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

The reaction of O(³P) atoms with propanehas been studied at temperatures near 300 K by using a discharge flow system. Oxygen atoms were generated in the absence of molecular oxygen by the reaction N + NO → N₂ + O, nitrogen atoms having been generated in a microwave discharge. Rate constants for the reaction were measured in two ways, either by measurement of O-atom decay in the presence of excess propane or by measuring the change in propane concentration after an appropriate time in the presence of an excess of oxygen atoms. The two methods were in good agreement, and the mean rate constant at 306 K is given by A study of the products of the reaction under conditions corresponding to complete removal of oxygen atoms has shown that an important product of the reaction in the early stages is propene. This is difficult to explain interms of a mechanism involving alkoxy radicals similar to that which has been proposed for some other O(³P)–hydrocarbon reactions. An alternative mechanism is proposed in terms of successive hydrogen abstraction reactions.     

The reaction of O(3P) with OClO

The laser photolysis-long path transient absorption technique was used to study the mechanism and products of the reaction O(³P) + OClO (→^M) Products (3) at 260 K in 100 to 400 torr of He, N₂, and Ar. ClO was not detected as a reaction product, <5% yield, from this reaction at 400 torr and 260 K. A broad UV absorption feature associated with a product of this reaction, with a peak located at 260 nm, was observed. The peak absorption cross section of this species was measured to be (1.72 ± 0.12) × 10⁻¹⁷ cm² molecule⁻¹. The rate coefficient for the appearance of this species was measured to be (1.69 ± 0.46) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ and was independent of pressure. The rate coefficient for the appearance of the species is ca. 10 times lower than that for the disappearance of O(³P), indicating that the observed species is not a direct product of the reaction of O(³P) with OClO. Mechanistic considerations and the possible identity of the absorber are discussed. © 1997 John Wiley & Sons, Inc.     

Reactions of atomic oxygen [O(3P)] with polymer films

The reactions of polymer films with oxygen atoms are reviewed. Emphasis is on work from this laboratory on polybutadienes having different amounts of 1,4 or 1,2 double bonds and their polyalkenamer homologues, on a polyimide (Kapton), and on a series of polyolefins with increasing fluorine content ranging from polyethylene (PE) to polytetrafluoroethylene (PTFE). Interest in this topic stems from a need to understand the mechanism(s) of surface erosion, or etching, that occurs in organic polymers when exposed to the low Earth orbital (LEO) environment, where ground-state atomic oxygen [O(³P)] is the most abundant constituent. The major findings for O atom reactions with the polyalkenamers were: (1) etch rates increase with decrease in -CH=CH-unsaturation, starting with 1,4-polybutadiene and reaching the maximum rate with PE or ethylene-propylene rubber; (2) in polybutadienes having both 1,4 and 1,2 double bonds, the rate of O(³P)-induced etching is lower the higher the 1,2 content; and (3) the reactions are confined to the polymer surface. Relative etch rates for various commercial polymer films (e.g., Kapton, Teflon, PE) exposed to O(³P) downstream from a low-pressure, radio-frequency O₂ plasma were compared with corresponding literature data for films exposed to O(³P) either during a Space Shuttle flight or in the plasma glow (where many species besides O(³P) are present). The use of O₂ discharge reactors – whether exposures are conducted “out of the glow” or “in the glow” – cannot serve as a routine, ground-based simulation of exposures of a polymer to the LEO environment. ESCA analysis of the surfaces of Kapton and other polymers, before and after reaction with O(³P), showed that there is a steady-state competition between surface recession and oxidation. In a series of fluorine-containing polyolefins exposed to O(³P), the maximum oxygen uptake increased regularly with decrease in fluorine content, ranging from a minimum in PTFE to a maximum in PE.     

The reaction of O(3P) with H2O2

The rate constant for the reaction of O(³P) with H₂O₂ was measured as a function of temperature and the [H₂O₂]₀/[O]₀ ratio. The numerical solution of the appropriate rate equations was used to arrive at a mechanism which adequately describes our results and the rather divergent data in the literature. A recommended expression for the temperature dependence of the absolute rate constant is presented from consideration of the available experimental data.     

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.     

Mechanism and kinetics properties for the reaction: Chloroethane with atomic O (P)

In this paper, we present direct dynamics calculations for the multiple-channel reaction of CH₃CH₂Cl with atomic O (³P) in a wide temperature range (200–3000 K), based on canonical variational transition state theory including small curvature corrections. Four distinct saddle points, one for α-abstraction and three for β-abstraction, have been located for this reaction. The potential energy surface information has been calculated at the MP2/6-311G(d,p) level. The energies along the minimum energy path have been further improved by single-point energy calculations at the G3MP2 level. In the β-abstraction channel, Jahn–Teller effect has been found. Changes of geometries, generalized normal-mode vibrational frequencies, and potential energies along the reaction paths for all channels have been discussed and compared. The calculated total rate constants match the available experimental values reasonable well over the measured temperature range. The results show the variational effect can be negligible and the small curvature tunneling contribution plays an important role for the calculation of the rate constant. At low temperature α-abstraction may be the major reaction channel, while β-abstraction will have more contribution to the whole reaction rate as the temperature increase.     

Kinetics of the reaction of O(3P) with CF3NO

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the reaction of O(³P) with CF₃NO (k₂) as a function of temperature. Our results are described by the Arrhenius expression k₂(T) = (4.54 ± 0.70) × 10^?12 exp[(?560± 46)/T] cm³molecule^?1 s^?1 (243 K ? T ? 424 K); errors are 2σ and represent precision only. The O(³P) + CF₃NO reaction is sufficiently rapid that CF₃NO cannot be employed as a selective quencher for O₂(a¹Δ_g) in laboratory systems where O(³P) and O₂(a¹Δ_g) coexist, and where O(³P) kinetics are being investigated. © 1995 John Wiley & Sons, Inc.     

The kinetics of the gas phase reaction of O(3P) with N2O5

The kinetics of the gas phase reaction between O(³P) atoms and N₂O₅ have been examined in a discharge-flow mass spectrometer at 4.5 torr (N₂) total pressure. At O(³P) concentrations (5–10) × 10¹⁴ molecules/cm³, the decay of N²O⁵ was very small and only slightly greater than the data scatter. From these data, upper limits to the rate constant of this reaction was obtained at 223 and 300 K: k²²³ and k³⁰⁰ ? 3 × 10^?16 cm³/molecule s.     

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.     

Isotope effect in the carbonyl sulfide reaction with O(3P)

The sulfur kinetic isotope effect (KIE) in the reaction of carbonyl sulfide (OCS) with O((3)P) was studied in relative rate experiments at 298 ± 2 K and 955 ± 10 mbar. The reaction was carried out in a photochemical reactor using long path FTIR detection, and data were analyzed using a nonlinear least-squares spectral fitting procedure with line parameters from the HITRAN database. The ratio of the rate of the reaction of OC(34)S relative to OC(32)S was found to be 0.9783 ± 0.0062 ((34)ε = (-21.7 ± 6.2)‰). The KIE was also calculated using quantum chemistry and classical transition state theory; at 300 K, the isotopic fractionation was found to be (34)ε = -14.8‰. The OCS sink reaction with O((3)P) cannot explain the large fractionation in (34)S, over +73‰, indicated by remote sensing data. In addition, (34)ε in OCS photolysis and OH oxidation are not larger than 10‰, indicating that, on the basis of isotopic analysis, OCS is an acceptable source of background stratospheric sulfate aerosol.     

The mechanism of O(3P) atom reaction with ethylene and other simple olefins

Reactions of oxygen atoms with ethylene, propene, and 2-butene were studied at room temperature under discharge flow conditions by resonance fluorescence spectroscopy of O and H atoms at pressures of 0.08 to 12 torr. The measured total rate constants of these reactions are K = (7.8 ± 0.6)·10^?13cm³s^?1,K = (4.3 ± 0.4) ± 10^?12 cm³ s^?1, K = (1.4 ± 0.4) · 10^?11 cm³ s^?1. The branching ratios of H atom elimination channels were measured for reactions of O atoms with ethylene and propene. No H-atom elimination was found for the reaction of O-atoms with 2-butene. A redistribution of reaction O + C₂ channels with pressure was found. A mechanism of the O + C₂ reaction was proposed and the possibility of its application to other olefins is discussed. On the basis of mechanism the pressure dependence of the total rate constant for reaction O + C₂ was predicted and experimentally confirmed in the pressure range 0.08–1.46 torr.     

Quantum chemical study of the reaction of trichloroethylene with O(3P)

The adiabatic mechanism of the reaction of trichloroethylene with O(³P), exploring the various O-atom addition and H-atom abstraction channels, is theoretically studied at the MP2/6-311++G(2d, 2p), MP2/aug-cc-pVTZ, CCSD/6-31G(d), G3, and CBS-QB3 levels of theory. From a kinetic point of view, the addition to the less substituted carbon atom of the double bond is more favorable than the addition to the more substituted carbon. Such O-atom addition reactions are favored over the one possible hydrogen-abstraction reaction. Calculations of the present study showed that five products are obtained: HCCl + C(O)Cl₂ (P1), Cl + ClC(O)CHCl (P2), H + ClC(O)CCl₂ (P3), Cl + HC(O)CCl₂ (P4), and CH(O)Cl + CCl₂ (P5). The products P2 and P4 are found to be the most favored ones. The kinetic calculations of rate constant in the range of 285–395 K are performed at the CBS-QB3 level of theory and are in conformity with the experimental outcomes.     

A kinetic study of the reaction of O(3P) with toluene

The absolute rate constant of the reaction of O(³P) with toluene was measured by the microwave discharge-fast-flow method to obtain k_T = 10^{9.7–2.7/2.303RT} l./mole·sec at 100–375°C. This was in good agreement with the rate constant calculated from the combination of the relative rate constant obtained by Jones and Cvetanovic with the recently determined absolute rate constants of the reaction of O(³P) + olefins. The extrapolation of the above Arrhenius plot to 27°C was also in good agreement with the absolute value of k_T = 4.5 × 10⁷ l./mole·sec determined recently by Atkinson and Pitts at 27°C. The rate constant of the reaction of chlorobenzene with O(³P), obtained at 238°C as 10^8.3 l./mole·sec by a competitive method, was smaller than k_T by a factor of about two at the same temperature.     

A theoretical study of the reaction of O(3P) with isobutene

The reaction of O((3)P) with isobutene ((CH(3))(2)C=CH(2)) is investigated using the unrestricted second-order M?ller-Plesset perturbation (UMP2) and complete basis set CBS-4M level methods. The minimum energy crossing point (MECP) between the singlet and triplet potential energy surfaces is located using the Newton-Lagrange method, and it is shown that the MECP plays a key role. The calculational results indicate that the site selectivity of the addition of O((3)P) to either carbon atom of the double bond of isobutene is weak, and the major product channels are CH(2)C(O)CH(3) + CH(3,) cis-/trans-CH(3)CHCHO + CH(3), (CH(3))(2)CCO + H(2), and CH(3)C(CH(2))(2) + OH, among which (CH(3))(2)CCO + H(2) is predicted to be the energetically most favorable one. The complex multichannel reaction mechanisms are revealed, and the observations in several recent experiments could be rationalized on the basis of the present calculations. The formation mechanisms of butenols are also discussed.     

The rate coefficient for the reaction of O(3P) with CH3OOH at 297 K

The absolute second-order rate coefficient for the reaction, O(³P) + CH₃OOH → products, was measured to be ??₁ = (1.06 ± 0.26) × 10^?14 cm³ molec^?1 s^?1 at 297 K, where the quoted error is 2σ including precision and estimated systematic errors. The possible presence of (CH₃CH₂)₂O in our sample of CH₃OOH leads to a large error in ??₁ which reflects the relatively large uncertainty indicated. O(³P) was generated in excess CH₃OOH by photolyzing a small amount of O₃ at 532 nm, where CH₃OOH does not photolyze. The rate of removal of O(³P) in the experiments was monitored by resonance fluorescence detection. The increased reactivity of O(³P) with CH₃OOH relative to H₂O₂ is interpreted as due to H abstraction from the CH₃ group.     

Formation of O(3P) atoms and epoxides from the gas- phase reaction of O3 with isoprene

The formation yields of 1,2-epoxy-2-methyl-3-butene and 1,2-epoxy-3-methyl-3-butene have been measured from the reaction of O₃ with isoprene at room temperature and one atmosphere total pressure of N₂ and air diluents, with and without cyclohexane to scavenge the OH radicals formed in this reaction system. In addition, a relative rate method was used to determine a rate constant for the gas-phase reaction of O₃ with 1,2-epoxy-2-methyl-3-butene of (2.5 ± 0.7) x 10^-18 cm³ molecules^-1 s^-1 at 296 ± 2 K. Our data show that the epoxide yields in N₂ and air diluents are the same, with formation yields of 1,2-epoxy-2-methyl-3-butene of 0.028 ± 0.007 and of 1,2-epoxy-3-methyl-3-butene of 0.011 ± 0.004. These data further show that the epoxides arise from the primary O₃ reaction with isoprene, and not via the formation of O(³P) atoms from the O₃ - isoprene reaction followed by reaction of these O(³P) atoms with isoprene.     

NaMn6(P2O7)2(P3O10) and KCd6(P2O7)2(P3O10)

The crystal structures of two new diphosphates, sodium hexamanganese bis­(diphosphate) triphosphate, NaMn₆(P₂O₇)₂(P₃O₁₀), and potassium hexacadmium bis­(diphosphate) triphosphate, KCd₆(P₂O₇)₂(P₃O₁₀), confirm the rigidity of the M₆(P₂O₇)₂(P₃O₁₀) matrix (M is Mn or Cd) and the relatively fixed dimensions of the tunnels extending in the a direction of the unit cell. The compounds are isomorphous; the P₂O₇^4? anion and the alkali metal cations lie on mirror planes. Bond‐valence analysis of the bonding details of the atoms found within the tunnels permits a prediction of the conductivity.     

The reaction of atomic oxygen and hydrogen with nitric acid

The reactions have been studied in a discharge-flow system. Kinetic studies were made using resonance fluorescence for the measurement of atom concentrations. Based on the rates of atom loss, the following upper limits were obtained for the rate constants: Observed reaction in the H? HNO₃ system is at least partially due to an autocatalytic chain removal of both reactants. Diagnostic tests have suggested that OH, NO₂, and NO₃ are the chain carriers.