O-atom Source Research Articles

Bioinspired Heterobimetallic Photocatalyst (RuIIchrom-FeIIIcat) for Visible-Light-Driven C-H Oxidation of Organic Substrates via Dioxygen Activation.

We report a bioinspired heterobimetallic photocatalyst RuIIchrom-FeIIIcat and its relevant applications toward visible-light-driven C-H bond oxidation of a series of hydrocarbons using O2 as the O-atom source. The RuII center absorbs visible light near 460 nm and triggers a cascade of electrons to FeIII to afford a catalytically active high-valent FeIV═O species. The in situ formed FeIV═O has been employed for several high-impact oxidation reactions in the presence of triethanolamine (TEOA) as the sacrificial electron donor.

Carbon-Electrode-Mediated Electrochemical Synthesis of Hypervalent Iodine Reagents Using Water as the O-Atom Source

Carbon-Electrode-Mediated Electrochemical Synthesis of Hypervalent Iodine Reagents Using Water as the O-Atom Source

Oxoiron(v) mediated selective electrochemical oxygenation of unactivated C-H and C[double bond, length as m-dash]C bonds using water as the oxygen source.

An efficient electrochemical method for the selective oxidation of C–H bonds of unactivated alkanes (BDE ≤97 kcal mol−1) and CC bonds of alkenes using a biomimetic iron complex, [(bTAML)FeIII-OH2]−, as the redox mediator in an undivided electrochemical cell with inexpensive carbon and nickel electrodes is reported. The O-atom of water remains the source of O-incorporation in the product formed after oxidation. The products formed upon oxidation of C–H bonds display very high regioselectivity (75 : 1, 3° : 2° for adamantane) and stereo-retention (RC ∼99% for cyclohexane derivatives). The substrate scope includes natural products such as cedryl acetate and ambroxide. For alkenes, epoxides were obtained as the sole product. Mechanistic studies show the involvement of a high-valent oxoiron(v) species, [(bTAML)FeV(O)]− formed via PCET (overall 2H+/2e−) from [(bTAML)FeIII-OH2]− in CPE at 0.80 V (vs. Ag/AgNO3). Moreover, electrokinetic studies for the oxidation of C–H bonds indicate a second-order reaction with the C–H abstraction by oxoiron(v) being the rate-determining step.

Epoxidations Catalyzed by Manganese(V) Oxo and Imido Complexes: Role of the Oxidant−Mn−Oxo (Imido) Intermediate

The manganese(V) oxo complex (TBP(8)Cz)Mn(V)(O) (1) is shown to catalyze the epoxidation of alkenes with a series of iodosylarenes (ArIO) as oxidants. Competition experiments reveal that the identity of ArIO influences the product ratios, implicating an unusual coordinated oxo-metal-ArIO intermediate (1-OIAr) as the active catalytic species. The isoelectronic manganese(V) imido complex (TBP(8)Cz)Mn(V)(NMes) (2) does not participate in NR transfer but does catalyze epoxidations with ArIO as the O-atom source, suggesting a mechanism similar to that seen for 1. Direct evidence (ESIMS) is obtained for 1-OIMes.

High-temperature oxidation of dispersed soot particles by O atoms

The reaction of O atoms with soot particles dispersed in 10 to 20 ppm N 2 O/Ar mixtures was studied at high temperatures behind reflected shock waves. Under the present conditions ( T >1900 K), N 2 O represents a fast thermal O-atom source, because it decomposes rapidly to form O atoms which in turn react with the dispersed soot particles. The progress in the heterogeneous reaction was determined by time-resolved measurements of O-atom concentration using the highly sensitive atomic resonance absorption spectroscopy. Besides the absorption of the OI radiation ( λ =130.5 nm) by O atoms, the absorption signal includes also particle light extinction. A separation of both influences is possible by performing additional experiments on pure soot/Ar aerosols. Laser light extinction measurements using an Ar + laser ( λ =488.0 nm) were performed simultaneously providing a relationship between the particle extinction at both wavelengths, which allows us to determine the pure O-atom absorption in aerosols. The reaction conditions were chosen such that the total amount of carbon is much higher than the total amount of atomic oxygen: that is, the reaction conditions are pseudo-first-order. A rate coefficient for the consumption of O atoms due to soot particle oxidation was determined. CO was identified to be the main reaction product. To determine the heterogeneous reaction probability from the O-atom consumption rate, it is essential to measure the reactive particle surface. This quantity can be determined from the soot volume fraction available from the laser light extinction.

High-temperature reactions of C 2 with atomic and molecular oxygen

The reaction of C2 radicals with atomic and molecular oxygen were studied behind reflected shock waves in the temperature range 2750 K≤T≤3950 K by using ring-dye-laser absorption spectroscopy. The shock-induced pyrolysis of acetylene was used as a well-defined C2 source, which was perturbated by the addition of N2O (O-atom source) or O2. The addition of these species results in specific changes in the C2 concentration compared to the unperturbated pyrolysis, mainly caused by the following reactions: {fx1} for which the rate coefficients {fx2} were obtained.

Discharge flow-tube studies of O(3P)+N2H4 reaction: The rate coefficient values over the temperature range 252–423 K and the OH(X 2Π) product yield at 298 K

The absolute second-order reaction rate coefficient, k1, for the gas phase reaction, O(3P)+N2H4→products, was studied in a discharge flow-tube apparatus. The reaction was studied under pseudo-first-order conditions in O(3P) concentration (i.e., [N2H4]≫[O(3P)]). The O atoms were generated by a microwave discharge of a suitable precursor gas in He in a fixed side-arm reactor upstream of the flow tube, or in the sliding inner injector of the flow tube. The hydrazine concentration was photometrically measured and introduced into the apparatus in a flow of He via the sliding injector or the fixed side-arm port, respectively. The kinetics of the O-atoms in the reaction was directly followed by 130.2–130.6 nm cw-resonance fluorescence detection of O(3P) at the fixed detector situated downstream of the flow tube. The Arrhenius expression, k1=(7.35±2.16)×10−13 exp[(640±60)/T] cm3 molec−1 s−1, in the temperature range 252–423 K, was fit to the data points. The rate coefficient at room temperature was, within experimental errors, independent of the He buffer gas pressure in the range 1.74 to 8.30 Torr, or the O-atom source reactor. The formation of OH(X 2Π) in the reaction, which can be vibrationally excited (v″≤2), was directly detected by pulsed laser-induced fluorescence. The total yield of OH in the reaction was determined to be (0.15±0.05) at 298 K, of which ∼50% is thought to be produced vibrationally hot. These results suggest that the single-H-atom removal channel is a minor process, in agreement with earlier molecular beam studies in which a direct two-H-atom removal channel was proposed to be the principal reaction mechanism by which O(3P) reacts with N2H4.

Deposition of single phase, homogeneous silicon oxynitride by remote plasma-enhanced chemical vapor deposition, and electrical evaluation in metal–insulator–semiconductor devices

Device-quality thin silicon oxynitride films were successfully prepared by combining remote plasma-enhanced chemical-vapor deposition and rapid thermal annealing techniques. The physical, chemical, and electrical properties of the films are determined by the relative flow rates of the N-atom and O-atom source gases, NH3 and N2O, respectively. Analysis of metal–insulator–semiconductor devices indicated the fixed charges in the dielectric (Qf) and the interface defect state densities (Dit), as determined from the analysis of conventional capacitance–voltage (C–V) measurements, were significantly reduced following post-deposition rapid thermal annealing. The approximate compositions of the films were obtained by on-line Auger electron spectroscopy, and the local bonding arrangements were determined by Fourier transform infrared spectroscopy. Correlations between the local bonding structures in the oxynitride films and the electrically active defective states at the Si–SiOyNz interface are also discussed.

Kinetics of the reaction between oxygen atoms and propargyl radicals

The kinetics of the reaction between C 3 H 3 (propargyl radical) and O( 3 P) has been studied using a heatable tubular reactor coupled to a photoionization mass spectrometer. Rate constants were determined in the temperature range 295–750 K, and information on the reaction mechanism was obtained. The reactants were generated homogeneously in the reactor by the simultaneous in situ photolysis of SO 2 (the O-atom source) and 1–3 butadiene or propargylbromide (the C 3 H 3 source). Initial conditions were chosen to yield [O] in excess, [O]/[C 3 H 3 ] o >20. The decay of C 3 H 3 was monitored in time-resolved experiments. The rate constant was found to be independent of temperature, having the value 2.3×10 −10 cm 3 molecule −1 s −1 , and also independent of density in the range used in this study, 6–12×10 16 molecule cm −3 . The reactivity of C 3 H 3 in this reaction is compared with that of other polyatomic free radicals whose reactions with atomic oxygen have been investigated.

Si-N Bonding at The SiO2/Si Interfaces During Deposition of SiO2 by the Remote Pecvd Process

Thin films of SiO2 were deposited on Si substrates by remote PECVD following an in-situ cleaning/surface passivation with atomic-H. Si-N bonds were found in the immediate vicinity of the SiO2/Si interface when nitrous oxide, N2O, was used as O-atom source gas in the remote PECVD process. Si-N bonds, as well as Si-surface roughening produced by H-atom etching, contribute to the formation of high densities of midgap trapping states, Dit ∼1011 cm−2 eV−1, at the SiO2/Si interface. Eliminating the H-atom processing step, and exposing the Si surface to plasma-generated O-atoms prior to the SiO2 deposition: i) eliminated Si-N bonding at the Si/SiO2 interface; ii) reduced midgap Dit to ∼ 1–3×1010 cm−2eV−1 iii) eliminated surface roughening; and iv) improved process latitude and reproducibility.

Si-N Bonding at the SiO2/Si Interfaces During Deposition of SiO2 by the Remote Pecvd Process

Thin films of SiO2 were deposited on Si substrates by remote PECVD following an in-situ cleaning/surface passivation with atomic-H. Si-N bonds were found in the immediate vicinity of the SiO2/Si interface when nitrous oxide, N2O, was used as O-atom source gas in the remote PECVD process. Si-N bonds, as well as Si-surface roughening produced by H-atom etching, contribute to the formation of high densities of midgap trapping states, Dit ∼1011 cm−2eV−1, at the SiO2/Si interface. Eliminating the H-atom processing step, and exposing the Si surface to plasma-generated O-atoms prior to the SiO2 deposition: i) eliminated Si-N bonding at the Si/SiO2 interface; ii) reduced midgap Dit to ∼1−3×1010 cm−2eV−1; iii) eliminated surface roughening; and iv) improved process latitude and reproducibility.

High temperature oxidation of suspended soot particles verified by CO and CO 2 measurements

The oxidation of soot particles suspended in different gas mixtures (O 2 /Ar, N 2 O/Ar, O 2 /Ar with traces of H 2 ) and ((H 2 /O 2 ) stoch ./Ar) was measured behind incident shock waves in the temperature range 1250 K ≤ T ≤ 3500 K. The progress in the heterogeneous reaction was determined by time resolved CO and CO 2 concentration measurements using a rapid tuning IR-Diode Laser. Additionally the particle properties during soot oxidation were also determined from in-situ laser light extinction measurements at three different wave lengths. It was possible to deduce the reaction probability for the surface oxidation by O 2 and O from the experimentally determined CO formation rates, whereby in the second case N 2 O served as a fast thermal O-atom source. In the two mixtures containing H 2 , the soot oxidation is coupled with the gas phase oxidation of hydrogen. The measured CO and CO 2 concentrations were interpreted on the basis of computer simulations. The well known H 2 /O 2 reaction kinetics was combined with global heterogeneous oxidation reactions of the suspended soot particles. A sensitivity analysis demonstrated the importance of the soot oxidation by OH under the present conditions. By varying the reaction probability of OH, good fits of the computed CO and CO 2 profiles to the measured ones were obtained.

A direct comparison of shock tube photolysis and pyrolysis methods in the determination of the rate coefficient for O + H 2 → OH + H

Mixtures of NO, N 2O, and H 2 in Ar were shock-heated and photolyzed with an ArF excimer laser. Measurements of O-atom profiles using atomic resonance absorption spectroscopy (ARAS) permitted the determination of the rate coefficient for the reaction, O + H 2 → OH + H, by two methods under near-identical conditions. The first used the pyrolysis of N 2O as an O-atom source, and the second used the photolysis of NO as the O-atom source. The rate coefficients determined by the two methods are indistinguishable and are well represented in the temperature range 2120 to 2750 K by the expression 8.13(± 10%) × 10 14 exp BRET9540[±800/ T (in K)] BRET1 cm 3 mol −1 s −1. This rate coefficient agrees well with those previously reported in the literature.

Kinetics of the reactions of chlorinated alkynes with atomic oxygen: The application of a new experimental technique

The kinetics of the reactions of oxygen atoms with acetylene and chlorinated acetylenes were investigated using a new experimental procedure which is described in detail. Both reactants were produced simultaneously in a tubular reactor coupled to a photoionization mass spectrometer by pulsed 193 nm laser photolysis. Sulfur dioxide was the O-atom source, and photolysis of halogenated olefins provided the appropriate alkyne. The decay of the alkyne was monitored in time-resolved experiments under pseudo first-order conditions (O atoms in large excess). The accuracy of the method is demonstrated with measurements of the O + C 2 H 2 rate constant between 370 and 876 K. Rate constants of the O + 1-chloroprop-1-yne reaction were determined between 298 and 876 K and fit to an Arrhenius expression: 3.9×10 −11 exp(−2.1 kcal mol −1 /RT) cm 3 molecule −1 s −1 . An upper limit for the O + C 2 Cl 2 rate constant at 823 K was also determined (1.6×10 −13 cm 3 molecule −1 s −1 ). The utility of the method for the study of the kinetics of other O-atom reactions with molecular reactants which can be produced by 193 nm photolysis of suitable precursors is discussed.

Primary products of the reaction between O(3P)-atoms and C2H4 studied with ESR- and LMR-detection

The primary products of the reaction between O(3P)-atoms and C2H4 O(3P)+C2H4→products (1) have been studied at room temperature in the gas phase. The products in the low pressure regime were investigated in two isothermal discharge flow systems with Electron Spin Resonance (ESR) detection of O- and H-atoms and Far Infrared Laser Magnetic Resonance (LMR) detection of O-atoms and HCO- as well as CH2-radicals. The absolute concentrations of these species were determined using several independent titration reactions. The experiments yielded the branching ratios for the following reaction channels: O+C2H4→C2H3O+H k1a/k1=(0.50±0.10) (1a) O+C2H4→CH3+HCO k1b/k1=(0.44±0.15) (1b) O+C2H4→CH2+H2CO k1c/k1=(0.06±0.03) (1c) The formation of the high pressure stabilization products ethylene oxide and acetaldehyde from the reaction channels O+C2H4+M→C2H4O+M (1d) O+C2H4+M→CH3CHO+M (1e) was observed in a static system at pressures between 5 bar ≦p≦70 bar using Hg*-sensitized decomposition of N2O as the O-atom source and gaschromatographic analysis. From the half pressure for CH3CHO production the lifetime of the addition complex from O+C2H4 with respect to redissociation and dissociation into fragments is concluded to be of the order 10−11 s. Thus, 1.2 H-atom migration in the addition complex has to proceed on this time scale.

A high flux source of energetic oxygen atoms for material degradation studies

A novel technique for generation of a high flux of energetic (nominal ~5 eV) oxygen atoms has been demonstrated in the laboratory. The generation technique involves a laser-induced breakdown of molecular oxygen followed by a rapid expansion of the recombining plasma, resulting in the production of a flux of nearly monoenergetic oxygen atoms. We have developed consistently high velocity streams of oxygen atoms in an evacuated hypersonic nozzle. The oxygen flow is introduced into the nozzle through the mediation of a fast (~ 100 IJLS) pulsing valve and broken down with a CO2 laser. We have measured average oxygen atom velocities of ~ 5 to 13 km/s with an estimated total production of 10 18 atoms per pulse over pulse durations of several jis. Although these were single pulse measurements, scaling to a repetitively pulsed device appears straightforward. The O-atom source was used to irradiate sample targets, including Al, Fe, and polyethylene, and target-materialspecific radiation was observed above the target surfaces, indicating energetic O-atom induced mass removal.

Description and Analysis of a Vacuum Ultraviolet Atomic Line Source

An intense vacuum uv atomic line source excited in a 2450-MHz microwave discharge cavity is described and analyzed. Additional features are a clean spectrum, very little window deterioration, little or no self-absorption, and easily interchangeable parts. (For the O-atom source only the 1300-A triplet shows up over the 1200-A to 1650-A region.) This source is suitable for the detection of various species by absorption measurement in fast occurring events such as those in ballistic ranges and shock tubes. Atomic line shape theory is used to analyze an O-atom line source with intensity ratios I(1306A): I(1305A): I(1302A):1:2.9:4.5. This source is described by a 2700 +/- 200 K temperature and certain self-absorption parameters as deduced from the measurement of their intensity ratios and their transmission characteristics through known concentrations of O-atoms. A good theoretical fit to the measured transmission validates the assumed uniform source temperature and excitation for the analysis. An f value (f = 0.052 +/- 0.005) for the O-atom 1300-A triplet absorption and a collision diameter of 5.5 +/- 1.0 A between the O-atom and the nitrogen molecule were determined in the course of this study.

The production of O ( 3P) atoms, free from excited molecules, and their reaction with O 3

Oxygen atoms in their ground state can be generated in the absence of vibrationally or electronically excited molecules by passing O 2 , O 3 , or N 2 O over a “Nernst” glower. The use of N 2 O has the additional advantage of providing the atoms free of molecular oxygen, N 2 being the only other dissociation product. Dissociations, up to 1% could be attained at 1 Torr total pressure. This O-atom source was used to study the rate of the reaction of O atoms with O 3 in a flow system. Ozone was introduced in large excess and the kinetics followed by measuring the rate of disappearance of the O atoms by means of a movable calorimetric probe consisting of a silver-or cobalt-covered platinum spiral. This detector was found to be un-affected by O 3 at gas temperatures below 50°C. Consistent values of the rate constant were found over a fourfold variation in initial ozone concentration. The rate constant at 23.2°C was found to be (1.46±0.06)×10 −14 cm 3 molecule −1 sec −1 This value is lower than direct methods using discharge sources for O atoms, presumably because O 3 is also dissociated by excited molecules in those systems. The value was within the relatively large range of values obtained indirectly from studies of the decomposition of O 3 .