Universiẗat, Diss., 2006--Frankfurt (Main).

University, Diss., 2000--München.

Universiẗat, Diss., 2001--München.

Kassel, Univ., Diss., 2001. / Dateien im PDF-Format

Universiẗat, Diss., 2000--München.

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Philosophiae Doctor - PhD / A variety of nitrogen based dendritic ligands have been synthesized and used in copper mediated Atom Transfer Radical Polymerization (ATRP) of MMA. These ligands were derived from the commercially available Generation 1 polypropyleneimine dendrimer DAB-(NH2)4. The first set of ligands was synthesized by reacting DAB-(NH2)4 with aromatic aldehydes such as 2-pyridinecarboxyaldhyde and 4-t-butyl benzaldehyde to form imine functionalized dendrimers. Analogous secondary amine functionalized dendrimers were also synthesized by reducing the abovementioned imine functionalized dendrimers using sodium borohydride. The ligands produced were characterized by 13C / 1H NMR, and infra-red spectroscopy as well as elemental analysis to confirm its structure. The ligands were then used in copper mediated ATRP of MMA. The resulting polymer solutions were analyzed by Gas Chromatography (GC) to monitor the monomer conversion while the isolated polymers were analyzed by gel permeation chromatography (GPC) for molecular weight determination. Results showed that the primary and secondary amine and imine dendritic ligands were not efficient in promoting ATRP reactions. This led to the modification of DAB-(NH2)4 using methyl methacrylate to replace the peripheral amino groups of the DAB-(NH2)4 with tertiary amine groups. A second generation tertiary amine dendrimer was also synthesized in a similar fashion. The ligands obtained were then characterized using 13C and 1H NMR spectroscopy. The tertiary amine dendrimers were used in copper mediated ATRP of MMA. The polymerization medium was analyzed over time using GC to monitor monomer conversion while GPC was used for molecular weight determination of the resulting polymers. The results obtained using the methyl methacrylate modified ligands indicated that in the case of MMA polymerization, these ligands essentially conformed to the requirements of a good ATRP system. However in the preliminary studies, when employed in copper mediated ATRP of styrene, these ligands did not perform well. Further investigation is needed to improve the performance of these ligands in styrene polymerization under ATRP conditions. / South Africa

Aziridines are important building blocks for the synthesis of a wide range of organic compounds, including biologically active compounds and pharmaceuticals. The development of more cost-effective catalysts for atom transfer reactions is a continuing area of research in chemistry. Transition metal complexes have been shown to catalyze the aziridination of olefins, however, most require expensive metal ions or complex ligands. Meso-tetra(N-methyl-4-pyridyl)porphyrin (TMPyP4) is a highly charged, planar ligand that has been used to support manganese(III) in complexes like Mn[TMPyP4]I5. Herein we report the optimization of the reaction conditions for the aziridination of olefins in water and buffered solutions catalyzed by Mn[TMPyP4]I5. The reaction conditions optimized include pH range, temperature, and reaction time. Additionally, nitrogen sources, nitrogen source/olefin ratios, and catalyst loading were optimized. In buffered solutions, Mn[TMPyP4]I5 can effectively catalyze the generation of aziridines from various aromatic and aliphatic olefins with Chloramine T in moderate to good yields (43-93 %).

An atomistic control of chemical bonds formation and cleavage holds the key to making molecular transformations more energy efficient and product selective. However, inherent scaling relations among binding strengths of adsorbates on various catalytic materials often give rise to volcano-shaped relationships between the catalytic activity and the affinity of critical intermediates to the surface. The optimal catalysts should bind the reactants 'just right', i.e., neither too strong nor too weak, which is the Sabatier's principle. It is extremely useful for searching promising catalysts, but also imposes serious constraints on design flexibility. Therefore, how to circumvent scaling constraints is crucial for advancing catalytic science. It has been shown that hot electrons can selectively activate the chemical bonds that are not responsive to phonon excitation, thus providing a rational approach beyond scaling limitation. Another emerging yet effective way to break the scaling constraint is single atom catalysis. Strong interactions of supported single atoms with supports dramatically affect the electronic structure of active sites, which reroutes mechanistic pathways of surface reactions. In my PhD research, we use CO oxidation reaction on metal-based active sites as a benchmark system to tailor mechanistic pathways through those two strategies 1) ultra-fast laser induced nonadiabatic surface chemistry and 2) oxide-supported single metal catalysis, with the aim to go beyond the Sabatier activity volcano in metal catalysis. / Doctor of Philosophy / Catalysis is the process of increasing the chemical reaction rate by lowering down the activation barrier. There are three different types of catalysis including enzyme, homogeneous, and heterogeneous catalysis. Heterogeneous catalytic reactions involve a sequence of elementary steps, e.g., adsorption of reactants onto the solid surface, transformation of adsorbed species, and desorption of the products. However, the existing scaling relations among binding energies of reaction intermediates on various catalytic materials lead to volcano-shaped relationships, which show the reaction activity versus the binding energy of critical intermediates. The optimal catalysts should bind the reaction intermediates neither too strong nor too weak. This is the Sabatier's principle, which provides useful guidance for searching promising catalysts. But it also imposes the constraint on the attainable catalytic performance. How to break the constraint to further improve the catalytic activity is an emerging problem. The recent studies have shown that the hot surface electrons on the metal surfaces induced by the ultra-fast laser can selectively activate the chemical bonds, thus providing a rational approach beyond scaling constraints. Another way to break the scaling constraint is single atom catalysis. The metal oxides are frequently used as the support to stabilize the single metal atoms. The strong interaction between the single metal atoms and the support affects the electronic structure of the catalysts. Thereby catalytic reactions on the single metal atoms catalyst are very different from that on metal surfaces. In my PhD research, we use CO oxidation reaction as a benchmark system, to tailor reaction pathways through those two strategies on 1) Ru(0001) under ultra-fast laser pulse and 2) Ir single metal atoms supported on spinel oxides, to go beyond Sabatier activity volcano in metal catalysis.

This dissertation describes new measurements of the van der Waals (vdW) potential energy for atoms near a surface. The measurements presented here were accomplished by studying diffraction a beam of atoms transmitted through a nanograting. I will describe how we improved precision by a factor of 10 over previous diffraction measurements by studying how different types of atoms interact with the same surface. As a result of this new precision, we were able to show for the first time the contribution of atomic core electrons to the atom-surface potential, and experimentally test different atomic structure calculation methods.In addition, this dissertation will describe how changing the width of the grating bars to achieve a particular "magic" grating bar width or rotating a grating to a particular "magic" angle allows us to determine both the atom-surface potential strength and the geometry of the grating. This represents an improvement over several recent studies where uncertainties in the nanograting geometry limited precision in the measurements of the vdW potential.For a complementary measurement, also discussed in this dissertation, we collaborated with the Vigue group in Toulouse, France. In this collaboration we used an atom interferometer to measure the phase shift due to transmission through a nanograting. By combining diffraction data from Tucson with interferometry data from Toulouse we improved the precision of interferometry measurements of the atom-surface potential of a single atomic species by almost a factor of 10 over previous interferometric measurements of the vdW potential. These interferometry measurements also serve to measure the shape of the vdW potential and set a limit on non-Newtonian gravitational interactions at 1-2 nm length scales.Finally, this dissertation will discuss how nanogratings with optimized geometry can improve atom interferometers, for example, with blazed gratings. We discuss next generation atom-surface potential measurements and examine new ways of analyzing diffraction data.