Kinetics and mechanism of the reaction of OH with ClO

The kinetics and mechanism of the following reactions have been studied in the temperature range 230–360 K and at total pressure of 1 Torr of helium, using the discharge‐flow mass spectrometric method: 1a : (1a) 1b : (1b) The following Arrhenius expression for the total rate constant was obtained from the kinetics of OH consumption in excess of ClO radical, produced in the Cl + O₃ reaction either in excess of Cl atoms or ozone: k₁ = (6.7 ± 1.8) × 10^?12 exp {(360 ± 90)/T} cm³ molecule^?1 s^?1 (with k₁ = (2.2 ± 0.4) × 10^?11 cm³ molecule^?1 s^?1 at T = 298 K), where uncertainties represent 95% confidence limits and include estimated systematic errors. The value of k₁ is compared with those from previous studies and current recommendations. HCl was detected as a minor product of reaction (1) and the rate constant for the channel forming HCl (reaction (1b)) has been determined from the kinetics of HCl formation at T = 230–320 K: k_1b = (9.7 ± 4.1) × 10^?14 exp{(600 ± 120)/T} cm³ molecule^?1 s^?1 (with k_1b = (7.3 ± 2.2) × 10^?13 cm³ molecule^?1 s^?1 and k_1b/k₁ = 0.035 ± 0.010 at T = 298 K), where uncertainties represent 95% confidence limits. In addition, the measured kinetic data were used to derive the enthalpy of formation of HO₂ radicals: Δ H_f,298(HO₂) = 3.0 ± 0.4 kcal mol^?1. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 587–599, 2001     

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.     

Fourier transform infrared study of the self reaction of C2H5O2 radicals in air at 295 K

Fourier transform infrared spectroscopy was used to identify and quantify products of the self reaction of ethylperoxy radicals, C₂H₅O₂, formed in the photolysis of Cl₂/C₂H₆ mixtures in 700 torr total pressure of synthetic air at 295 K. From these measurements, branching ratios for the reaction channels (1) of k_1a/(k_1a + k_1b) = 0.68 and k_1c/(k_1a + k_1b + k_1c) ? 0.06 were established. Additionally, using the relative rate technique, the rate constant for the reaction of Cl atoms with C₂H₅OOH was determined to be (1.07 ± 0.07) × 10^?10 × cm³ molecule^?1 s^?1. Results are discussed with respect to the previous kinetic and mechanistic studies of C₂H₅O₂ radicals.     

Kinetics of the reaction OH (v = 0) + O3 → HO2 + O2

The rate constant of the reaction OH (v = 0) + O₃

HO₂ + O₂ was measured over the temperature range from 220 to 450°K at total pressures between 2 and 5 torr using ultraviolet fluorescent scattering for the detection of OH radicals. An Arrhenius expression, k₁ = 1.3 × 10^?12 exp(?1900/RT) cm³/sec was obtained and the rate constant for the reaction HO₂ + O₃

OH + 2O₂ was inferred to be less than 0.1 k₁ over the entire temperature interval.     

The second limit of hydrogen + carbon monoxide + oxygen mixtures

Previous studies by Buckler and Norrish of the second limit of CO and O₂ mixtures containing small amounts (0.25–10%) of H₂ have been used to obtain the velocity constant of the reaction These estimates of k₃₃ = 3.9 × 10⁸ and 3.5 × 10⁸ liter² mole^?2 sec^?1 (M ? H₂) at 500° and 560°C, respectively, have been combined with other estimates over the range 300°–3500°K to give k₃₃ = 3.0 × 10⁸ exp (?3000/RT) for M ? Ar; the considerable scatter in the available points does not encourage any great confidence in this expression and may be attributed at least partly to the different molecules used as M by different workers. For KCl-coated and CsCl-coated vessels at 540°C, studies of the second limit of H₂ + O₂ mixtures, to which CO has been added, have indicated that with both the surfaces, the effect of CO on the limit is masked by changes in the surface nature. In the case of CsCl, the results have enabled a lower limit of about 0.6 to be obtained for the efficiency of CO relative to H₂ in the reaction Use of a computer treatment to interpret the second limit of CO + H₂ + O₂ mixtures in aged boric-acid-coated vessels at 500°C gives a value of m_CO = 0.74 ± 0.04 together with an estimate of k₃₂ (H + CO + M″ = HCO + M″)/k₄ = 0.022 ± 0.003, which leads to k₃₂ = 2.3 × 10⁸ liter² mole^?2 sec^?1 (M ? H₂) at 500°C.     

Kinetic study of the reactions of CL and BR with H2O2

The reactions of Cl and Br atoms with H₂O₂ have been studied in the range of 300–350 K using the very-low-pressure-reactor technique. It was found that metathesis to produce HX and HO₂ is the only significant process (≤99%). For the reaction of Br k₂ (300 K) = 1.3 ± 0.45 × 10^?14 and k₂ (350 K) = 3.75 ± 1.1 × 10^?14 cm³/molecules·s, with an activation energy of 4.6 ± 0.7 kcal/mol. Using an estimated A factor for A₂, we find suggesting that a best choice is E₂ = 3.9 ± 0.4 kcal/mol. The relation of these values to ΔH (HO₂) is discussed.     

Study of the kinetics of the reactions of NO3 with HO2 and OH

The Absolute rate constants for the gas-phase reactions of NO₃ with HO₂ and OH have been determined using the discharge flow laser magnetic resonance method (DF-LMR). Since OH was found to be produced in the reaction of HO₂ with NO₃, C₂F₃Cl was used to scavenge it. The overall rate constant, k₁, for the reaction, HO₂ + NO₃ → products, was measured to be k₁=(3.0 ± 0.7)×10^?12 cm³ molecule^?1 s^?1 at (297 ± 2) K and P=(1.4 – 1.9) torr. This result is in reasonable agreement with the previous studies. Direct detection of HO₂ and OH radicals and the use of three sources of NO₃ enabled us to confirm the existence of the channel producing OH:HO₂+NO₃→OH+NO₂+O₂ (1a); the other possible channel is HO₂+NO₃→HNO₃+O₂ (1b). From our measurements and the computer simulations, the branching ratio, k_1a/(k_1a + k_1b), was estimated to be (1.0). The rate coefficient for the reaction of OH with NO₃ was determined to be (2.1 ± 1.0) × 10^?11 cm³ molecule^?1 s^?1. © 1993 John Wiley & Sons, Inc.     

The rate constant of the reaction HO2 + COCO2 + OH

The high-temperature oxidation of formaldehyde in the presence of carbon monoxide was investigated to determine the rate constant of the reaction HO₂ + CO ? CO₂ + OH (10). In the temperature range of 878–952°K from the initial parts of the kinetic curves of the HO₂ radicals and CO₂ accumulation at small extents of the reaction, when the quantity of the reacted formaldehyde does not exceed 10%, it was determined that the rate constant k₁₀ is A computer program was used to solve the system of differential equations which correspond to the high-temperature oxidation of formaldehyde in the presence of carbon monoxide. The computation confirmed the experimental results. Also discussed are existing experimental data related to the reaction of HO₂ with CO.     

Estimation of the R-H bond energy from the activation energy for the reaction RH + O2 → R· + HO2·

Since the activation energy for the reaction RH + O₂ → R· + HO₂. is very close to its endothermicity, the R-H bond energy can be calculated from the activation energy for free radical formation by the reaction RH + O₂. The relation between E_i and Q_R–H was found empirically after measuring E_i by the method of inhibitors for the oxidation of cyclohexane, n? heptane, and toluene: The values of Q_R–H are calculated from these and earlier experimental data for five hydrocarbons, five phenols, and four aromatic amines.     

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.     

A flash photolysis-resonance fluorescence study of the reaction of atomic hydrogen with molecular oxygen H + O2 + M → HO2 + M

Using the technique of flash photolysis-resonance fluorescence, absolute rate constants have been measured for the reaction H + O₂ + M → HO₂+M over a temperature range of 220–360°K. Over this temperature range, the data could be fit to an Arrhenius expression of the following form: The units for k_Ar are cm⁶/mole-s. At 300°K the relative efficiencies for the third-body gases Ar:He:H₂:N₂:CH₄ were found to be 1.0:0.93:3.0:2.8:22. Wide variations in the photoflash intensity at several temperatures demonstrated that the reported rate constants were measured in the absence of other complex chemical processes.     

单分子水对H_2O_2+Cl气相反应影响的理论研究

在aug-cc-pVTZ基组下采用CCSD(T)和B3LYP方法,研究了H2O2+Cl反应,并考虑在大气中单个水分子对该反应的影响.结果表明,H2O2+Cl反应只存在一条生成产物为HO2+HCl的通道,其表观活化能为10.21kJ·mol-1.加入一分子水后,H2O2+Cl反应的产物并没有发生改变,但是所得势能面却比裸反应复杂得多,经历了RW1、RW2和RW3三条通道.水分子在通道RW1和RW2中对产物生成能垒的降低起显著的负催化作用,而在通道RW3中则起明显的正催化作用.利用经典过渡态理论(TST)并结合Wigner矫正模型计算了216.7-298.2 K温度范围内标题反应的速率常数.结果显示,298.2 K时通道R1的速率常数为1.60×10-13cm3·molecule-1·s-1,与所测实验值非常接近.此外,尽管通道RW3的速率常数kRW3比对应裸反应的速率常数kR1大了46.6-131倍,但该通道的有效速率常数k'RW3却比kR1小了10-14个数量级,表明在实际大气环境中水分子对H2O2+Cl反应几乎没有影响.     

Kinetics of the reactions of atomic chlorine with methanol and the hydroxymethyl radical with molecular oxygen at 298 K

The rate constants for the reactions Cl + CH₃OD → CH₂OD + HCl (1) and CH₂OH + O₂ → HO₂ + H₂CO (2) have been determined in a discharge flow system near 1 torr pressure with detection of radical and molecular species using collision-free sampling mass spectrometry. The rate constant k₁, determined from the decay of CH₃OD in the presence of excess Cl, is (5.1 ± 1.0) × 10^?11 cm³ s^?1. This is in reasonable agreement with the only previous measurement of k₁. The CH₂OH radical was produced by reaction (1) and its reaction with O₂ was studied by monitoring the decay of the CH₂OH radical in the presence of excess O₂. The result is k₂ = (8.6 ± 2.0) × 10^?12 cm³ s^?1. Previous estimates of k₂ have differed by nearly an order of magnitude, and our value for k₂ supports the more recent high values.     

The temperature dependence of the HO2 + HO2 reaction

The temperature dependence of the rate constant for the reaction HO₂ + HO₂ → H₂O₂ + O₂ (2k₁) has been determined using flash photolysis techniques, over the temperature range 298–510 K, in a nitrogen diluent at a total pressure of 700 Torr. The overall second order state constant is given by k₁ = (4.14 ± 1.15) × 10^?13 exp[(630 ± 115)/T] cm³ molecule^?1 s^?1, where the quoted errors refer to one standard deviation. This result is compared with previous findings and the negative activation energy is shown to be consistent with the observation that the rate constant is pressure dependent at 700 Torr.     

Determination of the rate coefficients of the SO2 + O + M → SO3 + M reaction

Rate coefficients of the title reaction R₃₁ (SO₂ + O + M → SO₃ + M) and R₅₆ (SO₂ + HO₂→ SO₃ + OH), important in the conversion of S(IV) to S(VI), were obtained at T = 970–1150 K and ρ_ave = 16.2 μmol cm^?3 behind reflected shock waves by a perturbation method. Shock‐heated H₂/O₂/Ar mixtures were perturbed by adding small amounts of SO₂ (1%, 2%, and 3%) and the OH temporal profiles were then measured using laser absorption spectroscopy. Reaction rate coefficients were elucidated by matching the characteristic reaction times acquired from the individual experimental absorption profiles via simultaneous optimization of k₃₁ and k₅₆ values in the reaction modeling (for satisfactory matches to the observed characteristic times, it was necessary to take into account R₅₆). In the experimental conditions of this study, R₃₁ is in the low‐pressure limit. The rate coefficient expressions fitted using the combined data of this study and the previous experimental results are k_31,0/[Ar] = 2.9 × 10³⁵ T^?6.0 exp(?4780 K/T) + 6.1 × 10²⁴ T^?3.0 exp(?1980 K/T) cm⁶ mol^?2 s^?1 at T = 300–2500 K; k₅₆ = 1.36 × 10¹¹ exp(?3420 K/T) cm³ mol^?1 s^?1 at T = 970–1150 K. Computer simulations of typical aircraft engine environments, using the reaction mechanism with the above k_31,0 and k₅₆ expressions, gave the maximum S(IV) to S(VI) conversion yield of ca. 3.5% and 2.5% for the constant density and constant pressure flow condition, respectively. Moreover, maximum conversions occur at rather higher temperatures (～1200 K) than that where the maximum k_31,0 value is located (～800 K). This is because the conversion yield is dependent upon not only the k_31,0 and k₅₆ values (production flux) but also the availability of H, O, and HO₂ in the system (consumption flux). © 2010 Wiley Periodicals, Inc. *

1 This article is a U.S. Government work and, as such, is in the public domain of the United States of America.

Int J Chem Kinet 42: 168–180, 2010     

Pressure dependence of the reaction Cl + C2H2 over the temperature range 230 to 370 K

The pressure dependence of reaction (1), Cl + C₂H₂ + M → C₂H₂Cl + M, has been measured by a relative rate technique using the pressure independent abstraction reaction (2), Cl + C₂H₆ → C₂H₅ + HCl, as the reference. Values of k₁/k₂ were measured at pressures between 25 and 1300 torr at four temperatures ranging from 252 to 370 K, using air, N₂, or SF₆ diluent gases. Low pressure measurements (10–50 torr) were performed at 230 K. Assuming a temperature-independent center broadening factor of 0.6 in the Troe formalism and using the established value of k₂, these data can be used to determine the temperature dependent high and low pressure limiting rate constants over the range of conditions studied in air for reaction (1): k_∞(1) = 2.13 × 10^?10 (T/300)^?1.045 cm³/molecule-s; and k₀(1) = 5.4 × 10^?30 (T/300)^?2.09 cm⁶/molecule²-s. Use of these expressions yields rate constants with an estimated 20% accuracy including uncertainty in the reference reaction. The data indicate that the rate constant for a typical stratospheric condition at 30 km altitude is approximately 50% of that previously estimated.     

Kinetics of the reaction HO2 + NO2(+M) = HO2NO2 using molecular modulation spectrometry

Rate constants for the reaction HO₂ + NO₂(+ M) = HO₂NO₂(+ M) have been obtained from direct observations of the HO₂ radical using the technique of molecular modulation ultraviolet spectrometry. HO₂ was generated by periodic photolysis of Cl₂ in the presence of excess H₂ and O₂, and k₁ was determined from the measured concentrations and lifetime of HO₂ with NO₂ present. k₁ increased with pressure in the range of 40–600 Torr, and a simple energy transfer model gave the following limiting second- and third-order rate constants at 283 K: k₁^∞ = 1.5 ± 0.5 × 10^?12 cm³/molec·sec and k₁^III = 2.5 ± 0.5 × 10^?31 cm⁶/molec·sec. The ultraviolet absorption spectrum of peroxynitric acid was also recorded in the range of 195–265 nm; it showed a broad feature with a maximum at 200 nm, σ_max = 4.4 × 10^?18 cm².     

Transient oxygen atom yields in H2?O2 ignition and the rate coefficient for O + H2 → OH + H

Time-resolved measurements of the oxygen atom concentration during shock-wave initiated combustion of low-density (25 ≤ p ≤ 175 kPa) H₂? O₂? CO? CO₂? Ar mixtures have been made by monitoring CO + O → CO₂ + hv (3 to 4 eV) emission intensity, calibrated against partial equilibrium conditions attained promptly at H₂:O₂ = 1. Significant transient excursions (“spikes”) of [O] above constant-mole-number partial-equilibrium levels were found from 1400 to 2000°K for initial H₂:O₂ ratios of 16 and 10 and below ± 1780°K for H₂:O₂ = 6; they did not occur in this range for H₂:O₂ ± 4. Numerical treatment of the H₂? O₂? CO ignition mechanism for our conditions showed [O] to follow a steady-state trajectory governed by large production and consumption rates from the reactions with a pronounced maximum in the production term k_a[H][O₂]. The measured spike concentration data determine k_b/k_a = 3.6 ± 20%, independent of temperature over 1400 ≤ T ≤ 1900°K, which with well-established k_a data yields This result reinforces the higher of several recent combustion-temperature determinations, and its correlation with results below 1000°K produces a distinctly concave upward Arrhenius plot which is closely matched by BEBO transition state calculations.     

FTIR study of the reaction of ethyl radicals with O2 and Cl2 in air at 295 K

Using Fourier transform infrared spectroscopy, the ethene yield from the reaction of C₂H₅ radicals with O₂ has been determined to be 1.50 ± 0.09%, 0.85 ± 0.11%, and <0.1% at total pressures of 25, 50, and 700 torr, respectively. Additionally, the rate constant of the reaction of C₂H₅ radicals with molecular chlorine was measured relative to that with molecular oxygen. (1) A ratio k₆/k₇ = 1.99 ± 0.14 was measured at 700 torr total pressure which, together with the literature value of k₇ = 4.4 × 10^?12 cm³ molecule^?1s^?1, yields k₆ = (8.8 ± 0.6) × 10^?12 cm³ molecule^?1s^?1. Quoted errors represent 2σ. These results are discussed with respect to previous kinetic and mechanistic studies of C₂H₅ radicals.     

The oxidation of HI at low temperatures and the heat of formation of HO2

The kinetics of the oxidation of hydrogen iodide (HI + O₂) at low temperature (414–499 K) in the gas phase by the method of iodination kinetics is complicated by a heterogeneous reaction between hydrogen iodide and oxygen. Present work leads to an upper limit for the bimolecular rate constant k₁ for the first and rate-determining step (1) These data are combined with an estimated A factor A₁ = 10^9.3±0.2 L/mol·s (assuming a tight linear I···H···O— transition state), to calculate the lower limit of the activation energy for the forward reaction E₁. This leads to a minimum value for the heat of formation of the HO₂ radical, ΔH_f298^°(HO₂) < 3.0 kcal/mol.