Surface reactions of molecular and atomic oxygen with carbon phosphide films

The surface reactions of atomic and molecular oxygen with carbon phosphide films have been studied using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Carbon phosphide films were produced by ion implantation of trimethylphosphine into polyethylene. Atmospheric oxidation of carbon phosphide films was dominated by phosphorus oxidation and generated a carbon-containing phosphate surface film. This oxidized surface layer acted as an effective diffusion barrier, limiting the depth of phosphorus oxidation within the carbon phosphide film to < 3 nm. The effect of atomic oxygen (AO) exposure on this oxidized carbon phosphide layer was subsequently probed in situ using XPS. Initially AO exposure resulted in a loss of carbon atoms from the surface, but increased the surface concentration of phosphorus atoms as well as the degree of phosphorus oxidation. For more prolonged AO exposures, a highly oxidized phosphate surface layer formed that appeared to be inert toward further AO-mediated erosion. By utilizing phosphorus-containing hydrocarbon thin films, the phosphorus oxides produced during exposure to AO were found to desorb at temperatures >500 K under vacuum conditions. Results from this study suggest that carbon phosphide films can be used as AO-resistant surface coatings on polymers.     

Self-assembly of stable monomolecular nucleic acid lipid particles with a size of 30 nm

The design of efficient nucleic acid complexes is key to progress in genetic research and therapies based on RNA interference. For optimal transport within tissue and across extracellular barriers, nucleic acid carriers need to be small and stable. In this Article, we prepare and characterize mono-nucleic acid lipid particles (mono-NALPs). The particles consist of single short double-stranded oligonucleotides or single siRNA molecules each encapsulated within a closed shell of a cationic-zwitterionic lipid bilayer, furnished with an outer polyethylene glycol (PEG) shield. The particles self-assemble by solvent exchange from a solution containing nucleic acid mixed with the four lipid components DOTAP, DOPE, DOPC, and DSPE-PEG(2000). Using fluorescence correlation spectroscopy, we monitor the formation of mono-NALPs from short double-stranded oligonucleotides or siRNA and lipids into monodisperse particles of approximately 30 nm in diameter. Small angle neutron and X-ray scattering and transmission electron microscopy experiments substantiate a micelle-like core-shell structure of the particles. The PEGylated lipid shell protects the nucleic acid core against degradation by nucleases, sterically stabilizes the mono-NALPs against disassembly in collagen networks, and prevents nonspecific binding to cells. Hence, PEG-lipid shielded mono-NALPs are the smallest stable siRNA lipid system possible and may provide a structural design to be built upon for the development of novel nucleic acid delivery systems with enhanced biodistribution in vivo.     

Heterogeneous reactions of singlet molecular oxygen with solid polymers

The effect of gas-phase singlet molecular oxygen (¹ΔO₂) upon several solid polymers was investigated by using electron paramagnetic resonance, infrared spectroscopy, and chemical detection techniques. The study was performed by use of ¹ΔO₂ produced by microwave discharge. The application of this method to polymer studies was closely examined. The saturated-chain polymers polystyrene, polyurethane, and polyethylene were found to be inert within the experimental conditions to reaction with ¹ΔO₂, while the unsaturated polymers cis-polybutadiene, trans-polybutadiene, and trans-polyisoprene were found to react quite readily in an apparently surface or near-surface limited reaction to produce hydroperoxide and/or peroxide groups. The introduction by homogeneous mixing of some known metal-chelate ¹ΔO₂ quenchers into the polymer trans-polyisoprene appeared to significantly decrease the rate of oxidation observed.     

The reactions of methyl radicals with atomic and molecular oxygen

The reaction of methyl radicals with atomic and molecular oxygen was studied with a photoionization mass spectrometer. The methyl radicals were generated by reacting oxygen atoms with ethylene in a fast-flow tube reactor. The rate constant for the reaction of methyl radicals with oxygen atoms was (1.0 ± 0.2) × 10^?10 cm³/molec · sec with no significant variation with temperature over the range of 259–341°K. The reaction of methyl radicals with molecular oxygen involves both a two-body reaction, having a rate constant \documentclass{article}\pagestyle{empty}\begin{document}$\begin{array}{*{20}c} {k_{{\rm 3a}} = (10^{- 12.54 \pm 0.35})\exp [(- 940 \pm 250)T^{- 1}]} & {{\rm cm}^{\rm 3} /{\rm molec} \cdot {\rm sec}} \end{array}$\end{document} and a three-body recombination having a negative temperature dependence. The methyl peroxy radical could be observed at its steady-state concentration. The rate constants determined at low pressures are compatible with the values determined at higher pressures by flash photolysis. Formaldehyde appears to be a major product of the two-body reaction of CH₃ with O₂, and also of the reaction of CH₃O₂ with oxygen atoms.     

The bimolecular reactions of trimethylenemethane diradicals with acrylonitrile and molecular oxygen

The interception and trapping by molecular oxygen and acrylonitrile of trimethylenemethane diradicals during their reversible thermal formation from stable methylenecyclopropane precursors is reported. The regiochemistry of acrylonitrile cycloaddition to a trimethylenemethane is also unequivocally demonstrated and provides insight into the trapping process.     

Chemical diffusion of oxygen in tin dioxide: Effects of dopants and oxygen partial pressure

Tin dioxide SnO_2−δ is a pronounced n-type electron conductor due to its oxygen deficiency. This study investigates the rate of chemical diffusion of oxygen in SnO_2−δ single crystals, which is a crucial step in the overall stoichiometry change of the material. The chemical diffusion coefficient D^δ was determined from conductivity- and EPR-relaxation methods. The temperature dependence was found to be . The dependence on crystal orientation, dopant content and oxygen partial pressure was below experimental error. The latter observation leads to the conclusion that the chemical diffusion coefficient is close to the diffusion coefficient of oxygen vacancies. Along with the relaxation process resulting from the chemical diffusion of oxygen, additional processes were observed. One of these was attributed to complications in the defect chemistry of the material. The relevance of the results for the kinetics of drift processes of Taguchi sensors is discussed.     

Visible light-induced partial oxidation of olefins on Cr-containing silica with molecular oxygen

Photocatalytic oxidation of olefins on Cr-containing silica with molecular oxygen by visible light irradiation (lambda > 400 nm) has been investigated. Cr-SiO(2) catalyst prepared by a conventional sol-gel method, containing highly dispersed chromate species, catalyzes efficient olefin oxidation with very high selectivity for partially oxidized products (>90%), whereas semiconductor TiO(2) promotes complete decomposition (CO(2) production). The Cr-SiO(2) catalyst shows much higher activity than Cr/SiO(2) prepared by an impregnation method or Cr proportional variant MCM-41 prepared by a templating method. ESR analysis reveals that photoirradiation of the chromate species with a tetrahedral coordination (T(d)(6+)) on Cr/SiO(2) and Cr proportional variant MCM-41 catalysts leads to the formation of excited state T(d)(5+) species (T(d)(5+*)), while irradiation to T(d)(6+) on Cr-SiO(2) produces T(d)(4+*) species. This can be explained by a homogeneous T(d)(6+) arrangement with Si species on the Cr-SiO(2) catalyst. On the strongly reduced T(d)(4+*), olefins are strongly attracted by an electron and/or proton donation, resulting in high oxidation activity. The Cr-SiO(2) catalyst is applicable to partial oxidation of various aliphatic and aromatic olefins with very high selectivity, and does not promote undesirable dimerization. The obtained findings suggest a potential use of Cr-SiO(2) as an efficient and recyclable heterogeneous photocatalyst for partial oxidation of olefins.     

“Increased-valence” formulae and metal-ion catalysed dehydrogenation reactions with molecular oxygen and azo compounds as oxidants

“Increased-valence” mechanisms are formulated for the generalized dehydrogenation reactions SH₂ + O₂ → S + H₂O₂ and SH₂ + RNNR → S + RNH-NHR, in which SH₂ is assumed to be coordinated to a transition metal ion. The role of the metal ion in these mechanisms is to assist with the homolytic breaking of the S-H and NN bonds     

Doping of fullerite with molecular oxygen at low temperature and pressure

Two methods are described for doping of fullerite C₆₀ with molecular oxygen at a pressure of ∼104 Pa and at temperature 20–30 °C. It was found by mass spectrometry using oxygen ¹⁸O as dopant that a portion of molecular oxygen absorbed by the pre-decontaminated fullerite (first method) is removed as CO and CO₂ at the heating temperature ≤200 °C. Doping during fullerite precipitation from the liquid phase (second method) makes it possible to prepare samples with the oxygen content ≥1.2 at.%. The fullerite doped with oxygen to this level is diamagnetic. The paramagnetic properties of an O₂ molecule disappear when O₂ is incorporated into the fullerene lattice. This is interpreted on the basis of quantum chemical calculations as a sequence of equilibrium formation of the adduct C₆₀O₂. Calculations showed that the subsequent chemical transformation of C₆₀O₂ resulting in the O-O bond cleavage is energetically favorable, enabling prerequisites for the formation of products of incomplete (CO) and deep (CO₂) oxidation of fullerene under mild conditions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 662–671, April, 2006     

Copper-mediated oxidative tandem reactions with molecular oxygen: synthesis of 2-arylbenzoxazinone derivatives from indoles

We developed an efficient method for the transformation of indoles by utilizing a copper catalyst and molecular oxygen as the oxidant. The transformation involves a tandem oxidative process of 2-arylindoles. Our reaction afforded a variety of N-benzoyl anthranilic acids and benzoxazinones. Our investigation revealed that the choice of solvent and additives is critical in these reactions.     

Mechanism of water oxidation to molecular oxygen with osmocene as photocatalyst: a theoretical study

In the present work, photoinduced O(2) evolution from the [Cp(2)Os-OH](+) complex in aqueous solution has been studied by the DFT, CASSCF, and CASPT2 methods. The CASPT2//CASSCF calculations predict that the S(3) state is initially populated and the subsequent deprotonation of [Cp(2)Os-OH](+) proceeds very easily along the T(1) pathway as a result of the efficient S(3) → T(1) intersystem crossing. It is found that the O-O bond is formed via the acid-base mechanism, which is different from the direct oxo-oxo coupling mechanism suggested in the experimental study. Formation of the O-O bond is the rate-determining step and has an activation energy and activation free energy of 81.3 and 90.4 kcal/mol, respectively. This is consistent with the low quantum yield observed for generating molecular oxygen upon irradiation at 350 nm (~ 82 kcal/mol). The O(2) release from an intermediate complex has to overcome a small barrier on the triplet pathway first and then pass through the triplet-singlet intersection, generating the O(2) molecules in either the lowest singlet or triplet state. The formed (3)O(2) molecule can be converted into the (1)O(2) molecule by the heavy atom effect in the Os complexes, which is probably the reason only the (1)O(2) molecule was detected experimentally.     

Internal pressure and solubility parameter as a function of pressure

The main goal of this work was to measure the solubility parameter of a complex mixture, such as a crude oil, especially as a function of pressure. Thus, its definition is explained, as well as the main approximations generally used in literature. Then, the internal pressure is investigated, since it is presented as an alternative of the solubility parameter. In this work, the assumption that internal pressure is a measure of the physical solubility parameter was made, i.e. representing the dispersion and polar forces. As for the pressure influence, it was seen that internal pressure reaches a maximum contrary to solubility parameter.An indirect method was chosen to estimate internal pressure, using thermal expansivities (determined by microcalorimetry) and isothermal compressibilities (determined by density measurements). The uncertainty is within 2% for the expansivity and 0.1% for the density. Five pure compounds (four hydrocarbons and 1 alcohol) were investigated at 303.15 K and up to 30 MPa, as well as a dead crude oil. The “physical” solubility parameter is slightly increasing with pressure (up to 0.8 MPa^1/2 for cyclohexane) and, at 0.1 MPa, the difference with literature data is less than 1 MPa^1/2 for hydrocarbons. On the contrary, the difference reaches 9 MPa^1/2 for ethanol as expected, due to the presence of hydrogen bonding. A dead crude oil was also studied and its solubility parameter is within the expected range.Two cubic equations of states (Peng–Robinson and Soave–Redlich–Kwong) were able to approximate the “physical” solubility parameter of n-heptane (within 0.2 MPa^1/2), providing that the volumes were measured and used as input. The Peng–Robinson equation gave somewhat better results.     

Visible-light-sensitive photocatalytic reaction on chromium-containing mesoporous silica: selective partial oxidation of propane with molecular oxygen

The photocatalytic reactivities of chromium-containing mesoporous silica molecular sieves (Cr-HMS) under visible light irradiation have been investigated. Cr-HMS involves tetrahedral chromium oxide (Cr-oxide) moieties which are highly dispersed and incorporated in the framework of molecular sieve with two terminal Cr=O groups. In the presence of propane with molecular oxygen, a partial oxidation proceeded under visible light irradiation to produce acetone and acrolein, with high selectivity, while a complete oxidation proceeded under UV light irradiation mainly to produce CO₂. The charge-transfer excited state of the tetrahedral Cr-oxide moieties plays a significant role in the photocatalytic reactions.     

Colloid chemistry of nanocatalysts: a molecular view

Recent advances of a colloidal chemistry can offer great opportunities to fabricate and design nanocatalysts. Comprehensive understanding of a basic concept and theory of the colloidal synthetic chemistry facilitates to engineer elaborate nano-architectures such as bi- or multi-metallic, heterodimers, and core/shell. This colloidal solution technique not only enables to synthesize high surface mesoporous materials, but also provides a versatile tool to incorporate nanoparticles into mesoporous materials or onto substrates. For green chemistry, catalysis research has been pursued to design and fabricate a catalyst system that produces only one desired product (100% selectivity) at high turnover rates to reduce the production of undesirable wastes. Recent studies have shown that several molecular factors such as the surface structures, composition, and oxidation states affect the turnover frequency and reaction selectivity depending on the size, morphology, and composition of metal nanoparticles. Multipath reactions have been utilized to study the reaction selectivity as a function of size and shape of platinum nanoparticles. In the past, catalysts were evaluated and compared with characterizations before and after catalytic reaction. Much progress on in situ surface characterization techniques has permitted real-time monitoring of working catalysts under various conditions and provides molecular information during the reaction.     

Rate constants for the reactions of HCCCO and NCCO radicals with molecular oxygen

Reactions of HCCCO and NCCO radicals with O₂ have been studied by a combination of pulsed laser photolysis and photoionization mass spectrometry. HCCCO was produced by 193‐nm photolysis of methylpropiolate or 3‐butyn‐2‐one, and NCCO was formed by 193‐nm photolysis of acetylcyanide. The rate constants obtained at 298 ± 3 K were (6.5 ± 0.7) × 10^?12 cm³ molecule^?1 s^?1 for the HCCCO + O₂ reaction, and no pressure dependence was observed between 1.5 and 16 Torr of N₂ as a bath gas. Because HCO and HCCO radicals were observed as reaction products, it was confirmed that the reaction proceeds by a two‐body reaction. On the other hand, the rate constants of NCCO with O₂ depended on the total pressure and were (5.4–8.8) × 10^?13 cm³ molecule^?1 s^?1 for total pressures 2.0–15.5 Torr of N₂, confirming that the reaction proceeds by a three‐body process. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 440–448, 2001     

Surface nanobubbles as a function of gas type

We experimentally investigate the nucleation of surface nanobubbles on PFDTS-coated silicon as a function of the specific gas dissolved in water. In each case, we restrict ourselves to equilibrium conditions (c = 100%, T(liquid) = T(substrate)). Not only is nanobubble nucleation a strong function of gas type, but there also exists an optimal system temperature of ～35 -40 °C where nucleation is maximized, which is weakly dependent on gas type. We also find that the contact angle is a function of the nanobubble radius of curvature for all gas types investigated. Fitting this data allows us to describe a line tension that is dependent on the type of gas, indicating that the nanobubbles sit on top of adsorbed gas molecules. The average line tension was τ ≈ -0.8 nN.     

Rare-earth quinolinates: infrared-emitting molecular materials with a rich structural chemistry

Near-infrared-emitting rare-earth chelates based on 8-hydroxyquinoline have appeared frequently in recent literature, because they are promising candidates for active components in near-infrared-luminescent optical devices, such as optical amplifiers, organic light-emitting diodes, .... Unfortunately, the absence of a full structural investigation of these rare-earth quinolinates is hampering the further development of rare-earth quinolinate based materials, because the luminescence output cannot be related to the structural properties. After an elaborate structural elucidation of the rare-earth quinolinate chemistry we can conclude that basically three types of structures can be formed, depending on the reaction conditions: tris complexes, corresponding to a 1:3 metal-to-ligand ratio, tetrakis complexes, corresponding to a 1:4 metal-to-ligand ratio, and trimeric complexes, with a 3:8 metal-to-ligand ratio. The intensity of the emitted near-infrared luminescence of the erbium(III) complexes is highest for the tetrakis complexes of the dihalogenated 8-hydroxyquinolinates.     

The domino chemistry approach to molecular complexity: high-yielding bis-hetero intramolecular Diels–Alder reactions with ketone components

The bis-keto-hetero-IMDA option, with two ketone components (both the heterodiene and heterodienophile moieties) has been examined in several representative domino templates with the aim of ultimately developing efficient methods for the synthesis of structurally complex natural products. The domino sequence could also be activated efficiently by utilizing the less toxic iodobenzene diacetate as the oxidative cleavage/[4+2] promoter while it is unbiased to the nature of substitution around the bicyclic framework.