The reaction of atomic oxygen O(3P) with isobutane

The reaction of O(³P) atoms with isobutane has been studied by using the discharge-flow system described previously [1]. The rate constant was measured from determinations of the isobutane concentration in the presence of an excess of O atoms and is given by k₁ = (7.9 ± 1.4) × 10⁷ dm³/mol·s at 307 K. In order to explain the observed reaction products, the mechanism requires that the principal process be the successive abstraction of H atoms from isobutane and from the t-butyl radical to give isobutene. A minor part of the reaction between O(³P) and the t-butyl radical gives the t-butoxy radical, which decomposes to acetone. The branching ratios are .     

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.     

The reactions of O(3P) with ozone and carbonyl sulfide

The competitive reactions between 2-trifluoromethylpropene (TMP) and OCS for O(³P) atoms were studied between 300° and 523°K, using the mercury-senstitized photolysis of N₂O as a source of O(³P). From the known value for the rate constant of the O(³P) + TMP reaction, k₃ was found to be 1.6 × 10^?11 exp (?4500/RT) cm³/particle-sec, where reaction (3) is Mixtures of O₃ and OCS were photolyzed at 197°, 228°, 273°, and 299°K with radiation above 4300 Å to produce O(³P) from the photolysis of O₃, and thus study the competition between reaction (3) and From the above value of k₃, k₁ could be computed. When combined with all the previous data, the best espression for k₁ is k₁ = 1.2 × 10^?11 exp (?4300/RT) cm³/particle-sec.     

Absolute rate constants for the reactions of O(3P) atoms with DCl and DBr

Earlier work on the reactions of O(³P) atoms with HCl and HBr has been extended by measuring rate constants for A flow-tube method was used with chemiluminescent monitoring of the removal of atomic oxygen. Rate constants were measured at temperatures between 340 and 489 K for (2a) and 295 and 419 K for (2b); they can be matched by the Arrhenius expressions: where the units are cm³ molecule^?1 sec^?1 and the errors correspond to a single standard deviation. The results of a quasiclassical trajectory study of collisions of O(³P) with HCl (v = 0,1, and 2) and DCl (v= 0,1, and 2) are also reported. These strengthen the conclusion that, although the rates of reactions (1a) and (2a) are selectively enhanced by vibrationally exciting HCl or DCl, molecules with 0 < v ? 2 are mainly removed in collisions with O(³P) atoms by nonreactive relaxation.     

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.     

The kinetics of the reactions of O(3P) atoms with dimethyl ether and methanol

The kinetics of the reaction of O + CH₃OCH₃ were investigated using fast-flow apparatus equipped with ESR and mass-spectrometric detection. The concentration of O(³P) atoms to CH₃OCH₃ was varied over an unusually large range. The rate constant for reaction was found to be k = (5.0 ± 1.0) × 10¹² exp [(?2850 ± 200/RT)] cm³ mole^?1 sec^?1. The reaction O + CH₃OH was studied using ESR detection. Based on an assumed stoichiometry of two oxygen atoms consumed per molecule of CH₃OH which reacts, we obtain a value of k = (1.70 ± 0.66) × 10¹² exp [(?2,280 ± 200/RT)] cm³ mole^?1 sec^?1 for the reaction The results obtained in this study are compared with the results from other workers on these reactions. The observation of essentially equal activation energies in these two reactions is indicative of approximately equal C? H bond strengths in CH₃OCH₃ and CH₃OH. This is in agreement with recent measurements of these bond energies.     

Kinetics and mechanism of the gas-phase reaction of O(3P) atoms with acetone

The kinetics of the reaction of O(³P) atoms with acetone were investigated using fast flow methods. The reaction was studied over a temperature range of 298 to 478°K. The specific rate constant obtained was (1.9 ± 0.4) × 10¹² exp(—5040 ± 180/1.987 T) cm³/mol·sec. The observation of a sizable primary H/D kinetic isotope effect in comparing rates of CH₃COCH₃ and CD₃COCD₃ led to the conclusion that the major reaction channel involves H atom abstraction, namely, The rather low Arrhenius preexponential factor obtained in this reaction is compared and contrasted with those reported for other reactions of O(³P) with low molecular weight compounds.     

A flash photolysis–resonance fluorescence kinetics study of ground-state sulfur atoms: I. Absolute rate parameters for reaction of S(3P) with O2(3Σ).

The flash photolysis–resonance fluorescence technique has been used to measure the reaction of ground-state sulfur atoms with molecular oxygen as a function of both temperature and total pressure. The most suitable source of S(³P) for this study was found to be COS in the presence of CO₂, as a diluent gas and with the photolysis flash filtered so as to remove all radiation of wavelengths below 1650 Å. Under these conditions, it was found that over the temperature range of 252–423°K the rate data could be fit to a simple Arrhenius-type equation of the form Units are cm³ molec^?1 s^?1. The small A-factor for this reaction, the lack of any pressure dependence, and the direct observation of the production of O(³P) with increasing reaction time suggest that the S(³P) atom attacks the O₂(³Σ) molecule end-on forming SOO which rapidly falls apart to form SO (³Σ) and O(³P).     

Absolute rate constants for the reactions O(3P) atoms with HCl and HBr

A flow tube method has been used to determine rate constants for the elementary reactions: Oxygen atoms were produced by adding a small excess of NO to a stream of partially dissociated nitrogen, and their reaction with hydrogen halide was monitored by observing the intensity of the NO + O afterglow. Experiments were carried out at temperatures from 293 to 440°K with HCl, and from 267 to 430°K with HBr. The role of secondary reactions was minimised and the residual effects were allowed for. The rate constants for the primary reactions could be matched by Arrhenius expressions: where the units are cm³/molec·sec and the errors correspond to a standard deviation.     

A study of the mechanism and kinetics of the reaction of O(3P) atoms with propane

The mechanism and kinetics of the reaction of O(³P) atoms with propane were investigated using molecular modulation spectroscopy, with the O(³P) atoms being generated by the Hg photosensitized decomposition of N₂O. The absorption spectrum of the X²II_3/2 state of OH was observed in the ultraviolet between 307 and 309 nm, and it was confirmed that OH was the product of the O(³P) reaction with propane. The rate constants for the reactions of O(³P) and OH with propane were determined to be 3.9±0.7±10¹⁰ and 1.19±0.05±10¹² cm³/mole·sec, respectively, at T=56±5°C.     

The oxidation of CFClCFCl and CF2CCl2

The oxidation of CFClCFCl and CF₂CCl₂ were studied at room temperature by chlorine- and oxygen-atom initiation. The chlorine-atom initiated oxidation of CFClCFCl yields CCl₂FCF(O) as the exclusive product. Its quantum yield is ～420, which gives k_3a/k_3b=210 where reactions (3a) and (3b) are The O(³P)? CFClCFCl reaction gives CClFO with a quantum yield of 0.80, polymer, and small amounts of an unidentified product which is probably cyclo-(CFCl)₃. Thereaction paths are with k_9a/k₉=0.80. The overall reaction of O(³P) with CFClCFCl proceed one fifth as fast as the O(³P)-C₂F₄ reaction. When O₂ is also present, the same free-radical chain oxidation occurs by O(³P)initiation as by chlorine-atom initiation. The chlorine-atom initiated oxidation of CF₂CCl₂ gives CF₂ClCCl(O) as the major product, with quantum yields ranging from 42 to 85. Smaller amounts of CF₂O and CCl₂O are produced in equal amounts with quantum yields of ～3.5. The reactions responsible for the products are The O(³P)-CF₂CCl₂interaction yields CF₂O and with quantum yields of 1.0 and ～0.85, respectively. In thepresence of O₂ the radical chain products are observed, but the mechanism is different than that for other chloroolefins.     

The photolysis of N2O at 2139 Å and 1849 Å

The method of chemical difference was utilized to accurately determine the relative importance of all the reaction steps in the direct photolysis of N₂O at 2139 Å (25° and 250°C) and 1849 Å (25° C), as well as in the Hg6(¹P₁)-sensitized photolysis of N₂O at 1849 Å (25°C). In all cases, the primary process is predominantly, if not exclusively, Experiments with trace amounts of C₃H₆ added showed a slight, but not significant, difference in product ratios (N₂ and O₂). From these experiments the quantum yield of O(³P) from all possible sources was estimated as 0.02 ± 0.02. Experiments with excess N₂ at 1849 Å indicated that O(¹S) was not produced in the direct photolysis. The O(¹S) yield is probably zero, and certainly <0.05. The O(¹D) atom can react with N₂O via The ratio k₂/k₃ was found to be 0.69 ± 0.05 in all cases. When combined with other data from our laboratory, the average value is 0.65 ± 0.07. This represents the value for translationally energetic O(¹D) atoms. When excess He was added to remove the excess translational energy, k₂/k₃ rose to 0.83 ± 0.06, which is in reasonable agreement with the value of 1.01 ± 0.06 found in another laboratory. We conclude that for O(¹D) atoms with no excess thermal energy, k₂/k₃ = 0.90 ± 0.10.     

The reaction of OH with NO2 and the deactivation of O(1D) by CO

NO₂ was photolyzed with 2288 Å radiation at 300° and 423°K in the presence of H₂O, CO, and in some cases excess He. The photolysis produces O(¹D) atoms which react with H₂O to give HO radicals or are deactivated by CO to O(³P) atoms The ratio k₅/k₃ is temperature dependent, being 0.33 at 300°K and 0.60 at 423°K. From these two points, the Arrhenius expression is estimated to be k₅/k₃ = 2.6 exp(?1200/RT) where R is in cal/mole – °K. The OH radical is either removed by NO₂ or reacts with CO The ratio k₂/k^α is 0.019 at 300°K and 0.027 at 423°K, and the ratio k₂/k⁰ is 1.65 × 10^?5M at 300°K and 2.84 × 10^?5M at 423°K, with H₂O as the chaperone gas, where k^α = k₁ in the high-pressure limit and k⁰[M] = k₁ in the low-pressure limit. When combined with the value of k₂ = 4.2 × 10⁸ exp(?1100/RT) M^?1sec^?1, k^α = 6.3 × 10⁹ exp (?340/RT)M^?1sec^?1 and k⁰ = 4.0 × 10¹²M^?2sec^?1, independent of temperature for H₂O as the chaperone gas. He is about 1/8 as efficient as H₂O.     

Kinetics of fluorine atom reactions using resonance absorption spectrometry in the far vacuum ultraviolet reactions F + HCl,CH4, CHCl3 CHCl2F,and CHClF2

Atomic resonance absorption spectrometry with a nonreversed fluorine resonance lamp (～95 nm) has been used to study the kinetics of elementary reactions of ground state F²P_J atoms in a discharge-flow system. The following rate constants (in cm³/molec·sec)

1 All rate constants are given with 1.5 σ.

were determined at 298° K: The reaction F ₂P_J + HCl(1) was found to give J-excited Cl ₂P_1/2 atoms with a product branching ratio [Cl ₂P_1/2]/[Cl ₂P_3/2] = 0.10.     

Some Reactions of OH(v = 1)

Reactions of OH(v = 1) with HBr, O, and CO have been studied at 295°K using a fast discharge flow apparatus: The reaction O + HBr → OH(v = 1) + Br was used as a source of OH(v = 1), and subsequent chemical reactions of the excited radical were followed using EPR spectroscopy. Rate constants for reactions (2b), (3b), and (6b) were measured as (4.5 ± 1.3) × 10^?11, (10.5 ± 5.3) × 10^?11, and <5 × 10^?12 cm³/molec·sec, respectively. The rate constant for physical deactivation of OH(v = 1) by CO was determined as <4 × 10^?13 cm³/molec·sec.     

Oxidation of S and SO by O2 in high-temperature pyrolysis and photolysis reaction systems

The reaction of S atoms with O₂ was studied behind reflected shock waves applying atomic resonance absorption spectroscopy (ARAS) for concentration measurements of S and O atoms. S atoms were generated either by laser-flash photolysis (LFP) of CS₂ or by the high-temperature pyrolysis of COS, respectively. The concentrations of O₂ in the mixtures ranged between 50 ppm and 400 ppm, and those of the S precursors, CS₂ and COS, between 5 and 25 ppm. The rate coefficient of the reaction was determined from the observed decay of the S absorption signals for temperatures 1220 K ? T ? 3460 K. The measured O-atom concentration profiles in COS/O₂/Ar reaction systems were evaluated, using simplified kinetic mechanism, to verify the given rate coefficient k₅. In experiments with the highest value of the [O₂]/[COS] ratio the measured O-atom concentrations were found to be sensitive to the reaction: The fitting of the calculated O-atom profiles to the measured ones results in mean value of: which is to be valid for the temperature range 2570 K ? T ? 2980 K. A first-order analysis of the observed S absorption decay in LFP shock wave experiments on CS₂/Ar gas mixtures resulted in a rate coefficient of the background reaction (R4): for temperatures 1260 K ? T ? 1820 K. © 1995 John Wiley & Sons, Inc.     

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     

Reaction of methoxy radicals with oxygen. I. Using dimethyl peroxide as a thermal source of methoxy radicals

Using dimethyl peroxide as a thermal source of methoxy radicals overthe temperature range of 110–160°C, and the combination of methoxy radicals and nitrogen dioxide as a reference reaction: a value was determined of the rate constant for the reaction of methoxy radicals with oxygen: is independent of nitrogen dioxide or oxygen concentration and added inert gas (carbon tetrafluoride). No heterogeneous effects were detected. The value of k₄ is given by the expression In terms of atmospheric chemistry, this corresponds to a value of 10^5.6 M^?1·sec^?1 at 298 K. Extrapolation to temperatures where the combustion of organic compounds has been studied (813 K) produces a value of 10^7.7 M^?1·sec^?1 for k₄. Under these conditions, reaction (4) competes with hydrogen abstraction or disproportionation reactions of the methoxy radical and its decomposition (3): In particular k₃ is in the falloff region under these conditions. It is concluded that reaction (4) takes place as the result of a bimolecular collision process rather than via the formation of a cyclic complex.     

The kinetics of the reaction of O(3P) and D atoms with cyclohexane

The kinetics of the reactions of O(³P) and D atoms with cyclohexane have been investigated using fast-flow techniques. The rates of reaction were computed by monitoring changes in both atom and cyclohexane concentrations using electron spin resonance and mass spectrometric methods, respectively. The O(³P) + C₆H₁₂ reaction was studied over a temperature range of 344 to 513°K and we obtain a specific rate constant of (3.2±0.6) × 10¹⁴ exp (?4400±400/RT) cm³/mole˙sec for this reaction. The only reaction product detectable mass spectrometrically under flow conditions of excess oxygen atoms is formaldehyde. The D + C₆H₁₂ reaction was studied over a temperature range of 297 to 596°K. A specific rate constant of (4.1±1.0) × 10¹³ exp (?4000±300/RT) cm³/mole˙sec was obtained for this reaction. On the basis of the results obtained in these studies, the important primary process in both the O(³P) and D atom reactions is concluded to be abstraction of a hydrogen atom from the cyclohexane molecule.