Study of the HNO + HNO and HNO + NO reactions by intracavity laser spectroscopy

Flash photolysis of CH₃CHO and H₂CO in the presence of NO has been investigated by the intracavity laser spectroscopy technique. The decay of HNO formed by the reaction HCO + NO → HNO + CO was studied at NO pressures of 6.8–380 torr. At low NO pressure HNO was found to decay by the reaction HNO + HNO → N₂O + H₂O. The rate constant of this reaction was determined to be k₁ = (1.5 ± 0.8) × 10^?15 cm³/s. At high NO pressure the reaction HNO + NO → products was more important, and its rate constant was measured to be k₂ = (5 ± 1.5) × 10^?19 cm³/s. NO₂ was detected as one of the products of this reaction. Alternative mechanisms for this reaction are discussed.     

Reactions of ozone with olefins: Ethylene,allene, 1,3-butadiene,and trans- 1,3-pentadiene

The reactions of O₃ with ethylene, allene, 1,3-butadiene, and trans-1,3-pentadiene have been studied in the presence of excess O₂ over the temperature range 232 to 298 K. The initial O₃ pressure was varied from 4–18 mtorr, and the olefin pressure was varied from 0.1 to 4.5 torr (ethylene), 2.8 to 39.6 torr (allene), 52.7 to 600 mtorr (1,3-butadiene) or 26.2 to 106 mtorr (trans-1,3-pentadiene). The O₃ decay was monitored by ultraviolet absorption. The reactions are first order in both O₃ and olefin, and the rate coefficients are independent of the O₂ pressure. For the O₃-ethylene system, various diluent gases (O₂, N₂, air) were used and the rate coefficients were found to be independent of the nature of the diluent gas. The various rate coefficients fit the Arrhenius expressions (k in cm³ s^?1): where the reported uncertainties are one standard deviation and R is in cal/mol K.     

Reactions of NO3-naphthalene adducts with O2 and NO2

The kinetics of the gas-phase reaction of the NO₃ radical with naphthalene have been investigated at 150 torr O₂ + 590 torr N₂ and 600 torr O₂ + 140 torr N₂ at 298 ± 2 K. Relative rate measurements were carried out in reacting NO₃? N₂O₅-naphthalene-propene-O₂? N₂ mixtures by longpath Fourier transform infrared absorption spectroscopy. A rate constant ratio for the reactions of O₂ and NO₂ with the NO₃-naphthalene adduct of k/k < 4 × 10^?7 was obtained from the competition between O₂ and NO₂ for reaction with the NO₃-naphthalene adduct and thermal decomposition of the adduct back to reactants. Atmospheric pressure ionization MS/MS measurements of the nitronaphthalene products of the NO₃ radical-initiated reaction of naphthalene are consistent with the proposed reaction mechanism, and the atmospheric implications of the data are discussed. © 1994 John Wiley & Sons, Inc.     

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 reaction of photochemically generated CF3O radicals with CO

CF₃O₂CF₃ was photolyzed at 254 nm in the presence of CO in 760 torr N₂ or air at 296 K in a static reactor. In N₂, the products CF₃OC(O)C(O)OCF₃ and CF₃OC(O)O₂C(O)OCF₃ were detected by FTIR spectroscopy. In air, the only observed products were CF₂O and CO₂ and a chain process, initiated by CF₃O, was invoked for the conversion of CO to CO₂. From both product studies, a mechanism for the CF₃O initiated oxidation of CO was derived, involving the addition reaction CF₃O₂ + CO → CF₃OC(O). The rate constant for the reaction CF₃O + CO at 296 K at a total pressure of 760 torr air was determined to be k(CF₃O + CO) = (5.0 ± 0.9) × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹. © 1997 John Wiley & Sons, Inc.     

Kinetics and mechanisms of the reaction of OH radicals with dimethyl sulfide

The rate constant of the reaction of OH with DMS has been measured relative to OH + ethene in a 420 l reaction chamber at 760 torr total pressure and 298 ± 3 K in N₂ + O₂ buffer gas using the 254 nm photolysis of H₂O₂ as the OH source. In agreement with a recent absolute rate determination of the reaction the measured effective rate constant was found to increase with increasing partial pressure of O₂ in the system, for 760 torr air a rate constant of (8.0 ± 0.5) × 10^?12 cm³ s^?1 was obtained. Product studies have been performed on the reaction in air using FTIR absorption spectrometry for detection of reactants and products. On a molar basis, SO₂ was formed with a yield of 70% and dimethyl sulfone (CH₃SO₂CH₃) with a yield of approximately 20%. These results are considerably different to those obtained in other product studies which were carried out in the presence of NO_x. These differences are compared and their relevance for the atmospheric oxidation mechanisms of DMS is discussed.     

Unusual H2-forming chain reaction in the 3130-Å photolysis of formaldehyde-oxygen mixtures at 25°C

A kinetic study has been made of the 3130-Å photolysis of CH₂O (8 torr) in O₂-containing mixtures (0.02–8 torr) and in the presence of added CO₂ (0–300 torr) at 25°C. Quantum yields of formation of H₂, CO, and CO₂ and the loss of O₂ were measured. Φ and Φ_CO were much above unity. In an explanation of these unexpected results, a new H-atom-forming chain mechanism was postulated involving HO₂ and HO addition to CH₂O: CH₂O + hν → H + HCO (1) H + CH₂O → H₂ + HCO (3) H + O₂ + M → HO₂ + M (6) HCO + O₂ → HO₂ + CO (8) HO₂ + CH₂O → (HO₂CH₂O) → HO + HCO₂H (15) HO + CH₂O → H₂O + HCO? (16); HCO? → H + CO (19) HO + CH₂O → H₂O + HCO (17) and HO + CH₂O → HCO₂H + H (18). When the results are rationalized in terms of this mechanism, the data suggest k₁₆ ? k₁₇ and k₁₆/k₁₈ ? 0.5. The data require that a reassessment of the relative rates of reactions (7) and (8) be made, since in the previous work HCO₂H formation was used as a monitor of the rate of reaction (7) HCO + O₂ + M → HCOO₂ + M (7). The present data from experiments at P = 8 torr and P = 1–4 torr give k₇[M]/(k₇[M] + k₈) ≥ 0.049 ± 0.017. These data coupled with the k₈ estimates of Washida and coworkers give k₇ ≥ (4.4 ± 1.6) × 10¹¹ l²/mol²·sec for M = CH₂O. The reaction sequence proposed here is consistent with the observed deterimental effect of O₂ addition on the laser-induced isotope enrichment in HDCO. In additional studies of CH₂O-O₂-isobutene mixtures it was found that Φ was equal to ?₂ as estimated in O₂-free CH₂O-isobutene mixtures. These results suggest that the increase in CO (ν = 1) product observed with O₂ addition in CH₂O photolysis does not result from perturbations in the fragmentation pattern of the excited CH₂O, but it is likely that it originates in the occurrence of the exothermic reaction HCO + O₂ → HO₂ + CO (ν = 1).     

Gas-phase ozonolysis of cis- and trans-dichloroethylene

Dichloroethylene (DCE), either cis or trans, was reacted with O₃ at 23°C in both N₂ and O₂ buffered mixtures. Both reactant consumption and product formation were monitored by infrared spectroscopy and, in some cases, O₃ consumption was monitored by ultraviolet absorption. For thoroughly dried mixtures, the initial products were only HCClO and O₂, but geometrical isomerization also occurred. The stoichiometry of the overall reaction always was The HCClO was unstable and disappeared slowly in a first-order reaction which was, at least in part, heterogeneous. The products were CO and HCl so that the stoichiometric reaction was The rate law was complex. The rate was always faster in N₂ than in O₂. In the N₂ buffered reaction, inhibition occurred as the reaction progressed and O₂ was produced. From the reactant and product decay curves, the following rate behavior was established: where high and low concentrations are relative terms for the initial pressure ranges covered ([DCE]₀ = 0.21?78.4 torr, [O₃]₀ = 0.30?6.76 torr). The rate coefficients k₂, k₃, and k₄ were larger for the trans-DCE than the cis-DCE, and for each isomer they were larger in N₂ than in O₂ buffered reactions. The ozonolysis can be explained in terms of the mechanism where R₂ is DCE, RO is HCClO, and RO₂ is HCClO₂. Rate ceofficients are computed. The isomerization is first order in [O₃] and approximately first order in [DCE] for the limited kinetic data we were able to obtain. The isomerization does not appear to be explained by the reverse reactions of reactions (6), (7), and (9). Presumably isomerization occurs through some other route.     

Kinetics and mechanism of the homogeneous reaction between some manganese(III)-Schiff base complexes and hydrogen peroxide in aqueous and sodium dodecyl sulphate solutions

Summary The kinetics of the reaction between H₂O₂ and some Schiff base complexes of Mn^III have been investigated in both aqueous and micellar sodium dodecyl sulphate (SDS) solution. The reaction rate is first order in both H₂O₂ and [complex], and inversely proportional to [H⁺]. The second-order rate constant increases in the sequence [Mn(salophen)(OAc)] > [Mn(salen)(OH₂)]-ClO₄ > [Mn(salen)(OAc)]H₂O, where salen = N,N-bis-(salicylidene)ethylenediamine and salophen = N,N-bis-(salicylidene)-o-phenylenediamine. At SDS concentrations below the critical micellar concentration, there is almost no effect on the rate of reaction whereas at higher concentrations the reaction rate increases slightly. A mechanism involving Mn^II and a peroxo intermediate is proposed.     

A spectroscopic study of the equilibrium NO2 + NO3 + M 2 N2O5 + M and the kinetics of the O3/N2O5/NO3/NO2/ air system

The equilibrium constant, K_eq of the reaction NO₂ + NO₃ + M 2 N₂O₅ + M has been determined for a small range of temperatures around room temperature in air at 740 torr by direct spectroscopical measurements of NO₂, NO₃, and N₂O₅. At 298 K, K_eq was determined as (3.73 ± 0.61) × 10⁻¹¹ cm³ molecule⁻¹. Averaging this and 11 other independent evaluations of K_eq yields K_eq = (3.31 ± 0.82) × 10⁻¹¹ cm³ molecule⁻¹, where the uncertainty is given as one standard deviation. The kinetics of the O₃/NO₂/N₂O₅/NO₃/ air system was studied in a static chamber at room temperature and 740 torr total pressure. Evidence of a unimolecular decay reaction of NO₃, NO₃ → NO + O₂, was found and its rate coefficient was estimated as (1.6 ± 0.7) × 10⁻³ s⁻¹ at 295 ± 2 K.     

The kinetics and the mechanism of the thermal decomposition of bis(trifluoromethyl) trioxide. The influence of carbonmonoxide on its decomposition

The kinetics of the thermal decomposition of CF₃O₃CF₃ has been investigated in the pressure range of 15–599 torr at temperatures between 59.8 and 90.3°C and also in the presence of CO between 42 and 7°C. The reaction is homogeneous. In the absence of CO the only reaction products are CF₃O₂CF₃ and O₂. The rate of reaction is strictly proportional to the trioxide pressure, and is not affected by the total pressure, the presence of inert gases, and oxygen. The following mechanism explains the experimental results: In the presence of CO there appear CO₂, (CF₃OCO)₂, and CF₃O₂C(O)OCF₃ as products. With increasing temperature the amount of peroxicarbonate decreases, while the amounts of oxalate and CO₂ increase. The rate of decomposition of the trioxide above a limiting pressure of about 10 torr CO is strictly first order and independent of CO pressure, total pressure, and the pressure of the products. The addition of larger amounts of O₂ to the CO containing system chaqnges the course of the reaction.     

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.     

The thermal decomposition of diimide in the gas phase: Kinetics and stoichiometry

The kinetics and stoiehiometry of the decomposition of N₂H₂ and N₂D₂ have been studied as a function of sample size, pressure, and temperature. The reaction follows a single first order kinetic expression over most of its time course. It is suggested that the rate-determining step in the mechanism is a first-order homogeneous gas-phase isomerization of trans-diimide with rate constants:k = 1.8 exp (-4.2 kcal/mol/RT) sec^?1 and k = 1 exp (-4.4 kcal/mol/RT) sec^?1. The detailed mechanism of this isomerization, however, is not evident. At temperatures above room temperature, self-heating has been observed which leads to an initial fast decay. At room temperature the reaction exhibits autocatalysis with the rate increasing as the reaction proceeds. This has been attributed to enhancement by a surface decay process involving adsorbed hydrazine. The only significant products from the decomposition of N₂H₂ are N₂, H₂, and N₂H₄, and the results are interpreted in terms of two parallel reactions: The decomposition of N₂D₂ occurs almost completely by the single reaction giving N₂ + N₂D₄. No azide formation has been detected from either N₂D₂, or N₂D₂, and limits have been put on the yield of ammonia. Extinction coefficients at 365 nm of 3.9 ± 0.2 for N₂H₂ and 3.3 ± 0.1 for N₂D₂ have been measured. Both the rate of decay and the stoichiometry of products show pressure dependence below 150 torr, and this is suggested to be due to direct decomposition of cis-N₂H₂ on the surface.     

Thermal reduction of NO by H2: Kinetic measurement and computer modeling of the HNO + NO reaction

The kinetics and mechanism of the thermal reduction of NO by H₂ have been investigated by FTIR spectrometry in the temperature range of 900 to 1225 K at a constant pressure of 700 torr using mixtures of varying NO/H₂ ratios. In about half of our experimental runs, CO was introduced to capture the OH radical formed in the system with the well-known, fast reaction, OH + CO → H + CO₂. The rates of NO decay and CO₂ formation were kinetically modeled to extract the rate constant for the rate-controlling step, (2) HNO + NO → N₂O + OH. Combining the modeled values with those from the computer simulation of earlier kinetic data reported by Hinshelwood and co-workers (refs. [3] and [4]), Graven (ref.[5]), and Kaufman and Decker (ref. [6]) gives rise to the following expression: . This encompasses 45 data points and covers the temperature range of 900 to 1425 K. RRKM calculations based on the latest ab initio MO results indicate that the reaction is controlled by the addition/stabilization processes forming the HN(O)NO intermediate at low temperatures and by the addition/isomerization/decomposition processes producing N₂O + OH above 900 K. The calculated value of k₂ agrees satisfactorily with the experimental result. © 1995 John Wiley & Sons, Inc.     

Chemical lasers produced from O(3P) atom reactions. III. 5-μm CO laser emission from the O + CH reaction

Very strong laser emission at 5 μm was detected when SO₂ and CHBr₃ were flash photolyzed in the vacuum ultraviolet (λ ≥ 165 nm) in the presence of a large amount of diluent (SF₆, He, or Ar). About 110 vibration–rotation transitions ranging from Δv = 18 → 17 to 3 → 2, except 16 → 15, were identified. The primary reactions leading to the CO stimulated emission are as follows: The product analysis results and the variation of laser intensity with flash energy and SO concentration indicate that the following side reactions are also occurring. Addition of a small amount of O₂ enhances the laser output by both eliminating these side reactions and simultaneously producing vibrationally excited CO via reaction (8), which has been previously shown to generate CO stimulated emission. The effects of various reactive (NO and H₂) and inert (He, Ar, SF₆, CO, N₂, N₂O, and CO₂) gases have been examined. All additives (P ≤ 20 torr), except NO and H₂, increase the total laser output. N₂O enhances the power most efficiently, whereas CO, N₂, and CO₂ are less effective and have similar efficiencies. The enhancement of the laser intensity by these near-resonant gases is ascribed to the depletion of CO population at lower levels which thus increases the rates cascading from higher levels. NO and H₂ quench the laser output by chemically reducing the concentration of the CH radical.     

Reactions of the copper dimer,Cu2, in the gas phase

Reactions of Cu₂ with several small molecules have been studied in the gas phase, under thermalized conditions at room temperature, in a fast-flow reactor. They fall into one of two categories. Cu₂ does not react with O₂, N₂O, N₂, H₂, and CH₄ at pressures up to 6 torr. This implies bimolecular rate constants of less than 5 × 10^?15 cm³ s^?1 at 6 torr He. Cu₂ reacts with CO, NH₃, C₂H₄, and C₃H₆ in a manner characteristic of association reactions. Second-order rate constants for all four of these reagents are dependent on total pressure. The reactions with CO, NH₃, and C₂H₄ are in their low pressure limit at up to 6 torr He buffer gas pressure. The reaction with C₃H₆ begins to show fall-off behavior at pressures above 3 torr. Limiting low-pressure, third-order rate constants are 0.66 ± 0.10, 8.8 ± 1.2, 9.3 ± 1.4, and 85 ± 15 × 10^?30 cm⁶ s^?1 in He for CO, NH₃, C₂H₄, and C₃H₆, respectively. Modeling studies of these rate constants imply that the association complexes are bound by at least 20 kcal mol^?1 in the case of C₂H₄ and C₃H₆ and at least 25 kcal mol^?1 in the other cases. The implications of these results for Cu-ligand bonding are developed in comparison with existing work on the interactions of these ligands with Cu atoms, larger clusters, and surfaces. © 1994 John Wiley & Sons, Inc.     

Stopped-flow study of the gas-phase reaction of ozone with organic sulfides: Dimethyl sulfide

The autoinhibiting reaction of ozone with dimethyl sulfide (DMS), has been studied at 296°K and 1.1 kPa (8 torr) as a function of the concentrations of both reactants. The major products of the reaction are H₂CO, H₂O, CO, and SO₂. The specific rate of primary attack of O₃ on DMS is immeasurably slow. It is suggested that the rapid overall rate observed for this reaction is due to a chain reaction initiated by the very slow primary reaction. It is concluded that reaction (1) cannot be important under atmospheric conditions and that the major loss process for DMS in the atmosphere is probably reaction with photochemically generated free radicals.     

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.     

Rate constant for the OH + CO reaction: Pressure dependence and the effect of oxygen

The effect of pressure on the rate constant of the OH + CO reaction has been measured for Ar, N₂, and SF₆ over the pressure range 200–730 torr. All experiments were at room temperature. The method involved laser-induced fluorescence to measure steady-state OH concentrations in the 184.9 nm photolysis of H₂O-CO mixtures in the three carrier gases, combined with supplementary measurements of the CO depletion in these same carrier gases in the presence and absence of competing reference reactants. The effect of O₂ on the pressure effect was determined. A pressure enhancement of the rate constant was observed for N₂ and SF₆, but not for Ar, within an experimental error of about 10%. The pressure effect for N₂ was somewhat lower than previous literature reports, being about 40% at 730 torr. For SF₆ a factor of two enhancement was seen at 730 torr. In each case it was found that O₂ had no effect on the pressure enhancement. The roles of the radical species HCO and HOCO were evaluated.     

Absolute rate constant for the reaction of Phenyl radical with Acetylene

The absolute rate constant for the reaction of phenyl radical with acetylene has been measured at 20 torr total pressure in the temperature range of 297 to 523 K using the cavity-ring-down technique. These new kinetic data could be quantitatively correlated with the data obtained earlier with a relative rate method under low-pressure (10^?3–10^?2 torr) and high-temperature (1000–1330 K) conditions. These kinetic data were analyzed in terms of the RRKM theory employing the thermochemical and molecular structure data computed with the BAC-MP4 technique. The calculated results reveal that the total rate constant for the C₆H₅ + C₂H₂ reaction (k_t) is pressure-independent, whereas those for the formation of C₆H₅C₂H (k_b) and the C₆H₅C₂H₂ adduct (k_c) are strongly pressure-dependent. A least-squares analysis of the calculated values for 300–2000 K at the atmospheric pressure of N₂ or Ar can be given by and all in units of cm³/s. The latter equation effectively represents the two sets of experimental data. © 1994 John Wiley & Sons, Inc.