Singlet O2 Research Articles

Neutrophils Generate Microparticles during Exposure to Inert Gases Due to Cytoskeletal Oxidative Stress

This investigation was to elucidate the mechanism for microparticle (MP) formation triggered by exposures to high pressure inert gases. Human neutrophils generate MPs at a threshold of ∼186 kilopascals with exposures of 30 min or more. Murine cells are similar, but MP production occurs at a slower rate and continues for ∼4 h, whether or not cells remain under pressure. Neutrophils exposed to elevated gas but not hydrostatic pressure produce MPs according to the potency series: argon ≃ nitrogen > helium. Following a similar pattern, gases activate type-2 nitric-oxide synthase (NOS-2) and NADPH oxidase (NOX). MP production does not occur with neutrophils exposed to a NOX inhibitor (Nox2ds) or a NOS-2 inhibitor (1400W) or with cells from mice lacking NOS-2. Reactive species cause S-nitrosylation of cytosolic actin that enhances actin polymerization. Protein cross-linking and immunoprecipitation studies indicate that increased polymerization occurs because of associations involving vasodilator-stimulated phosphoprotein, focal adhesion kinase, the H(+)/K(+) ATPase β (flippase), the hematopoietic cell multidrug resistance protein ABC transporter (floppase), and protein-disulfide isomerase in proximity to short actin filaments. Using chemical inhibitors or reducing cell concentrations of any of these proteins with small inhibitory RNA abrogates NOS-2 activation, reactive species generation, actin polymerization, and MP production. These effects were also inhibited in cells exposed to UV light, which photoreverses S-nitrosylated cysteine residues and by co-incubations with the antioxidant ebselen or cytochalasin D. The autocatalytic cycle of protein activation is initiated by inert gas-mediated singlet O2 production.

Gold nanoshells-mediated bimodal photodynamic and photothermal cancer treatment using ultra-low doses of near infra-red light

Previously, gold nanoshells were shown to be able to effectively convert photon energy to heat, leading to hyperthermia and suppression of tumor growths in mice. Herein, we show that in addition to the nanomaterial-mediated photothermal effects (NmPTT), gold nanoshells (including, nanocages, nanorod-in-shell and nanoparticle-in-shell) not only are able to absorb NIR light, but can also emit fluorescence, sensitize formation of singlet oxygen and exert nanomaterial-mediated photodynamic therapeutic (NmPDT) complete destruction of solid tumors in mice. The modes of NmPDT and NmPTT can be controlled and switched from one to the other by changing the excitation wavelength. In the in vitro experiments, gold nanocages and nanorod-in-shell show larger percentage of cellular deaths originating from NmPDT along with the minor fraction of NmPTT effects. In contrast, nanoparticle-in-shell exhibits larger fraction of NmPTT-induced cellular deaths together with minor fraction of NmPDT-induced apoptosis. Fluorescence emission spectra and DPBF quenching studies confirm the generation of singlet O2 upon NIR photoirradiation. Both NmPDT and NmPTT effects were confirmed by measurements of reactive oxygen species (ROS) and subsequent sodium azide quenching, heat shock protein expression (HSP 70), singlet oxygen sensor green (SOSG) sensing, changes in mitochondria membrane potential and apoptosis in the cellular experiments. In vivo experiments further demonstrate that upon irradiation at 980 nm under ultra-low doses (∼150 mW/cm2), gold nanocages mostly exert NmPDT effect to effectively suppress the B16F0 melanoma tumor growth. The combination of NmPDT and NmPTT effects on destruction of solid tumors is far better than pure NmPTT effect by 808 nm irradiation and also doxorubicin. Overall, our study demonstrates that gold nanoshells can serve as excellent multi-functional theranostic agents (fluorescence imaging + NmPDT + NmPTT) upon single photon NIR light excitation under ultra-low laser doses.

Tunable Oxygen Activation for Catalytic Organic Oxidation: Schottky Junction versus Plasmonic Effects

The charge state of the Pd surface is a critical parameter in terms of the ability of Pd nanocrystals to activate O2 to generate a species that behaves like singlet O2 both chemically and physically. Motivated by this finding, we designed a metal-semiconductor hybrid system in which Pd nanocrystals enclosed by {100} facets are deposited on TiO2 supports. Driven by the Schottky junction, the TiO2 supports can provide electrons for metal catalysts under illumination by appropriate light. Further examination by ultrafast spectroscopy revealed that the plasmonics of Pd may force a large number of electrons to undergo reverse migration from Pd to the conduction band of TiO2 under strong illumination, thus lowering the electron density of the Pd surface as a side effect. We were therefore able to rationally tailor the charge state of the metal surface and thus modulate the function of Pd nanocrystals in O2 activation and organic oxidation reactions by simply altering the intensity of light shed on Pd-TiO2 hybrid structures.

Tunable Oxygen Activation for Catalytic Organic Oxidation: Schottky Junction versus Plasmonic Effects

AbstractThe charge state of the Pd surface is a critical parameter in terms of the ability of Pd nanocrystals to activate O2 to generate a species that behaves like singlet O2 both chemically and physically. Motivated by this finding, we designed a metal–semiconductor hybrid system in which Pd nanocrystals enclosed by {100} facets are deposited on TiO2 supports. Driven by the Schottky junction, the TiO2 supports can provide electrons for metal catalysts under illumination by appropriate light. Further examination by ultrafast spectroscopy revealed that the plasmonics of Pd may force a large number of electrons to undergo reverse migration from Pd to the conduction band of TiO2 under strong illumination, thus lowering the electron density of the Pd surface as a side effect. We were therefore able to rationally tailor the charge state of the metal surface and thus modulate the function of Pd nanocrystals in O2 activation and organic oxidation reactions by simply altering the intensity of light shed on Pd–TiO2 hybrid structures.

Air–water ‘tornado’-type microwave plasmas applied for sugarcane biomass treatment

The production of cellulosic ethanol from sugarcane biomass is an attractive alternative to the use of fossil fuels. Pretreatment is needed to separate the cellulosic material, which is packed with hemicellulose and lignin in cell wall of sugarcane biomass. A microwave ‘tornado’-type air–water plasma source operating at 2.45 GHz and atmospheric pressure has been applied for this purpose. Samples of dry and wet biomass (∼2 g) have been exposed to the late afterglow plasma stream. The experiments demonstrate that the air–water highly reactive plasma environment provides a number of long-lived active species able to destroy the cellulosic wrapping. Scanning electron microscopy has been applied to analyse the morphological changes occurring due to plasma treatment. The effluent gas streams have been analysed by Fourier-transform infrared spectroscopy (FT-IR). Optical emission spectroscopy and FT-IR have been applied to determine the gas temperature in the discharge and late afterglow plasma zones, respectively. The optimal range of the operational parameters is discussed along with the main active species involved in the treatment process. Synergistic effects can result from the action of singlet O2(a 1Δg) oxygen, NO2, nitrous acid HNO2 and OH hydroxyl radical.

Thiocyanate Blocks Peroxidase-Dependent Extracellular Trap (ET) Formation By PMN and Eosinophils: Heme Is a Potent New Agonist For The ET Pathway

PMN and eosinophils (EO) activated by physiologic agonist such as LPS, C5a, GM-CSF and IL-5 form extracellular traps (ET) comprised of extracellular DNA, histones, and active secondary granule constituents that retain microbicidal capacity and thus contribute to host defense but also participate in the pathogenesis of sepsis, microangiopathy, vasculitis, asthma and thrombosis. Previous studies have implicated superoxide, H2O2 and myeloperoxidase (MPO) in PMN ET formation, which reflects a novel death pathway dubbed “ETosis.” Thiocyanate (SCN-) is the principle physiologic substrate for eosinophil peroxidase (EPO) and a major substrate for MPO. Because previous studies of ET were all performed in the absence of SCN- and HOSCN, the product of SCN- peroxidation, is a weak, sulfhydryl-specific oxidant markedly less toxic than HOCl or HOBr, we hypothesized SCN- blocks PMN and EO ET formation. We also hypothesized that heme, elevated levels of which occur in intravascular hemolytic states such as sickle cell disease, induces PMN ETosis because both LPS and heme signal by engaging TLR4. We assessed ET formation by human PMN stimulated with either GM-CSF + C5a or 10 µM heme in RPMI one hour using glass slide-attached leukocytes with both cell impermeant (Sytox Orange) and permeant (Syto13) DNA stains and IF localization of secondary granule proteins using confocal microscopy. ET formation in response to to both stimuli was 5-20% of PMN in stimulated cells (vs. 0% in control). The NADPH oxidase inhibitor DPI, MPO inhibitor 4-ABAH and 1 mM SCN- all diminished ET formation by >80% in response to both stimuli. Heme-dependent ET formation was inhibited 80% by the TLR4 antagonist TAK242 (see figure below in which intracellular DNA stains green and extracellular and membrane-compromised PMN DNA stains orange). PMA-induced PMN ET formation requires singlet O2 resulting from the secondary reaction of HOCl with excess H2O2. In PMN stimulated with GM-CSF + C5a, the singlet O2 scavenger edaravone (10 µM) inhibited ETs by 80%. ETs also formed in 4-8% of EOs stimulated by IL-5 + C5a, vs. 0% in control EO and 22.5% in unstimulated EO supplemented with the alternative EPO substrate 1 mM Br-. The EPO inhibitor resorcinol decreased by >90% EO ET formation stimulated with by IL-5 + C5a or by adding Br-, as did addition of SCN- or edaravone. These studies show that both PMN and EO ETosis depend upon MPO- and EPO- generated oxidants, respectively, and the presence of an alternative physiologic substrate, SCN-, markedly antagonizes ETosis. We propose that SCN- inhibits ET formation by generating HOSCN that reacts with H2O2 to yield cyanate, not singlet O2. In addition, we also demonstrate that heme is a potent inducer of PMN ETosis through a mechanism dependent upon TLR4, NADPH oxidase and MPO, raising the possibility that this pathway may directly promote the inflammation and thrombosis that contribute of the pathogenesis of sickle cell disease. Based on these findings, we speculate that dietary augmentation of SCN- to normal or supranormal levels might have clinically beneficial therapeutic effects in a variety of inflammatory states, including sickle cell disease. [Display omitted] Disclosures:No relevant conflicts of interest to declare.

Numerical study of the enhancement of combustion performance in a scramjet combustor due to injection of electric-discharge-activated oxygen molecules

A comprehensive analysis of the efficiency of an approach based on the injection of a thin oxygen stream, subjected to a tailored electric discharge, into a supersonic H2–air flow to enhance the combustion performance in the mixing layer and in the scramjet combustor is conducted. It is shown that for such an approach there exist optimal values of reduced electric field E/N and transversal dimension d of the injected oxygen stream, which provide the minimal length of induction zone in the mixing layer. The optimal values of E/N and d depend on air flow parameters and the specific energy put into the oxygen. The injection of a thin oxygen stream (d = 1 mm) subjected to an electric discharge with E/N = 50–100 Td, which produces mostly singlet oxygen O2(a 1Δg) and molecules and atomic oxygen, allows one to arrange stable combustion in a scramjet duct at an extremely low air temperature Tair = 900 K and pressure Pair = 0.3 bar even at a small specific energy put into the oxygen Es = 0.2 J ncm−3, and to provide rather high combustion completeness η = 0.73. The advance in the energy released during combustion is much higher (hundred times), in this case, than the energy supplied to the oxygen stream in the electric discharge. This approach also makes it possible to ensure the rather high combustion completeness in the scramjet combustor with reduced length. The main reason for the combustion enhancement of the H2–air mixture in the scramjet duct is the intensification of chain-branching reactions due to the injection of a small amount of cold non-equilibrium oxygen plasma comprising highly reactive species, O2(a 1Δg) and molecules and O atoms, into the H2–air supersonic flow.

Electronic states of porphycene-O2 complex and photoinduced singlet O2 production

Porphycene (PC), a structural isomer of porphine, is a promising photosensitizer for photodynamic therapy. Its excited states can be quenched by molecular oxygen, generating singlet O2. The electronic structures of PC and of the PC⋯O2 complex were investigated using complete active space perturbation theory. It is shown that singlet oxygen generation involves 12 electronic states of the complex, with singlet, triplet, and quintet multiplicities. Two scenarios for singlet-O2 yield are analyzed: (I) quenching of triplet states of PC and (II) quenching of singlet states of PC. In the first scenario, which is favored under low O2 concentration, singlet-O2 yield is limited by the relatively low triplet quantum yield of PC. We discuss how the singlet-O2 yield would be busted if conditions for occurrence of the second scenario could be achieved.

Time-Resolved EPR Study of Singlet Oxygen in the Gas Phase

X-band EPR spectra of singlet O2((1)Δg) and triplet O2((3)Σg(-)) were observed in the gas phase under low molecular-oxygen pressures PO2 = 0.175-0.625 Torr, T = 293-323 K. O2((1)Δg) was produced by quenching of photogenerated triplet sensitizers naphthalene C8H10, perdeuterated naphthalene, and perfluoronaphthalene in the gas phase. The EPR spectrum of O2((1)Δg) was also observed under microwave discharge. Integrated intensities and line widths of individual components of the EPR spectrum of O2((3)Σg(-)) were used as internal standards for estimating the concentration of O2 species and PO2 in the EPR cavity. Time-resolved (TR) EPR experiments of C8H10 were the main focus of this Article. Pulsed irradiation of C8H10 in the presence of O2((3)Σg(-)) allowed us to determine the kinetics of formation and decay for each of the four components of the O2((1)Δg) EPR signal, which lasted for only a few seconds. We found that the kinetics of EPR-component decay fit nicely to a biexponential kinetics law. The TR EPR 2D spectrum of the third component of the O2((1)Δg) EPR spectrum was examined in experiments using C8H10. This spectrum vividly presents the time evolution of an EPR component. The largest EPR signal and the longest lifetime of O2((1)Δg), τ = 0.4 s, were observed at medium pressure PO2 = 0.4 Torr, T = 293 K. The mechanism of O2((1)Δg) decay in the presence of photosensitizers is discussed. EPR spectra of O2((1)Δg) evidence that the spin-rotational states of O2((1)Δg) are populated according to Boltzmann distribution in the studied time range of 10-100 ms. We believe that this is the first report dealing with the dependence of O2((1)Δg) EPR line width on PO2 and T.

Spin effects in oxygen electrocatalysis: A discussion

The reduction of molecular oxygen in triplet and singlet spin states at metal electrodes is analyzed in the framework of quantum mechanical theory of charge transfer. Both outer- and inner-sphere mechanism is considered. Singlet oxygen is argued to be considerably more active in electron transfer processes. It is demonstrated that spin polarization may play a catalytic role, parallel with the effect of overlap of reactant orbitals with the d-band of a metal electrode. Our model is based on two main assumptions: (i) some metal surfaces favor the existence of singlet molecular oxygen in adsorbed state; and (ii) short-living singlet O2 molecules may appear as intermediates at some reduction steps. These two reasons are expected to increase the local concentration of active singlet molecular oxygen in reaction layer.

Esterified Dendritic TAM Radicals with Very High Stability and Enhanced Oxygen Sensitivity

In this work, we have developed a new class of dendritic TAM radicals (TG, TdG, and dTdG) through a convergent method based on the TAM core CT-03 or its deuterated analogue dCT-03 and trifurcated Newkome-type monomer. Among these radicals, dTdG exhibits the best EPR properties with sharpest EPR singlet and highest O(2) sensitivity due to deuteration of both the ester linker groups and the TAM core CT-03. Like the previous dendritic TAM radicals, these new compounds also show extremely high stability toward various reactive species owing to the dendritic encapsulation. The highly charged nature of these molecules resulting from nine carboxylate groups prevents concentration-dependent EPR line broadening at physiological pH. Furthermore, we demonstrate that these TAM radicals can be easily derivatized (e.g., PEGylation) at the nine carboxylate groups and the resulting PEGylated analogue dTdG-PEG completely inhibits the albumin binding, thereby enhancing suitability for in vivo applications. These new dendritic TAM radicals show great potential for in vivo EPR oximetric applications and provide insights on approaches to develop improved and targeted EPR oximetric probes for biomedical applications.

Morphology dependent photosensitization and formation of singlet oxygen (1Δg) by gold and silver nanoparticles and its application in cancer treatment.

Singlet oxygen is a very important reactive oxygen species (ROS) involved in peroxidation of olefins and polymers, as well as in clinical photodynamic therapy treatments of tumors. Previously, it was reported that singlet oxygen can be formed via sensitization by spherical metal nanoparticles upon photo-excitation of the surface plasmon resonance (SPR) bands. In this paper, we report that sensitization and formation of singlet O2 is strongly dependent on the morphologies of gold and silver nanostructures. For example, singlet O2 can be generated via photo-irradiation and sensitization of silver decahedrons and silver triangular nanoplates, but not by silver nanocubes and gold decahedrons. The sensitization patterns of silver and gold nanoparticles are the reverse of each other. In the case of gold nanorods, singlet O2 can be generated via photo-excitation at the longitudinal SPR band, but not by excitation at the transverse SPR band. The controlling factors for such a morphology dependent singlet O2 sensitization will be discussed. Furthermore, we also demonstrate in vitro morphology dependent sensitization behaviour of silver nanoparticles in the photodynamic cancer treatment. Our results indicate that metal nanoparticles with certain morphologies are potentially very promising dual functional nanomaterials with capabilities of simultaneously serving as near infrared (NIR) activatable photodynamic therapy and photothermal therapy reagents for cancer treatments.

Low temperature oxidation of linseed oil: a review

Abstract This review analyses and summarises the previous investigations on the oxidation of linseed oil and the self-heating of cotton and other materials impregnated with the oil. It discusses the composition and chemical structure of linseed oil, including its drying properties. The review describes several experimental methods used to test the propensity of the oil to induce spontaneous heating and ignition of lignocellulosic materials soaked with the oil. It covers the thermal ignition of the lignocellulosic substrates impregnated with the oil and it critically evaluates the analytical methods applied to investigate the oxidation reactions of linseed oil. Initiation of radical chains by singlet oxygen (1Δg), and their propagation underpin the mechanism of oxidation of linseed oil, leading to the self-heating and formation of volatile organic species and higher molecular weight compounds. The review also discusses the role of metal complexes of cobalt, iron and manganese in catalysing the oxidative drying of linseed oil, summarising some kinetic parameters such as the rate constants of the peroxidation reactions. With respect to fire safety, the classical theory of self-ignition does not account for radical and catalytic reactions and appears to offer limited insights into the autoignition of lignocellulosic materials soaked with linseed oil. New theoretical and numerical treatments of oxidation of such materials need to be developed. The self-ignition induced by linseed oil is predicated on the presence of both a metal catalyst and a lignocellulosic substrate, and the absence of any prior thermal treatment of the oil, which destroys both peroxy radicals and singlet O2 sensitisers. An overview of peroxyl chemistry included in the article will be useful to those working in areas of fire science, paint drying, indoor air quality, biofuels and lipid oxidation.

Theoretical study on the oxidation of zigzag silicon carbide nanotubes (SiCNTs) by singlet O2

Singlet O 2 produced upon photoexcitation is a very important oxidative reagent. The study on its reaction with nanotube might be useful not only to evaluate the stability of the nanotube upon air exposure and sunlight, but also to modify the properties of the nanotube. Considering the unique properties and wide applications of silicon carbide nanotube (SiCNT), in this paper, we performed extensive density functional theory (DFT) calculations to study the oxidation of a series of zigzag ( n ,0) SiCNTs ( n =6 to 12) by singlet O 2 . It is found that the reaction process contains two steps, namely, (i) [2+2] cycloaddition of a singlet O 2 to the Si–C bond, followed by (ii) the dissociation of the O–O bond, leading to the formation of an epoxide configuration with a highly exothermicity (>4.00 eV). Compared with pure SiCNT, the cycloaddition of singlet O 2 on tube leads to the decrease of the band gap, while the formation of the stable epoxy structure render band gap increase. Our results indicate that the SiCNT is more prone to be degraded after exposure to air and sunlight.

Total synthesis of angelone enabled by a remarkable biomimetic sequence

The natural product angelone was readily accessed with a remarkable biomimetic journey that sequentially features carbonyl formation–elimination, 6π-electrocyclization, visible light-promoted singlet O2 Diels–Alder reaction, Kornblum–DeLaMare-type peroxide rearrangement, 6π-electrocyclic ring-opening, and conjugative addition–elimination as the key steps. With key intermediates isolated and characterized, this study stands to reveal a rare system in which bio-inspired synthetic strategies were found to mirror experimental realities.

Metal Nanoparticles Sensitize the Formation of Singlet Oxygen

Singlet oxygen (O2) is known to play an indispensible role in the photodynamic therapy (PDT) treatment of cancer, and is an important oxidant for hydroperoxidation of olefins in organic synthesis. Singlet O2 is conventionally formed by sensitization by organic photosensitizers, such as Rose Bengal, silicon phthalocyanine, etc. These organic or organometallic dyes are, however, prone to photoinduced degradation and enzymatic degradation, which becomes problematic in PDT treatments, and reduces the efficiency of the generation of singlet O2. [5,8] It is, therefore, important to search for photosensitizers with highly efficient singlet O2 generation and large absorption coefficients that are photochemically more stable and less prone to enzymatic degradation. Previously, it was reported that the yield of singlet oxygen production by a photosensitizer, namely, Rose Bengal, was enhanced by a silver island film through the metal-enhanced absorption of photosensitizer. It was also reported that a gold nanodisk could enhance the phosphorescence decay rate of singlet oxygen, leading to a larger characteristic phosphorescence emission band of singlet oxygen at 1270 nm. In another two studies, it was observed that the quantum yield of singlet O2 formation generated by phthalocyanine photosensitizers can be enhanced by the presence of gold nanoparticles. Herein we report an unprecedented observation that singlet oxygen can be formed through direct sensitization by metal nanoparticles (M NPs, M=Ag, Pt, and Au) without the presence of any organic photosensitizers. Unambiguous experimental evidence includes direct observation of singlet oxygen emission at roughly 1268 nm, hydroperoxidation of cyclohexene, green fluorescence from a selective singlet oxygen fluorescent sensor, namely, Singlet Oxygen Sensor Green (SOSG, Molecular Probe), and quenching of singlet oxygen phosphorescence by sodium azide. As shown in Figure 1, photoexcitation of M NPs at the surface plasmon resonance absorption bands of Ag (d= 55, 42 nm), Pt (10 nm), and Au (22 nm) in D2O results in characteristic singlet oxygen emission at 1264 and 1268 nm, respectively. Control experiments show that in the absence of metal nanoparticles, photoexcitation of poly(vinyl pyrrolidone (PVP) in D2O using either 254 or 508 nm light did not result in any detectable singlet O2 emission signal (see the

Theoretical study on energy transfer from the excited C60 to molecular oxygen

Singlet oxygen is used as an important oxidizer for various organic reactions, on the other hand, it has toxicity for the human body, since it destroys biological molecules such as DNA. Fullerene C60 is known to act as a photosensitizer of singlet oxygen molecule, and it has been understood that energy transfer from the excited triplet C60 to the ground triplet O2 forming the singlet O2 is a biologically important reaction. In the present study, we have analyzed these reaction pathways by means of the DFT (density functional theory) calculation.

Mechanically induced generation of highly reactive excited-state oxygen molecules in cluster scattering

Molecular electronic excitation in (O(2))(n) clusters induced by mechanical collisions via the "chemistry with a hammer" is investigated by a combination of molecular dynamics simulations and quantum chemistry calculations. Complete active space self-consistent field augmented with triple-zeta polarizable basis set quantum chemistry calculations of a compressed (O(2))(2) cluster model in various configurations reveal the emergence of possible pathways for the generation of electronically excited singlet O(2) molecules upon cluster compression and vibrational excitation, due to electronic curve-crossing and spin-orbit coupling. Extrapolation of the model (O(2))(2) results to larger clusters suggests a dramatic increase in the population of electronically excited O(2) products, and may account for the recently observed cluster-catalyzed oxidation of silicon surfaces, via singlet oxygen generation induced by cluster impact, followed by surface reaction of highly reactive singlet O(2) molecules. Extensive molecular dynamics simulations of (O(2))(n) clusters colliding onto a hot surface indeed reveal that cluster compression is sufficient under typical experimental conditions for nonadiabatic transitions to occur. This work highlights the importance of nonadiabatic effects in the "chemistry with a hammer."

Post‐Hartree‐Fock (MP2 and MP4) study on decomposition of nitrous oxide on the nonframework AlO(+) site in ZSM‐5 zeolite

AbstractThe reaction mechanism for nitrous oxide (N2O) decomposition has been studied on AlO(+) site in AlO‐ZSM‐5 zeolite using the MP2/6‐31+G(d) method. The active centers were taken to be mononuclear [AlO]+ and [AlO2]+, and the surrounding portion of the zeolite was represented by a 3T cluster, namely [AlSi2O4H8]−. The first elementary step of N2O decomposition involves the formation of [AlO2]+ and the release of N2. The metal‐peroxo species produced in this step then reacts with N2O again, to release N2 and form [AlO3]+. The calculated activation energies at MP2 level for N2O dissociation on AlO‐ZSM‐5 and AlO2‐ZSM‐5 are 26.9 and 38.2 kcal/mol at 298 K, respectively. The calculated reaction enthalpies at MP2 level for N2O dissociation on AlO‐ZSM‐5 and AlO2‐ZSM‐5 are −25.1 and −21.3 kcal/mol at 298 K, respectively. Four‐order perturbation theory (MP4//MP2) predicts that the activation barriers for N2O dissociation at 298 K on AlO‐ZSM‐5 and AlO2‐ZSM‐5 are 12.9 and 29.5 kcal/mol, respectively. The calculated energy for desorption of singlet O2 from the 3T−[Al(O)3]+ cluster at MP2 level is 46.4 kcal/mol. When one takes into account the entropy gained on desorption of singlet O2, the contribution of entropy to the free energy of desorption is TΔS = 11.1 kcal/mol at 298 K. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Theoretical Study of O2 Molecular Adsorption and Dissociation on Silicon Carbide Nanotubes

The adsorption/dissociation of the O2 molecule on the surface of silicon carbide nanotubes (SiCNTs) was investigated by density functional theory. We found several adsorption configurations, including chemisorption and cycloaddition configurations, for triplet and singlet O2. Unlike the case for carbon nanotubes, the chemisorption of triplet O2 on SiCNTs is exothermic with remarkable charge transfer from nanotubes to the O2 molecule. Singlet O2 adsorption on the surface of SiCNTs can yield cycloaddition structures with large binding energies and sizable charge transfer. The reaction mechanism studies show that for triplet O2, the chemisorption configuration is favorable, but the cycloaddition configuration is preferred for singlet O2. For singlet O2, we also studied the dissociation of the O2 molecule, and a two-step mechanism was presented. The dissociation of molecular O2 results in formation of two three-membered rings with large binding energies. The key to the dissociation process of singlet O2 on the SiCNT surface is the first step with a barrier energy of 0.40 eV. Finally, the electronic properties of SiCNTs with adsorbed triplet and singlet O2 are shown to be dramatically influenced.