 ± 82)/T]. The Arrhenius plot for the isoprene + OH reaction indicates curvature in this relationship and is given by k = (3.47 ± 0.14) × 10^?17 T² exp [(1036 ± 14)/T]. Our measured rate constant for the β‐myrcene + OH reaction at 298 K is higher, but not significantly, than current literature values. This is the first report of β‐myrcene's rate constant with OH as a function of temperature. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 407–413, 2009     

Laser-induced chemical kinetics: Absolute rate constants for the reactions Ċ2F2 + Br2 → C2F5Br + Ḃr and n-Ċ3F7 + Br2 → n-C3F7Br + Ḃr

Absolute rate constants at room temperature for the metathesis reaction have been measured under VLPP conditions: k₁ = (2.0 ± 0.5) × 10⁸M^?1·s^?1, k₂ = (3.0 ± 0.7) × 10⁸M^?1·s^?1. The radicals were generated through collisionless infrared-multiphoton decomposition of the corresponding iodides by irradiation from a high-power CO₂-TEA laser. The reaction of ?₂F₅ and ?₃F₇ with \documentclass{article}\pagestyle{empty}\begin{document}$$\mathop {\rm N}\limits^{\rm .} {\rm O}_{\rm 2} $$\end{document} are briefly discussed in relation to the reaction of ?₃ with \documentclass{article}\pagestyle{empty}\begin{document}$$\mathop {\rm N}\limits^{\rm .} {\rm O}_{\rm 2} $$\end{document}, which had been measured previously.     

Rate constants for the elementary reactions between CN radicals and CH4, C2H6, C2H4, C3H6, and C2H2 in the range: 295 ⩽ T/K ⩽ 700

Pulsed laser photolysis, time-resolved laser-induced fluorescence experiments have been carried out on the reactions of CN radicals with CH₄, C₂H₆, C₂H₄, C₃H₆, and C₂H₂. They have yielded rate constants for these five reactions at temperatures between 295 and 700 K. The data for the reactions with methane and ethane have been combined with other recent results and fitted to modified Arrhenius expressions, k(T) = A′(298) (T/298)ⁿ exp(?θ/T), yielding: for CH₄, A′(298) = 7.0 × 10^?13 cm³ molecule^?1 s^?1, n = 2.3, and θ = ?16 K; and for C₂H₆, A′(298) = 5.6 × 10^?12 cm³ molecule^?1 s^?1, n = 1.8, and θ = ?500 K. The rate constants for the reactions with C₂H₄, C₃H₆, and C₂H₂ all decrease monotonically with temperature and have been fitted to expressions of the form, k(T) = k(298) (T/298)ⁿ with k(298) = 2.5 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.24 for CN + C₂H₄; k(298) = 3.4 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.19 for CN + C₃H₆; and k(298) = 2.9 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.53 for CN + C₂H₂. These reactions almost certainly proceed via addition-elimination yielding an unsaturated cyanide and an H-atom. Our kinetic results for reactions of CN are compared with those for reactions of the same hydrocarbons with other simple free radical species. © John Wiley & Sons, Inc.     

Kinetics of the thermal dimerization and isomerization of cis,trans-1,5-cyclooctadiene in the gas phase and of related reactions. A simple algorithm to determine the rate constants of competing first- and second-order reactions

In the gas phase, cis,trans-1,5-cyclooctadiene (\documentclass{article}\pagestyle{empty}\begin{document}$ {\mathop 1\limits_\sim} $\end{document}) undergoes a unimolecular rearrangement to cis,cis-1,5-cyclooctadiene (\documentclass{article}\pagestyle{empty}\begin{document}$ {\mathop 2\limits_\sim} $\end{document}) and bimolecular formation of dimers \documentclass{article}\pagestyle{empty}\begin{document}$ {\mathop 3\limits_\sim}-{\mathop 5\limits_\sim} $\end{document} $\end{document}. The Arrhenius parameters are E_A = 135.7 ± 4.4 kJ mole^?1 and log(A/sec^?1) = 12.9 ± 0.6 for the first reaction and E_A = 66.1 ± 6.0 kJ mole^?1 and log[A/(liter mole^?1 sec^?1)] = 5.5 ± 0.8 for the second reaction. Using thermochemical kinetics, the first reaction is shown to proceed via a rate determining Cope rearrangement of \documentclass{article}\pagestyle{empty}\begin{document}$ {\mathop 1\limits_\sim} $\end{document} to cis? 1,2-divinylcyclobutane (\documentclass{article}\pagestyle{empty}\begin{document}$ {\mathop 6\limits_\sim} $\end{document}), E_A = 136.2 - 4.4 kJ mole^?1 and log(A/sec^?1) = 13.0 ± 0.6. The corresponding back reaction, \documentclass{article}\pagestyle{empty}\begin{document}$ {\mathop 6\limits_\sim}{\rightarrow}{\mathop 1\limits_\sim} $\end{document}, which was investigated separately, shows E_A = 110.2 ± 1.2 kJ mole^?1 and log(A/sec^?1) = 10.9 ± 0.2. The heat of formation of \documentclass{article}\pagestyle{empty}\begin{document}$ {\mathop 6\limits_\sim} $\end{document} is determined to 188 ± 5.5 kJ mole^?1. The mechanism of formation of dimers \documentclass{article}\pagestyle{empty}\begin{document}$ {\mathop 3\limits_\sim}-{\mathop 5\limits_\sim} $\end{document} is discussed. To allow the formal analysis of the kinetic problem, a simple algorithm to obtain the rate constants of competing first- and second-order reactions was developed.     

Gas-phase elimination kinetics of 1-substituted ethyl acetates. Effect of polar substituents at the α carbon of secondary acetates

The gas-phase elimination of several polar substituents at the α carbon of ethyl acetates has been studied in a static system over the temperature range of 310–410°C and the pressure range of 39–313 torr. These reactions are homogeneous in both clean and seasoned vessels, follow a first-order rate law, and are unimolecular. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: 2-acetoxypropionitrile, log k₁ (s^?1) = (12.88 ± 0.29) – (203.3 ± 2.6) kJ/mol (2.303RT)^?1; for 3-acetoxy-2-butanone, log ±₁(s^?1) = (13.40 ± 0.20) – (202.8 ± 2.4) kJ/mol (2.303RT)^?1; for 1,1,1-trichloro-2-acetoxypropane, log ?₁ (s^?1) = (12.12 ± 0.50) – (193.7 ± 6.0) kJ/mol (2.303RT)^?; for methyl 2-acetoxypropionate, log ?₁ (s^?1) = (13.45 ± 0.05) – (209.5 ± 0.5) kJ/mol (2.303RT)^?1; for 1-chloro-2-acetoxypropane, log ?₁ (s^?1) = (12.95 ± 0.15) – (197.5 ± 1.8) kJ/mol (2.303RT)^?1; for 1-fluoro-2-acetoxypropane, log ?₁ (s^?1) = (12.83 ± 0.15)– (197.8 ± 1.8) kJ/mol (2.303RT)^?1; for 1-dimethylamino-2-acetoxypropane, log ?₁ (s^?1) = (12.66 ± 0.22) –(185.9 ± 2.5) kJ/mol (2.303RT)^?1; for 1-phenyl-2-acetoxypropane, log ?₁ (s^?1) = (12.53 ± 0.20) – (180.1 ± 2.3) kJ/mol (2.303RT)^?1; and for 1-phenyl?3?acetoxybutane, log ?₁ (s^?1) = (12.33 ± 0.25) – (179.8 ± 2.9) kJ/mol (2.303RT)^?1. The C_α? O bond polarization appears to be the rate-determining process in the transmition state of these pyrolysis reactions. Linear correlations of electron-releasing and electron-withdrawing groups along strong σ bonds have been projected and discussed. The present work may provide a general view on the effect of alkyl and polar substituents at the C_α? O bond in the gas-phase elimination of secondary acetates.     

Kinetics of the reaction Cl + CH4 → CH3+ HCl from 200° to 500deg;K

The rate constant for the reaction \documentclass{article}\pagestyle{empty}\begin{document}${\rm Cl} + {\rm CH}_4 \mathop {\longrightarrow}\limits^1 {\rm CH}_3 + {\rm HCl}$\end{document} has been determined over the temperature range of 200°–500°K using a discharge flow system with resonance fluorescence detection of atomic chlorine under conditions of large excess CH₄. For 300° > T > 200°K the data are best fitted to the expression k₁ = (8.2 ± 0.6) × 10^?12 exp[?(1320 ± 20)/T] cm³/sec. Curvature is observed in the Arrhenius plot such that the effective activation energy increases from 2.6 kcal/mol at 200° < T < 300°K to 3.5 kcal/mol at 360° < T < 500°K. The data over the entire range may be fitted by the expression k₁ = 8.6×10^?18 T^2.11 exp[?795/T]. These results are compared with other experimental studies and with a semiempirical transition state calculation. Their atmospheric significance is discussed.     

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.