Le dioxyde de carbone occupe une place particulière dans l'amplification anthropogénique de l'effet de serre. En particulier, son utilisation dans la synthèse de carbonates organiques à partir d'alcools constituerait une alternative à l'emploi actuel du phosgène ou du monoxyde de carbone. L'insertion de CO2 dans Sn-OCH3 liaison de dialkyldimethoxystannanes est reconnue comme la première étape pour la formation du carbonate de diméthyle (DMC) à partir du méthanol et CO2. L'identification des étapes ultérieures est cruciale pour améliorer l'activité et est encore l’objet de débats car les espèces étain ont la propension à l’oligomérisation. Nous avons utilisé des calculs basés sur la Théorie de la Fonctionnelle de la Densité pour donner un aperçu sur le mécanisme de réaction. L'insertion de CO2 est promue par l’interaction acide-base concertée de Lewis de CO2 avec l'étain et l'atome d'oxygène du ligand méthoxy. Le chemin principal de la réaction pour la formation de DMC est proposé selon un réarrangement intramoléculaire des espèces monomériques, Me2Sn[OC(O)OCH3]2. Le processus conduit à un transfert d'un groupe méthyle d'un carbonate de méthyle à l'autre via un anneau de 4 atomes ou 6 atomes formant un fragment Sn-CO3. Dans une deuxième étape, le méthanol réagit avec un intermédiaire et conduit à la formation de DMC et un complexe de trimère qui peut permettre la régénération des réactifs. Mais un complexe décanucléaire d’étain peut être aussi produit par une réaction latérale. Enfin, un cycle catalytique pour produire le DMC a pu être élaboré. Les calculs DFT sont en accord avec les données expérimentales et permettent une analyse comparative des chemins de réaction / Carbon dioxide occupies a special place in the amplification of the anthropogenic greenhouse effect. In particular, its use in the synthesis of organic carbonates from alcohols, constitute an alternative to the current application of phosgene or carbon monoxide. The insertion of CO2 into the Sn-OCH3 bond of dialkyldimethoxystannanes is recognized as the first step to dimethyl carbonate (DMC) formation from methanol and carbon dioxide. The identification of the subsequent steps is crucial for activity improvements and is still under debate as the tin species have the propensity for oligomerization. We have used density functional theory calculations to provide insight into the reaction mechanism. The CO2 insertion into the Sn-OCH3 bond is promoted by the concerted Lewis acid-base interaction of CO2 with tin and the oxygen atom of the methoxy ligand. The major reaction pathway to DMC is proposed to occur via an intramolecular rearrangement of the monomeric species, Me2Sn[OC(O)OCH3]2. The process results in the transfer of a methyl group from one methyl carbonate to the other via a 4-membered or 6-membered ring forming the Sn-CO3 moiety. In a second stage, methanol reacts with one intermediate and leads to DMC formation and a trimer compound which may allow the regeneration of the reactant. Also a decanuclear tin complex is produced due to the lateral reaction. Finally a catalytic cycle for DMC production in methanol can be elaborated. DFT calculations are in agreement with the experimental data and allow a comparative analysis of reaction channels

The current dissertation thesis is focused on the investigation of metal-catalyzed reactions using electrospray ionization mass spectrometry as the primary research technique. However, other gas- phase methods such as tandem mass spectrometry, infrared multiphoton dissociation spectroscopy and quantum chemical calculations have also been involved to unravel and support the findings and proposals. As organometallic chemistry is a very broad and complicated topic, this thesis is only focused on a few projects. The fist of them is dedicated to copper acetate speciation in organic solvents, the second - to the mechanistic investigation of copper catalyzed aerobic cross coupling of thiol esters and arylboronic acids, the third studied coordination and bond activation of nickel(II) - phenylpyridine complexes and the last investigated carboxylate assisted C-H activation reactions.

No description available.

First- principles modeling is used to predict hydrogen permeability through Palladium (Pd)-rich binary alloy membranes as a function of temperature and H2 pressure. We introduce a simplified model that incorporates only a few factors and yields quantitative prediction. This model is used to predict hydrogen permeability in a wide range of binary alloy membranes and to find promising alloys that have high hydrogen permeability. We show how our efficient Density Functional Theory (DFT)-based model predicts the chemical stability and proton conductivity of doped barium zirconate (BaZrO3), barium stannate (BaSnO3), and barium hafnate (BaHfO3). Our data is also used to explore the physical origins of the trends in chemical stability and proton conductivity among different dopants. We also study potassium tantalate (KTaO3), which is a prototype perovskite, to examine the characteristics of undoped perovskites. Specifically, we study the impacts of isotope effects, tunneling effects, and native point defects on proton mobility in KTaO3. It is important to find and develop solid-state Li-ion electrolyte materials that are chemically stable and have high ionic conductivities for high performance Li-ion batteries. We show how we predict the chemical stability of Li7La3Zr2O12, Li7La3Sn2O12, and Li7La3Hf2O12 with respect to carbonate and hydroxide formation reactions.

This dissertation presents studies of the synthetic processes and applications of siloxane-based materials. Kinetic investigations of bridged organoalkoxysilanes that are precursors to organic-inorganic hybrid polysilsesquioxanes are a primary focus. Quick gelation despite extensive cyclization is found during the polymerization of bridged silane precursors except for silanes with certain short bridges. This work is an attempt to characterize and understand some of the distinct features of bridged silanes using experimental characterization, kinetic modeling and simulation. In addition to this, the dissertation shows how the properties of siloxane- materials can be engineered for drug delivery and adsorption. The phase behavior of polymerizing mixtures is first investigated to identify the solutions that favor kinetic characterization. Microphase separation is found to cause gradual loss of NMR signal for certain initial compositions. Distortionless Enhancement by Polarization Transfer 29Si NMR is employed to identify the products of polymerization of some short-bridged silanes under no signal loss conditions. This technique requires knowing indirect 29Si-1H scalar coupling constants which sometimes cannot be measured due to second-order effects. However, the B3LYP density functional method with 6-31G basis set is found to predict accurate 29Si-1H coupling constants of organoalkoxysilanes and siloxanes. The scalar coupling constants thus estimated are employed to resolve non-trivial coupled NMR spectra and quantitative kinetic modeling is performed using the DEPT Si NMR transients. In order to investigate the role of the organic bridging group, the structural evolution of bridged and non-bridged silanes are compared using Monte Carlo simulations. Kinetic and simulation models suggest that cyclization plays a key role right from the onset of polymerization for bridged silanes even more than in non-bridged silanes. The simulations indicate that the carbosiloxane rings formed from short-bridged precursors slow down but do not prevent gelation. The tuning of siloxane-based materials for adsorption technologies are also discussed here. In the first example, antioxidant enzyme loading is investigated as a means to reduce oxidative stress generated by silica nanoparticle drug carriers. Materials are engineered for promising enzyme loading and protection from proteolysis. Second, the potential of copper sulfate impregnation to enhance adsorption of ammonia by silica is explored by molecular simulation.

The graphene moiré superstructure formed on Ru(0001) (g/Ru(0001)) has shown the potential as a template to self-assemble super-lattices of metal nanoparticles as model catalysts. To explore the possibility of rational catalyst design on g/Ru(0001), detailed density functional theory (DFT) calculations have been performed to investigate the adsorption and diffusion of Rh and Au adatoms on g/Ru(0001). The consequences of different hopping rates for cluster nucleation have been explored by performing Monte Carlo-based statistical analysis, which suggests that diffusing species other than adatoms need to be taken into account to develop an accurate description of cluster nucleation and growth on this surface. DFT calculations have also been carried out to investigate the adsorption and diffusion of 18 4d (Y-Ag) and 5d (La-Au) transition metal adatoms on g/Ru(0001). Given the necessity to study larger diffusing species than adatoms, DFT calculations have been performed to study the adsorption and diffusion of Rh and Au dimers and trimers on g/Ru(0001). It was shown that the mobility of Rh clusters decreases with the increase of cluster size, while for Au, dimers diffuse faster than monomers and trimers on the moiré surface. We then used a genetic algorithm combined with DFT calculations to predict the lowest energy structure of a Au8 cluster on g/Ru(0001). Our prediction leads us to propose that Au clusters aggregates through Oswald ripening with Au dimer being the major diffusing species. Finally, we examined the morphology of a Cu19 cluster on g/Cu(111) using MD simulations with COMB3 potential. We also studied the mobility of Cu clusters on g/Cu(111) at elevated temperatures. The analysis suggests that g/Cu(111) may not be a suitable substrate for the formation and growth of isolated Cu clusters. All these calculation results have provided us a better understanding and useful insights into the nucleation and growth mechanism of metal clusters on graphene moiré.

Made available in DSpace on 2015-05-14T13:21:31Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 2255792 bytes, checksum: 5043ebf08321332b3a5629d90122a0e8 (MD5) Previous issue date: 2013-08-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Oxidative stress is a biological condition in which there is an imbalance between the release of reactive species and their reuptake by biological systems, leading to the production of peroxides and free radicals that damage different cellular components, including proteins, lipids and DNA. Porphyrins and their metal complexes can be used as mimics of the enzyme superoxide dismutase (SOD) in the modulation of oxidative stress. The in vivo efficacy of porphyrins is related, among other factors, to their bioavailability, which is strongly influenced by lipophilicity and can also be related to different structural and electronic properties of these molecules. In this context, the present work has the aim of analyzing the influence of certain molecular properties on the lipophilicity of a series of N-pyridylporphyrins, by means of theoretical calculations, intending to provide useful information for the design of new porphyrin derivatives. Initially, the logarithm of the octanol/water partition coefficient (log P) of these molecules was estimated by using different free software, as well as based on the difference of octanol/water free energy of solvation, employing the Density Functional Theory (DFT), with different hybrid functionals (B3LYP, M06 and M062X). From DFT calculations, electronic and structural properties were also obtained, such as atomic partial charges, molecular orbitals, dipole moment, polarizability, surface area and molecular volume. The obtained data were compared to the available experimental measure of lipophilicity to this series of porphyrins, the chromatographic retention factor (Rf). The results show that traditional and well established methods for log P calculation, such as fragmental methods, do not provide reliable results for the studied porphyrins, unlike the calculations based on free energy of solvation, which have provided similar trends to the experimental log P of manganese porphyrins analogous to the studied ones. Furthermore, the obtained results suggests that charge distribution on the porphyrin structure may be related to the observed differences in lipophilicity and, consequently, with the passage of these molecules across the cellular membrane. It also became evident that the molecular volume is strongly related to lipophilicity, since a larger relaxation of the structures in the n-octanol phase is observed due to the lipophilic character of the long alkyl chains attached to the pyridyl group. Low correlations between Rf values and the other molecular properties calculated were obtained, indicating that the lipophilicity of porphyrins can only be modulated by the size of alkyl changes and the introduction of groups that favorably alter the charge distribution. / O estresse oxidativo é uma condição biológica na qual há um desequilíbrio entre a liberação de espécies reativas e a sua captação pelos sistemas biológicos, podendo provocar a produção de peróxidos e radicais livres que danificam diferentes componentes celulares, incluindo proteínas, lipídios e o DNA. As porfirinas e seus complexos metálicos podem ser usados como mímicos da enzima superóxido dismutase (SOD) na modulação do estresse oxidativo. A eficácia in vivo das porfirinas está relacionada, entre outros fatores, com sua biodisponibilidade, que é fortemente influenciada pela lipofilicidade e pode também ser relacionada a diferentes propriedades estruturais e eletrônicas dessas moléculas. Neste contexto, o presente trabalho teve o intuito de analisar a influência de determinadas propriedades moleculares na lipofilicidade de uma série de N-piridilporfirinas, por meio de cálculos teóricos, visando fornecer informações úteis para o planejamento de novos derivados de porfirinas. Inicialmente, o logaritmo do coeficiente de partição octanol/água (log P) dessas moléculas foi estimado utilizando diferentes programas computacionais livres, bem como com base na diferença de energia livre de solvatação octanol/água, empregando a Teoria do Funcional da Densidade (DFT), com diferentes funcionais híbridos (B3LYP, M06 e M062X). A partir dos cálculos DFT foram obtidas, ainda, propriedades eletrônicas e estruturais, tais como cargas parciais atômicas, orbitais moleculares, momento de dipolo, polarizabilidade, área superficial e volume molecular. Os dados obtidos foram comparados à medida experimental de lipofilicidade disponível para esta série de porfirinas, o fator de retenção cromatográfica (Rf). Os resultados mostram que métodos tradicionais e bem estabelecidos de cálculo do log P, como os métodos fragmentais, não fornecem resultados confiáveis para as porfirinas estudadas, ao contrário dos cálculos baseados na energia livre de solvatação, que forneceram tendências semelhantes aos valores experimentais de log P de porfirinas de manganês análogas às estudadas. Além disso, os resultados obtidos sugerem que a distribuição de cargas na estrutura das porfirinas pode estar relacionada com as diferenças observadas na lipofilicidade e, consequentemente, com a passagem destas moléculas através da membrana celular. Também ficou evidente que o volume molecular está fortemente relacionado à lipofilicidade, uma vez que se observa um maior relaxamento da estrutura na fase n-octanol devido ao caráter lipofílico das longas cadeias alquílicas ligadas ao grupo piridil. Baixas correlações entre os valores de Rf e as demais propriedades moleculares calculadas foram obtidas, indicando que a lipofilicidade das porfirinas pode ser modulada apenas pelo tamanho das cadeias alquílicas e pela introdução de grupos que alterem favoravelmente a distribuição de cargas.

Au cours de la dernière décennie, l’usage intensif des herbicides en agriculture a provoqué plusieurs crises sanitaires et environnementales. La recherche de nouveaux herbicides bio-inspirés est donc devenue une urgence, en particulier afin de réduire les risques de pollution. Les porphyrines, naturellement présentes dans les végétaux, sont des molécules photosensibles. En présence d’oxygène, leur photo-activation conduit à la production d’Espèces Réactives de l’Oxygène capables d’induire la mort cellulaire. Ce principe utilisé en thérapie photodynamique peut être transféré aux plantes, par exemple à l’aide de porphyrines chargées. Nous avons étudié les propriétés physicochimiques (absorption UV-Visible, émission de fluorescence, photo-stabilité et production d’EROs) ainsi que les effets sur des cellules de tabac TBY-2 d’une série de porphyrines chargées. Pour étudier les mécanismes d’action des porphyrines en tant qu’herbicides, ces molécules doivent être tracées et localisées dans la plante. Dans ce but, nous avons synthétisé des porphyrines liées de manière covalente à un marqueur fluorescent par plusieurs bras espaceurs ; ces derniers ont été choisis grâce à une étude en modélisation moléculaire de leur flexibilité conformationnelle. Les propriétés photo-physiques de ces nouvelles dyades ont été étudiées expérimentalement et théoriquement. / Over the past decade, intensive use of herbicide in agriculture has caused several sanitary and environmental problems. Finding new bio-inspired herbicides preventing pollution has appeared crucial. Naturally present in plants, porphyrins are photosensitive. In the presence of oxygen, their photo-activation leads to production of Reactive Oxygen Species, which induce cell death. Already used in Photodynamic Therapy, this effect can be used to plant. In that purpose, a series of charged porphyrins (commercial and synthesized) were selected, and their physicochemical properties (e.g. UV-Vis absorption, fluorescence emission, photostability, ROS production) as well as their effects on TBY-2 (Tobacco Bright Yellow) cells were evaluated. Second, localizing molecules in plants is mandatory to understand mechanisms of the herbicide action. In this context, porphyrins were covalently grafted to a fluorescent marker, by a series of spacers that were chosen according to a preliminary molecular modeling evaluation of their conformational flexibility. The new dyads obtained were thoroughly studied both theoretically and experimentally for their photophysical properties.

No description available.

Gold catalysis has recently experienced a sustained upswing in interest from scientific community. The amount of new reactions catalysed by gold is so significant that little is known about mechanisms of most of these reactions. Research into mechanisms of (not only) gold catalysed reactions is therefore very significant area of interest and important to the continued improvement of gold catalysed reactions. Formation of cationic π-complexes is considered a first mechanistic step in reactions catalysed by gold(I). The bond dissociation energies of gas phase cationic π-complexes were investigated by mass spectrometry and theory calculations in this thesis. These complexes consisted of differently substituted unsaturated hydrocarbons (alkenes, alkynes, alkadienes and allenes) and complex cations of silver and gold containing second ligand (triphenylphosphine, acetonitrile). On the basis of the results obtained from this study, a possible origin of the "silver effect" in gold(I) catalysis is discussed. Key words catalysis, gold, silver, mass spectrometry, DFT calculations, reaction intermediates