beta Phase transformations in the Cu-11mass%Al alloy with Ag additions

In the Cu-Al system, due to the sluggishness of the beta a dagger" (alpha + gamma(1)) eutectoid reaction, the beta phase can be retained metastably. During quenching, metastable beta alloys undergo a martensitic transformation to a beta' phase at Al low content. The ordering reaction beta a dagger" beta(1) precedes the martensitic transformation. The influence of Ag additions on the reactions containing the beta phase in the Cu-11mass%Al alloy was studied using differential scanning calorimetry and in situ X-ray diffractometry. The results indicated that, on cooling, two reactions are occurring in the same temperature range, the beta -> (alpha + gamma(1)) decomposition reaction and the beta -> beta(1) reaction, with different reaction mechanisms (diffusive for the former and ordering for the latter) and, consequently, with different reaction rates. For lower cooling rates, the dominant is the decomposition reaction and for higher cooling rates the ordering reaction prevails. on heating, the (alpha + gamma(1)) -> beta reverse eutectoid reaction occurs with a resulting beta phase saturated with alpha. The increase of Ag concentration retards the beta -> (alpha + gamma(1)) decomposition reaction and the beta -> beta(1) ordering reaction, which occurs in the same temperature range, becomes the predominant process.

Xylo-oligosaccharides from lignocellulosic materials: Chemical structure, health benefits and production by chemical and enzymatic hydrolysis

Currently, there is worldwide interest in the technological use of agro-industrial residues as a renewable source of food and biofuels. Lignocellulosic materials (LCMs) are a rich source of cellulose and hemicellulose. Hemicellulose is rich in xylan, a polysaccharide used to develop technology for producing alcohol, xylose, xylitol and xylo-oligosaccharides (XOSs). The XOSs are unusual oligosaccharides whose main constituent is xylose linked by β 1-4 bonds. The XOS applications described in this paper highlight that they are considered soluble dietary fibers that have prebiotic activity, favoring the improvement of bowel functions and immune function and having antimicrobial and other health benefits. These effects open a new perspective on potential applications for animal production and human consumption. The raw materials that are rich in hemicellulose include sugar cane bagasse, corncobs, rice husks, olive pits, barley straw, tobacco stalk, cotton stalk, sunflower stalk and wheat straw. The XOS-yielding treatments that have been studied include acid hydrolysis, alkaline hydrolysis, auto-hydrolysis and enzymatic hydrolysis, but the breaking of bonds present in these compounds is relatively difficult and costly, thus limiting the production of XOS. To obviate this limitation, a thorough evaluation of the most convenient methods and the opportunities for innovation in this area is needed. Another challenge is the screening and taxonomy of microorganisms that produce the xylanolytic complex and enzymes and reaction mechanisms involved. Among the standing out microorganisms involved in lignocellulose degradation are Trichoderma harzianum, Cellulosimicrobium cellulans, Penicillium janczewskii, Penicillium echinulatu, Trichoderma reesei and Aspergillus awamori. The enzyme complex predominantly comprises endoxylanase and enzymes that remove hemicellulose side groups such as the acetyl group. The complex has low β-xylosidase activities because β-xylosidase stimulates the production of xylose instead of XOS; xylose, in turn, inhibits the enzymes that produce XOS. The enzymatic conversion of xylan in XOS is the preferred route for the food industries because of problems associated with chemical technologies (e.g., acid hydrolysis) due to the release of toxic and undesired products, such as furfural. The improvement of the bioprocess for XOS production and its benefits for several applications are discussed in this study. © 2012 Elsevier Ltd.

Antifungal applications of Ag-decorated hydroxyapatite nanoparticles

Pure hydroxyapatite (HA) and hydroxyapatite decorated with silver (HA@Ag) nanoparticles were synthesized and characterized. The antifungal effect of HA@Ag nanoparticles in a distilled water solution was evaluated against Candida albicans. The origin of the antifungal activity of the HA@Ag is also discussed. The results obtained showed that the HA nanorod morphology remained the same with Ag ions decorations on the HA structure which were deposited in the form of nanospheres. Interaction where occurred between the structure and its defect density variation in the interfacial HA@Ag and intrafacial HA region with the fungal medium resulted in antifungal activity. The reaction mechanisms involved oxygen and water adsorption which formed an active complex cluster. The decomposition and desorption of the final products as well as the electron/hole recombination process have an important role in fungicidal effects. © 2013 C. A. Zamperini et al.

Ativação da ligação c-h no metano por espécies MOm n+ (M = Nb, Fe; m = 0, 1; n = 0, 1, 2): estrutura eletrônica, mecanismo de reação e reatividade

The need for renewal and a more efficient use of energy resources has provided an increased interest in studies of methane activation processes in the gas phase by transition metal oxides. In this respect, the present work is an effort to assess , by means of a computational standpoint, the reactivity of NbOm n+ and FeOm n+ (m = 1, 2, n = 0, 1, 2) oxides in the activation process of the methane C-H bond, which corresponds to the first rate limiting step in the process of converting methane to methanol. These oxides are chosen, primarily, because the iron oxides are the most experimentally studied, and iron ions are more abundant in biological mediums. The main motive for choosing niobium oxides is the abundance of natural reserves of this mineral in Brazil (98%), especially in Minas Gerais. Initially, a thorough investigation was conducted, using different theoretical methods, to analyze the structural and electronic properties of the investigated oxides. Based on these results, the most reliable methodology was selected to investigate the activation process of the methane C-H bond by the series of iron and niobium oxides, considering all possible reaction mechanisms known to activate the C-H bond of alkanes. It is worth noting that, up to this moment and to our knowledge, there are no papers, in literature , investigating and comparing all the mechanisms considered in this work. I n general, the main results obtained show different catalytic tendencies and behaviors throughout the series of monoxides and dioxides of iron and niobium. An important and common result found in the two studies is that the increase in the load on the metal center and the addition of oxygen atoms to the metal, clearly favor the initial thermodynamics of the reaction, i.e., favor the approach of the metal center to methane, distorting its electron cloud and, thereby, decreasing its inertia. Comparing the two sets of oxides, we conclude that the iron oxides are the most efficient in activating the methane C-H bond. Among the iron oxides investigated, FeO + showed better kinetic and thermodynamic performance in the reaction with methane, while from the niobium oxides and ions NbO 2+ and NbO2 2+, showed better catalytic efficiency in the activation of the methane C-H bond.

Metodologia computacional baseada em DFT, para compreensão de processos eletródicos em fases intermetálicas ordenadas

Pós-graduação em Ciência e Tecnologia de Materiais - FC

Searching for resonances in the unbound Be-6 nucleus by using a radioactive Be-7 beam

Knowledge of the He-3(He-3,2p)He-4 reaction is important for understanding stellar burning and solar neutrino production. Previous measurements have found a surprisingly large rise in the cross section at low energies that could be due to a low-energy resonance in the He-3 + He-3 (Be-6) system or electron screening. In the Be-6 nucleus, however, no excited states have been observed above the first 2(+) state at E (x) = 1.67 MeV up to 23 MeV, even though several are expected. The H-2(Be-7,H-3)Be-6 reaction has been studied for the first time to search for resonances in the Be-6 nucleus that may affect our understanding of the He-3(He-3,2p)He-4 reaction. A 100-MeV radioactive Be-7 beam from the Holifield Radioactive Ion Beam Facility (HRIBF) was used to bombard CD2 targets, and tritons were detected by using the silicon detector array (SIDAR). A combination of reaction mechanisms appears to be necessary to explain the observed triton energy spectrum.

Sequential Methyl-Fluorine Exchange Reactions of Siloxide Ions in the Gas Phase

Air Force Office of Scientific Research (AFOSR)

Gas-phase reactivity of 2-hydroxy-1,4-naphthoquinones: a computational and mass spectrometry study of lapachol congeners

In order to understand the influence of alkyl side chains on the gas-phase reactivity of 1,4-naphthoquinone derivatives, some 2-hydroxy-1,4-naphthoquinone derivatives have been prepared and studied by electrospray ionization tandem mass spectrometry in combination with computational quantum chemistry calculations. Protonation and deprotonation sites were suggested on the basis of gas-phase basicity, proton affinity, gas-phase acidity (?Gacid), atomic charges and frontier orbital analyses. The nature of the intramolecular interaction as well as of the hydrogen bond in the systems was investigated by the atoms-in-molecules theory and the natural bond orbital analysis. The results were compared with data published for lapachol (2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone). For the protonated molecules, water elimination was verified to occur at lower proportion when compared with side chain elimination, as evidenced in earlier studies on lapachol. The side chain at position C(3) was found to play important roles in the fragmentation mechanisms of these compounds. Copyright (c) 2012 John Wiley & Sons, Ltd.

(18O,16O) Two-neutron transfer reactions for spectroscopic studies.

A systematic study of the response of different nuclei to the (18O, 16O) two-neutron transfer reaction at 84 MeV incident energy was pursued at the INFN-LNS in Catania (Italy). The experiments were performed using several solid targets from light (9Bc, 11 B, 12,13C, 16O, 28Si) to heavier ones (58,64Ni, 120Sn, 208Pb). The 16O ejectiles were detected at forward angles by the MAGNEX magnetic spectrometer and identified without the need of time of flight measurements. Exploiting the large momentum (≈ 25%) and angular (50 msr) acceptance of the spectrometer, energy spectra were obtained with a relevant yield up to about 20 MeV excitation energy. A common feature of the light nuclei spectra is the strong population of states with well known configuration of two-particle over a core and the appearance of unknown resonant structures in the continuum. These latter can reveal the excitation of a collective mode connected with the transfer of a pair. For the heavier nuclei as 66Ni a completely different behaviour is observed indicating the presence of more dissipative processes in the reaction mechanisms that hide the spectroscopic information.

Theoretical studies of mononuclear non-heme iron active sites

The quantum chemical investigations presented in this thesis use hybrid density functional theory to shed light on the catalytic mechanisms of mononuclear non-heme iron oxygenases, accommodating a ferrous ion in their active sites. More specifically, the dioxygen activation process and the subsequent oxidative reactions in the following enzymes were studied: tetrahydrobiopterin-dependent hydroxylases, naphthalene 1,2-dioxygenase and α-ketoglutarate-dependent enzymes. In light of many experimental efforts devoted to the functional mimics of non-heme iron oxygenases, the reactivity of functional analogues was also examined. The computed energetics and the available experimental data served to assess the feasibility of the reaction mechanisms investigated. Dioxygen activation in tetrahydrobiopterin- and α-ketoglutarate-dependent enzymes were found to involve a high-valent iron-oxo species, which was then capable of substrate hydroxylation. In the case of naphthalene 1,2-dioxygenase, the reactivity of an iron(III)-hydroxperoxo species toward the substrate was investigated and compared to the biomimetic counterpart.

Acqua: solvente elettivo in Organocatalisi e Biocatalisi

Water is a safe, harmless, and environmentally benign solvent. From an eco-sustainable chemistry perspective, the use of water instead of organic solvent is preferred to decrease environmental contamination. Moreover, water has unique physical and chemical properties, such as high dielectric constant and high cohesive energy density compared to most organic solvents. The different interactions between water and substrates, make water an interesting candidate as a solvent or co-solvent from an industrial and laboratory perspective. In this regard, organic reactions in aqueous media are of current interest. In addition, from practical and synthetic standpoints, a great advantage of using water is immediately evident, since it does not require any preliminary drying process. This thesis was found on this aspect of chemical research, with particular attention to the mechanisms which control organo and bio-catalysis outcome. The first part of the study was focused on the aldol reaction. In particular, for the first time it has been analyzed for the first time the stereoselectivity of the condensation reaction between 3-pyridincarbaldehyde and the cyclohexanone, catalyzed by morpholine and 4-tertbutyldimethylsiloxyproline, using water as sole solvent. This interest has resulted in countless works appeared in the literature concerning the use of proline derivatives as effective catalysts in organic aqueous environment. These studies showed good enantio and diastereoselectivities but they did not present an in depth study of the reaction mechanism. The analysis of the products diastereomeric ratios through the Eyring equation allowed to compare the activation parameters (ΔΔH≠ and ΔΔS≠) of the diastereomeric reaction paths, and to compare the different type of catalysis. While morpholine showed constant diasteromeric ratio at all temperatures, the O(TBS)-L-proline, showed a non-linear Eyring diagram, with two linear trends and the presence of an inversion temperature (Tinv) at 53 ° C, which denotes the presence of solvation effects by water. A pH-dependent study allowed to identify two different reaction mechanisms, and in the case of O(TBS)-L-proline, to ensure the formation of an enaminic species, as a keyelement in the stereoselective process. Moreover, it has been studied the possibility of using the 6- aminopenicillanic acid (6-APA) as amino acid-type catalyst for aldol condensation between cyclohexanone and aromatic aldehydes. A detailed analysis of the catalyst regarding its behavior in different organic solvents and pH, allowed to prove its potential as a candidate for green catalysis. Best results were obtained in neat conditions, where 6-APA proved to be an effective catalyst in terms of yields. The catalyst performance in terms of enantio- and diastereo-selectivity, was impaired by the competition between two different catalytic mechanisms: one via imine-enamine mechanism and one via a Bronsted-acid catalysis. The last part of the thesis was dedicated to the enzymatic catalysis, with particular attention to the use of an enzyme belonging to the class of alcohol dehydrogenase, the Horse Liver Alcohol Dehydrogenase (HLADH) which was selected and used in the enantioselective reduction of aldehydes to enantiopure arylpropylic alcohols. This enzyme has showed an excellent responsiveness to this type of aldehydes and a good tolerance toward organic solvents. Moreover, the fast keto-enolic equilibrium of this class of aldehydes that induce the stereocentre racemization, allows the dynamic-kinetic resolution (DKR) to give the enantiopure alcohol. By analyzing the different reaction parameters, especially the pH and the amount of enzyme, and adding a small percentage of organic solvent, it was possible to control all the parameters involved in the reaction. The excellent enatioselectivity of HLADH along with the DKR of arylpropionic aldehydes, allowed to obtain the corresponding alcohols in quantitative yields and with an optical purity ranging from 64% to >99%.

Stabilizing molecular self-assemblies via on-surface reactions

Für die Realisierung zukünftiger Technologien, wie z.B. molekulare Elektronik, werden Strategien benötigt, um funktionale Strukturen direkt auf Oberflächen zu erzeugen. Für die Bewältigung dieser Aufgabe ist die molekulare Selbstanordnung ein äußerst vielversprechender Bottom-up-Ansatz. Hierbei ist eine der größten Herausforderungen das Zusammenspiel aus intramolekularer Wechselwirkung und der Wechselwirkung zwischen Substrat und Molekülen in ein Gleichgewicht zu bringen. Da jedoch die wirkenden Kräfte der molekularen Selbstanordnung ausschließlich reversibler Natur sind, ist eine langfristige Stabilität fragwürdig. Somit ist die kovalente Verknüpfung der gebildeten Strukturen durch Reaktionen direkt auf der Oberfläche unerlässlich, um die Stabilität der Strukturen weiter zu erhöhen. Hierzu stellt die vorliegende Arbeit eine ausführliche Studie zu molekularer Selbstanordnung und der zielgerichteten Modifikation ebensolcher Strukturen dar. Durch den Einsatz von hochauflösender Rasterkraftmikroskopie im Ultrahochvakuum, welche es erlaubt einzelne Moleküle auf Nichtleitern abzubilden, wurde der maßgebliche Einfluss von Ankerfunktionalitäten auf den Prozess der molekularen Selbstanordnung gezeigt. Des Weiteren konnte die Stabilität der selbst angeordneten Strukturen durch neue Oberflächenreaktionskonzepte entschieden verbessert werden. Der Einfluss von Ankerfunktionen, die elektrostatische Wechselwirkung zwischen Molekül und Substrat vermitteln, auf den Strukturbildungsprozess der molekularen Selbstanordnung wird eingehend durch den Vergleich eines aromatischen Moleküls und seines vierfach chlorierten Derivates gezeigt. Für diese beiden Moleküle wurde ein deutlich unterschiedliches Verhalten der Selbstanordnung beobachtet. Es wird gezeigt, dass die Fähigkeit zur Bildung selbst angeordneter, stabiler Inseln entscheidend durch die Substituenten und die Abmessungen des Moleküls beeinflusst wird. Auch wird in dieser Arbeit die erste photochemische Reaktion organischer Moleküle auf einem Isolator gezeigt. Qualitative und quantitative Ergebnisse liefern ein detailliertes Bild darüber, wie die Abmessungen des Substratgitters die Richtung der Reaktion gezielt beeinflussen. Des Weiteren wird ein allgemeines Konzept zur selektiven Stabilisierung selbstangeordneter Molekülstrukturen durch den kontrollierten Transfer von Elektronen präsentiert. Durch die gezielte Steuerung der Menge an Dotierungsatomen wird die Desorptionstemperatur der molekularen Inseln signifikant erhöht und das Desorptionsverhalten der Inseln entschieden verändert. Diese Arbeit präsentiert somit erfolgreich durchgeführte Strategien um den Prozess der molekularen Selbstanordnung zu steuern, sowie entscheidende Mechanismen um die Stabilisierung und Modifizierung von selbst angeordneten Strukturen zu gewährleisten.

EVOLUTION OF SELECTED ISOPRENE OXIDATION PRODUCTS IN DARK AQUEOUS AMMONIUM SULFATE

The aqueous phase processing of glyoxylic acid, pyruvic acid, oxalic acid and methylglyoxal was studied simulating dark and radical free atmospheric aqueous aerosol. A novel observation of the cleavage of a carbon-carbon bond in pyruvic acid and glyoxylic acid leading to their decarboxylation was made in the presence of ammonium salts but no decarboxylation was observed from oxalic acid. The empirical rate constants for decarboxylation were determined. The structure of the acid, ionic environment of solution and concentration of species found to affect the decarboxylation process. A tentative set of reaction mechanisms was proposed involving nucleophilic attack by ammonia on the carbonyl carbon leading to fragmentation of the carbon-carbon bond between the carbonyl and carboxyl carbons. Whereas, the formation of high molecular weight organic species was observed in the case of methylglyoxal. The elemental compositions of the species were determined. It was concluded that, additional pathways that are not currently known likely contribute to aqueous phase processing leading to high molecular weight organic species. Under similar conditions in atmospheric aerosol, the aqueous phase processing will markedly impact the physicochemical properties of aerosol.

Diurnal cycle of fossil and nonfossil carbon using radiocarbon analyses during CalNex

Radiocarbon (14C) analysis is a unique tool to distinguish fossil/nonfossil sources of carbonaceous aerosols. We present 14C measurements of organic carbon (OC) and total carbon (TC) on highly time resolved filters (3–4 h, typically 12 h or longer have been reported) from 7 days collected during California Research at the Nexus of Air Quality and Climate Change (CalNex) 2010 in Pasadena. Average nonfossil contributions of 58% ± 15% and 51% ± 15% were found for OC and TC, respectively. Results indicate that nonfossil carbon is a major constituent of the background aerosol, evidenced by its nearly constant concentration (2–3 μgC m−3). Cooking is estimated to contribute at least 25% to nonfossil OC, underlining the importance of urban nonfossil OC sources. In contrast, fossil OC concentrations have prominent and consistent diurnal profiles, with significant afternoon enhancements (~3 μgC m−3), following the arrival of the western Los Angeles (LA) basin plume with the sea breeze. A corresponding increase in semivolatile oxygenated OC and organic vehicular emission markers and their photochemical reaction products occurs. This suggests that the increasing OC is mostly from fresh anthropogenic secondary OC (SOC) from mainly fossil precursors formed in the western LA basin plume. We note that in several European cities where the diesel passenger car fraction is higher, SOC is 20% less fossil, despite 2–3 times higher elemental carbon concentrations, suggesting that SOC formation from gasoline emissions most likely dominates over diesel in the LA basin. This would have significant implications for our understanding of the on-road vehicle contribution to ambient aerosols and merits further study.