Molecular Oxygen 1O2 Research Articles

Laser-induced desorption of atomic and molecular fragments from a tin dioxide surface modified by a thin organic covering of copper phthalocyanine

The systematic features of laser-induced desorption from an SnO2 surface exposed to 10-ns pulsed neodymium laser radiation are studied at the photon energy 2.34 eV, in the range of pulse energy densities 1 to 50 mJ/cm2. As the threshold pulse energy 28 mJ/cm2 is achieved, molecular oxygen O2 is detected in the desorption mass spectra from the SnO2 surface; as the threshold pulse energy 42 mJ/cm2 is reached, tin Sn, and SnO and (SnO)2 particle desorption is observed. The laser desorption mass spectra from the SnO2 surface coated with an organic copper phthalocyanine (CuPc) film 50 nm thick are measured. It is shown that laser irradiation causes the fragmentation of CuPc molecules and the desorption of molecular fragments in the laser pulse energy density range 6 to 10 mJ/cm2. Along with the desorption of molecular fragments, a weak desorption signal of the substrate components O2, Sn, SnO, and (SnO)2 is observed in the same energy range. Desorption energy thresholds of substrate atomic components from the organic film surface are approximately five times lower than thresholds of their desorption from the atomically clean SnO2 surface, which indicates the diffusion of atomic components of the SnO2 substrate to the bulk of the deposited organic film.

Measurement of NADPH Oxidase Activity in Plants

NADPH oxidase is a membrane-bound enzyme that generates (O2-) by transferring electrons from NADPH to molecular oxygen O2. O2-is spontaneously dismasted to the more stable form H2O2. Both O2-and H2O2 are forms ofreactive oxygen species (ROS), which are involved in regulation of many cellular activities such as transcription, intracellular signaling, and host defense. The NADPH oxidase - dependent generation of O2- in total membrane fraction of plant tissue has been determined by the reduction of the tetrazolium salt XTT by O2-. In the presence of O2-, XTT generates a soluble yellow formazan that can be quantified spectrophotometrically.

Interaction Mechanisms between ArO2 Post‐discharge and Biphenyl

AbstractInteractions between a late ArO2 post‐discharge and biphenyl (C6H5)2 are studied. Thin films grown by spin‐coating are efficiently etched by the post‐discharge to get a clean surface after treatment. Thin films are strongly preferentially oriented. But this orientation does not depend on the way the sample is pretreated. The pretreatment can create cracks and modify the etching rate. The etching of the biphenyl occurs by interaction with the singlet state of molecular oxygen O2(a1Δg). [4 + 2] cycloaddition is assumed to be the main process leading to ring opening. Next, a large variety of compounds including alcohols, ketones, acids, and aldehydes are created. Atomic oxygen does not seem to play a significant role in the etching process but it functionalizes the biphenyl by creating alcohol groups.magnified image

Electronic kinetics of molecular nitrogen and molecular oxygen in high-latitude lower thermosphere and mesosphere

Abstract. Total quenching rate coefficients of Herzberg states of molecular oxygen and three triplet states of molecular nitrogen in the collisions with O2 and N2 molecules are calculated on the basis of quantum-chemical approximations. The calculated rate coefficients of electronic quenching of O2* and N2* molecules show a good agreement with available experimental data. An influence of collisional processes on vibrational populations of electronically excited N2 and O2 molecules is studied for the altitudes of high-latitude lower thermosphere and mesosphere during auroral electron precipitation. It is indicated that molecular collisions of metastable nitrogen N2(A3Σu*) with O2 molecules are principal mechanism in electronic excitation of both Herzberg states c1Σu&minus, A'3Δu, A3Σu+ and high vibrational levels of singlet states a1Δg and b1Σg+ of molecular oxygen O2 at these altitudes.

On the Potential Use of Squaraine Derivatives as Photosensitizers in Photodynamic Therapy: A TDDFT and RICC2 Survey.

A time-dependent density functional theory (TDDFT) and the second-order approximated coupled-cluster model with the resolution of identity approximation (RICC2) studies are reported here for some classes of squaraine derivatives. These compounds have a sharp electronic band, ranging from the visible to near-red part of the spectrum, with an high molar absorption coefficient. These features make them potential photosensitizers in the photodynamic therapy of cancer (PDT), in which a light source, a photosensitizer, and molecular oxygen ((3)O2) are combined to give cytotoxic singlet oxygen ((1)O2) as a final result in a photochemical process. For the examined structures, the introduction of different substituents (electron donating, electron withdrawing, or fused rings) in the parent molecule, in order to give different squaraine derivatives, changes the maximum absorption wavelength (λmax) from 620 to 730 nm, giving a near-red absorbing photosensitizer that can better penetrate human tissue to damage tumor cells. Theoretical results, obtained from both TDDFT/PBE0 and RICC2, are able to reproduce qualitatively the substitution effect on λmax, resulting in a useful tool for testing different structure modifications and, in general, for the molecular design of PDT photosensitizers. Calculated vertical excitation energies (singlet-singlet transitions) generally agree with experimental data within 0.3 eV. The singlet oxygen generation ability of these compounds requires that their triplet energy, for a type II reaction mechanism, should be greater than 0.98 eV. Theoretical triplet energies from the RICC2 method suggests that this requisite is fulfilled for all compounds, though the results are generally overestimated with respect to experiment by 0.7 eV, whereas TDDFT/PBE0 triplet energies, which are underestimated within 0.2 eV in few cases, lie close to the above-mentioned limit and can be considered suitable for PDT applications.

Seasonal features in the response of the mesopause temperature and emission intensities to solar activity variations

The seasonal dependences of the response of the hydroxyl ((6–2) band) and molecular oxygen O2(b1Σg+) ((0–1) band) emission intensities, temperature, and density indicator in the region of the hydroxyl emission maximum (87 km) to solar activity have been obtained based on the spectral observations of the mesopause emissions at Zvenigorod observatory during 2000–2007. The ratio of the OH (7–3) and (9–4) band intensities, characterizing the behavior of the vibrational temperature, has been used as an indicator of density. It has been established that the response of the studied mesopause characteristics to solar activity is positive in all seasons. In winter the response is maximal in the intensities and temperature and is minimal in the density indicator. The main mechanisms by which solar activity affects the mesopause characteristics have been considered. The behavior of the internal gravity waves with periods of 0.33–7 h depending on solar activity has been studied. It has been noted that these waves become more active at a minimum of the 11-year solar cycle.

“Inversion substitution” reactions with participation of molecular oxygen: ZH3I + O2 → O2ZH3 + I (Z = C, Si). The potential energy surface and rate constants

A new class of reactions of molecular oxygen O2 + ZH3I → O2ZH3 + I (Z = C, Si) proceeding by the mechanism of “inversion substitution” was investigated by quantum chemistry methods and the transition state theory (TST). The profiles of the potential energy surfaces (PES) along the reaction coordinate and the characteristics of transition states were calculated using the DFT approach with the B3LYP hybrid functional and the DZVP basis set. The characteristics of the transition states were then used for TST calculations of the rate constants for the direct and reverse “inversion substitution” reactions and their temperature dependences in the temperature interval 273–2000 K. The activation barriers to the substitution reactions under study were found to be substantially lower than the barriers to the abstraction reactions O2 + ZH3I → ZH2I + HO2 (by 16.3 kcal mol−1 for Z = C and by 7.2 kcal mol−1 for Z = Si). The results obtained show that the “inversion substitution” reactions dominate over the abstraction reactions in the interaction of molecular oxygen with carbon- and silicon-centered iodides as well as (probably) many other substrates.

Oxidation of 2‘-Deoxyguanosine 5‘-Monophosphate Photoinduced by Pterin: Type I versus Type II Mechanism

UV-A radiation (320-400 nm) induces damage to the DNA molecule and its components through different photosensitized reactions. Among these processes, photosensitized oxidations may occur through electron transfer or hydrogen abstraction (type I) and/or the production of singlet molecular oxygen ((1)O2) (type II). Pterins, heterocyclic compounds widespread in biological systems, participate in relevant biological processes and are able to act as photosensitizers. We have investigated the photosensitized oxidation of 2'-deoxyguanosine 5'-monophosphate (dGMP) by pterin (PT) in aqueous solution under UV-A irrradiation. Kinetic analysis was employed to evaluate the participation of both types of mechanism under different pH conditions. The rate constant of (1)O2 total quenching (k(t)) by dGMP was determined by steady-state analysis of the (1)O2 NIR luminescence, whereas the rate constant of the chemical reaction between (1)O2 and dGMP (k(r)) was evaluated from kinetic analysis of concentration profiles obtained by HPLC. The results show that the oxidation of dGMP photosensitized by PT occurs through two competing mechanisms that contribute in different proportions depending on the pH. The dominant mechanism in alkaline media involves the reaction of dGMP with (1)O2 produced by energy transfer from the PT triplet state to molecular oxygen (type II). In contrast, under acidic pH conditions, where PT and the guanine moiety of dGMP are not ionized, the main pathway for dGMP oxidation involves an initial electron transfer between dGMP and the PT triplet state (type I mechanism). The biological implications of the results obtained are also discussed.

Oxidation of Flame Retardant Tetrabromobisphenol A by Singlet Oxygen

Wide use of flame retardants can pose an environmental hazard, and it is of interest to investigate how they may degrade. We report here that 3,3',5,5'-tetrabromobisphenol A (TBBPA) is subject to photosensitized oxidation involving singlet molecular oxygen ((1)O2). By using visible light and rose bengal or methylene blue as 102 photosensitizers, we have found that TBBPA is a 102 quencher. The quenching rate constant, k(q), depends on TBBPA ionization (pK = 7.4). In acetonitrile, where TBBPA is undissociated, the kq value is 6.1 x 10(5) M(-1) s(-1) for a TBBPA monomer and decreases to 2.9 x 10(4) M(-1) s(-1) for TBBPA dimers and/or aggregates. TBBPA dissociates in aqueous solutions, and its kq value is 1.44 x 10(9) M(-1) s(-1) in alkaline solution, decreasing to 3.9 x 10(8) M(-1) s(-1) at pH 7.2. The strong 102 quenching by TBBPA anion initiates an efficient oxidation of TBBPA, which results in oxygen consumption in aqueous micellar (e.g., Triton X-100) solutions containing photosensitizer. This oxygen consumption is mediated by transient radical species, which we detected by using EPR spectroscopy. We observed two major radicals and one minor radical generated from TBBPA by reaction with 102 at pH 10. One was identified as the 2,6-dibromo-p-benzosemiquinone radical (a2H = 2.36 G, g = 2.0056). A second radical (aH = 2.10 G, g = 2.0055) could not be identified butwas probably a 2,6-dibromo-p-benzosemiquinone radical containing an EPR-silent substituent at the 3-position. Spin trapping with 5,5-dimethyl-1-pyrroline N-oxide (DPMO) showed that other minor radicals (hydroxyl, carbon-centered) are also generated during the reaction of TBBPA with (1)O2. The photosensitized production of radicals and oxygen consumption were completely inhibited by the azide anion, an efficient physical (1)O2 quencher. Because TBBPA is a stable compound that at neutral pH does not absorb much of the atmosphere-filtered solar radiation, its photosensitized oxidation by (1)O2 may be the key reaction initiating or mediating TBBPA degradation in the natural environment.

Retrieval of total ozone in the mesosphere with a new model of electronic-vibrational kinetics of O3 and O2 photolysis products

The paper presents a new model of electronic-vibrational kinetics of the products of ozone and molecular oxygen photodissociation in the terrestrial middle atmosphere. The model includes 45 excited states of the oxygen molecules O2(b 1, Σ + ,v= 0−2), O2 (a 1Δ g , v= 0−5), and O2(X 3Σ − , v= 1−35) and of the metastable atom O (1 D) and over 100 aeronomic reactions. The model takes into account the dependence of quantum yields of the production of O2(a 1Δ g , v= 0−5) in a singlet channel of ozone photolysis in the Hartley band on the wavelength of photolytic emission. Taking account of the electronic-vibrational kinetics is important in retrieval of the vertical profiles of ozone concentration from measured intensities of the Atm and IR Atm emissions of the oxygen bands above 65 km and leads to an increase in the ozone concentration retrieved from the 1.27-µm emission, in contrast to the previous model of pure electronic kinetics. Sensitivity analysis of the new model is made for variations in the concentrations of atmospheric constituents ([O2], [N2], [O(3P)], [O3], [CO2]), the gas temperature, rate constants of the reactions, and quantum yields of the reaction products. A group of reactions that most strongly affect the uncertainty of ozone retrieval from measured intensities of atmospheric emissions of molecular oxygen O2(b 1Σ + , v) and O2(a 1Δ g , v) has been determined.

Biocatalytic Single‐Step Alkene Cleavage from Aryl Alkenes: An Enzymatic Equivalent to Reductive Ozonization

O2 can do: Innocuous molecular oxygen O2 is the only reagent needed to perform highly chemoselective biocatalytic single-step alkene-cleavage reactions (see scheme). The products are analogous to those of (reductive) ozonization and related metal-based methods. In contrast neither special equipment nor an additional reducing agent is required. The biocatalytic reaction can be performed at ambient temperature. Depending on the substrate, aldehydes or ketones are obtained.

Exploration of Mars in the SPICAM-IR experiment onboard the Mars-Express spacecraft: 2. Nadir observations: Simultaneous observations of water vapor and O2 glow in the Martian atmosphere

The SPICAM experiment onboard the Mars-Express spacecraft includes sounding the Martian atmosphere in the ultra-violet (118–320 nm) and near IR (1–1.7 μm) ranges. The infrared spectrometer operates in the range of 1–1.7 μm with a resolution of 3.5 cm−1 in the mode of nadir observations and solar and stellar occulations. This paper is devoted to analyzing the basic results of nadir observations of the infra-red SPICAM channel during the first Martian year of the instrument operation: from January 2004 to November 2005. One of the primary goals of SPICAM-IR is water vapor monitoring in the atmosphere of Mars in the band of 1.37 μm and ozone abundance determination from the day-time airglow of molecular oxygen O2(a 1Δg) in the band of 1.27 μm. Simultaneous measurements of these minor constituents of the planet are necessary for understanding photochemical processes in the Martian atmosphere. The degree of their anticorrelation and a comparison with the results of photochemical modeling of the atmosphere will contribute to our knowledge of the Martian atmosphere stability.

Effects of temperature and flux on oxygen bubble formation in Li borosilicate glass under electron beam irradiation

Oxygen bubble formation and evolution under a 300keV electron beam are analyzed in a Li borosilicate glass under different irradiation conditions: temperature, flux, and dose. Oxygen bubbles are observed to form in a delimited flux and temperature region with a threshold requirement. This region ranges between 100 and 300°C for the temperatures and between 1019 and 1021em−2s−1 for the electron fluxes. In situ transmission electron microscopy allows the bubble evolution kinetics to be described as a four-step process. An incubation step is followed by a growth phase of sigmoid type. For high values of temperature and/or flux the saturation growth and the subsequent dissolution of the bubbles are also observed. The bubble evolution kinetics can be explained by considering the production and diffusion mechanisms of the molecular oxygen O2 as a function of the temperature and the electron flux.

Unusually Large Dynamic Electron Polarization in an O2(1Δg)−2,2,6,6-Tetramethylpiperidine-1-oxyl Radical System

Net absorptive (Abs) chemically induced dynamic electron polarization (CIDEP) is observed in the system of the lowest excited singlet molecular oxygen O2(1Δg) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical in benzene by means of the time-resolved ESR (TR-ESR) spectroscopy. As a result of (1) the analysis of CIDEP time profiles by the modified Bloch and kinetic equations and (2) the quenching of CIDEP by triethylamine, it is confirmed that the Abs CIDEP is created through the interaction of O2(1Δg) with TEMPO. The creation of Abs CIDEP is reasonably interpreted in terms of the radical-triplet pair mechanism (RTPM) of the O2(3Σg-)−TEMPO pair with doublet precursor. The absolute magnitude of net Abs CIDEP created on TEMPO is experimentally estimated to be 340 ± 40 in the unit of Boltzmann polarization. To our knowledge, this is one of the largest magnitudes of CIDEP measured so far by the TR-ESR method. Based upon the RTPM theory, the origin of this unusually large CIDEP is attributed to the large ...

Photodissociation of O2 via the Herzberg continuum: Measurements of O-atom alignment and orientation

Irradiation of molecular oxygen O2 in the region of the Herzberg continuum between 218 nm and 239 nm results in the production of open-shell photofragments O(3P)+O(3P). Product O(3Pj; j=0,1,2) atoms were ionized using resonantly enhanced multiphoton ionization (2+1 REMPI) near 225 nm and the ions collected in a velocity-sensitive time-of-flight mass spectrometer. By controlling the polarization of the photolysis and ionization radiation, we have measured alignment and orientation parameters of O-atom electronic angular momentum (j) in the molecule frame. The results show alignment from both parallel and perpendicular transitions that are cylindrically symmetric about the velocity (v) of the recoiling O atom. We also observe electronic alignment that is noncylindrically symmetric about v, resulting from coherence between multiply excited dissociative states. Photodissociation with linearly polarized light is shown to produce O atoms that are oriented in the molecule frame, resulting from interference between parallel and perpendicular dissociative states of O2. Semiclassical calculations that include spin–orbit coupling between six excited states reproduce closely the observed polarization.

Singlet molecular oxygen quenching ability of carotenoids in a reverse-micelle membrane mimetic system.

The influence of the medium heterogeneity upon the bimolecular rate constants for the physical quenching, kq, and chemical quenching, kr, of singlet molecular oxygen O2(1deltag) by seven natural and three synthetic carotenoids (CAR) with different substituent patterns was studied in a reverse micelle system of sodium bis(2-ethylhexyl)sulfosuccinate, hexane and water. Because O2(1deltag) was generated inside the water pools of the reverse micelles by photosensitization of the water-soluble dye rose bengal and the CAR are mainly located in the external hexane pseudophase, the quenching process was interpreted using a pseudophase model for the partition of 02(1deltag) between the water pools and the organic pseudophases. The kq values were mainly dependent on the extent of the double-bond conjugation of the CAR, as demonstrated by a good empirical relationship between log(k(q)) and the energy E(S) of the longest wavelength transition pi-->pi* of the CAR. In contrast, the kr values were almost independent of the extent of the double-bond-conjugated system and about four orders of magnitude lower than kq. However, in all cases, CAR photobleaching was observed with the formation of various oxidation products, depending on the photosensitization time. Chromatographic and spectroscopic product analysis for the reaction products of beta-carotene with O2(1deltag) indicated the formation of the beta-carotene-5,8-endoperoxide as the primary oxidation product.

Time-resolved spectroscopy of sulfur- and carboxy-substituted N-alkylphthalimides.

The photophysical and photochemical properties of N-phthaloyl-methionine (1), S-methyl-N-phthaloyl-cysteine methyl ester (2) and N-phthaloyltranexamic acid (3) were studied by time-resolved UV/Vis spectroscopy, using laser pulses at 248 or 308 nm. The quantum yield of fluorescence is low (phi(f)< 10(-2)) for 1-3 in fluid and glassy media, whereas that of phosphorescence is large (0.3-0.5) in ethanol at - 196 degrees C. The triplet properties were examined in several solvents, at room temperature and below. The spectra and decay kinetics are similar, but the population of the pi(pi*) triplet state, as measured by T-T absorption, is much lower for 1 and 2 than for 3 or N-methyltrimellitimide (5') at ambient temperatures. The quantum yield (phi(delta)) of singlet molecular oxygen O2(1deltag) formation is substantial for 3 and 5' in several air- or oxygen-saturated solvents at room temperature, but small for 2 and 1. The quantum yield of decomposition is substantial (0.2-0.5) for 3 and small (<0.05) for 2 and 1. It is postulated that photoinduced charge separation in the spectroscopically undetectable 3n,pi* state may account for the cyclization products of 1 and 2. In aqueous solution, this also applies for 3, whereas in organic solvents cyclization involves mainly the lower lying 3pi,(pi*) state. Triplet acetone, acetophenone and xanthone are quenched by 1-3 in acetonitrile; the rate constant is close to the diffusion-controlled limit, but smaller for benzophenone. While the energy transfer from the triplet ketone occurs for 3, a major contribution of electron transfer to the N-phthalimide derivative is suggested for 1 and 2, where the radical anion of benzophenone or 4-carboxybenzophenone is observed in alkaline aqueous solution.

Charge transfer induced quenching of triplet sensitizers by ground state oxygen and of singlet oxygen by ground state sensitizers: A common deactivation channel

The photosensitized production of singlet molecular oxygen O2(1Δg) during oxygen quenching of excited triplet states, T1, and the deactivation of O2(1Δg) by the sensitizer have been investigated for a set of biphenyl derivatives, in carbon tetrachloride, dichloromethane and acetonitrile. The rate constants of the quenching of T1 by ground state oxygen, O2(3Σg−), leading to O2(3Σg−) and of O2(1Δg) quenching by ground state sensitizers, S0, have been subjected to a common analysis. It is shown that the rate constants for both processes are described by one Marcus-type plot in each solvent. This indicates that both processes proceed ia exciplexes (T13Σ) and (S01Δ) of the same partial charge transfer (pCT) from sensitizer to O2 . These findings are in accordance with results obtained previously with a series of naphthalene derivatives. Further comparative analysis leads to a relation for the estimation of the average charge transfer character of the (T13Σ) and (S01Δ) pCT complexes, which is consistent with the existing concept of the reorganization energy. The average charge transfer character varies only slightly between the series of biphenyl and naphthalene derivatives, and is shown to increase in each case with solvent polarity. The overall reorganization energies could be separated into constant intramolecular and solvent-dependent outer reorganization contributions. The small but significant differences in the reorganization energy indicate that the structure of pCT complexes varies between these two series.

Proton and electron impact on molecular and atomic oxygen: I. High resolution fluorescence spectra in the visible and VUV spectral range and emission cross-sections for dissociative ionisation and excitation of

Molecular oxygen O2 was dissociated in collisions with protons and electrons in the intermediate velocity range (p+-energies: 17-800 keV, e--energies: 0.2-2 keV). Fluorescence from excited atomic and singly ionised fragments and from singly ionised molecules was detected in the VUV and in the visible and near UV spectral range. Highly resolved spectra are presented for the VUV (46-131 nm) and the near UV/visible (340-605 nm) spectral range. Absolute emission cross-sections have been determined for dissociative ionisation and excitation leading to fluorescence in the VUV. Results are compared with published data.

Tetra(1,1,4,4-tetramethyl-6,7-tetralino)porphyrazine as a novel luminescence sensor of laser-induced singlet oxygen generation in solutions

Absorption and fluorescence spectra and quantum yields of fluorescence and singlet oxygen generation were studied in air-saturated solutions of a newly synthesised dye, tetra(1,1,4,4-tetramethyl-6,7-tetralino)porphyrazine (TMTP), in benzene. Photophysical properties of TMTP are similar to those of previously studied tetra(4-tert-butyl) phthalocyanine (TBPc). However, the TMTP absorption and fluorescence bands are shifted to the longer wavelengths compared with those of TBPc. The laser-induced generation of singlet molecular oxygen 1O2 in TMTP solutions caused strong singlet oxygen-sensitised delayed fluorescence of the dye. The efficiency of singlet-oxygen-sensitised formation of excited TMTP molecules is two times higher than that of TBPc. It is shown that TMTP can serve as an efficient luminescence sensor of singlet oxygen.