Theses and Dissertations at Montana State University (MSU)
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Item A computational study of a high-spin iron(I) complex for possible dinitrogen reduction to ammonia(Montana State University - Bozeman, College of Letters & Science, 2023) Pollock, Charlie Jeananne; Chairperson, Graduate Committee: Martin A. MosqueraA series of high-spin, low coordinate, paramagnetic iron complexes bearing a phenyltris((tert- butylthio)methyl)borate ligand were computationally modeled with density functional theory (DFT) and complete active space self-consistent field theory (CASSCF). The iron complexes examined in this research were inspired by nitrogenase, a naturally occurring, dinitrogen- fixating, iron-containing metalloenzyme. DFT and CASSCF offer a convenient way to explore reactions, complexes, and molecular orbitals without an immediate need to perform synthetic experiments. Our computational work can be used to guide synthetic efforts as well as urge future theoretical work in related research. DFT was utilized to compute two different thermodynamic properties: bond dissociation free energy (BDFE N-H) and Gibb's free energy. The conductor-like polarizable continuum model (CPCM) was applied to examine the solution phase of the system, and all BDFE and DeltaG values found were endothermic in tetrahydrofuran (THF). The methods, BP86 and BP86 ZORA, examined the gas phase of the system. The BDFE and DeltaG values calculated when using those two methods were largely inconsistent, which lead to the conclusion that the solution phase model is the most appropriate method for computing values of the dinitrogen complex ([Fe] 2(Mu-N 2)) and its related complexes. An N 2 vibrational mode was found (1915.30 cm -1) for [Fe] 2(Mu-N 2), which reflects a strongly coordinated dinitrogen bridge (Fe-N identical to N-Fe). Broken symmetry DFT (BSDFT) was used to examine the exchange coupling, which was found to have positive values (JAB =82.51 cm -1, 61.88 cm -1, 81.36 cm -1), and implied that [Fe] 2(Mu-N 2) is ferromagnetically coupled. Lastly, CASSCF and DFT were applied to plot and characterize certain molecular orbitals of [Fe] 2(Mu-N 2). The plotted and characterized molecular orbitals reflected moderate (DFT) to strong (CASSCF) covalent bonding between iron and dinitrogen. All this data reflected the synthetic plausibility of dinitrogen coordination to the bridged, Fe(I) complex ([Fe] 2(Mu-N 2)) that can be reduced through the dinitrogen cleavage mechanism.Item Development and exploration of nitrogen heterocycle methodologies : experimental and theoretical investigations(Montana State University - Bozeman, College of Letters & Science, 2003) Jones, Thomas Nicholas; Chairperson, Graduate Committee: Cynthia K. McClureItem Development of density functional methods for iron-sulfur clusters : application to the elusive structure of the iron-molybdenum cofactor of nitrogenase(Montana State University - Bozeman, College of Letters & Science, 2011) Harris, Travis Victor; Chairperson, Graduate Committee: Robert K. SzilagyiIron-sulfur proteins are found in all forms of life, performing key functions in many of the most important biological processes, such as nitrogen fixation, respiration, DNA repair, and photosynthesis. The functional centers of these metalloproteins are iron-sulfur clusters, which are efficient electron-transfer agents, and provide a range of other functions, from catalysis to iron sensing. The main focus of this dissertation is to eliminate the structural uncertainties of one of the most complex iron-sulfur clusters, the iron-molybdenum cofactor of nitrogenase. The long-term goal of nitrogenase research is to understand all of the molecular-level details that enable biological nitrogen fixation, which may lead to alternatives to the fossil fuel-reliant, high temperature and pressure Haber-Bosch process for industrial nitrogen fixation. The research presented here is all based on computational chemistry, using primarily density functional theory with comprehensive experimental support from the literature. Computational chemistry can help explain experimental findings, and it can be used to probe structural and mechanistic details that cannot be observed through experiment; however, methods for theoretical modeling of iron-sulfur clusters must be developed before accurate results can be obtained. In this work, the best available methods (functionals, basis sets, and population analyses) are developed by comparing calculated properties of a series of model iron-sulfur complexes with experimental reference data. The newly recommended methods are then applied to a protein-embedded cluster, and the effects of the network of hydrogen bonds, dipoles, and electrostatic interactions are systematically probed. In addition to new insights regarding the role of the protein in tuning iron-sulfur cluster properties, an algorithm is derived for constructing computational models that capture all significant environmental effects. Future work will benefit from the foundations developed in these first two studies. The final work is a computational evaluation of the uncertain composition, charge, magnetic structure, and protonation state of the iron-molybdenum cofactor, based on a series of criteria defined by all available experimental data. The results show that the [Mo IV -2Fe II -5Fe III -9S ²- -C ⁴-] composition with hydroxyl-protonated homocitrate ligand is favored over the [Mo IV -4Fe II -3Fe III -9S ²- -N ³- ] composition preferred in the literature, adding new context to the many proposed mechanisms for biological nitrogen fixation.