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‣ Preparation of recoverable Ru catalysts for liquid-phase oxidation and hydrogenation reactions

Jacinto, Marcos José; Santos, Osvaldo H C F; Jardim, Renato de Figueiredo; LANDERS, Richard; Rossi, Liane Marcia
Fonte: ELSEVIER SCIENCE BV Publicador: ELSEVIER SCIENCE BV
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
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We here report the synthesis, characterization and catalytic performance of new supported Ru(III) and Ru(0) catalysts. In contrast to most supported catalysts, these new developed catalysts for oxidation and hydrogenation reactions were prepared using nearly the same synthetic strategy, and are easily recovered by magnetic separation from liquid phase reactions. The catalysts were found to be active in both forms, Ru(III) and Ru(0), for selective oxidation of alcohols and hydrogenation of olefins, respectively. The catalysts operate under mild conditions to activate molecular oxygen or molecular hydrogen to perform clean conversion of selected substrates. Aryl and alkyl alcohols were converted to aldehydes under mild conditions, with negligible metal leaching. If the metal is properly reduced, Ru(0) nanoparticles immobilized on the magnetic support surface are obtained, and the catalyst becomes active for hydrogenation reactions. (c) 2009 Elsevier B.V. All rights reserved.; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Brazilian agencies FAPESP; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); CNPq; Instituto Nacional de Ciencia a Tecnologia de Catalise em Sistemas Moleculares a Nanoestruturados (INCT-CMN); Instituto Nacional de Ciencia a Tecnologia de Catalise em Sistemas Moleculares a Nanoestruturados (INCT-CMN)

‣ Phototransformation of hematoporphyrin in aqueous solution: Anomalous behavior at low oxygen concentration

PRATAVIEIRA, S.; SANTOS, P. L. A.; MENEZES, P. F. C.; KURACHI, Cristina; SIBATA, C. H.; JARVI, M. T.; WILSON, B. C.; BAGNATO, Vanderlei Salvador
Fonte: MAIK NAUKA/INTERPERIODICA/SPRINGER Publicador: MAIK NAUKA/INTERPERIODICA/SPRINGER
Tipo: Artigo de Revista Científica
Português
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The photoactivation of a photosensitizer is the initial step in photodynamic therapy (PDT) where photochemical reactions result in the production of reactive oxygen species and eventually cell death. In addition to oxidizing biomolecules, some of these photochemical reactions lead to photosensitizer degradation at a rate dependent on the oxygen concentration among other factors. We investigated photodegradation of Photogem A (R) (28 mu M), a hematoporphyrin derivative, at different oxygen concentrations (9.4 to 625.0 mu M) in aqueous solution. The degradation was monitored by fluorescence spectroscopy. The degradation rate (M/s) increases as the oxygen concentration increases when the molar ratio of oxygen to PhotogemA (R) is greater than 1. At lower oxygen concentrations (< 25 mu M) an inversion of this behavior was observed. The data do not fit a simple kinetic model of first-order dependence on oxygen concentration. This inversion of the degradation rate at low oxygen concentration has not previously been demonstrated and highlights the relationship between photosensitizer and oxygen concentrations in determining the photobleaching mechanism(s). The findings demonstrate that current models for photobleaching are insufficient to explain completely the effects at low oxygen concentration.; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); CEPID; FAPESP; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); CAPES; Canadian Cancer Society/NCIC; Canadian Cancer Society/NCIC

‣ Pentacloreto de nióbio como ácido de Lewis em reações de cicloadição [2+2] e [4+2]; Niobium Pentachloride as Lewis acid in [2 + 2] and [4 + 2] cycloadditions reactions

Silva Filho, Luiz Carlos da
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 12/06/2006 Português
Relevância na Pesquisa
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O objetivo deste trabalho foi investigar o uso de NbCl5 como ácido de Lewis em reações de cicloadição. Foram estudadas algumas reações de cicloadição [2 + 2], Reações de Diels-Alder entre enonas e ciclopentadieno e Reações da aza-Diels-Alder com bases de Schiff, avaliando o efeito da temperatura e da concentração molar do NbCl5. A comparação dos rendimentos, dos produtos formados e do tempo de reação com NbCl5, também foi um ponto fundamental da pesquisa. As reações de cicloadição [2 + 2] foram realizadas entre ésteres propiólicos e diferentes tipos de alcenos (éteres enólicos de silício e alcenos alifáticos). Nas reações com os éteres enólicos de silício não foi verificada a formação dos respectivos adutos de ciclobuteno, pois o NbCl5 promove a quebra da ligação oxigênio-silício, não levando à formação do produto desejado. Nas reações com os alcenos alifáticos foi possível obter o respectivo aduto, porém com rendimentos menores que com outros ácidos de Lewis. Quanto às reações de Diels-Alder, foram investigadas as reações entre diferentes ciclo-enonas (dienófilos de baixa reatividade) com ciclopentadieno (dieno) na presença de NbCl5. Os resultados obtidos indicam que o NbCl5 é um bom ácido de Lewis para ativar reações de Diels-Alder...

‣ Oxidação do triptofano pelo oxigênio molecular no estado singlete [O2 (1Δg)]: estudos mecanísticos envolvendo marcação isotópica, espectrometria de massa e quimiluminescência; Tryptophan oxidation by singlet molecular oxygen [(O2(1Δg): mechanistic studies using isotopic labelling, mass spectrometry analysis and chemiluminescence

Ronsein, Graziella Eliza
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 07/05/2008 Português
Relevância na Pesquisa
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As proteínas são consideradas importantes alvos para os oxidantes, devido à abundância em sistemas biológicos e às altas constantes de reações com estas espécies. Adicionalmente, têmse demonstrado que o triptofano (W) é um aminoácido extremamente susceptível a oxidação, inclusive pelo oxigênio singlete (1O2). A reação do W com o 1O2 tem despertado o interesse de diversos pesquisadores. Recentemente, esta reação tem atraído mais atenção, uma vez que produtos de oxidação do W tais como N-formilquinurenina (FMK) e quinurenina (kn) têm sido associados com algumas condições patológicas, tais como o desenvolvimento de catarata e a formação de agregados covalentes da superóxido dismutase envolvidos na esclerose lateral amiotrófica. Entretanto, há poucos trabalhos enfocando detalhadamente as reações, com estudos de estabilidade, identificação de subprodutos e propostas mecanísticas. Desta forma, pretendemos com este trabalho contribuir no esclarecimento do mecanismo de oxidação do triptofano pelo 1O2, através da análise e caracterização de produtos de oxidação gerados. Com este intuito, dois hidroperóxidos de triptofano isômeros (WOOH cis e trans, em relação ao grupamento carboxila) foram completamente caracterizados por análises de HPLC/espectrometria de massa e RMN como os principais produtos de oxidação do W pelo 1O2. Estes hidroperóxidos demonstraram ser relativamente estáveis às temperaturas ambiente e fisiológica...

‣ Geração de ozônio isotopicamente marcado com átomo de oxigênio-18, (18O3), formando oxigênio-18 molecular singlete, 18O2 (1Δg), e modificações na 2'- desoxiguanosina; Isotopically labeled ozone, 18O3, generate 18O-labeled singlet molecular oxygen, 18O2 (1Δg), and oxidation of product of the purine moiety of 2'-deoxyguanosine.

Motta, Flavia Daniela
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 28/07/2011 Português
Relevância na Pesquisa
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O ozônio (O3) é um poderoso oxidante e quantidades significativas podem ser formadas em ambientes urbanos, como resultado de uma série de eventos fotoquímicos, sendo um risco para a saúde humana. Devido a sua reatividade química, o ozônio é capaz de promover modificações oxidativas em diversas biomoléculas, tais como, DNA, proteínas e lipídios. As reações do O3 com biomoléculas geram quantidades significativas de O2 (1Δg). Sendo assim, essas reações são caracterizadas pela transferência de um átomo de oxigênio do O3 ao substrato oxidado. Devido à regra de conservação do Spin, isto requer que o dioxigênio gerado nesta reação esteja no seu estado singlete. Neste específico mecanismo, a formação do hidrotrióxido tem sido frequentemente assumida como um importante intermediário da ozonização. Ainda, constatou-se o elevado potencial mutagênico do O3 sobre o DNA, levando, principalmente, à substituição de suas bases. A frequência das substituições das bases foi essencialmente localizada no par G: C's (75%), uma característica das espécies reativas de oxigênio, como o O2 (1Δg). No entanto, os mecanismos pelos quais O3 causa danos ao DNA ainda não foram completamente elucidados. No presente trabalho...

‣ Reações envolvendo NOx mediadas por Fe-heme em alimentos e sistemas biológicos; Reactions involving NOx mediated by heme iron in foods and biological systems

Zawadzki, Andressa de
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 15/03/2013 Português
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Os ânions inorgânicos nitrato (NO3-) e nitrito (NO2-) por muito tempo foram considerados produtos finais inertes do metabolismo do óxido nítrico (NO) e constituintes indesejáveis da dieta. Entretanto, é crescente o interesse das potenciais reações que podem se processar em meio fisiológico e, que envolvem especialmente o íon nitrito mediadas por complexos de metais de transição. Ferro-heme presente em proteínas como a mioglobina podem formar complexos porfirínicos com NO, gerando espécies que podem atuar principalmente como catalisadores em diversas vias biológicos importantes. Complexos com NOx coordenado a ferro-heme estão envolvidos tanto no processo da cura da carne (pigmentação da carne), como em processos redox, nos quais são capazes de oxidar substratos indiretamente pela produção de espécies reativas ou diretamente por reações de transferência de átomo de oxigênio (TAO). Os lipídeos, constituintes celulares fundamentais na composição de membranas e lipoproteínas, podem servir como substratos para essas reações e, sua oxidação pode ocasionar danos ao organismo. Tióis, importantes antioxidantes e constituintes de proteínas, também podem participar desse tipo de reações e, conseqüentemente gerar danos ao organismo. Considerando que o mecanismo e os parâmetros cinéticos e termodinâmicos destas reações ainda não foram completamente estabelecidos para explicar a vasta funcionalidade destes complexos...

‣ Oxygen interstitial dynamics in Bi2Sr2CaCu 208+δ oxides measured by mechanical spectroscopy

Grandim, C. R.; Ruiz, S. L M; Silva, M. R.; Gimenez, J. M A; Rubo, E. A A
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Conferência ou Objeto de Conferência Formato: 634-638
Português
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Many researchers became interested in the discovery of Bi 2Sr2CaCu2O8+5 oxides with critical temperature of around 80 K. It is known that the critical temperature is related to the CuO2 planes of the material. For this reason, the study of the interstitial oxygen in these oxides is of great relevance. The samples were prepared by means of conventional solid state reactions, through the stoichiometric mixture of precursory powders. After the sinterization, the samples were submitted to measurements of density, electrical resistivity, x-ray diffraction, scanning electron microscopy and energy dispersion spectroscopy, with the objective of performing their characterization. The measurements of mechanical spectroscopy were performed by a torsion pendulum. The results show three relaxation processes in the temperature range of 200 and 700 K, with activation energy of approximately 0.9 eV, which has been attributed to the dynamics of the interstitial oxygen present in the material.

‣ Evaporation and carbonic anhydrase activity recorded in oxygen isotope signatures of net CO2 fluxes from a Mediterranean soil

Wingate, Lisa; Seibt, Ulli; Maseyk, Kadmiel; Ogée, Jérome; Almeida, Pedro; Yakir, Dan; Pereira, João Santos; Mencuccini, Maurizio
Fonte: Blackwell Publicador: Blackwell
Tipo: Artigo de Revista Científica
Publicado em //2008 Português
Relevância na Pesquisa
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The oxygen stable isotope composition (d18O) of CO2 is a valuable tool for studying the gas exchange between terrestrial ecosystems and the atmosphere. In the soil, it records the isotopic signal of water pools subjected to precipitation and evaporation events. The d18O of the surface soil net CO2 flux is dominated by the physical processes of diffusion of CO2 into and out of the soil and the chemical reactions during CO2–H2O equilibration. Catalytic reactions by the enzyme carbonic anhydrase, reducing CO2 hydration times, have been proposed recently to explain field observations of the d18O signatures of net soil CO2 fluxes. How important these catalytic reactions are for accurately predicting large-scale biosphere fluxes and partitioning net ecosystem fluxes is currently uncertain because of the lack of field data. In this study, we determined the d18O signatures of net soil CO2 fluxes from soil chamber measurements in a Mediterranean forest. Over the 3 days of measurements, the observed d18O signatures of net soil CO2 fluxes became progressively enriched with a well-characterized diurnal cycle. Model simulations indicated that the d18O signatures recorded the interplay of two effects: (1) progressive enrichment of water in the upper soil by evaporation...

‣ LaNiO3-based catalyst in gas diffusion electrodes: activity and stability for oxigen reactions

Silva, R. A.; Soares, C. O.; Carvalho, M. D.; Jorge, M. E. Melo; Gomes, A.; Rangel, C. M.; Pereira, M. I. da Silva
Fonte: Laboratório Nacional de Energia e Geologia Publicador: Laboratório Nacional de Energia e Geologia
Tipo: Conferência ou Objeto de Conferência
Publicado em //2013 Português
Relevância na Pesquisa
47.339263%
Perovskite-type oxides are potential catalysts for next generation of regenerative fuel cells. In particular, LaNiO3 has been recognised as one of the most promising oxygen electrodes. In this work LaNiO3 perovskite-type oxides, prepared by a self-combustion method [1, 2], have been used for the preparation of porous gas-diffusion electrodes (GDE). Electrodes were prepared on Toray carbon paper (CP) substrates, consisting of a diffusion layer, a catalyst layer and a Nafion® layer. The gas diffusion layers were prepared using Vulcan XC-72R. The catalyst ink was prepared by suspending the material in isopropanol, stirring the mixture in an ultrasonic bath to thoroughly disperse it. Ink slurries were also pasted onto glassy carbon discs and used as working electrodes for full kinetic studies at potential domains for the oxygen reduction (ORR) and oxygen evolution (OER) reactions. A systematic study on the effect of the oxide loading (OL) on the electrodes surface area was done by cyclic voltammetry. It was found a quasi linear variation between the electrodes surface area and the oxide loading. Roughness values varying from 106±3 to 307±6 were obtained for OL between 1 and 5 mg cm-2 respectively. The results show that the peak current density increases with the increasing on oxide loading as shown in Fig. 1. Higher current densities for ORR were obtained for the electrodes prepared using LaNiO3-based perovskite with partial substitution of Ni by Cu. Stability studies of the GDEs...

‣ Mechanisms of Oxidation with Oxygen

Taube, Henry
Fonte: The Rockefeller University Press Publicador: The Rockefeller University Press
Tipo: Artigo de Revista Científica
Publicado em 01/09/1965 Português
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Several topics are dealt with in discussing the reactions of molecular oxygen, but a common goal is pursued in each: to try to understand the reactions in terms of the fundamental properties of the oxygen molecule, and of the other reactants. The paper first describes the electronic structure of oxygen and of two low-lying electronically excited states. Concern with the low-lying electronically excited states is no longer the sole property of spectroscopists; recently, evidence has been presented for the participation of such activated molecules in chemical reactions. The chemistry of oxygen is dominated by the fact that the molecule in the ground state has two unpaired electrons, whereas the products of oxidation in many important reactions have zero spin. In its reactions with transition metal ions the restrictions imposed by the spin state of the oxygen molecule are easily circumvented. A number of reactions of oxygen with metal ions have been studied in considerable detail; conclusions on basic aspects of the reaction mechanism are outlined. Among the most interesting reactions of oxygen are those in which it is reversibly absorbed by reducing agents. Reversible absorption to form a peroxide in the bound state is possible; some of the conditions which must be fulfilled by a reducing system to qualify as storing oxygen in this way are reasonably well understood and are here enunciated. Little has been done on the formation of oxygen from water; some factors involved in this process are discussed.

‣ Copper Proteins and Oxygen : Correlations between structure and function of the copper oxidases

Frieden, Earl; Osaki, Shigemasa; Kobayashi, Hiroshi
Fonte: The Rockefeller University Press Publicador: The Rockefeller University Press
Tipo: Artigo de Revista Científica
Publicado em 01/09/1965 Português
Relevância na Pesquisa
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A comprehensive survey of the interaction of the copper proteins and oxygen is presented including a correlation of structure, function, and other properties of the known copper oxidases and of hemocyanin. The origin of their blue color and the structure of copper complexes and copper proteins are related to the oxidation state of copper ion and relevant electronic transitions probably arising from the formation of charge transfer complexes. The oxygen reactions of hemocyanin, ceruloplasmin, and cytochrome oxidase show half-saturation values far below the other Cu enzymes. The formation of hydrogen peroxide as a reaction product is associated with the presence of one Cu atom per oxidase molecule or catalytic system. Water is the corresponding product of the other Cu oxidases with four or more Cu atoms per molecule, except for monoamine oxidase. Mechanisms for the oxidase action of the two and four electron transfer Cu oxidases and tyrosinase are proposed. These reactions account for the number, the oxidation-reduction potential, and the oxidation state of Cu in the resting enzyme, the cyclical change from Cu(II) to Cu(I), the diatomic nature of O2, the sequence of the oxidation and reduction reactions, and other salient features. The catalytic reactions involved in the oxidation of ascorbic acid by plant ascorbate oxidase...

‣ Singlet Oxygen Reactions with Flavonoids. A Theoretical – Experimental Study

Morales, Javier; Günther, Germán; Zanocco, Antonio L.; Lemp, Else
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Publicado em 10/07/2012 Português
Relevância na Pesquisa
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Detection of singlet oxygen emission, λmax = 1270 nm, following laser excitation and steady-state methods were employed to measure the total reaction rate constant, kT, and the reactive reaction rate constant, kr, for the reaction between singlet oxygen and several flavonoids. Values of kT determined in deuterated water, ranging from 2.4×107 M−1s−1 to 13.4×107 M−1s−1, for rutin and morin, respectively, and the values measured for kr, ranging from 2.8×105 M−1s−1 to 65.7×105 M−1s−1 for kaempferol and morin, respectively, being epicatechin and catechin chemically unreactive. These results indicate that all the studied flavonoids are good quenchers of singlet oxygen and could be valuable antioxidants in systems under oxidative stress, in particular if a flavonoid-rich diet was previously consumed. Analysis of the dependence of rate constant values with molecular structure in terms of global descriptors and condensed Fukui functions, resulting from electronic structure calculations, supports the formation of a charge transfer exciplex in all studied reactions. The fraction of exciplex giving reaction products evolves through a hydroperoxide and/or an endoperoxide intermediate produced by singlet oxygen attack on the double bond of the ring C of the flavonoid.

‣ Application of thermal ion-molecule reactions in tackling spectroscopic inferences in inductively coupled plasma mass spectrometry

Wang, Yong
Fonte: Brock University Publicador: Brock University
Português
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Part I: Ultra-trace determination of vanadium in lake sediments: a performance comparison using O2, N20, and NH3 as reaction gases in ICP-DRC-MS Thermal ion-molecule reactions, targeting removal of specific spectroscopic interference problems, have become a powerful tool for method development in quadrupole based inductively coupled plasma mass spectrometry (ICP-MS) applications. A study was conducted to develop an accurate method for the determination of vanadium in lake sediment samples by ICP-MS, coupled with a dynamic reaction cell (DRC), using two differenvchemical resolution strategies: a) direct removal of interfering C10+ and b) vanadium oxidation to VO+. The performance of three reaction gases that are suitable for handling vanadium interference in the dynamic reaction cell was systematically studied and evaluated: ammonia for C10+ removal and oxygen and nitrous oxide for oxidation. Although it was able to produce comparable results for vanadium to those using oxygen and nitrous oxide, NH3 did not completely eliminate a matrix effect, caused by the presence of chloride, and required large scale dilutions (and a concomitant increase in variance) when the sample and/or the digestion medium contained large amounts of chloride. Among the three candidate reaction gases at their optimized Eonditions...

‣ Online oxygen kinetic isotope effects using membrane inlet mass spectrometry can differentiate between oxidases for mechanistic studies and calculation of their contributions to oxygen consumption in whole tissues

Cheah, Mun Hon; Millar, A. Harvey; Myers, Ruth C; Day, David A; Roth, Justine; Hillier, Warwick; Badger, Murray R
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica Formato: 8 pages
Português
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The reduction chemistry of molecular oxygen underpins the energy metabolism of multicellular organisms, liberating free energy needed to catalyse a plethora of enzymatic reactions. Measuring the isotope signatures of 16O and 18O during O2 reduction can provide insights into both kinetic and equilibrium isotope effects. However, current methods to measure O2 isotope signatures are time consuming and disruptive. This paper describes the application of membrane inlet mass spectrometry (MIMS) to determining the oxygen isotope discrimination of a range of O2-consuming reactions, providing a rapid and convenient method for determining these values. A survey of oxygenase and oxidase reactions provides new insights into previously uncharacterised amino acid oxidase enzymes. Liquid and gas phase measurements show the ease of assays using this approach for purified enzymes, biological extracts and intact tissues.; We acknowledge funding from the Australian Research Council (FT110100242, CE0561495, DP120100872).

‣ Singlet Oxygen Reactions with Flavonoids. A Theoretical – Experimental Study

Zanocco Loyola, Antonio Luis; Lemp Miranda, Else Mónica; Günther Sapunar, German Ricardo; Morales, Javier
Fonte: Universidade do Chile Publicador: Universidade do Chile
Tipo: Artículo de revista
Português
Relevância na Pesquisa
37.200105%
Artículo de publicación ISI; Detection of singlet oxygen emission, lmax = 1270 nm, following laser excitation and steady-state methods were employed to measure the total reaction rate constant, kT, and the reactive reaction rate constant, kr, for the reaction between singlet oxygen and several flavonoids. Values of kT determined in deuterated water, ranging from 2.46107 M21s21 to 13.46107 M21s21, for rutin and morin, respectively, and the values measured for kr, ranging from 2.86105 M21s21 to 65.76105 M21s21 for kaempferol and morin, respectively, being epicatechin and catechin chemically unreactive. These results indicate that all the studied flavonoids are good quenchers of singlet oxygen and could be valuable antioxidants in systems under oxidative stress, in particular if a flavonoid-rich diet was previously consumed. Analysis of the dependence of rate constant values with molecular structure in terms of global descriptors and condensed Fukui functions, resulting from electronic structure calculations, supports the formation of a charge transfer exciplex in all studied reactions. The fraction of exciplex giving reaction products evolves through a hydroperoxide and/or an endoperoxide intermediate produced by singlet oxygen attack on the double bond of the ring C of the flavonoid.

‣ Determination of chemical rate constants in singlet molecular oxygen reactions by using 1,4-dimethylnaphthalene endoperoxide

Zanocco Loyola, Antonio Luis; Lemp Miranda, Else Mónica; Günther Sapunar, Germán
Fonte: ELSEVIER SCIENCE SA Publicador: ELSEVIER SCIENCE SA
Tipo: Artículo de revista
Português
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Thermal decomposition of 1,4-dimethylnaphthalene endoperoxide (DMNE) as a source of singlet oxygen was used to measure chemical rate constants, k(R), for reactions between singlet oxygen, O-2((1)Delta(g)), and various substrates. Time resolved O-2((1)Delta(g)) IR luminescence detection and steady-state experiments were used to monitor the decomposition of the endoperoxide and the rate of singlet oxygen production. Only 25% of oxygen from thermal decomposition of DMNE, in acetonitrile at 20degreesC, is detected as O-2((1)Delta(g)). Values of k(R) for reactions of O-2((1)Delta(g)) with 1,4-diphenylisobenzofurane (DPBF) and rubrene measured by this method are similar to values obtained by photosensitization. Values of k(R) for the chemical reaction of O-2((1)Delta(g)) with the antiinflammatory drugs piroxicam and tenoxicam, of (6.1 +/- 0.4) x 10(6) and (1.6 +/- 0.2) 10(7) M-1 s(-1), respectively, are close to those for the total singlet oxygen deactivation rate. Thermal decomposition of aromatic endoperoxides is a convenient source of singlet oxygen for measurements of rate constants in reactions of O-2((1)Delta(g)) where photosensitization cannot be employed. However, experimental conditions and approaches involved determine the method's limitations and applicability in a given system.

‣ Understanding the influence of Lewis acids in the regioselectivity of the Diels–Alder reactions of 2-methoxy-5-methyl-1,4-benzoquinone: A DFT study

Contreras, Renato; Domingo, Luis R.; Soto-Delgado, Jorge
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artículo de revista
Português
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The mechanisms of the Diels–Alder (DA) reactions of 2-methoxy-5-methyl-1,4-benzoquinone 1 with 2- methyl-1,3-butadiene 2, in the absence and in the presence of LA catalysts, have been studied using the DFT method at the B3LYP/6-31G(d) level of theory. The uncatalyzed DA reactions between 1 and 2 take place via synchronous concerted TSs. The large activation barrier as well as the low stereo and regioselectivity associated with the uncatalyzed process are in clear agreement with the non-polar character of the cycloaddition. Coordination of the LA catalysts, BF3 or SnCl4, to the oxygen atoms of the benzoquinone 1 produces a large acceleration of the reaction, which can be associated with the large polar character of the cycloaddition. The different coordination modes of BF3 and SnCl4 LA catalysts to the oxygen atoms of benzoquinone 1 allow explaining the reverse para/meta regioselectivity observed in these LA-catalyzed DA reactions. The analysis based on the global and local electrophilicity indices of the reagents correctly explains the polar nature of the title reactions, as well as the change of regioselectivity experimentally observed.; We acknowledge support from FONDECYT Grant No. 1070715. J.S.D. acknowledges MECESUP fellowship No. 0408. L.R.D. thanks the Spanish Government for financial support through project CTQ2006-14297/BQU.

‣ Towards stable bifunctional oxygen electrodes and corrosion resistant gas diffusion layers for regenerative fuel cells

Silva, R. A.; Soares, C. O.; Carvalho, M. D.; Jorge, M. E. Melo; Gomes, A.; Pereira, M. I. da Silva; Rangel, C. M.
Fonte: Laboratório Nacional de Energia e Geologia Publicador: Laboratório Nacional de Energia e Geologia
Tipo: Conferência ou Objeto de Conferência
Publicado em //2013 Português
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57.27187%
ABSTRACT: Regenerative fuel cells (RFCs) can provide very high energy storage at minimal weight in a dual mode system, by combining an electrolyzer and a fuel cell. Although RFCs are an appealing technology their development is still at an early stage. One key issue is the search for highly active electrocatalysts for both oxygen reduction and water oxidation. Presently, platinum is the best electrocatalyst for the oxygen reduction reaction (ORR), but has a poor oxygen evolution (OER) performance while metal oxides catalyze the OER but not the ORR. Yet, the search for the development of bi-functional oxygen electrodes is also associated to structurally stable gas diffusion layers - they must be capable of withstanding high potentials when cells are operated in the electrolyzer mode and in addition, mass transport limitations when used as a cathode in fuel cell mode. A novel approach is used in this work to tackle the issue, focussing on the development of stable gas diffusion electrodes for the oxygen reactions, having as a base high surface area LaNiO3. Previous work by the authors has optimised the synthesis of the mentioned perovskite-type oxide, prepared by a self-combustion method. The high electrochemical surface area and low porosity of the oxide has been indicated by electrochemical impedance spectroscopy (EIS) and BET measurements. A full characterization has been the subject of recent publications [1...

‣ Redox stability and bifunctionality of LaNiO3-based oxygen electrodes

Silva, R. A.; Soares, C. O.; Carvalho, M. D.; Jorge, M. E. Melo; Gomes, A.; Rangel, C. M.; Pereira, M. I. da Silva
Fonte: Laboratório Nacional de Energia e Geologia Publicador: Laboratório Nacional de Energia e Geologia
Tipo: Conferência ou Objeto de Conferência
Publicado em //2013 Português
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One key issue in the development of Regenerative fuel cells (RFCs) is the availability of cheap, highly active electrocatalysts for both oxygen reduction and water oxidation. Perovskite-type oxides, with the general formula ABO3, are potential catalysts for next generation of regenerative fuel cells. In particular, LaNiO3 has been recognised as one of the most promising oxygen electrodes. In this work LaNiO3 perovskite-type oxides, prepared by a self-combustion method [1, 2], have been simultaneously optimized for activity and stability as an anode and cathode material for water oxidation and oxygen reduction reaction (ORR), respectively. Extremely high surface area has been measured by BET analysis with matching electrochemical activity estimated by cyclic voltammetry and electrochemical impedance spectroscopy. A full electrochemical study has been conducted in order to kinetically characterize the prepared electrodes in alkaline media, using a Ni foam and carbon paper as support material for the electrodes. For LaNiO3 deposits on Ni foam, low contact resistance between the oxide and support, possibility of high metal oxide loadings and dimensional stability were accomplished with remarkable stability in the region of oxygen evolution. For LaNiO3 deposits on carbon paper...

‣ Oxygen Interstitial Dynamics in Bi(2)Sr(2)CaCu(2)O(8+delta) Oxides Measured by Mechanical Spectroscopy

Grandini, Carlos Roberto; Ruiz, S. L M; Silva, M. R.; Gimenez, J. M. A.; Rubo, Elisabete Aparecida Andrello
Fonte: Trans Tech Publications Ltd Publicador: Trans Tech Publications Ltd
Tipo: Conferência ou Objeto de Conferência Formato: 634-638
Português
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Many researchers became interested in the discovery of Bi(2)Sr(2)CaCu(2)O(8+delta) oxides with critical temperature of around 80 K. It is known that the critical temperature is related to the CuO2 planes of the material. For this reason, the study of the interstitial oxygen in these oxides is of great relevance. The samples were prepared by means of conventional solid state reactions, through the stoichiometric mixture of precursory powders. After the sinterization, the samples were submitted to measurements of density, electrical resistivity, x-ray diffraction, scanning electron microscopy and energy dispersion spectroscopy, with the objective of performing their characterization. The measurements of mechanical spectroscopy were performed by a torsion pendulum. The results show three relaxation processes in the temperature range of 200 and 700 K, with activation energy of approximately 0.9 eV, which has been attributed to the dynamics of the interstitial oxygen present in the material.