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     

The rate constant of NO + O3 → NO2 + O2 in the temperature range of 283–443 K

The reaction NO + O₃ → NO₂ + O₂ has been studied in a 220-m³ spherical stainless steel reactor under stopped-flow conditions below 0.1 mtorr total pressure. Under the conditions used, the mixing time of the reactants was negligible compared with the chemical reaction time. The pseudo-first-order decay of the chemiluminescence owing to the reaction of ozone with a large excess of nitric oxide was measured with an infrared sensitive photomultiplier. One hundred twenty-nine decays at 18 different temperatures in the range of 283–443 K were evaluated. A weighted least-squares fit to the Arrhenius equation yielded k = (4.3 ± 0.6) × 10^?12 exp[-(1598 ± 50)/T] cm³/molecule sec (two standard deviations in brackets). The Arrhenius plot showed no curvature within experimental accuracy. Comparison with recent results of Birks and co-workers, however, suggests that a nonlinear fit, as proposed by these authors, is more appropriate over an extended temperature range.     

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.     

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.     

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.     

High-pressure range of the recombination O + O2 → O3

The recombination reaction O + O₂ → O₃ was studied by laser flash photolysis of pure O₂ in the pressure range 3–20 atm, and of N₂O? O₂ mixtures in the bath gases Ar, N₂, (CO₂, and SF₆) in the pressure range 3–200 atm. Fall-off curves of the reaction have been derived. Low-pressure rate coefficients were found to agree well with literature data. A high-pressure rate coefficient of k_∞ = (2.8 ± 1.0) × 10^?12 cm³ molecule^?1 s^?1 was obtained by extrapolation.     

A study of the reaction NO3 + NO2 + M → N2O5 + M(M = N2, O2)

The gas-phase reaction of the NO₃ radical with NO₂ was investigated, using a flash photolysis-visible absorption technique, over the total pressure range 25–400 Torr of nitrogen or oxygen diluent at 298 ± 2 K. The absolute rate constants determined (in units of 10^?13 cm³ molecule^?1 s^?1) at 25, 100, and 400 Torr total pressure were, respectively, (4.0 ± 0.5), (7.0 ± 0.7), and (10 ± 2) for M = N₂ and (4.5 ± 0.5), (8.0 ± 0.4), and (8.8 ± 2.0) for M = O₂. These data show that the third-body efficiencies of N₂ and O₂ are identical, within the error limits, and that previous evaluations for M = N₂ are applicable to the atmosphere. In addition, upper limits were determined for the rate constants of the reactions of the NO₃ radical with methanol, ethanol, and propan-2-ol of ?6 × 10^?16, ?9 × 10^?16, and ?2.3 × 10^?15 cm³ molecule^?1 s^?1, respectively, at 298 ± 2 K.     

Temperature and third-body dependence of the rate constant for the reaction O + O2 + M → O3 + M

The flash photolysis resonance fluorescence technique was used to measure the rate constants of the reaction O + O₂ + M → O₃ + M (M = N₂, O₂, Ar, and He) as a function of temperature. The results for the rate constants are given by The activation energies with N₂, O₂, and Ar as third bodies are equal within the experimental error, (?1370 → 340 cal/mol), and the relative third-body efficiencies at 298 K for N₂, O₂, Ar, and He are 1.00, 0.99, 0.69, and 0.60, respectively.     

Rate constant of OH + OCS reaction over the temperature range 255–483 K

The rate constant of the reaction between OH and OCS in helium over the temperature range 255–483 K has been determined using the discharge flow-resonance fluorescence technique. The OCS has been carefully purified to avoid interference from H₂S and CO impurities. An FTIR with a multireflection cell was used to determine the impurity concentrations and the purified sample was found to contain less than 0.005% of H₂S. At 300 K, the rate constant was determined to be (2.0 ±^0.4_0.8) × 10^?15 cm³ molecule^?1 s^?1. Although the rate constants showed slight positive deviation at lower temperatures, thev can be satisfactorily fitted by the Arrhenius equation, k = 1.13 × 10^?13 exp(?1200/T) cm³ molecule^?1 s^?1. No pressure dependence was observed at all temperatures, nor was O₂ enhancement observed under our experimental conditions.     

Rate constant for the reaction Br + O3 → BrO + O2 from 248 to 418 K: Kinetics and mechanism

The rate constant for the Br + O₃ → BrO + O₂ reaction was measured by the discharge flow technique, employing resonance fluorescence detection of Br. Over the temperature range 248 to 418 K, in 1 to 3 torr of He, decays of Br in excess O₃ yield the value k₁ = (3.28 ± 0.40) × 10^?11 e^[?944±30]/T cm³ molecule^?1 s^?1. Cited uncertainties are at the 95% confidence level and include an estimate of the systematic errors. The rate constants for the reactions of O₃ with Br, Cl, F, OH, O, and N correlate with the electron affinities of the radicals suggesting that the reactions proceed through early transition states dominated by transfer of electron density from the highest occupied molecular orbital of ozone to the singly occupied radical MO. The implications of this new measurement of k₁ for stratospheric chemistry are discussed.     

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.     

Measurements of the bimolecular rate constants for S + O2 → SO + O and CS2 + O2 → CS + SO2 at high temperatures

The bimolecular reactions in the title were measured behind shock waves by monitoring the O-atom production in COS? O₂? Ar and CS₂? O₂? Ar mixtures over the temperature range between 1400 and 2200 K. A value of the rate constant for S + O₂ → SO + O was evaluated to be (3.8 ± 0.7) × 10¹² cm³ mol^?1 s^?1 between 1900 and 2200 K. This was connected with the data at lower temperatures to give an expression k₂ = 10^10.85 T^0.52 cm³ mol^?1 s^?1 between 250 and 2200 K. An expression of the rate constant for CS₂ + O₂ → CS + SO₂ was obtained to be k₂₁ = 10^12.0 exp(?32 kcal mol^?1/RT) cm³ mol^?1 s^?1 with an error factor of 2 between 1500 and 2100 K.     

Measurement of the rate coefficient of the reaction CH+O2 → products in the temperature range 2200 to 2600 K

The rate coefficient of the reaction CH+O₂ → products was determined by measuring CH-radical concentration profiles in shock-heated 100–150 ppm ethane/1000 ppm O₂ mixtures in Ar using cw, narrow-linewidth laser absorption at 431.131 nm. Comparing the measured CH concentration profiles to ones calculated using a detailed kinetics model, yielded the following average value for the rate coefficient independent of temperature over the range 2200–2600 K: The experimental conditions were chosen such that the calculated profiles were sensitive mainly to the reactions CH+O₂ → products and CH₃+M → CH+H₂+M. For the methyl decomposition reaction channel, the following rate-coefficient expression provided the best fit of the measured CH profiles: Additionally, the rate coefficient of the reaction CH₂+H→CH+ H₂ was determined indirectly in the same system: © 1997 John Wiley & Sons, Inc.     

O(3P) + HOONO2 → products: Temperature-dependent rate constant

The title reaction was studied in a stirred-flow reactor at six temperatures ranging from 228 to 297 K and for pressures near 2 torr. The experiments were performed under O-atom-rich conditions, and the HOONO₂ concentration was monitored with a modulated molecular-beam mass spectrometer. O-atom concentrations were measured by titration with NO₂ and by monitoring the portion of O₂ dissociated in the microwave discharge. A weighted least-squares analysis gives (k ± 1σ) = (7.0 ± 12.2) × 10^?11 exp(-3369 ± 489/T) cm³/s, where the uncertainties are 1 standard deviation (six temperatures) the covariance was σ_AB = 5.97 × 10^?8. Due to the possible presence of systematic errors, the uncertainty in the rate constant could be as great as a factor of 2 over the entire temperature range.     

Determination of the rate constant for the OH(X2Pi) + OH(X()Pi) --> O(3P) + H2O reaction over the temperature range 293-373 K

The rate constant for the reaction OH(X2Pi) + OH(X2Pi) --> O(3P) + H2O has been measured over the temperature range 293-373 K and pressure range 2.6-7.8 Torr in both Ne and Ar bath gases. The OH radical was created by 193 nm laser photolysis of N2O to produce O(1D) atoms that reacted rapidly with H2O to produce the OH radical. The OH radical was detected by quantitative time-resolved near-infrared absorption spectroscopy using Lambda-doublet resolved rotational transitions of the first overtone of OH(2,0) near 1.47 microm. The temporal concentration profiles of OH were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of residuals between the experimental profiles and the model calculations. At 293 K the rate constant for the title reaction was found to be (2.7 +/- 0.9) x 10(-12) cm(3) molecule(-1) s(-1), where the uncertainty includes an estimate of both random and systematic errors at the 95% confidence level. The rate constant was measured at 347 and 373 K and found to decrease with increasing temperature.     

Experimental measurement of the rate of the reaction o(3P) + H2(v = 0) → OH(v = 0) + H at T = 298 K

Measurement of the rate of the reaction is reported. The measurements were made in a flow tube apparatus. The result is based on data for the absolute density of OH(v = 0) obtained from laser-induced fluorescence measurements in the (0–0) band of the OH(A²Σ⁺ → X²II) system. The density of oxygen atoms was varied by changing the flow rate of NO which is consumed in the reaction N + NO → O + N₂. We find that k₁ (298 K) = (5.5 ± 3.0) × 10⁶ cm³/mol sec. This result was obtained with consideration and control of the effect of reaction (2): for which vibrationally excited hydrogen is created by energy transfer in the presence of active nitrogen. It was found that the addition of N₂ or CO₂ effectively suppressed the excitation of H₂(v = 1). Measurements of the density of H₂(v = 1) were made by VUV absorption in the Lyman band system of H₂. All of the reports of low-temperature measurements and some recent theoretical calculations for k₁ are discussed. The present result confirms and extends the growingevidence for significant curvature in the low-temperature end of a modified Arrhenius plot of k₁ (T).     

Kinetic study of the recombination reaction OD + NO2 + M → DNO3 + M

Rate constants for the recombination reaction OD + NO₂ + M → DNO₃ + M have been determined in the falloff region (5–500 torr) and at 297 ± 2 K, in the presence of He, N₂, and SF₆ as third bodies, by using a pulsed laser photolysis-resonance absorption technique. Values of k₀, k_x and the falloff parameter F_c have been estimated. Our rate constants were, within the experimental uncertainty, the same as those reported for the reaction of OH radicals with NO₂.     

Classical dynamics of the O + ClO → Cl + O2 and Cl + O3 → ClO + O2 reactions

An explicit function has been derived for the potential-energy surface of the ground state of ClO₃ with the six interatomic distances as variables. This surface is valid over all configurations of the atoms. The surface has been used to calculate classical trajectories for the reactions R₁: O(³P₂)+ClO(²Π_3/2)→ O₂(³∑)+Cl(²)P_3/2 and R₂: Cl(²P_3/2)+O₃(¹A₁)→ClO(²Π_3/2)+O₂(³∑). An appreciable fraction (～1/3) of the reactive trajectories for R₁ go through a long-lived complex ClOO(²A″). The cross section decreases with increasing rotational state of the ClO; and 37% of the energy release is vibrational. The calculated rate constant at 298°K is 2.6 × 10^?11 cm³/molecule sec. For reaction R₂ there is no evidence of long-lived complexes. The product ClO is predominantly found in the backward-scattering direction. Most of the internal energy is carries off by ClO but O₂ carried off most translational energy. An Arrhenius expression has been deduced from calculations at 220 and 300°K to give an A factor of 2.488 × 10^?11 cm³/molecule sec and an activation energy of 1.543 kJ/mol.     

A reinvestigation of the temperature dependence of the rate constant for the reaction O + O2 + M → O3 + M (for M = O2, N2, and Ar) by the flash photolysis resonance fluorescence technique

The flash photolysis resonance fluorescence technique has been used to reinvestigate the kinetics of the oxygen atom–oxygen molecule combination reaction. Third-order rate constants for O₂, N₂, and Ar as deactivant molecules were determined over the temperature range of 219–368 K. The results presented herein are the most extensive data sets available for atmospheric modeling and are used to formulate a recommendation for such purposes. The recommended rate expressions are or Comparisons of these results with existing literature data are presented.     

K2[Hg(SO3)2]·2.25H2O

The characteristic feature of the structure of the title compound, dipotassium bis(sulfito‐κS)mercurate(II) 2.25‐hydrate, is a layered arrangement parallel to (001) where each of the two independent [Hg(SO₃)₂]²⁻ anions are grouped into centrosymmetric pairs and are surrounded by two K⁺ cations to give the overall layer composition {K₂[Hg(SO₃)₂]₂}²⁻. The remaining cations and the uncoordinated water molecules are situated between these layers. Within the [Hg(SO₃)₂]²⁻ anions, the central Hg atoms are twofold coordinated by S atoms, with a mean Hg—S bond length of 2.384 (2) Å. The anions are slightly bent [174.26 (3) and 176.99 (3)°] due to intermolecular O...Hg interactions greater than 2.8 Å. All coordination polyhedra around the K⁺ cations are considerably distorted, with coordination numbers ranging from six to nine. Although the H atoms of the five water molecules (one with symmetry 2) could not be located, O...O separations between 2.80 and 2.95 Å suggest a system of medium to weak O—H...O hydrogen bonds which help to consolidate the structural set‐up. Differences and similarities between the bis(sulfito‐κS)mercurate(II) anions in the title compound and those in the related salts (NH₄)₂[Hg(SO₃)₂] and Na₂[Hg(SO₃)₂]·H₂O are discussed.