Theses and Dissertations at Montana State University (MSU)
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Item Development of palladium L-edge x-ray absorption spectroscopy and its application on chloro palladium complexes(Montana State University - Bozeman, College of Letters & Science, 2005) Boysen, Ryan Bradley; Chairperson, Graduate Committee: Robert K. SzilagyiX-ray Absorption Spectroscopy (XAS) is a synchrotron-based experimental technique that can provide information about geometric and electronic structures of transition metal complexes with unoccupied d orbitals. Combination of metal L-edge and ligand K-edge XAS has the potential to define the experimental ground state electronic structure. We developed a quantitative treatment for Pd L-edge spectroscopy based on the already established Cl K-edge XAS for a series of chloro palladium complexes, which are precatalysts in numerous organic transformations. We found that Pd-Cl bonds are highly covalent (23% per Cl in [PdCl₄]²⁻, 34% per Cl in [PdCl₆]²⁻, and 46% in PdCl₂). Dipole integrals for Pd(2p..4d) transitions of 42 eV for Pd(II) and 48 eV for Pd(IV) LIII-edges and 39 eV and 35 eV, respectively, at these Pd oxidation states for the LII-edges were determined. Application of the metal-ligand covalencies and transition dipole integrals by describing the ground state bonding in PdCl₂ with bridging Cl ligands was demonstrated. In future studies, a similar approach will be utilized for palladium phosphine, allyl, olefin complexes in order to define their experimental electronic structure and correlate this with their observed reactivity.Item Multi-edge X-ray absorption spectroscopy and electronic structure calculations of biomimetic model complexes of the H-cluster of [FeFe]-hydrogenase(Montana State University - Bozeman, College of Letters & Science, 2012) Giles, Logan James; Chairperson, Graduate Committee: Robert K. Szilagyi; Alexios Grigoropoulos and Robert K. Szilagyi were co-authors of the article, 'Multi-edge x-ray absorption spectroscopy part I: Xanes analysis of a biomimetic model complex of [FeFe]-hydrogenase' in the journal 'Journal of physical chemistry B' which is contained within this thesis.; Alexios Grigoropoulos and Robert K. Szilagyi were co-authors of the article 'Electron and spin density topology of the H-cluster and its biomimetic complexes' in the journal 'European journal of inorganic chemistry' which is contained within this thesis.FeFe-hydrogenases are members of a family of metalloenzymes that catalyze the conversion of protons and electrons to dihydrogen at a remarkable rate. The catalytic center of this enzyme, the H-cluster, contains a classical [4Fe-4S] cluster that is covalently and magnetically coupled through a cysteine residue to a 2Fe-subcluster. The 2Fe-subcluster contains normally biotoxic carbonyl and cyanide ligands and a dithiolate ligand that is unique in biology. Many biomimetic model complexes have been synthesized that attempted to mimic the H-cluster reactivity, but none have been successful at as low of a reduction potential and as high of a reaction rate as the metalloenzyme. Thus the goal of this research is to develop a blueprint for understanding the electronic structure of the H-cluster, through functionally analogous model complexes. The first step towards this goal is to carry out multi-edge X-ray absorption spectroscopic measurements and electronic structure calculations. We first developed the multi-edge X-ray absorption spectroscopy method for a prototypical biomimetic complex, Fe 2(u-S(CH 2) 3S)(CO) 6. This allowed for the complete definition of the orbital composition for the unoccupied frontier orbitals. We used this information to calibrate our computational results in order to accurately describe similar biomimetic model complexes. We used the multi-edge X-ray absorption spectroscopic approach and the calibrated computational models to analyze four structural features of the 2Fe-subcluster of the H-cluster through representative biomimetic model complexes. We find unique trends for each series that helped to develop an understanding of how each compositional feature contribute to structure. These insights can be used for optimizing model complexes with potential to match the reactivity of the FeFe-hydrogenase enzymes. We also used our calibrated electronic structure method to analyze the spin density at the bridgehead position of the unique dithiolate ligand and dissect the intricate details of the electronic structure for the protein-environment embedded H-cluster model.Item Synchrotron radiation-based spectroscopic investigation of the electronic and geometric structures of iron-sulfur clusters, particles, and minerals(Montana State University - Bozeman, College of Letters & Science, 2012) Gardenghi, David Jeremiah; Chairperson, Graduate Committee: Robert K. SzilagyiIron-sulfur systems are ubiquitous in biological and geological environments. They range from molecular scale [2Fe-2S] clusters to nanometer scale particles to micrometer scale minerals. Across the length scale, each system has unique structural and functional roles with respect to its environment, such as metalloproteins or hydrothermal vents. Therefore, it is of great interest to understand the electronic and geometric structural properties that give a wide range of reactivity. The main focus of this dissertation is to investigate the possible connections among the different size scales using a primary technique, X-ray absorption spectroscopy (XAS), by gaining an understanding into the relationship between the system size and properties. X-ray absorption spectroscopy is a powerful tool to probe geometric and electronic structures across the entire length scale. In this work, extend x-ray absorption fine structure (EXAFS) analysis method was applied to determine geometric structure of the protein bound, molecular iron-sulfur cluster of HydA. It was found to contain a preformed [4Fe-4S] cluster. This method combined with Fe/S K-edge XANES analysis was applied to spore photoproduct lyase metalloprotein to structurally characterize the [4Fe-4S] cluster and its interaction with SAM. The Fe K-edge EXAFS and Fe/S K-edge XANES provide evidence for a cluster distortion upon interacting with SAM as a new EXAFS feature, indicating the presence of longer Fe-Fe distances and this provides new insights into the structure of radical SAM enzymes. At the nanometer scale, mineral and protein encapsulated particles were investigated with a possible link between molecular and micrometer scale. A reference library of FeS systems was established to describe the variation in bonding and structure. Also, a considerable amount was learned about XAS detection methods, and this was applied to the micrometer scale systems. From EXAFS and XANES analysis, the modified surface of pyrite was revealed to have an intermediate layer of Fe(I)-S phase with a metallic iron surface, and a reaction scheme was proposed. These studies of the different size iron-sulfur systems provide insights into the change in the electronic and geometric structures, and a model was proposed to describe the effects of size on the electronic and geometric structure.