UMD Theses and Dissertations
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Item MECHANISMS AND RATIONAL CATALYST DESIGN OF ORGANIC TRANSFORMATIONS FOR THE SYNTHESIS OF NEW C-C AND C-X BONDS(2021) Rotella, Madeline Elizabeth; Gutierrez, Osvaldo; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The creation of new C-C or C-X bonds, where X can be oxygen, nitrogen or fluorine, is vital to organic synthesis and the discovery of new methods for complex molecule synthesis. In many cases, the mechanism of these transformations is not investigated, although an understanding of the underlying mechanism would allow for rational design of new catalysts and would lead to the development of novel reactivity. Computational studies probing the mechanisms of valuable synthetic methods including C-H oxidation, organocatalysis, nickel photocatalysis, alkyne metathesis and multicomponent reactions are presented. Specifically, computational methods were used in the development of a novel tetradentate amine iron (II) catalyst for the promotion of C(sp3)-H oxidation (Chapter 1). Next, the mechanism of an organocatalyzed amination was studied thoroughly with density functional theory (DFT) calculations in combination with molecular dynamics simulations to develop a predictive model for reactivity for use in the creation of new catalysts in the field of amination chemistry (Chapter 2). Additionally, the mechanism of a regio- and enantioselective iridium-catalyzed asymmetric fluorination was studied, with an emphasis on determining the role of the trichloroacetimidate group in the reaction (Chapter 3). Further, the mechanisms of various transition metal-catalyzed C-C bond formations were studied through computationally. First, a photoredox/nickel-dual catalyzed Tsuji-Trost reaction was studied through DFT and DLPNO-CCSD(T) calculations to investigate the stereoselectivity of the reaction as well as the order of reaction events. Next, a photoredox/nickel-dual catalyzed C-C bond formation using oxanorbornadienes as electrophilic coupling partners was investigated computationally (Chapter 4). Additionally, the mechanism of tungsten- and molybdenum-catalyzed alkyne metathesis as well as the difference in reactivity between the two metals was explored (Chapter 5). A nickel-catalyzed diarylation of alkenes was studied computationally, with particular emphasis on the role of the phosphine ligand in controlling regioselectivity (Chapter 6). Finally, an iron-catalyzed dicarbofunctionalization of vinyl ethers with aryl Grignard reagents and alkyl halides or (fluoro)alkyl halides was developed experimentally (Chapter 7).Item Multiple sulfur isotope fractionations in inorganic aqueous systems(2016) Eldridge, Daniel Lee; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)New constraints on isotope fractionation factors in inorganic aqueous sulfur systems based on theoretical and experimental techniques relevant to studies of the sulfur cycle in modern environments and the geologic rock record are presented in this dissertation. These include theoretical estimations of equilibrium isotope fractionation factors utilizing quantum mechanical software and a water cluster model approach for aqueous sulfur compounds that span the entire range of oxidation state for sulfur. These theoretical calculations generally reproduce the available experimental determinations from the literature and provide new constraints where no others are available. These theoretical calculations illustrate in detail the relationship between sulfur bonding environment and the mass dependence associated with equilibrium isotope exchange reactions involving all four isotopes of sulfur. I additionally highlight the effect of isomers of protonated compounds (compounds with the same chemical formula but different structure, where protons are bound to either sulfur or oxygen atoms) on isotope partitioning in the sulfite (S4+) and sulfoxylate (S2+) systems, both of which are key intermediates in oxidation-reduction processes in the sulfur cycle. I demonstrate that isomers containing the highest degree of coordination around sulfur (where protonation occurs on the sulfur atom) have a strong influence on isotopic fractionation factors, and argue that isomerization phenomenon should be considered in models of the sulfur cycle. Additionally, experimental results of the reaction rates and isotope fractionations associated with the chemical oxidation of aqueous sulfide are presented. Sulfide oxidation is a major process in the global sulfur cycle due largely to the sulfide-producing activity of anaerobic microorganisms in organic-rich marine sediments. These experiments reveal relationships between isotope fractionations and reaction rate as a function of both temperature and trace metal (ferrous iron) catalysis that I interpret in the context of the complex mechanism of sulfide oxidation. I also demonstrate that sulfide oxidation is a process associated with a mass dependence that can be described as not conforming to the mass dependence typically associated with equilibrium isotope exchange. This observation has implications for the inclusion of oxidative processes in environmental- and global-scale models of the sulfur cycle based on the mass balance of all four isotopes of sulfur. The contents of this dissertation provide key reference information on isotopic fractionation factors in aqueous sulfur systems that will have far-reaching applicability to studies of the sulfur cycle in a wide variety of natural settings.Item A PHYSICAL CHEMIST'S GUIDE TO APPLIED COMPUTATIONAL CHEMISTRY: PRACTICAL CALCULATION OF POLYPROTIC ACID PKA VALUES, MERCURY HALIDES, THIOLS, AND METHYLMERCURY ANALOGUES' STABILITIES AND STRUCTURES, AND RAMAN SPECTRA OF MYO-INOSITOL HEXAKIS PHOSPHATE.(2010) Zimmermann, Merle; Tossell, John A; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we present both ab-initio investigation of the series of compounds HgClxy and the charges of each system running x=(0,1,2,3,4) and y=(+2,+1,0,-1,-2). We investigate the energies of formation using Gaussian 03 (G03), a quantum chemistry package. In our calculations, HgCl3-1 was most stable in the gas phase, and HgCl20 the most stable in the polarizable continuum model water-solvated phase. The addition of a solvent layer of H2O molecules did not significantly affect the results. DFT calculations on the series running between HgCl+, through HgCl20, and HgCl3-1 compounds done with the Amsterdam Density Functional (ADF) program from Scientific Computing and Modeling (SCM) yielded absolute Hg NMR shieldings with a Δ of approximately -1000 ppm for each additional atom of Chlorine bonding to the Mercury for the first two additions. We also investigate H3PO4, H3AsO4, and the HClOx acid series with x=(1,2,3,4). We have succeeded in determining pKas with theoretical quality results within 2 kcal/mol of experimental measurement for the majority of the systems examined by use of a discovered linear correlation between experimental and calculated pKa values. Finally, we present our contribution to a joint project involving myo-inositol hexakis phosphate with an experimental group, confirming the observed experimental trends seen in the Raman spectra.