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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Measurement and prediction of nonlinear harmonics as a tool for dynamic characterization of electrochemical systems /

Wilson, Jamie Robyn, January 2007 (has links)
Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (p. 134-140).
2

The influence of alkali cations on the photodimerization of cinnamate salts and cinnamic acid derivatives intercalated hydrotalcite studied by solid-state NMR

Zahan, Marufa 10 January 2024 (has links)
The requirements of information storage exclusively continue to increase in our daily lives and future, will need even more memory space which should be very small in size but hold the capacity to store a huge volume of information. For that reason, modern research is focusing on molecular memory or optical switches devices where nanoparticles undergoing light-induced transformations (e.g. photodimerization) between structural phases with different optical properties are key components. Hence, the UV reversible photodimerization of the cinnamate groups have the potential to be used as switching segments in optical memory devices. The dissertation deals with the solid state NMR investigation on the influence of alkali cations on the photodimerization of cinnamate salts and cinnamic acid derivatives intercalated in hydrotalcite LDH (layered double hydroxides). The packing of molecules determines reactivity and stereo product, so the motivation is to modify molecular packing through noncovalent interactions by using different cations to influence packing and photodimerization products as well as photodimerization kinetics. The alkali metal salts of m-bromo/chloro cinnamic acid and their photodimerization products are analyzed by solid-state NMR. Cesium, rubidium, and potassium salts show well resolved signals in the 13C CPMAS spectra, whereas for ammonium and sodium salts broader lines are obtained. The size of the cation and with that the packing arrangement in the crystal has a significant influence on the resulting spectra. Cesium, rubidium, and potassium salts photoreact to the corresponding truxinates with a non-planar cyclobutane ring but disorder and lower crystallinity are found in the ammonium and sodium photoproducts. In case of divalent cations, both show good crystal packing in the reactant but Ca di-trans cinnamate shows better photoreaction than Mg di-trans cinnamate. Analysis of the kinetics by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation shows that the reaction rate is faster with smaller catcationsd in general chlorine salts react faster than the corresponding bromine salts. Furthermore, dethe termination of the chemical shift anisotropy (CSA) tensor for reactant and product provides information for required atomic reorientations in consequence of photoreaction. Results are explained based on these necessary movements of atoms betweethe n reactant and product. Finally, m-bromo cinnamate and K m-bromo cinnamate are successfully incorporated into the LDH to investigate photoreactivity in a confine system. These samples also undergo [2+2] photodimerization upon irradiation with UV light to yield truxinates the , but high degree of disorder in the ipso, cyclobutane and carboxylate spectral region of the photoproduct is found which indicates low crystallinity in both samples.:Bibliographische Beschreibung III Table of contents V List of Figures VII List of Tables XIX Abbreviations XXI 1 Motivation 1 2 Introduction 4 2.1 [2+2] photodimerization of cinnamic acid 5 2.2 Cinnamic acid as optical memory or molecular switches 7 2.3 Layered Double Hydroxide (LDH) 9 2.4 Previous studies of cinnamic acid derivatives by SSNMR 10 3 Experimental 12 3.1 NMR techniques 12 3.1.1 NMR interactions in Solid State NMR 12 3.1.2 Chemical Shift: 13 3.1.3 Dipole-Dipole Interaction: 13 3.1.4 Magic-angle spinning (MAS) 14 3.1.5 Relaxation times 15 3.2 Pulse sequences 16 3.2.1 13C CP (Cross polarization) 16 3.2.2 Dipolar decoupling 17 3.2.3 13C cross polarization dephasing experiment 17 3.2.4 Dipolar coupling 18 3.2.5 2D PASS 19 3.2.6 FSLG 13C-1H CP HETCOR 20 3.3 Typical measurement parameters 20 3.4 Sample preparation 22 3.4.1 Preparation of different cinnamate salt 22 3.4.2 Incorporation of cinnamic compounds into LDH 24 3.5 Sample irradiation by an UV lamp 24 3.6 Computational methods 28 3.7 Single crystal structure data 28 4 Results and discussion 30 4.1 m-Br cinnamate salts of different cations 30 13C CPMAS Spectra of Truxinates 42 Summary 57 4.2 m-Cl cinnamate salts of different cations 57 13C CPMAS Spectra of Truxinates 69 Summary 83 4.3 Influence of divalent cation on cinnamic acid 84 Ca di-trans-cinnamate 84 Mg di-trans-cinnamate 91 Summary 95 4.4 Photoreaction kinetics of cinnamate salts 96 Analysis of reaction kinetics 96 Summary 119 4.5 Intercalation of cinnamic acid derivatives in LDH 120 5 Conclusion 131 6 Outlook 135 7 Appendix 136 8 References 146
3

Stockage solide et génération d’hydrogène : du borohydrure de sodium NaBH4 à l’hydrazine borane N2H4BH3 : catalyse, cinétique et mécanismes / Solid-state hydrogen storage and generation : from sodium borohydride NaBH4 to hydrazine borane N2H4BH3 : catalysis, kinetic and mechanisms

Hannauer, Julien 12 December 2011 (has links)
Parmi les procédés de stockage d’hydrogène étudiés actuellement, le stockage solide de l’hydrogène dans les hydrures chimiques, associée à sa génération par une réaction de solvolyse, est une technologie prometteuse. La première partie de cette thèse s’articule donc autour de l’étude de la solvolyse de deux composés étudiés ces dernières années, le borohydrure de sodium NaBH4 et l’ammoniaborane NH3BH3. Le dégagement contrôlé d’hydrogène peut alors se faire par des réactions d’hydrolyse. La comparaison de la cinétique de la réaction d’hydrolyse du NaBH4 avec celle de la méthanolyse du NaBH4 nous a permis de décrire ces réactions avec le modèle de Langmuir-Hinshelwood. Concernant la réaction d’hydrolyse du NH3BH3, nos recherches se sont focalisées sur la préparation in situ de catalyseurs présentant de fortes activitéspar l’étude des hydrolyses spontanées et catalysées de mélanges NH3BH3-NaBH4. La seconde partie de la thèse est consacrée au développement d’un nouveau système N2H4BH3-eau pour la génération d’hydrogène. Les premiers essais, réalisés avec des métaux de transition comme catalyseur, nous ont permis de mettre en évidence que cette réaction se faisait en deux étapes catalytiques, l’hydrolyse de BH3, puis la décomposition de N2H4.Une faible sélectivité pour la décomposition complète de N2H4 étant atteinte dans ces conditions, la suite de l’étude a porté sur la préparation de catalyseurs sélectifs. La stratégie adoptée a été l’utilisation de matériaux bimétalliques Ni-Pt. La sélectivité de la réaction est alors dépendante de la teneur en Pt et une sélectivité maximum de 93 % pour la seconde étape de la réaction a été obtenue avec Ni0,89Pt0,11 / Hydrogen use as a potential alternative solution to fossil fuels is hindered by engineering problems, its storage being one of the most prominent. Various storage methods are under investigation but solid-state storage in chemical hydrides appears to be convenient with regards to their storage capacities, safety and cost. The first part of this thesis deals with the solvolysis reaction of two well known compounds, sodium borohydride NaBH4 and ammonia borane NH3BH3. The hydrogen can be easily released by hydrolysis at ambient temperature. We focused on understanding the kinetics and reaction mechanisms of NaBH4 hydrolysis. Thus, we compared this reaction with NaBH4 methanolysis, and found that the Langmuir-Hinshelwood model well captures the kinetics of the reaction. Concerning the NH3BH3 hydrolysis reaction, we concentrated our efforts on the in situ preparation of highly-active catalysts. This was achieved by studying the spontaneous and catalyzed hydrolysis of NaBH4-NH3BH3 mixtures. The second part of the thesis is dedicated to the development of the N2H4BH3-water system for hydrogen generation. Initial tests using transition metals as catalysts allowed us to determine that the reaction takes place in two steps, the hydrolysis of BH3 and the N2H4 decomposition. Since Rh as catalyst exhibits only a 29 % selectivity for the complete decomposition of N2H4, the strategy was set up to use Ni-Pt bimetallic nanoparticles. It has been found that the selectivity for the reaction is dependent on the Pt content in the Ni-Pt alloy and a selectivity of 93 % was reached in the presence of Ni0,89Pt0,11 nanoparticles
4

Biosynthèse des flavan-3-ols chez Vitis vinifera : structure, mécanisme catalytique et première approche cinétique de la leucoanthocyanidine réductase

Mauge, Chloé 01 July 2010 (has links)
Les flavan-3-ols et leurs polymères, les proanthocyanidines, appartiennent à une famille de flavonoïdes appelée les tannins condensés. Ces composés polyphénoliques jouent un rôle essentiel dans la qualité phytosanitaire des baies de raisin ainsi que dans les propriétés organoleptiques du vin (flaveur, astringence et couleur). La connaissance des mécanismes qui régissent leur biosynthèse est donc primordiale afin de mieux comprendre la mise en place de la typicité d’un vin. Ce mémoire est consacré à l’étude de l’une des enzymes responsables de la synthèse des flavan-3-ols : la leucoanthocyanidine réductase 1 de Vitis vinifera (VvLAR1). Dans une première partie, nous décrivons les conditions d’expression, de purification et de stabilité de l’enzyme recombinante. L’activité enzymatique est démontrée et la configuration 2R,3S des produits réactionnels caractérisée. La seconde partie de ce manuscrit décrit l’étude structurale de l’enzyme. Des monocristaux d’apoenzyme, de complexes binaires (VvLAR1 / NADPH) et d’un complexe ternaire (VvLAR1 / NADPH / (+)-catéchine) ont été obtenus. Les différentes structures permettent de décrire les modifications structurales associées à la fixation du coenzyme puis du produit. Un mécanisme catalytique basé sur les interactions intermoléculaires observées au sein du complexe ternaire est proposé. La troisième partie de ce mémoire est consacrée à la recherche de conditions expérimentales permettant la production et la stabilisation de la leucocyanidine en solution, un des substrats naturels de l’enzyme. Les résultats obtenus permettent une première approche de l’étude des propriétés cinétiques de la VvLAR1. / Flavan-3-ols and their polymerisation products, proanthocyanidins, belong to a flavonoid family named condensed tannins. These polyphenolic compounds play a major role in the protection of grape berries to intruders and in the organoleptic properties of wine (flavour, astringency and colour). Knowledge of the mechanisms which govern their biosynthesis is thus essential to better understand wine typical composition. The present thesis is devoted to the investigation of one of the two enzymes which catalyse the flavan-3-ols formation: leucoanthocyanidin reductase 1 from Vitis vinifera (VvLAR1). In the first part of the manuscript, we describe the expression, purification and stability conditions of the recombinant enzyme. The enzyme activity is demonstrated and the reaction product 2R,3S configuration is characterised. The second part of this report describes the structural studies of the enzyme. Monocrystals of the apoenzyme and of binaries (VvLAR1 / NADPH) and a ternary (VvLAR1 / NADPH / (+)-catechin) complexes were obtained. These different structures allow the description of the structural changes associated with the coenzyme and then the substrate binding. A catalytic mechanism, based on the intermolecular interactions within the ternary complex, is proposed. The last part of the work is devoted to the investigation of the experimental conditions leading to the stability of leucocyanidin in solution, one of the natural substrates of the enzyme. The results obtained allow a first study of the VvLAR1 kinetic properties.

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