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.     

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      

The reaction of OH with CO

Hydroxyl radicals were prepared from the photolysis of N₂O at 213.9 nm in the presence of excess H₂. The O(¹D) produced in the primary photolytic act reacts with H₂ to produce OH radicals. If CO is also present, then OH can react either with H₂ or CO: The competition between reactions (1) and (2) was measured by measuring the CO₂ yield at various values of the ratio [CO]/[H₂] at 217–298°K. At 298°K the ratio of the rate coefficients k₁/k₂ increased with pressure from a low-pressure limiting value of 14 to a high-pressure limiting value of 50. The low-pressure limiting value agrees well with the low-pressure values found by others. At lower temperatures our high-pressure values of k₁/k₂ were larger than deduced from the accepted low-pressure Arrhenius expression and could be fitted to the expression The mechanism which seems to fit the results best is with k₁° = k_ak_b/k_-a and k₁^∞ = k_a.     

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 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.     

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      

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      

The gas-phase reaction of O3 with H2CO

Explosions occur when O₃ and H₂CO are mixed in a fresh vessel, even in the presence of several hundred torr of N₂ or O₂. However, in an aged vessel the reaction is well behaved. The reaction between O₃ and H₂CO was studied at room temperature in an aged vessel in the presence of about 400 torr of either N₂ or O₂. The initial rate of O₃ decay in the presence of N₂ is about 10³ times faster than in the presence of O₂, and very small amounts of O₂ quickly reduce the initial rate of O₃ decay in the N₂ case. A chain mechanism is postulated to account for the results in which chain initiation can occur both by thermal decomposition of O₃, followed by reaction of O(³P) with H₂CO to produce HO and HCO, as well as by which may occur both homogeneously and heterogeneously. The rate coefficient k₁ ? 2.1 × 10^?24 cm³/molec · sec represents an upper limit (to within a factor of 2 uncertainty) to the direct gas-phase reaction between O₃ and H₂CO.     

H2 formation in the reaction of O(1D) with CH4

The reaction of O(¹D) with CH₄ was studied to determine the efficiency of H₂ production in a direct process, and it was found to be 0.11 ± 0.02. Thus the two channels which account for all of the reaction between O(¹D) and CH₄ in the gas phase are      

N2O Dissociation behind reflected shock waves

The dissociation of N₂O/Ar mixtures, with and withoutadded CO, has been studied by monitoring both infrared and ultraviolet emissions behind reflected shock waves. Initial temperatures ranged from 1850 to 2535°K, and the total concentrations were 1.94–2.40 × 10¹⁸ molecule/cm³. The infrared emission, corrected if necessary for CO, was observed to decay exponentially, and an apparent rate constant K_app was obtained. Addition of CO had no effect upon k_app and all the data can be described by the followingArrhenius parameters (in units of cm³/molecule.sec): log A=?9.31±0.12 and E_A=219.1±5.2 kJ/mole. Ultraviolet emission data, in runs with added CO, indicate that the atomic oxygen concentration reached a constant value at t < 600 μsec for T₀ > 2050°K. Numerical integration of the mechanism allowed comparison of calculated and observed parameters relating to both infrared and ultraviolet data. A consistent fit to these data was obtained with k₁=1.3×10^?9 exp (?238 kJ/RT) and k₂=k₃=1.91×10^?11 exp(?105 kJ/RT). The concentration of atomic oxygen produced by N₂O dissociation is shown to be a sensitive function of k₁ through k₃. Upper limits are also set for the rate constants of the following reactions:      

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.     

Kinetics and mechanism of the reaction H + CH3ONO

The reaction of hydrogen atoms with methyl nitrite was studied in a fast-flow system using photoionization mass spectrometry and excess atomic hydrogen. The associated bimolecular rate coefficient can be expressed by in the temperature range of 223-398°K. NO, CH₃OH, CH₄, C₂H₆, CH₂O, and H₂O are the main products; OH and CH₃ radicals were detectable intermediates. The mechanism was deduced from the observed product yields using normal and deuterated reactants. The primary reaction steps were identified as followed by a rapid unimolecular decomposition of CH₂ONO into CH₂O and NO. Since the extent of reaction channel (1b) could not be determined independently, only extreme limits could be obtained for the individual contributions of the two channels of reaction (3) which follows the generation of CH₃O radicals: The most probable values, k_3a/k₃ = 0.31 ± 0.30 and k_3b/k₃ = 0.69 ± 0.30, support the previous results on this reaction, although the range of uncertainties is much greater here.     

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.     

The reaction of O(1D) with CH4

The room-temperature photolysis of N₂O (10–100 torr) at 2139 Å to produce O(¹D) has been studied in the presence of CH₄ (10–891 torr). The reactions of O(¹D) with CH₄ were found to be The method of chemical difference was used to measure the rate constant ratio k₄/(k₂ + k₃), where reactions (2) and (3) are The CH₃ radicals produced in reaction (4) react with the O₂ and NO produced in reactions (2) and (3). Thus, near the endpoint of the internal titration, ?{C₂H₆} gives an accurate measure of k₄/(k₂ + k₃). For the translationally energetic O(¹D) atoms produced in the photolysis, k₄/(k₂ + k₃) = 2.28 ± 0.20. However, if He is added to remove the excess translational energy, then k₄/(k₂ + k₃) drops to 1.35 ± 0.3.     

Kinetics of the reaction of cyclopentane with trichloroethylene

The kinetics of the thermally and radiation initiated chain reaction between trichloroethylene and cyclopentane to produce 1,1-dichlorovinylcyclopentane and hydrogen chloride have been investigated in the temperature range 250–360°C at high pressure in the gas phase. The rate governing step in the chain is (k₃ = 3.3 × 10⁹ exp ?(4800/RT) cc mole^?1 sec ^?1). The rate of the unimolecular decomposition of trichloroethylene is 1.4 × 10¹⁴ exp ?(61,200/RT) sec^?1.     

Reaction of CF3 radicals with H2O and D2O

The reaction of CF₃ radicals with H₂O (D₂O) has been studied over the range of 533–723 K using the photolysis and the pyrolysis of CF₃I as the free radical source. Arrhenius parameters for the reactions where X = H or D, relative to CF₃ radical recombination are given by where k/k is in cm^3/2/mol^1/2·s^1/2 and θ = 2.303RT/cal/mol. The activation energy and the primary kinetic isotope effect have been compared with those derived from the BEBO method.     

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.     

The determination of the arrhenius parameters for the reaction Cl + C2F5I → ICl + C2F5 using a competitive method

The reactions have been studied competitively over the range of 28–182°C by photolysis of mixtures of Cl₂ + C₂F₅I+ CH₄. We obtain where θ = 2.303RT J/mol. The use of published data on reaction (2) leads to log (k₁cm³/mol sec) = (13.96 ± 0.2) ? (11,500 ± 2000)/θ.     

The reaction of CH3O2 with SO2

Mixtures of Cl₂, CH₄, and O₂ were flash photolyzed at room temperature and pressures of ∽60–760 Torr to produce CH₃O₂. The CH₃O₂ radicals decay by the second-order process with k₆ = (3.7 ± 0.3) × 10^?13 cm³/sec in good agreement with other studies. This value ignores any removal by secondary radicals produced as a result of reaction (6), and therefore the true value might be as much as 30% lower. The value is independent of total pressure or the presence of H₂O vapor. With SO₂ also present, the CH₃O₂ decay becomes pseudo first order at sufficiently high SO₂ pressure which indicates the reaction The value of (8.2 ± 0.5) × 10^?15 cm³/sec at about 1 atm total pressure (mostly CH₄) was found for CH₃O₂ removal by SO₂, in good agreement with another recent measurement. This value can be equated with k₁, unless the products rapidly remove another CH₃O₂ radical, in which case k₁ would be a factor of 2 smaller.     

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.