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Experimental and theoretical studies of nitrated polycyclic aromatic hydrocarbonsOnchoke, Kefa Karimu, January 2006 (has links)
Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 355-382).
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Computational Studies of C–H/C–C Manipulation Utilizing Transition Metal ComplexesPardue, Daniel B. 05 1900 (has links)
Density Functional Theory (DFT) is an effective tool for studying diverse metal systems. Presented herein are studies of a variety of metal systems, which can be applied to accomplish transformations that are currently difficult/impossible to achieve. The specific topics studied utilizing DFT include: 1) C–H bond activation via an Earth-abundant transition metal complex, 2) C–H bond deprotonation via an alkali metal superbase, 3) and amination/aziridination reactions utilizing a CuI reagent. Using DFT, the transformation to methanol (CH3OH) from methane (CH4) was examined. The transition metal systems studied for this transformation included a model FeII complex. This first-row transition metal is an economical, Earth-abundant metal. The ligand set for this transformation includes a carbonyl ligand in one set of complexes as well as a phosphite ligand in another. The 3d Fe metal shows the ability to convert alkyls/aryls to their oxidized counterpart in an energetically favorable manner. Also, “superbasic” alkali metal amides were investigated to perform C—H bond cleavage. Toluene was the substrate of interest with Cs chosen to be the metal of interest because of the highly electropositive nature of this alkali metal. These highly electrophilic Cs metal systems allow for very favorable C—H bond scission with a toluene substrate. Finally, the amination and aziridination of C–H and C=C bonds, respectively, by a CuI reagent was studied. The mechanism was investigated using DFT calculations. Presently, these mechanisms involving the use of coinage metals are debated. Our DFT simulations shed some insight into how these transformations occur and ultimately how they can be manipulated.
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Applications of Single Reference Methods to Multi-Reference ProblemsJeffrey, Chris C. 05 1900 (has links)
Density functional theory is an efficient and useful method of solving single-reference computational chemistry problems, however it struggles with multi-reference systems. Modifications have been developed in order to improve the capabilities of density functional theory. In this work, density functional theory has been successfully applied to solve multi-reference systems with large amounts of non-dynamical correlation by use of modifications. It has also been successfully applied for geometry optimizations for lanthanide trifluorides.
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Theoretical study of magnetic odering of defects in diamondBenecha, Evans Moseti 11 1900 (has links)
Magnetic ordering of dopants in diamond holds the prospect of exploiting diamond’s unique
properties in the emerging field of spintronics. Several transition metal defects have been
reported to order ferromagnetically in various semiconductors, however, low Curie
temperatures and lack of other fundamental material properties have hindered practical
implementation in room temperature spintronic applications. In this Thesis, we consider the
energetic stability of 3d transition metal doped-diamond and its magnetic ordering properties
at various lattice sites and charge states using ab initio Density Functional Theory methods.
We find the majority of 3d transition metal impurities in diamond at any charge state to be
energetically most stable at the divacancy site compared to substitutional or interstitial lattice
sites, with the interstitial site being highly unstable (by ~8 - 10 eV compared to the divacancy
site). At each lattice site and charge state, we find the formation energies of transition metals
in the middle of the 3d series (Cr, Mn, Fe, Co, Ni) to be considerably lower compared to
those early or late in the series. The energetic stability of transition metal impurities across
the 3d series is shown to be strongly dependent on the position of the Fermi level in the
diamond band gap, with the formation energies at any lattice site being lower in p-type or ntype
diamond compared to intrinsic diamond.
Further, we show that incorporation of isolated transition metal impurities into diamond
introduces spin polarised impurity bands into the diamond band gap, while maintaining its
semiconducting nature, with band gaps in both the spin-up and spin-down channels. These
impurity bands are shown to originate mainly from s, p-d hybridization between carbon sp
3
orbitals with the 3d orbitals of the transition metal. In addition, the 4p orbitals contribute
significantly to hybridization for transition metal atoms at the substitutional site, but not at
the divacancy site. In both cases, the spin polarisation and magnetic stabilization energies are
critically dependent on the lattice site and charge state of the transition metal impurity.
By allowing magnetic interactions between transition metal atoms, we find that ferromagnetic
ordering is likely to be achieved in divacancy Cr+2, Mn+2, Mn+1 and Co0 as well as in
substitutional Fe+2 and Fe+1, indicating that transition metal-doped diamond is likely to form
a diluted magnetic semiconductor which may successfully be considered for room
temperature spintronic applications. In addition, these charge states correspond to p-type
diamond, except for divacancy Co0, suggesting that co-doping with shallow acceptors such as
B (
will result in an increase of charge concentration, which is likely to
enhance mediation of ferromagnetic spin coupling. The highest magnetic stabilization energy
occurs in substitutional Fe+1 (33.3 meV), which, also exhibits half metallic ferromagnetic
ordering at the Fermi level, with an induced magnetic moment of 1.0 μB per ion, thus
suggesting that 100 % spin polarisation may be achieved in Fe-doped diamond. / Physics / D. Litt. et Phil. (Physics)
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Investigation of chemical shielding property and its relationship to structure of biomacromolecules using NMR and density functional theory methods. / CUHK electronic theses & dissertations collectionJanuary 1999 (has links)
Xu, Xiao-ping. / "March 1999." / Thesis (Ph.D.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (p. 152-166). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.
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Confinement effect on semiconductor nanowires propertiesNduwimana, Alexis 02 November 2007 (has links)
Confinement effect on semiconductor nanowires properties.
Alexis Nduwimana
100 pages
Directed by Dr. Mei-Yin Chou
We study the effect of confinement on various properties of semiconductor
nanowires. First, we study the size and direction dependence of the band gap of
germanium nanowires. We use the density functional theory in the local density approximation. Results shows that the band gap decreases with the diameter The susceptibility of these nanowires is also computed. Second, we look at the confinement effect on the piezoelectric coefficients of ZnO and AlN nanowires. The Berry phase method is used. It is found that depending on passivation, thepiezoelectric effect can decrease or increase. Finally, we study the size and direction dependence of the melting temperature of silicon nanowires. We use the molecular dynamics with the Stillinger Weber potential. Results indicate that the melting temperature increases with the nanowire diameter and that it is direction dependent.
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Computational modeling of materials in polymer electrolyte membrane fuel cellsBrunello, Giuseppe 16 September 2013 (has links)
Fuel cells have the potential to change the energy paradigm by allowing more efficient use of energy. In particular, Polymer Electrolyte Membrane Fuel Cells (PEMFC) are interesting because they are low temperature devices. However, there are still numerous challenges limiting their widespread use including operating temperature, types of permissible fuels and optimal use of expensive catalysts. The first two problems are related mainly to the ionomer electrolyte, which largely determines the operating temperature and fuel type. While new ionomer membranes have been proposed to address some of these issues, there is still a lack of fundamental knowledge to guide ionomer design for PEMFC.
This work is a computational study of the effect of temperature and water content on sulfonated poly(ether ether ketone) and the effect of acidity on sulfonated polystyrene to better understand how ionomer material properties differ. In particular we found that increased water content preferentially solvates the sulfonate groups and improves water and hydronium transport. However, we found that increasing an ionomer’s acid strength causes similar effects to increasing the water content.
Finally, we used Density Functional Theory (DFT) to study platinum nano-clusters as used in PEMFCs. We developed a model using the atom’s coordination number to quickly compute the energy of a cluster and therefore predict which platinum atoms are most loosely held. Our model correctly predicted the energy of various clusters compared to DFT. Also, we studied the interaction between the various moieties of the electrolyte including the catalyst particle and developed a force field.
The coordination model can be used in a molecular dynamics simulation of the three phase region of a PEMFC to generate unbiased initial clusters. The force field developed can be used to describe the interaction between this generated cluster and the electrolyte.
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Foundation of Density Functionals in the Presence of Magnetic FieldLaestadius, Andre January 2014 (has links)
This thesis contains four articles related to mathematical aspects of Density Functional Theory. In Paper A, the theoretical justification of density methods formulated with current densities is addressed. It is shown that the set of ground-states is determined by the ensemble-representable particle and paramagnetic current density. Furthermore, it is demonstrated that the Schrödinger equation with a magnetic field is not uniquely determined by its ground-state solution. Thus, a wavefunction may be the ground-state of two different Hamiltonians, where the Hamiltonians differ by more than a gauge transformation. This implies that the particle and paramagnetic current density do not determine the potentials of the system and, consequently, no Hohenberg-Kohn theorem exists for Current Density Functional Theory formulated with the paramagnetic current density. On the other hand, by instead using the particle density as data, we show that the scalar potential in the system's Hamiltonian is determined for a fixed magnetic field. This means that the Hohenberg-Kohn theorem continues to hold in the presence of a magnetic field, if the magnetic field has been fixed. Paper B deals with N-representable density functionals that also depend on the paramagnetic current density. Here the Levy-Lieb density functional is generalized to include the paramagnetic current density. It is shown that a wavefunction exists that minimizes the "free" Hamiltonian subject to the constraints that the particle and paramagnetic current density are held fixed. Furthermore, a convex and universal current density functional is introduced and shown to equal the convex envelope of the generalized Levy-Lieb density functional. Since this functional is convex, the problem of finding the particle and paramagnetic current density that minimize the energy is related to a set of Euler-Lagrange equations. In Paper C, an N-representable Kohn-Sham approach is developed that also include the paramagnetic current density. It is demonstrated that a wavefunction exists that minimizes the kinetic energy subject to the constraint that only determinant wavefunctions are considered, as well as that the particle and paramagnetic current density are held fixed. Using this result, it is then shown that the ground-state energy can be obtained by minimizing an energy functional over all determinant wavefunctions that have finite kinetic energy. Moreover, the minimum is achieved and this determinant wavefunction gives the ground-state particle and paramagnetic current density. Lastly, Paper D addresses the issue of a Hohenberg-Kohn variational principle for Current Density Functional Theory formulated with the total current density. Under the assumption that a Hohenberg-Kohn theorem exists formulated with the total current density, it is shown that the map from particle and total current density to the vector potential enters explicitly in the energy functional to be minimized. Thus, no variational principle as that of Hohenberg and Kohn exists for density methods formulated with the total current density. / <p>QC 20140523</p>
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First-principles study of the li adsorption on various carbon hybrid systemsKoh, Wonsang 29 June 2011 (has links)
Recent carbon allotropes such as carbon nanotubes (CNTs), fullerenes (C60s) and graphene have attracted great interests in both science and engineering due to their unique properties such as excellent electrical and mechanical properties as well as its vast surface area, and have led to many commercial applications. Especially, CNTs have been considered to be one of the promising candidates in the Li ion battery system because of its outstanding properties. However, the experimental results in the pristine CNT system have shown just slight improvement than original graphitic carbon material, which has been attributed to the weak adsorption of Li on CNTs. In this study, we investigated two types of CNT-C60 hybrid system consisting of CNTs and C60s to improve Li adsorption capabilities and predict its performance through quantum mechanical (QM) computations. First, we investigated adsorption energy of lithium (Li) on dilute CNT-C60 hybrid and CNT-C60 nanobud system as well as various electronic properties such as band structure, density of states (DOS), molecular orbital and charge distribution. Then, we expanded our interest to the more realistic condensed structure of CNT-C60 hybrid and nanobud system to examine actual electrochemical characteristics. The study of the condensed structure has been expanded to the very unique CNT-C60 nano-network system and examined mechanical strength as well as electronic properties. Finally, Li adsorption on other carbon allotropes system such as graphene-C60 hybrid and graphene-C60 bud system was investigated in order to provide fundamental understanding of electronic interaction between carbon allotrope and effect of Li adsorption.
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Funcionais orbitais: investigação de estratégias de implementação no contexto da formulação Kohn-Sham da Teoria do Funcional da Densidade / Orbital functionals: implementation strategies in the context of the Kohn-Sham formulation of Density Functional TheoryBento, Marsal Eduardo 16 December 2014 (has links)
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Previous issue date: 2014-12-16 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The development of Density Functional Theory (DFT) has been focused primarily on two main pillars: (1) the pursuit of more accurate exchange-correlation (XC) density functionals; (2) the feasibility of computational implementation when dealing with many-body systems. In this context, this work is aimed on using one-dimensional quantum systems as theoretical laboratories to investigate the implementation of orbital functionals (OFs) of density. By definition, OFs are those which depend only implicitly on the density, via an explicit formulation in terms of Kohn-Sham orbitals. Typical examples are the XC functionals arising from the Perdew-Zunger self-interaction correction (PZSIC). Formally, via Kohn-Sham equations, the implementation of OFs must be performed by means of the optimized effective potential method (OEP), which is known by requiring an excessive computational effort even when dealing with few electrons systems (N ̴ 10). Here, we proceed a systematical investigation aiming to simplify or avoid the OEP procedure, taking as reference the implementation of the PZSIC correction applied to one-dimensional Hubbard chains. / O desenvolvimento da Teoria do Funcional da Densidade (DFT) tem se concentrado, sobretudo, em dois pilares fundamentais: (1) a busca por funcionais de troca e correlação (XC) mais precisos; (2) a viabilidade de implementação computacional diante de sistemas com muitos elétrons. Nesse contexto, o objetivo principal deste trabalho consiste em utilizar sistemas quânticos unidimensionais, mais simples de serem tratados numericamente, como laboratórios teóricos para o desenvolvimento de alternativas de implementação numérica de funcionais orbitais (OFs) da densidade. Por definição, OFs são todos aqueles que dependem apenas implicitamente da densidade, via formulação explícita em termos dos orbitais Kohn-Sham. Exemplos típicos são os funcionais XC advindos da correção de auto-interação de Perdew e Zunger (PZSIC). Formalmente, via equações de Kohn-Sham, a implementação de OFs deve ser procedida por meio do método do potencial efetivo otimizado (OEP) que, no contexto computacional, é conhecido por se tornar demasiadamente custoso, inclusive para sistemas com poucos elétrons (N ̴ 10). Sendo assim, investigamos, de forma sistemática, alternativas de simplificar ou evitar o procedimento OEP, tomando como referência a implementação da correção PZSIC aplicada a cadeias de Hubbard unidimensionais.
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