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      

Shock tube studies of the N2O/Ar and N2O/H2/Ar systems

N₂O decay has been monitored via infrared emission for a series of mixtures containing N₂O/Ar and N₂O/H₂/Ar. These mixtures were studied behind reflected shock waves in the temperature interval of 1950–3075°K with total concentrations ranging from 1.2 to 2.5 × 10¹⁸ molec/cm³. In all cases the N₂O decayed exponentially, and a rate constant k_obs was obtained. Runs without added H₂ could be described by the following Arrhenius parameters: log A = ?9.72 ± 0.08 (in units of cm³/molec · sec) and E_A = 203.5 ± 3.6 kJ/mole. Addition of 0.01% and 0.1% H₂ was observed to increase the decay rate; the largest increase occurred between 2250 and 2500°K with 0.1% H₂, where k_obs doubled. Mixtures with no added H₂ were analyzed by numerical integration of the following reactions: Quantitative agreement between calculations and observations were obtained with both high and low choices for k₂ and k₃. The additional reactions were included in the analysis of the mixtures containing H₂. Here agreement was obtained only when low values were assigned to k₂ and k₃. The combinations of k₁ ← k₃ which agreed with all the data were k₁ = 3.25 × 10^?10 exp (?215 kJ/RT) and k₂ = k₃ = 1.91 × 10^?11 exp (-105 kJ/RT).     

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      

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 OH + OH → H2O + O reaction

The rate coefficient for the reaction has been determined in mixtures of nitric acid (HNO₃) and argon in incident shock wave experiments. Quantitative OH time-histories were obtained by cw narrow-linewidth uv laser absorption of the R₁(5) line of the A² σ⁺ ← X² Π_i (0,0) transition at 32606.56 cm^?1 (vacuum). The experiments were conducted over the temperature range 1050–2380 K and the pressure range 0.18–0.60 atm. The second-order rate coefficient was determined to be with overall uncertainties of +11%, ?16% at high temperatures and +25%, ?22% at low temperatures. By incorporating data from previous investigations in the temperature range 298–578 K, the following expression is determined for the temperature range 298–2380 K © 1994 John Wiley & Sons, Inc.     

CH and C-atom time histories in dilute hydrocarbon pyrolysis: Measurements and kinetics calculations

CH and C-atom concentration-time histories were measured during pyrolysis of highly dilute mixtures (6 to 100 ppm) of ethane or methane in argon behind reflected shock waves over the temperature range 2500 to 3800 K and pressure range 0.5 to 1.3 atm. CH was detected using narrow-linewidth laser absorption at 431 nm. C-atom concentrations were measured using atomic resonance absorption spectroscopy (ARAS) at 156.1 nm. These data allow improved understanding of dilute hydrocarbon pyrolysis. A pyrolysis reaction mechanism was developed which fits essential characteristics of the CH and C-atom profiles (time to peak, peak concentration, and 50% decay time) within ±25%. Critical reactions for which rate coefficient data were not previously available are: Best-fit rate coefficients, valid over the range 2500 to 3800 K, are: k₄ = 5.0 × 10¹⁵ exp(?42800 K/T), k₅ = 4.0 × 10¹⁵ exp(?41800 K/T), k₆ = 1.3 × 10¹⁴ exp(?29700 K/T), and k₇ = 1.9 × 10¹⁴ exp(?33700 K/T) cm³ mol^?1 s^?1     

The oxidation of methyl radicals at room temperature

Using the technique of molecular modulation spectrometry, we have measured directly the rate constants of several reactions involved in the oxidation of methyl radicals at room temperature: k₁ is in the fall-off pressure regime at our experimental pressures (20–760 torr) where the order lies between second and third and we obtain an estimate for the second-orderlimit of (1.2 ± 0.6) × 10^?12 cm³/molec · sec, together with third-order rate constants of (3.1 ± 0.8) × 10^?31 cm⁶/molec² · sec with N₂ as third body and (1.5 ± 0.8) × 10^?30 with neopentane; we cannot differentiate between k_2a and k_2c and we conclude k_2a + (k_2c) = (3.05 ± 0.8) × 10^?13 cm³/molec · sec and k_2b = (1.6 ± 0.4) × 10^?13 cm³/molec · sec; k₃ = (6.0 ± 1.0) × 10^?11 cm³/molec · sec.     

The reaction of O(1D) with H2O and the reaction of OH with C3H6

N₂O was photolyzed at 2139 Å to produce O(¹D) atoms in the presence of H₂O and CO. The O(¹D) atoms react with H₂O to produce HO radicals, as measured by CO₂ production from the reaction of OH with CO. The relative importance of the various possible O(¹D )–H₂O reactions is The relative rate constant for O(¹D) removal by H₂O compared to that by N₂O is 2.1, in good agreement with that found earlier in our laboratory. In the presence Of C₃H₆, the OH can be removed by reaction with either CO or C₃H₆: From the CO₂ yield, k₃/k₂ = 75,0 at 100°C and 55.0 at 200°C to within ± 10%. When these values are combined with the value of k₂ = 7.0 × 10^?13exp (–1100/RT) cm³/sec, k₃ = 1.36 × 10^?11 exp (–100/RT) cm³/sec. At 25°C, k₃ extrapolates to 1.1 × 10^?11 cm³/sec.     

Reactions of alkoxy radicals with O2. III. i-C4H9O radicals

i-C₄H₉ONO was photolyzed with 366-nm radiation at ?8, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yield of i-C₃H₇CHO, Φ{i-C₃H₇CHO}, was measured as a function of reaction of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?₁ = 0.24 ± 0.02 independent of temperature. The i-C₄H₉O radicals can react with NO by two routes The i-C₄H₉O radical can decompose via or react with O₂ via Values of k₄/k₂ ? k_4b/k₂ were determined to be (2.8 ± 0.6) × 10¹⁴, (1.7 ± 0.2) × 10¹⁵, and (3.5 ± 1.3) × 10¹⁵ molec/cm³ at 23 55, and 88°C, respectively, at 150-torr total pressure of N₂. Values of k₆/k₂ were determined from ?8 to 120°C. They fit the Arrhenius expression: For k₂ ? 4.4 × 10¹¹ cm³/s, k₆ becomes (3.2 ± 2.0) × 10^?13 exp{?(836 ± 159)/T} cm³/s. The reaction scheme also provides k_4b/k₆ = 3.59 × 10¹⁸ and 5.17 × 10¹⁸ molec/cm³ at 55 and 88°C, respectively, and k_8b/k₈ = 0.66 ± 0.12 independent of temperature, where      

High temperature determination of the rate coefficient for the reaction H2O + CN → HCN + OH

The rate coefficient, k, of the reaction has been determined in the temperature range 2460–2840 K using a shock tube technique. C₂N₂? H₂O? Ar mixtures were heated behind incident shock waves and the CN and OH concentration time histories were monitored simultaneously using broad-band absorption near 388 nm (CN) and narrow-line laser absorption at 306.67 nm (OH). The rate coefficient expression providing the best fit to the data was with uncertainty limits of about ±45% in the temperature range 2460–2840 K. The rate coefficient of the reverse reaction was calculated using detailed balancing, and its extrapolation to lower temperatures was compared with previously published results.     

Reaction of H with C2N2 at pressures near 1 torr

The rate of disappearance of C₂N₂ in the presence of a large excess of H atoms has been measured in a discharge-flow system at pressures near 1 torr and temperatures in the range of 282–338 K. Under these conditions the reaction has a small negative temperature coefficient. A transition from second-order to third-order kinetics with decreasing pressure occurs at pressures near 1 torr. The results are discussed in terms of the mechanism where k₇ = (1.5 ± 0.2) × 10^–15 cm³/molec¹·sec is found for the forward rate of reaction (7). The results also give k₇k₈/k_?7 = 3.7 × 10^?31 cm⁶/molec²·sec and k₇k₉/k_?7 = 3.0 × 10^?32 cm⁶/molec²·sec, the first being probably an upper limit and the second probably a lower limit; hence k₈/k₉ = 12 is found as an upper limit.     

The reaction of OH with NO

Mixtures of N₂O, CO, and NO in excess H₂ were photolyzed at 213.9 nm and 298°K. The initially formed O(¹D) atoms from the photolysis of N₂O abstract an H atom from H₂ permitting a study of the competition: From the CO₂ yield the relative rate coefficient k₁/k₂ is obtained. It is found to be slightly dependent on pressure for total pressures (mainly H₂) of 95.5 to 768 torr. However, the values are near the high-pressure limiting value which is found by extrapolation to give k₁^∞ = 1.2 × 10^?11 cm³/sec based on k₂^∞ = 3.55 × 10^?13 cm³/sec.     

The reaction of O(3P) with C2HCl3

The reaction of O(³P), prepared from the Hg photosensitization of N₂O, with C₂HCl₃ was studied at 25°C. The products of the reaction in the absence of O₂ were CO, CHCl₃, and polymer (as well as N₂ from the N₂O). The quantum yields of CO and CHCl₃ were 0.23 ± 0.01 and 0.14 ± 0.05, is respectively independent of reaction conditions. The reaction mechanism is with k_14a/k₁₄ = 0.23, where k_14a + k_14b. Most of the HCl and CCl₂ combine to form CHCl₃, but some other products must also be formed to account for the difference in the CO and CHCl₃ quantum yields. The C₂HCl₃O* adduct polymerizes without involving additional C₂HCl₃ molecules, since the quantum yield of C₂HCl₃ disappearance, ? Φ{C₂HCl₃}, was about 1.0 at high values of [N₂O]/[C₂HCl₃]. The rate coefficient for the reaction of O(³P) with C₂HCl₃ is 0.10 that for the reaction of O(³P) with C₂F₄. In the presence of O₂ the free radical chain oxidation occurs because of the reaction The main product is CHCl₂CCl(O) with smaller amounts of CO and CCl₂O, and some CO₂. The chain lengths were long and values of ? Φ {C₂HCl₃} up to 90 were observed.     

Reactions of O 3PJ atoms with halogens: The rate constants for the elementary reactions O + BrCl,O + Br2, and O + Cl2

Direct determinations of the rate constants (cm³/molec · sec) k₁, k₂, and k₃ from 298 to 299°K are reported, using atomic resonance fluorescence in discharge flow systems:

1 One standard deviation.

The rate constant k₁, which has not been determined previously, was found to possess an insignificant temperature coefficient (E_A = (0 ± 700) J/mole) in the range of 299 to 619°K. The present result for k₂ agrees well with reinterpreted values from the one previous determination. Measurements of O atom consumption rates and Br atom production rates in the O + Br₂ reaction are interpreted to give an estimate of the rate constant k₄, which has not been reported previously, at 298°K: k₃ has been measured at three temperatures between 299 and 602°K. The present and previous results for k₃ were combined to give the following rate expression:      

Transferts Protoniques de Sels d'Ammonium Substitues—VIII: Sels de Pyridinium 4-Substitues

The deprotonation rate 1/τ of the title compounds, [4 – R – Py H]⁺, where R = NH₂, t-Bu, Me, Cl, Br or CN, is measured using the coalescence of the pyridinic α-protons, in a mixture CF₃COOH/H₂O/HClO₄ of variable acidity Ho, at 38°C. 1/τ is a linear function k/ho of the acidity 1/ho. k is approximately proportional to the water content and independent of the salt concentration, which seems to be evidence for an exchange with an intermediate pyridine hydrate, according to: . After a preliminary ionisation step: k values, like K_A, fit a Hammett relationship (ρ = 5,05), except for R ? NH₂, and are very sensitive to the nature of R (k = 3,44 × 10² for R = NH₂ and k = 3,14 × 10⁸ M^?1 s^?1 for R ? CN), while k_H values (10¹⁰ s^?1) are not.     

Temperature dependence of the reactions of HO2 with NO and NO2

Mixtures of N₂O, H₂, O₂, and trace amounts of NO and NO₂ were photolyzed at 213.9 nm, at 245°–328°K, and at about 1 atm total pressure (mostly H₂). HO₂ radicals are produced from the photolysis and they react as follows: Reaction (1b) is unimportant under all of our reaction conditions. Reaction (1a) was studied in competition with reaction (3) from which it was found that k_1a/k₃^1/2 = 6.4 × 10^?6 exp { z?(1400 ± 500)/RT} cm^3/2/sec^1/2. If k₃ is taken to be 3.3 × 10^?12 cm³/sec independent of temperature, k_1a = 1.2 × 10^?11 exp {?(1400 ± 500)/RT} cm³/sec. Reaction (2a) is negligible compared to reaction (2b) under all of our reaction conditions. The ratio k_2b/k₁ = 0.61 ± 0.15 at 245°K. Using the Arrhenius expression for k_1a given above leads to k_2b = 4.2 × 10^?13 cm³/sec, which is assumed to be independent of temperature. The intermediate HO₂NO₂ is unstable and induces the dark oxidation of NO through reaction (?2b), which was found to have a rate coefficient k_?2b = 6 × 10¹⁷ exp {?26,000/RT} sec^?1 based on the value of k_1a given above. The intermediate can also decompose via Reaction (10b) is at least partially heterogeneous.     

Some reactions of the isopropyl radical

The decomposition of ethane sensitized by isopropyl radicals was studied in the temperature range of 496–548°K. The rate of formation of n-butane, isopentane, and 2,3-dimethylbutane were measured. The expression k₁/k₂^½ was found to be where k₁ and k₂ are rate constants of The decomposition of propylene sensitized by isopropyl radicals was studied between 494 and 580°K by determination of the initial rates of formation of the main products. The ratio of k₁₃/k₂^1/2 was evaluated to be where k₁₃ is the rate constant for The isomerization of the isopropyl radical was investigated by studying the decomposition of azoisopropane. The decomposition of the iso-C₃H₇ radical into C₂H₄ and CH₃ was followed by measuring the rate of formation of C₂H₄. On the basis of the experimental data, obtained in the range of 538–666° K, k₁₅/k₂^½ was found: where k₁₅ is the rate constant of      

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.     

Oxidation studies with peroxomonophosphoric acid. II. A kinetic and mechanistic study of dimethyl sulfoxide oxidation in acid and alkaline media

The kinetics of dimethyl sulfoxide (DMSO) oxidation by peroxomonophosphoric acid (PMPA) in aqueous medium at 308 K and I = 0.4 mol/dm³ follow the rate expressions In the pH range from 0 to 2, where k₁ and k₂ are 5.092 × 10^?1 dm³/mol sec and ? 0, respectively; in the pH range from 4 to 7, where k₂ = 8.127 × 10^?3 and k₃ = 2.90 × 10^?3 dm³/mol sec; and in the pH range from 10 to 13.6, where k₄ ? 0, and k₅ = 3.08 × 10^?2 dm³/mol sec. The reaction is interpreted in terms of mechanisms involving an electrophilic and a nucleophilic attack of the peroxomonophosphoric acid species, respectively, in acid and alkaline regions, on the sulfur atom of the sulfoxide molecule giving rise to S-type transition states followed by oxygen-oxygen bond fission to form the products.     

Kinetics and mechanism of oxidation of hydrazinium ion with peroxomonophosphoric acid in acid perchlorate solutions and role of trace iodide ions

The reaction of peroxomonophosphoric acid and hydrazinium ion in acid perchlorate solutions occurs as per stoichiometry (i), and the rate law (ii) at large [N₂H₅ ⁺], where K′_d is the first acid dissociation constant of H₃PO₅ and k ₁ and k ₂ are rate constants found to be 2.6 × 10^?4 s^?1 and 5.0 × 10^?2 M^?1 s^?1, respectively, at 35°. The reaction is greatly catalyzed by iodide ions. The mechanism involves a redox cycle I^?/I₂ and the rate is independent of [N₂H₅ ⁺] in the presence of iodide ions. K′_d was found to be 0.55 M^?1 and independent of temperature.