Rate Constants For O2 Research Articles

Numerical investigation of the hybrid pulse–DC plasma assisted ignition and NO[formula omitted] emission of NH[formula omitted]/N[formula omitted]/O[formula omitted] mixture

The ignition delay and NO emission characteristics of the NH3/N2/O2 mixture under the influence of pulse–DC hybrid plasma discharges have been studied. The plasma and combustion chemistry mechanisms for NH3/N2/O2 mixture are reviewed, an improved kinetics mechanism is proposed with a well validated electron scattering cross section data. An improved coupled plasma and combustion chemistry code allowing sensitivity/pathway analysis is developed. The role of key species on the ignition delay time and the effects of low E/N discharge over long periods of time (DC stage) on NOx emission are studied. The results show that, NO emission will be strongly enhanced due to the increased HNO, NH and N density in NH3 containing mixtures in the DC stage, NO density increases with the E/N value in DC stage due to the reduction of NH and NH2 species. The role of O2(a1Δg) in promoting ignition was studied using different rate constant of O2(a1Δg)+NO→O2+NO: O2(a1Δg) mainly reacts with H2 and H to produce radicals such as O, H and OH instead of quenched by NO. The vibrationally excited NH3(v2) molecule plays the dominating role in heat release through V–T relaxation with NH3.

O2(b1Σg+) Quenching by O2, CO2, H2O, and N2 at Temperatures of 300-800 K.

Rate constants for the removal of O2(b1Σg+) by collisions with O2, N2, CO2, and H2O have been determined over the temperature range from 297 to 800 K. O2(b1Σg+) was excited by pulses from a tunable dye laser, and the deactivation kinetics were followed by observing the temporal behavior of the b1Σg+-X3Σg- fluorescence. The removal rate constants for CO2, N2, and H2O were not strongly dependent on temperature and could be represented by the expressions kCO2 = (1.18 ± 0.05) × 10-17 × T1.5 × exp[Formula: see text], kN2 = (8 ± 0.3) × 10-20 × T1.5 × exp[Formula: see text], and kH2O = (1.27 ± 0.08) × 10-16 × T1.5 × exp[Formula: see text] cm3 molecule-1 s-1. Rate constants for O2(b1Σg+) removal by O2(X), being orders of magnitude lower, demonstrated a sharp increase with temperature, represented by the fitted expression kO2 = (7.4 ± 0.8) × 10-17 × T0.5 × exp[Formula: see text] cm3 molecule-1 s-1. All of the rate constants measured at room temperature were found to be in good agreement with previously reported values.

Catalytic Consequences of the Thermodynamic Activities at Metal Cluster Surfaces and Their Periodic Reactivity Trend for Methanol Oxidation

AbstractThe periodic reactivity trend and the connection of kinetics to the thermodynamic activity of oxygen are established for the oxidation of methanol on metal clusters. First‐order rate coefficients are a single‐valued function of the O2‐to‐CH3OH ratio, because this ratio, together with the rate constants for O2 and CH3OH activation, determine the oxygen chemical potential, thus the relative abundance of active sites and bulk chemical state of the clusters. CH3OH activation rate constants on oxygen‐covered Ag, Pt, and Pd and on RuO2 clusters vary with the metal–oxygen binding strength in a classical volcano‐type relation, because the oxygen‐binding strength directly influences the reactivities of oxygen as H abstractors during the kinetically relevant CH3OH activation step. The differences in oxygen thermodynamic activity lead to five orders of magnitude variation in rates (Pt>Pd>RuO2>Ag, 373 K), because of its strong effects on the activation enthalpy and more prominently activation entropy in CH3OH activation.

Catalytic consequences of the thermodynamic activities at metal cluster surfaces and their periodic reactivity trend for methanol oxidation.

The periodic reactivity trend and the connection of kinetics to the thermodynamic activity of oxygen are established for the oxidation of methanol on metal clusters. First-order rate coefficients are a single-valued function of the O2 -to-CH3OH ratio, because this ratio, together with the rate constants for O2 and CH3OH activation, determine the oxygen chemical potential, thus the relative abundance of active sites and bulk chemical state of the clusters. CH3OH activation rate constants on oxygen-covered Ag, Pt, and Pd and on RuO2 clusters vary with the metal-oxygen binding strength in a classical volcano-type relation, because the oxygen-binding strength directly influences the reactivities of oxygen as H abstractors during the kinetically relevant CH3OH activation step. The differences in oxygen thermodynamic activity lead to five orders of magnitude variation in rates (Pt>Pd>RuO2>Ag, 373 K), because of its strong effects on the activation enthalpy and more prominently activation entropy in CH3OH activation.

Kinetics of superoxide ion in dimethyl sulfoxide containing ionic liquids

The chemical generation of superoxide ion (O2 •−) by dissolving potassium superoxide was investigated. The chemically generated O2 •− was then used to study the long-term stability of O2 •− in dimethyl sulfoxide (DMSO) containing ionic liquids (ILs) and determine the rate constants based on pseudo 1st- and 2nd-order reactions between the IL and O2 •− generated in DMSO. O2 •− was unstable in DMSO containing [S222][TFSI], [DMIm][MS], [BDMIm][TfO], [EMIm][TFSI], [EMIm][MS], [P14,666][TPTP], [P14,666][TFSI], and all pyridinium-based ILs. In contrast, O2 •− was very stable in DMSO containing [MOPMPip][TFSI], [BMPyrr][DCA], [BMPyrr][TFSI], [BMPyrr][TfO], [HMPyrr][TFSI], [MOEMPip][TPTP], [BMPyrr][TFA], [N112,1O2][TFSI], and [MOEMMor][TFSI]. This shows that these ILs can be used for different applications involving O2 •−. The reaction rate constant, total consumption, and consumption rate of O2 •− were determined. The rate constants of O2 •− in some ILs were found to follow pseudo 1st-order reaction, while in others, pseudo 2nd-order reaction kinetics were observed.

Singlet oxygen O2(b1Σ g + ) production at altitudes of the polar ionosphere

The relative rate constants of O2(b1Σ + , v= 1–4) production at an inelastic interaction between electronically excited N(2D) atoms and O2(X3Σ − , v = 0) oxygen molecules have been calculated. It was shown that an increase in equilibrium distances between oxygen atoms in NO2 quasi-molecule, produced during the interaction, substantially increases the calculated relative production rates of O2(b1Σ + , v > 1). The obtained coefficients are used to calculate the O2(b1Σ + , v= 1–4) relative populations at 110 km (T = 250 K) and 150 km (T = 500 K) altitudes of the polar ionosphere. The calculated populations have been compared with the results of the published measurements of the Atmospheric system band luminosity intensities, and satisfactory agreement has been obtained for low altitudes.

The rate constants of singlet oxygen collision-induced emission at 634 and 703 nm wavelengths

Absolute spectral luminosity from an O2–O2(a)-H2O gas flow formed by a chemical singlet oxygen generator was measured at 600–800 and 1230–1310 nm wavelengths. The results were used to determine the rate constants for O2(a, 0) + O2(a, 0) → O2(X, 0) + O2(X, 0) + hν (λ = 634 nm) and O2(a, 0) + O2(a, 0) → O2(X, 1) + O2(X, 0) + hν (λ = 703 nm) collision-induced emission ((6.72 ± 0.8) × 10−23 and (7.17 ± 0.8) × 10−23 cm3/s, respectively).

Effects of Aqueous Temperature on Sonolysis of Bisphenol A: Rate Constants Increasing with Temperature under Oxygen

Sonolysis of bisphenol A (BPA), a representative endocrine disrupting chemical, has been studied at different temperatures ranging from 5 to 70°C in aqueous solutions under various gases at 489 kHz. The degradation rate constant increased significantly with temperature under O2 and air up to 50°C and 30°C, respectively. In this temperature range, the rate constants for O2 and air were significantly higher than those for Ar and N2, under which they hardly changed with temperature. The degradation rate constants decreased in the order O2 > air > Ar > N2. These results are in contrast with the thermal theory of sonochemistry. By adding tert-butanol as an OH radical scavenger, the degradation rate decreased significantly, revealing that OH radical reactions are responsible for the temperature effects on BPA sonolysis.

Reactivity of Superoxide Radical Anion and Hydroperoxyl Radical with α-Phenyl-N-tert-butylnitrone (PBN) Derivatives

Nitrones have exhibited pharmacological activity against radical-mediated pathophysiological conditions and as analytical reagents for the identification of transient radical species by electron paramagnetic resonance (EPR) spectroscopy. In this work, competitive spin trapping, stopped-flow kinetics, and density functional theory (DFT) were employed to assess and predict the reactivity of O(2)(*-) and HO(2)(*) with various para-substituted alpha-phenyl-N-tert-butylnitrone (PBN) spin traps. Rate constants of O(2)(*-) trapping by nitrones were determined using competitive UV-vis stopped-flow method with phenol red (PR) as probe, while HO(2)(*) trapping rate constants were calculated using competition kinetics with 5,5-dimethylpyrroline N-oxide (DMPO) by employing EPR spectroscopy. The effects of the para substitution on the charge density of the nitronyl-carbon and on the free energies of nitrone reactivity with O(2)(*-) and HO(2)(*) were computationally rationalized at the PCM/B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level of theory. Theoretical and experimental data show that the rate of O(2)(*-) addition to PBN derivatives is not affected by the polar effect of the substituents. However, the reactivity of HO(2)(*) follows the Hammett equation and is increased as the substituent becomes more electron withdrawing. This supports the conclusion that the nature of HO(2)(*) addition to PBN derivatives is electrophilic, while the addition of O(2)(*-) to PBN-type compounds is only weakly electrophilic.

Pressure scaling of an electro-discharge singlet oxygen generator (ED SOG)

This work is devoted to the study of the possibility of obtaining the highest O2(a 1Δg) yield in ED SOG at the high absolute O2(a 1Δg) concentration needed for developing a powerful oxygen–iodine laser pumped by electric discharge. A singlet oxygen was produced in a transversal rf discharge in the pressure range 10–30 Torr of pure oxygen in the small-diameter (7 mm) quartz tube with HgO coating of the inner walls for removing atomic oxygen to eliminate fast O2(a 1Δg) quenching. It is shown that pd scaling (p—pressure, d—tube diameter) of the rf discharge actually allows an increase of the absolute O2(a 1Δg) density. The increase in the rf frequency from 13.56 to 81 MHz results in the essential increase of the O2(a 1Δg) yield (beyond 15% at such a high oxygen pressure as 15 Torr), but the subsequent transfer to the higher rf frequency of 160 MHz only slightly influences the maximally obtained O2(a 1Δg) yield. The effect of the NO admixture on the O2(a 1Δg) production has been also studied. The rate constant of O2(a 1Δg) quenching by was directly measured. The NO admixture (up to 20%) resulted in the noticeable increase in the O2(a 1Δg) yield mainly at low energy inputs. But this gain in the O2(a 1Δg) concentration drops with increasing energy input. Nevertheless it is shown that by combining the O2 + NO mixture with the HgO coating of the discharge tube walls one can provide the O2(a 1Δg) yield on the level of ∼21% at 10 Torr, ∼17% at 20 Torr and ∼13% at 30 Torr of O2 with the efficiency of ∼4–6%. The analysis of the NO admixture influence on the discharge structure and O2(a 1Δg) production has been carried out by using the 2D model. It was found that at the low energy input the NO admixture acts as an easily ionized species that enlarges the region occupied by plasma. Thus, in the O2 + NO discharge the normal current density is lower than in the pure oxygen discharge. As a result a higher energetic efficiency of O2(a 1Δg) production is also observed in the case of the O2 + NO mixture and the low energy input. In order to provide the optimal conditions for O2(a 1Δg) production (with regard to the yield and efficiency) in the continuous wave transversal VHF discharge at such high oxygen pressures as of 10–30 Torr it is necessary to find out the range of energy inputs where the VHF discharge operates in the regime of normal current density on the boundary with the abnormal regime and to remove atomic oxygen produced in the discharge by some volume or surface processes.

Iron Porphyrin−Cyclodextrin Supramolecular Complex as a Functional Model of Myoglobin in Aqueous Solution

The 1:1 inclusion complex of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato iron(II) (Fe(II)TPPS) and an O-methylated beta-cyclodextrin dimer having a pyridine linker (1) binds dioxygen reversibly in aqueous solution. The O2 adduct was very stable (t(1/2) = 30.1 h) at pH 7.0 and 25 degrees C. ESI-MS and NMR spectroscopic measurements and molecular mechanics (MM) calculations indicated the inclusion of the sulfonatophenyl groups at the 5- and 15-positions of Fe(III)TPPS or Fe(II)TPPS into two cyclodextrin moieties of 1 to form a supramolecular 1:1 complex (hemoCD1 for the Fe(II)TPPS complex), whose iron center is completely covered by two cyclodextrin moieties. Equilibrium measurements and laser flash photolysis provided the affinities ( and ) and rate constants for O2 and CO binding of hemoCD1 (k(O2)(on), k(O2)(off), k(CO)(on), and k(CO)(off)). The CO affinity relative to the O2 affinity of hemoCD1 was abnormally high. Although resonance Raman spectra suggested weak back-bonding of d(pi)(Fe) --> pi(CO) and hence a weak CO-Fe bond, the CO adduct of hemoCD1 was very stable. The hydrophobic CO molecule dissociated from CO-hemoCD1 hardly breaks free from a shallow cleft in hemoCD1 surrounded by an aqueous bulk phase leading to fast rebinding of CO to hemoCD1. Isothermal titration calorimetry furnished the association constant (K(O2)), DeltaH degrees , and DeltaS degrees for O2 association to be (2.71 +/- 0.51) x 10(4) M(-1), -65.2 +/- 4.4 kJ mol(-1), and -133.9 +/- 16.1 J mol(-1) K(-1), respectively. The autoxidation of oxy-hemoCD1 was accelerated by H+ and OH-. The inorganic anions also accelerated the autoxidation of oxy-hemoCD1. The O2-Fe(II) bond is equivalent to the O2.--Fe(III) bond, which is attacked by the inorganic anions or the water molecule to produce met-hemoCD1 and a superoxide anion.

Quantitative Determination of 1Σg+ and 1Δg Singlet Oxygen in Solvents of Very Different Polarity. General Energy Gap Law for Rate Constants of Electronic Energy Transfer to and from O2 in the Absence of Charge Transfer Interactions

The quenching of excited triplet states of sufficient energy by O2 leads to O2(1sigma(g)+) and O2(1delta(g)) singlet oxygen and O2(3sigma(g)-) ground-state oxygen as well. The present work investigates the question whether in the absence of charge transfer (CT) interactions between triplet sensitizer and O2 the rate constants of formation of the three different O2 product states follow a generally valid energy gap law. For that purpose, lifetimes of the upper excited O2(1sigma(g)+) have been determined in a mixture of 7 vol % benzene in carbon tetrachloride, in chloroform, and in perdeuterated acetonitrile. They amount to 1.86, 1.40, and 0.58 ns, respectively. Furthermore, rate constants of O2(1sigma(g)+), O2(1delta(g)), and O2(3sigma(g)-) formation have been measured in these three solvents for five pi pi* triplet sensitizers with negligible CT interactions. The rate constants are independent of solvent polarity. After normalization for the multiplicity of the respective O2 product state, the rate constants follow a common dependence on the excess energies of the respective product channels. This empirical energy gap relation describes also quantitatively the rate constants of quenching of O2(1delta(g)) by 28 carotenoids. Therefore, it represents in the absence of CT interactions a generally valid energy gap law for the rate constants of electronic energy transfer to and from O2.

Antioxidative Properties of Martynoside: Pulse Radiolysis and Laser Photolysis Study

Free radical reactions of Martynoside (MAR), a phenylpropanoid glycoside, with a variety of oxidants were studied in the aqueous solution by laser photolysis and pulse radiolysis techniques. The pKa value of MAR in aqueous solution was measured from the pH dependent changes of the UV absorption at 384 nm with value of pKa=9.2. The phenoxyl radical of MAR which exhibits maximum absorption at 360 nm was generated by one-electron transfer to N3• or Br2•-. Other important properties of phenoxyl radical such as extinction coefficient, formation and decay rate constants were also determined. The reaction rate constant of O2•- with MAR, k=8.5×104 dm3 · mol-1 · s-1, was measured by the method of competition kinetics. By measuring time-resolved luminescence emission at 1270 nm, the quenching rate constant of singlet oxygen by MAR was obtained to be 3.3×106 dm3 · mol-1 · s-1. Reduction potential of the MAR couple (MAR•/MAR), determined using rutin as reference compound, gave a value E=0.66 V vs. NHE. The antioxidative properties of MAR were compared with those of some well-known antioxidants.

On the antioxidant properties of therapeutic drugs: quenching of singlet molecular oxygen by aminosalicylic acids

The ability of the widely employed therapeutic drugs 4-aminosalicylic acid and 5-aminosalicylic acid to act as singlet molecular oxygen (O2(1Δg)) scavengers was investigated at pH 7 and pH 12. The isomer 3-aminosalicylic acid was also included in the study for comparative purposes. All three compounds quench photochemically generated O2(1Δg) with rate constants in the range of 107-108 M-1s-1, depending on the experimental conditions. No chemical reaction (oxidation of the aminosalicylic acids) was detected at the neutral pH, whereas at pH 12 both chemical and physical interactions with O2(1Δg) operated. The physical process implies the de-activation of the oxidant species without destruction of the aminosalicylic acid. The quotients between the overall and reactive rate constants for O2(1Δg) quenching at pH 12 (kr/kt ratios), which account for the actual effectiveness of photodegradation, were relatively low (0.22, 0.04, and 0.06 for 3-, 4- and 5-aminosalicylic acids, respectively). This indicates that the drugs, particularly the 4- and 5-amino derivatives, de-activate the excited oxygen species, at both pH values studied, mainly in a physical fashion, preventing its photodegradation and providing an antioxidative protection for possible photo-oxidizable biological targets in the surroundings.

Rate Constants for O2 and CO Binding to the α and β Subunits within the R and T States of Human Hemoglobin

Despite a large amount of work over the past 30 years, there is still no universal agreement on the differential reactivities of the individual alpha and beta subunits in human hemoglobin. To address this question systematically, we prepared a series of hybrid hemoglobins in which heme was replaced by chromium(III), manganese(III), nickel(II), and magnesium(II) protoporphyrin IXs in either the alpha or beta subunits to produce alpha2(M)beta2(Fe)1 and alpha2(Fe)beta2(M) tetramers. None of the abnormal metal complexes react with dioxygen or carbon monoxide. The O2 affinities of the resultant hemoglobins vary from 3 microM-1 (Cr(III)/Fe(II) hybrids) to 0.003 microM-1 (Mg(II)/Fe(II) hybrids), covering the full range expected for the various high (R) and low (T) affinity quaternary conformations, respectively, of human hemoglobin A0. The alpha and beta subunits in hemoglobin have similar O2 affinities in both quaternary states, despite the fact that the R to T transition causes significantly different structural changes in the alpha and beta heme pockets. This functional equivalence almost certainly evolved to maintain high n values for efficient O2 transport.

Collisional removal of O2 (c 1Σ−u, ν=9) by O2, N2, and He

The collisional removal of O2 molecules in selected vibrational levels of the c 1Σ−u state is studied using a two-laser double-resonance technique. The output of the first laser excites the O2 to ν=9 or 10 of the c 1Σ−u state, and the ultraviolet output of the second laser monitors specific rovibrational levels via resonance-enhanced ionization. The temporal evolution of the c 1Σ−u state vibrational level is observed by scanning the time delay between the two pulsed lasers. Collisional removal rate constants for c 1Σ−u, ν=9 colliding with O2, N2, and He are (5.2±0.6)×10−12, (3.2±0.4)×10−12, and (7.5±0.9)×10−12 cm3 s−1, respectively. As the rate constants for O2 and N2 are similar in magnitude, N2 collisions dominate the removal rate in the earth’s atmosphere. For ν=10 colliding with O2, we find a removal rate constant that is 2–5 times that for ν=9 and that single quantum collision cascade is an important pathway for removal.

Laboratory evidence for a possible non‐LTE mechanism of stratospheric ozone formation

Experimental evidence is given that supports the possibility of a previously unknown non‐LTE mechanism for stratospheric ozone formation, which could have a significant impact on the stratospheric ozone budget even if the quantum yield for production of highly vibrationally excited O2 in reaction (1), (averaged over all wavelengths shorter than 243nm) were as low as 0.2%. Stimulated emission pumping enabled preparation of individual vibrational states of O2(X³Σg−,19≤v≤27) and laser induced fluorescence was used to follow the time evolution of the prepared states and thereby determine the vibrational‐state‐specific total‐removal rate‐constants for relaxation by O2 and N2 at 295K. Self‐relaxation shows a sharp threshold for enhanced relaxation near the energy of O2(X³Σg−, v=26) which is coincident with the energetic threshold for reaction (2). The magnitudes of the self‐relaxation rate constants for O2(X³Σg−, v=26 and 27) are quantitatively consistent with the kinetic parameters of reaction (2). Relaxation by N2, while important for lower O2 vibrational states, is shown to be about 10 and 200 times slower than self relaxation for v=26 and v=27, respectively. These are the first two vibrational states of O2 that could form O3 via reaction (2).

Improvement of a direct spectrophotometric assay for routine determination of superoxide dismutase activity

The growing interest in measuring superoxide dismutase (EC 1.15.1.1) in many diseases calls for useful routine assays. For this purpose, the direct spectrophotometric method of Marklund (J Biol Chem 1976;251:7504-7) was improved to offer an alternative to the imprecise, indirect assays currently used. The decay of O2.- (from KO2) at pH 9.5 was monitored as the decrease in delta A (delta A = A250nm-A360nm). Superoxide dismutase was determined from the pseudo-first-order rate constant of O2.- dismutation. The precision of the assay was improved by increasing the concentration of O2.- and expanding the interval for measurements of O2.- concentrations to 4-16 mumol/L. Other assay characteristics, including temperature, were also optimized. In hemolysate the assay had a within-day CV of 5.5-13% and a between-day CV of 4%. Mn-superoxide dismutase and some superoxide dismutase mimics are inhibited at alkaline pH. Therefore, the method is primarily recommended for Cu,Zn-superoxide dismutase.

Rate constants of O2(1Δg)

AbstractThe rate constants of O2(1Δg) with aliphatic alcohols, terpenes, unsaturated hydrocarbons, chlorinated hydrocarbons, oxygen, and diamines have been studied in thepresence of NO2. The rate constants for oxygen, 1,2‐ethane diamine, and 1,2‐propane diamine are (9.9 ± 0.4) × 102, (8.7 ± 0.7) × 104, and (1.4 ± 0.3) × 104 1/mol/s, respectively. The rate constants for all other compounds are less than the oxygen rate constant.

A reinvestigation of the temperature dependence of the rate constant for the reaction O + O2+ M → O3+ M (for M = O2, N2, and Ar) by the flash photolysis resonance fluorescence technique

AbstractThe flash photolysis resonance fluorescence technique has been used to reinvestigate the kinetics of the oxygen atom–oxygen molecule combination reaction. Third‐order rate constants for O2, N2, and Ar as deactivant molecules were determined over the temperature range of 219–368 K. The results presented herein are the most extensive data sets available for atmospheric modeling and are used to formulate a recommendation for such purposes. The recommended rate expressions areorComparisons of these results with existing literature data are presented.