UMD Theses and Dissertations
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Item Mechanistic Studies and Rational Catalyst Design of Nickel/Photoredox Dual-Catalyzed C–C Cross-Coupling Reactions(2022) Yuan, Mingbin; Gutierrez, Osvaldo; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The merging of photoredox and nickel catalysis has revolutionized the field of C–C cross-coupling. However, in comparison to the development of synthetic methods, detailed mechanistic investigations of these catalytic systems are lagging. In this vein, computational tools have been applied to elucidate the mechanistic pictures, allowing for the rational design of new catalysts and the development of novel reactivity. Based on the reported studies, it appears that the mechanistic picture of catalytic systems is not generally applicable, but is rather dependent on the specific choice of substrate, ligands, photocatalysts, etc. Therefore, the challenges and opportunities of investigating the mechanisms of Ni/photoredox dual-catalyzed C–C cross-coupling reactions were first discussed (Chapter 1). Using both quantum mechanics and molecular dynamics simulations, the mechanism of the tertiary radical cross-coupling between alkyl trifluoroborates and aryl bromides was then investigated, revealing the effect of ligands and the properties of alkyl radicals (Chapter 2). After exploring the mechanism of two-component Ni/photoredox dual-catalyzed C(sp2)–C(sp3) cross-couplings, further mechanistic investigation was conducted for multicomponent cross-coupling reactions, revealing the factors controlling reactivity and selectivity in these complex catalytic transformations. In a multicomponent C–H activation/cross-coupling reaction, the origin of chemoselectivity between two-component versus three-component products was studied, showcasing the effect of intramolecular H-bonding (Chapter 3). Moreover, the mechanism of a novel enantioselective olefin difunctionalization was computationally investigated, identifying the radical addition step as the enantioselectivity-determining step (Chapter 4). Expanded to a wider scope of catalytic systems and reagents, the catalytic cycles of an enantioselective dicarbofunctionalization of vinylphosphonate were explored, demonstrating the origin of stereo- and enantioselectivity of this transformation (Chapter 5). Given the complexity of the mechanistic pictures of these nickel metallaphotoredox systems, the need for more accurate computational methods, readily available and user-friendly dynamics simulation tools, and data-driven approaches is clear in order to understand at the molecular level of these transformations.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 Computational Study of Solid-Cathode Interfaces and Coatings for Lithium-Ion Batteries(2021) Nolan, Adelaide M; Mo, Yifei; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)All-solid-state batteries, which use a solid electrolyte, are a promising technology for improving the safety of currently commercialized batteries based on liquid electrolytes. However, to enable all-solid-state batteries with high energy densities, we need to integrate solid electrolytes with high-voltage and high-capacity cathodes. The interface between solid electrolytes and high-energy cathodes is often thermodynamically unstable, which can lead to reactions and the formation of decomposition products which cause high interfacial resistance. One solution to improve resistance and poor contact at the interface is the application of a coating layer, which can act as a physical barrier between the solid electrolyte and the cathode and prevent decomposition. I performed first-principles computation and thermodynamic analyses to study the thermodynamic stability and Li-ion transport in coating layers for solid-solid interfaces. I used a high-throughput systematic analysis of phase diagrams based on a materials database to study the decomposition energy and products of reactions between coating layer chemistries and layered and high-voltage cathodes. My thermodynamic stability analysis revealed that the strong reactivity of lithiated and delithiated cathodes greatly limits the possible choice of materials that are stable with the cathode under high-voltage cycling. The computation results reaffirmed previously demonstrated coating chemistries and identified several new chemistries for high energy cathodes. In particular, I found that lithium quaternary phosphates and lithium ternary fluorides were two promising materials classes, with good stability with high-voltage cathodes and sufficient lithium content to enable Li-ion transport. I next studied the interface stability between solid electrolytes and common cathodes. The lithium garnet solid electrolytes are promising among known solid electrolytes because of their high temperature stability, good stability in air and moisture, and wide electrochemical window, but have limited stability against a variety of cathodes. To guide the development of coating layers for the garnet-cathode interface, I analyzed the stability of garnet with families of lithium ternary oxide (Li-M-O) coating chemistries and revealed factors governing the stability of materials with LLZO garnet and high-energy NMC cathodes. In addition to classifying known coating layers, I provide detailed guiding tables for coating layer selection and identify and discuss several new promising coating layer materials for stabilizing the interface between garnet and high-capacity cathodes. The crystal structure of a coating material plays a major role in transport properties such as Li-ion diffusivity and conductivity, which are required in the coating layer to achieve low interfacial resistance and good battery performance. Alumina is widely used as a coating layer in batteries and other applications, and has decent stability against a wide range of solid electrolytes and cathodes. I used first-principles molecular dynamics simulations and nudged-elastic band calculations to study Li-ion transport and migration barriers in several crystalline polymorphs and amorphous alumina. I found structural features in the Al framework, specifically the Li-Al distance variation, determined migration barriers in both crystalline and amorphous structures. Based on this structure-property relationship, I investigated how Li content, defects and off-stoichiometry changed the Li-ion transport within selected polymorphs, and suggest lowering the Al-ion content as a strategy to achieve stable and Li-diffusive alumina coatings. With this work, I provide an understanding of trends in stability between coating layers, cathodes, and the garnet solid electrolyte, new promising coating layer materials and families, and rational guidance for coating layer design and interfacial engineering for energy dense all-solid-state batteries. My thesis provides guiding principles for selecting materials with long-term cycling stability and good Li diffusivity as coatings for energy dense Li-ion batteries.Item ORBITAL-FREE DENSITY FUNCTIONAL THEORY OF ATOMS, MOLECULES, AND SOLIDS(2005-11-23) Chai, Jeng-Da; Weeks, John D.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Density functional (DF) theory has proved to be a powerful way to determine the ground state energy of atoms, molecules, and extended systems. An important part of the theory requires one to determine the kinetic energy of the ground state of a system of N noninteracting electrons in a general external field. Kohn and Sham showed how this can be numerically calculated very accurately using a set of N orbitals. However this prevents the simple linear scaling in N that would arise if the kinetic energy could be directly expressed as a functional of the electron density, as is done with other components of the total energy like the exchange-correlation energy. Orbital free methods attempt to calculate the noninteracting kinetic energy directly by approximating the universal but unknown kinetic energy density functional. However simple local approximations are inaccurate and it has proved very difficult to devise generally accurate nonlocal approximations. We focus instead on the kinetic potential, the functional derivative of the kinetic energy DF, which appears in the Euler equation for the electron density. We argue the kinetic potential is more amenable to simple physically motivated approximations in many relevant cases. We propose a family of nonlocal orbital free kinetic potentials that reduce to the known exact forms for both slowly varying and rapidly varying perturbations and also reproduce exact results for the linear response of the density of the homogeneous system to small perturbations. A simple and systematic approach for generating accurate and weak ab initio local pseudopotentials describing a smooth slowly varying valence component of the electron density is proposed for use in orbital free DF calculations of molecules and solids. The use of these local pseudopotentials further minimizes the possible errors arising from use of the approximate kinetic potentials. A linear scaling method for treating large extended systems is proposed for fast computations. Our theory yields results for the total energies and ionization energies of atoms, and for the shell structure in the atomic radial density profiles that are in very good agreement with calculations using the full Kohn-Sham theory. We describe the first use of nonlocal orbital free methods to determine the ground-state bond lengths and binding energies of diatomic molecules. These results and the ground-state lattice parameters, and total energy of bulk aluminum and bulk silicon are in generally good agreement with detailed calculations using the full Kohn-Sham theory.