Graduate

Thesis (M. Sc. (Chemistry)) --University of Limpopo, 2018 / The dissertation describes the use of 2-benzenesulfonyloxyquinoxaline as a good coupling partner for different amine substrates. The palladium-mediated cross- coupling of aryl electrophiles and amines has become a widely used method of constructing arylamine frameworks. The formation of carbon-nitrogen bonds was accomplished via palladium-catalysed Buchwald-Hartwig amination employing different amine substrates to yield substituted quinoxaline-2-amines compounds in good to moderate yields. Buchwald ligands (Xphos, tButylxphos and BrettPhos), were varied with different amine substrates in an attempt of improving the yields. Compounds 81a N-phenylquinoxalin-2-amine and 82b, N-benzylquinoxalin-2-amine were obtained with the yield over 70 % employing Xphos as the ligand. Significant attention has also been given to the application of cross coupling reaction protocols in substrates bearing electron withdrawing substituents. The presence of deactivating groups on the arylamine such as fluoro, nitro and iodo proved to be a challenge as only few compounds were synthesised in moderate yields. Compound 81b, N-(4-fluorophenyl)quinoxalin-2-amine which has electronegative atom attached, showed significant improvement when employing tButyl-Xphos ligand rather than XPhos since the yield improved from 10 % to 71 %. Furthermore, nucleophilic substitution on Buchwald-Hartwig coupled compounds by treating them with alkyl iodides was successful when using methyl and ethyl electrophiles on the N-H group of 81a 2-quinoxalineamine. The synthesised quinoxaline derivatives comprised 7 novel compounds. The in vitro analysis on anti-tubercular screening against H37RvMA strains of Mycobacterium tuberculosis was conducted on 9 compounds. The results revealed none of the compounds to have promising inhibition percentages against Mycobacterium tuberculosis when compared with rifampicin which was used as a positive control. Screening against malaria with chloroquine as the control also did not yield any active compounds.

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This PhD thesis presents a study of structure-composition correlations of bioactive glasses (BGs) by employing solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. Silicate-based Na2O−CaO−SiO2−P2O5 BGs are utilized clinically and are extensively investigated for bone regeneration purposes. Once implanted in the human body, they facilitate bone regeneration by partially dissolving in the body fluids, followed by the formation of a biomimetic surface-layer of calcium hydroxy-carbonate apatite (HCA). Eventually, the implanted BG totally integrates with the bone. The bioactivity of melt-prepared BGs depends on their composition and structure, primarily on the phosphorus content and the average silicate-network connectivity (NC). We explored these composition-structure relationships for a set of BGs for which the NC and phosphorus contents were varied independently. The short-range structural features of the glasses were explored using 29Si and 31P magic-angle-spinning (MAS) NMR spectroscopy. 31P MAS NMR revealed that the orthophosphate content is directly proportional to the total P content of the glass, with a linear correlation observed between the orthophosphate content and the silicate network connectivity. The bearings of the results for future BG design are discussed. By using multiple-quantum coherence-based 31P NMR experiments, the spatial distribution of orthophosphate groups was probed in the melt prepared BGs, as well as in two mesoporous bioactive glasses prepared by an evaporation-induced self-assembly technique. The results evidence randomly distributed orthophosphate groups in the melt-prepared BGs, whereas the pore-walls of the mesoporous bioactive glasses constitute nanometer-sized clusters of calcium phosphate. The distribution of Na+ ions among the phosphate/silicate groups were studied by heteronuclear dipolar-based 23Na−31P NMR experiments, verifying that sodium is dispersed nearly randomly in the glasses. The phosphorus and proton environments in biomimetically grown HCA were investigated by using 1H and 31P MAS NMR experiments. Our studies revealed that the biomimetic HCA shared many local structural features with synthetic and well-ordered hydroxy-apatite. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Accepted.</p>

Models of geometry that are intrinsically quantum-mechanical in nature arise from the recoupling theory of space-time symmetry groups. Roger Penrose constructed such a model from SU(2) recoupling in his theory of spin networks; he showed that spin measurements in a classical limit are necessarily consistent with a three-dimensional Euclidian vector space. T. Regge and G. Ponzano expressed the semi-classical limit of this spin model in a form resembling a path integral of the Einstein-Hilbert action in three Euclidian dimensions. This thesis gives new proofs of the Penrose spin geometry theorem and of the Regge-Ponzano decomposition theorem. We then consider how to generalize these two approaches to other groups that give rise to new models of quantum geometries. In particular, we show how to construct quantum models of four-dimensional relativistic space-time from the re-coupling theory of the Poincare group.

La spectroscopie de RMN des solides est une méthode de choix pour la caractérisation de la structure et de la dynamique à l'échelle atomique des matériaux ordonnés et désordonnés. Cependant, l'utilisation de cette technique est limitée par son manque de sensibilité qui empêche l'observation de la surface des matériaux, souvent responsable de leurs propriétés chimiques. Il a été récemment montré que la Polarisation Nucléaire Dynamique (en anglais, Dynamic Nuclear Polarization, DNP) dans les conditions de rotation à l'angle magique (en anglais Magic-Angle Spinning, MAS) permet de surmonter cette limitation. Cette technique permet d'augmenter la sensibilité de la RMN de plusieurs ordres de grandeur. Elle consiste à transférer la polarisation élevée des électrons non-appariés vers les noyaux grâce une irradiation micro-onde. L'objectif de cette thèse consiste à appliquer la MAS-DNP pour sonder la structure de matériaux nanostructurés inorganiques et hybrides. Ces nouvelles informations faciliteront l'amélioration raisonnée de leurs propriétés. Deux classes de matériaux ont été étudiées : des nanoparticules (NP) de silice fonctionnalisées avec des chaînes siloxane et deux formes d'alumine. Les NP de silice fonctionnalisées permettent d'accroître la durée de vie des piles à combustible. Grâce au gain en sensibilité offert par la DNP, il a été possible de sonder les connectivités et les proximités 29Si-29Si dans ces matériaux et ainsi d'élucider le mode de condensation des chaînes siloxane à la surface des NP de silice. La seconde classe de matériaux étudiés comprend deux formes d'alumine : l'alumine- et l'alumine mésoporeuse. La première est largement utilisée dans l'industrie comme catalyseur, support de catalyseur et adsorbant, tandis que la seconde est un matériau prometteur du fait de sa porosité contrôlée et de son accessibilité élevée. Néanmoins, la structure de ces alumines est toujours largement débattue car elles ne forment pas des monocristaux. Grâce à une meilleure compréhension des performances de la MAS-DNP, conduisant notamment à une optimisation de la préparation des échantillons, il a été possible de compenser la très faible efficacité des expériences 27Al sélectives de la surface. La structure de la surface d'alumine a été sondée par des expériences RMN avancées à deux dimensions et une nouvelle expérience a été proposée pour l'observation sélective du cœur de l'alumine. Afin d'obtenir davantage d'informations sur les proximités 27Al-27Al, nous avons cherché à mieux comprendre les séquences de recouplage dipolaire homonucléaire pour des noyaux 27Al. Pour ce faire, la dynamique de spin au cours de ces séquences a été analysée par la théorie de l'hamiltonien moyen et des simulations numériques. En résumé, au cours de cette thèse, nous avons montré comment la MAS-DNP ouvre de nouvelles perspectives pour l'étude des matériaux nanostructurés. / Solid-state NMR spectroscopy is a powerful analytical technique to characterize the atomic-level structure and dynamics of both ordered and disordered materials. However, its main limitation is the lack of sensitivity, particularly preventing studies on the surface of materials, an important region determining their chemical properties. It has been recently shown that Magic Angle Spinning Dynamic Nuclear Polarization (MAS-DNP) could overcome this difficulty. This technique can provide an enhancement of NMR sensitivity of many orders of magnitude. It is based on the partial microwave-driven transfer of the large intrinsic polarization of electron spins to nuclear spins, making impractical NMR experiments feasible. The aim of this work is to use this MAS-DNP technique to help gain new insights into the structure of inorganic and hybrid nanostructured materials. Such knowledge will facilitate the rational improvement of their properties. Two classes of materials are investigated. The first ones are siloxane-functionalized silica nanoparticles (NPs), which can be used to extend the working durability of fuel cells. Owing to the sensitivity enhancement achieved by MAS-DNP, the condensation network structure of siloxanes bound to the surface of silica NPs could be elucidated using 29Si-29Si homonuclear correlation NMR experiments. The second class of investigated systems encompasses two forms of aluminas, -alumina and mesoporous alumina. The former is widely used in industry as a catalyst, catalyst support, and adsorbent, whereas the latter is a promising material owing to its highly controlled porosity and its high surface accessibility. Nevertheless, their structures are still under heavy investigation since they do not form single crystals. Due to an improved comprehension of MAS-DNP performance, including optimized sample preparation, the obstacle of extremely low efficiency for surface-selective 27Al NMR experiments is circumvented. Sophisticated two-dimensional NMR experiments are employed to provide selective insights into structures on the surface and a new experiment is proposed to study only the bulk of these materials. For achieving further information on the spatial proximities between different 27Al sites, a thorough understanding of homonuclear dipolar recoupling pulse sequences for half-integer quadrupolar nuclei is required. In order to do this, Average Hamiltonian theory and numerical simulations are used to analyze the spin dynamics resulting from these pulse sequences, giving insights into their relative performances. Overall, it is shown that the use of MAS-DNP can be crucial for the characterization of state-of-the-art materials, highlighting the future importance of this technique.

Organic-inorganic nanostructured composites are nowadays integrated in the field of material science and technology. They are used as advanced materials directly or as precursors to novel composites with potential applications in optics, mechanics, energy, catalysis and medicine. Many properties of these complex materials depend on conformational rearrangements in their inherently dynamic organic parts. The focus of this thesis is on the study of the molecular mobility in ordered nanostructured composites and lyotropic mesophases and also on the development of relevant solid-state NMR methodologies. In this work, a number of new experimental approaches were proposed for dipolar NMR spectroscopy for characterizing molecular dynamics with atomic-level resolution in complex solids and liquids. A new acquisition scheme for two-dimensional dipolar spectroscopy has been developed in order to expand the spectral window in the indirect dimension while using limited radio-frequency power. Selective decoupling of spin-1 nuclei for sign-sensitive determination of the heteronuclear dipolar coupling has been described. A new dipolar recoupling technique for rotating samples has been developed to achieve high dipolar resolution in a wide range of dipolar coupling strength. The experimental techniques developed herein are capable of delivering detailed model-independent information on molecular motional parameters that can be directly compared in different composites and their bulk analogs. Solid-state NMR has been applied to study the local molecular dynamics of surfactant molecules in nanostructured organic-inorganic composites of different morphologies. On the basis of the experimental profiles of local order parameters, physical motional models for the confined surfactant molecules were put forward. In layered materials, a number of motional modes of surfactant molecules were observed depending on sample composition. These modes ranged from essentially immobilized rigid states to highly flexible and anisotropically tumbling states. In ordered hexagonal silica, highly dynamic conformationally disordered chains with restricted motion of the segments close to the head group have been found. The results presented in this thesis provide a step towards the comprehensive characterization of the molecular states and understanding the great variability of the molecular assemblies in advanced nanostructured organic−inorganic composite materials. / <p>QC 20150225</p>

No description available.

Le risk management d’entreprise (ERM) est un ensemble de politiques, de structures et de pratiques infuses d’idéaux sociaux de bonne gouvernance et de contrôlabilité. Nous nous demandons ici dans quelles mesures le travail des acteurs permet l’adoption durable de l’ERM dans l’organisation. Dans le cas du Groupe La Poste, l’appropriation des pressions externes par les auditeurs internes a été cruciale. Une fois introduit dans l’organisation postale, l’ERM a d’abord été confronté à l’indifférence du management. Les contenus réglementaires et normatifs ont été adoptés sans être mis en œuvre. Des structures partielles d’ERM ont été peuplées de risk managers détenant les compétences nécessaires pour répondre aux attentes externes. Ces risk managers ont ensuite travaillé à légitimer et structurer le dispositif conformément aux « meilleures pratiques » en réalisant un travail de recouplage – action intentionnelle des individus visant à aligner le risk management organisationnel avec les traits externes et abstraits de l’ERM. L’étude du cas La Poste suggère ainsi une explication organisationnelle au fait que l’ERM ne puisse être révoqué une fois en place. / Enterprise-wide risk management (ERM) is a set of policies, structures and practices infused with social ideals of good governance and controllability. In this research, we seek to know how actors’ work allows ERM lasting adoption in the organization. In La Poste, the enactment of external pressures by internal auditors turned out to be critical. When introduced in the postal organization, ERM first encountered management disregard. As a consequence, ERM regulatory and normative meanings and categories were adopted but not implemented in the business. Partial ERM structures were created and inhabited by risk managers who hold necessary skills to help organizational compliance with external expectations. Newly appointed risk managers worked internally to legitimize and structure the device consistently with external “best practices” performing what we named recoupling work - the purposive action of individuals and organizations aimed at aligning organizational risk management with external ERM abstract features. Thus, our case suggests an organizational explanation to the fact that risk management could not be rescinded after its initial settlement.

La mesure de couplage dipolaire permet d’accéder à la structure tridimensionnelle d’un composé solide. Cependant, en présence d’une forte densité de spins couplés, le phénomène de troncature dipolaire rend difficile l’obtention de ces informations par RMN du solide. Ce problème peut être affranchi par l’étude de spins rares en abondance naturelle. En effet, avec une abondance naturelle de 1.1 %, la probabilité que trois 13C soient couplés, et avec elle la troncature dipolaire, devient négligeable. Une méthodologie basée sur la séquence de recouplage dipolaire POST-C7 permet d’accéder à des informations structurales d’échantillons en abondance naturelle sensibles à la fois à la conformation moléculaire et à l’empilement cristallin par mesure de couplages dipolaires 13C-13C. La sensibilité de détection des signaux RMN 13C est augmentée à l’aide la polarisation dynamique nucléaire ce qui permet de réduire considérablement les temps d’expériences. De plus, la séquence de recouplage R20_9_2 aidée de supercycles s’est montrée être plus robustes que POST-C7 face à de fortes anisotropies de déplacement chimique ou de forts couplages hétéronucléaires 1H-13C. La seconde problématique abordée concerne l’attribution de signaux 13C. En effet, il existe seulement quelques exemples de détermination de connectivités 13C -13C en abondance naturelle. Nous montrons ici que des spectres de corrélations dipolaires 13C-13C peuvent être obtenus en quelques jours à l’aide de la séquence de recouplage R20_9_2. Contrairement aux méthodologies basées sur le couplage J, notre séquence requiert un temps d’excitation DQ plus court ce qui la rend adaptée à l’étude de solides désordonnés. / Measurment of dipolar coupling provides 3D structural information of powder samples. However, in practice, the high density of spins in organic compounds prevents the measurements of long-range dipolar couplings in solid-state NMR by the so-called dipolar truncation effect. The study of rare spins on natural abundance allows to overcome this problem. In fact, with a natural abundance of 1.1 %, the probability for three 13C to be coupled is negligible. We developed a methodology based either on the dipolar recoupling NMR pulse sequence POST-C7 or on the dramatic increase in sensitivity provided by dynamic nuclear polarization. We demonstrated that its methodology provides a measure of 13C-13C dipolar couplings in natural abundance powder samples and that the so-obtained distance information is sensitive to both molecular conformation and crystal packing of powder samples. Moreover, we show that the recoupling pulse sequence R20_9_2 is more robust to strong chemical shift anisotropy and also to strong 1H-13C heteronuclear dipolar couplings than POST-C7. The second challenge involves 13C signal assignment for natural abundance. In fact, there are only a few examples of 13C-13C correlation spectra obtained for natural abundance samples. Here, we show that 13C-13C correlation spectra sequence based on the reintroduction of 13C−13C dipolar couplings can be obtained with standard MAS probe and within few days using R20_9_2 pulse sequence. Contrary to pulse sequences based on 13C-13C J coupling, our pulse sequence requires shorter DQ excitation time and hence, is more suitable for samples having short T2 relaxation times such as amorphous solids.