Thesis (Ph. D.)--University of Missouri-Columbia, 2008. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on June 19, 2009) Includes bibliographical references.

<p> This thesis describes an attempt to develop a potentially useful route to seven-membered carbocycles. α-Diketones and their enol ether derivatives were proposed to be suitable three-carbon cycloaddition dienophiles when complexed with a Lewis acid. Reaction with 1,3-dienes should then lead to the desired cycloheptyl system.</p> <p> 1H-NMR and 13C-NMR studies were used to determine the stoichiometry and charge delocalization of complexes formed between 2,3-butanedione, 1,2-cyclohexanedione, 2-methoxy-cyclohex-2-en-1-one and the Lewis acids TiCl4 and SnCl4. Observations from these studies suggested that 1,2-cyclohexanedione and 2-methoxy-cyclohex-2-en-1one could behave as substituted allyl cations when complexed with TiCl4 or SnCl4.</p> <p> Reaction of the 1,2-cyclohexanedione/TiCl4 and 2-methoxy-cyclohex-2-en-1one/ TiCl4 complexes with 1 ,3-butadiene gave rise to six-membered carbocycles from [4C+2C] cycloadditions. The reaction of 2,3-butanedione-mono-trimethylsilyl enol ether/TiCl4 complex with 1,3-butadiene gave a mixture of the [4C+2C] and [4C+3C] cycloaddition products.</p> / Thesis / Master of Science (MSc)

No description available.

The elemental composition within structured insect populations was tested by creating a new method to analyze how variables (survival, growth and fecundity) within a population matrix could potentially affect the stoichiometric regime of a structured population at steady state. This was done by focusing on if the growth rate hypothesis, which states that there is a linear relationship between an individual growth rate and the percent of phosphorus within the individual, works at a population level. This was analyzed by creating and combining two matrices: the matrix-population containing the variables and a matrix containing the element phosphorus and dry weight. Data from a beetle species, Chrysomela tremulae F., was used as a guideline to create eight stoichiometric generic populations, where survival, growth and fecundity were tested in each of the eight generic populations. The results showed deviations from the growth rate hypothesis, suggesting that the hypothesis does not always work within structured populations. However, more research is needed to predict exactly how this hypothesis works in populations. Overall, this new method for analyzing stoichiometry within structured populations is a useful analytical tool, but there is a need for analyzing the results from these models in a more efficient way.

Orientador: Fernando Rogério de Paula / Banca: Rafael Zadorozny / Banca: Fernando Henrique Cristovan / Resumo: Com os supercondutores de alta temperatura crítica (Tc) descobertos na década de 80, os estudos dos materiais supercondutores se intensificaram na tentativa de se obter materiais com Tc's cada vez maiores, ou seja, acima da temperatura do nitrogênio líquido (77 K). No entanto, alguns materiais supercondutores de alta Tc são difíceis de serem obtidos, como no caso do supercondutor cerâmico composto de Bi-Sr-Ca-Cu-O (BSCCO) que apresenta quatro fases predominantes: Bi2Sr2CuO6 (Bi-2201), Bi2Sr2Ca1Cu2O8 (Bi-2212), Bi2Sr2Ca2Cu3O10 (Bi-2223) e Bi2Sr2Ca3Cu4O12 (Bi-2234), com suas Tc's, respectivamente, na faixa de 7-22 K, 75-94 K, 100-125 K e 90-100 K. Infelizmente este supercondutor apresenta instabilidade na fase de maior Tc (Bi-2223). Uma das soluções utilizadas para melhorar a estabilidade desta fase consiste na substituição de elementos químicos. Desta forma, neste trabalho estudamos a estabilização da fase Bi-2223, utilizando o método de substituição onde o Bi foi substituído parcialmente pelo Pb. Para isso, a estequiometria utilizada foiBi2-xPbxSr2Ca2Cu3Oycom x = 0,00, 0,30, 0,35, 0,40, 0,45, 0,50 mol/g. As soluções precursoras foram preparadas baseadas no método Pechini de síntese, na qual se obteve uma resina polimérica que foi tratada termicamente a 200°C por 10 h. Em seguida, a resina foi macerada e o pó obtido nesse procedimento foi novamente tratado termicamente entre 400 e 810°C. Com o pó resultante, foram preparadas pastilhas e, logo após, submetidas a um novo tratamento térmico a 845°C/60 h. Nas pastilhas foram feitas as caracterizações estruturais por meio da técnica de Difratometria de Raios - X (DRX); caracterização elétrica pelo método de quatro pontas dc e química pela técnica de Espectroscopia por Energia Dispersiva de Raios - X (EDX).Os resultados mostraram que nas amostras preparadas com as concentrações de 0,00; 0,30 e 0,35 mols/g de Pb predominou a fase Bi-2212, no entanto para 0,40... / Abstract: With high critical temperature superconducting discovered in the 80s, the studies of superconducting materials have intensified in an attempt to obtain materials with Tc increasing, above the temperature of liquid nitrogen (77 K). However, some superconducting materials of high Tc are difficult to be obtained, as in the case of superconducting ceramic compositesof Bi- Sr-Ca-Cu-O (BSCCO) that presents predominant four phases: Bi2Sr2CuO6 (Bi-2201), Bi2Sr2Ca1Cu2O8 (Bi 2212), Bi2Sr2Ca2Cu3O10 (Bi-2223) and Bi2Sr2Ca3Cu4O12 (Bi-2234), with its Tc respectively in the range of 7-22 K, 75-94 K, 100-125 K and 90-100 K. Unfortunately the high-Tc phase (Bi-2223) in this high-temperature superconductor is thermodynamically unstable. One of the used solutions to improve this phase stability is the chemical element substitution. Thus, in this work we study the stabilization of the Bi-2223 phase using the method of chemical element substitution in which the Bi was replaced partially by Pb. For this, the stoichiometry used was Bi2-xPbxSr2Ca2Cu3Oywith x = 0.00; 0.30; 0.35; 0.40; 0.45; 0.50 mol/g. The solutions were prepared by the polymeric precursor method based on the Pechini method. The polymeric resin obtained was thermally treated at 200°C for 10 h.Then the resin was macerated and the powder obtained in this procedure was again thermally treated between 400°C and 810°C. With the resulting powder, pastilles were prepared and soon after were subjected to a new thermal treatment to 845°C for 60 h. Structural characterizations of the samples were investigated by the technique of X-ray diffraction - (XRD); electrical characterization by four-point probe dc conductivity method and chemical characterization by Energy-dispersive X-ray spectroscopy (EDX).The results showed that in the samples prepared with concentrations of 0.00, 0.30 and 0.35 mol/g Pb prevailed the phase Bi -2212, however for the concentration of 0.40 mol/g of Pb the Bi-2223 phase was the... / Mestre

Three biologically-active aminothiols cysteamine (CA), DL-cysteine (CYSH) and DL-homocysteine, were studied in this thesis. These aminothiols react with nitrous acid (HNO2), prepared in situ, to produce S-nitrosothiols (RSNOs): S-nitrosocyteamine (CANO), S-nitrosocysteine (CYSNO) and S-nitrosohomocysteine (HCYSNO). They also react with S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) through a transnitrosation reaction to produce their corresponding RSNOs. A detailed kinetics and mechanistic study on the formation of these RSNOs and their subsequent decomposition to release nitric oxide (NO) were studied. For all three aminothiols the stoichiometry of their reaction with nitrous acid is strictly 1:1 with the formation of one mole of RSNO from one mole of HNO2. In all cases, the nitrosation reaction is first order in nitrous acid, thus implicating it as a nitrosating agent in mildly acidic pH conditions. Acid catalyzes nitrosation after nitrous acid has saturated, implicating another nitrosating agent, the nitrosonium cation, NO+ ( which is produced from the protonation of nitrous acid) as a contributing nitrosating species in highly acidic environments. The acid catalysis at constant nitrous acid concentrations suggests that the nitrosonium cation nitrosates at a much higher rate than nitrous acid. Nitric oxide itself was not detected as a nitrosant. Bimolecular rate constants for the nitrosation of CA, CYSH and HCYSH were deduced to be 17.9, 6.4, 0.09 M-1 s-1 for the nitrosation by nitrous acid and 8.25 x 1010, 2.89 x 1010 and 6.57 x 1010 M-1 s-1 for the nitrosation by nitrosonium cation respectively. A linear correlation was obtained between the rate constants and the pKa of the sulfur center of the aminothiols for nitrosation by NO+. The stabilities of the three RSNOs were found to be affected by metal ions. They were unstable in the presence of metal ions, with half-lives of few seconds. However, in the presence of metal ion chelators, they were found to be relatively stable with half-lives of 10, 30 and 198 hours for CYSNO, CANO and HCYSNO respectively. The relative stability of HCYSNO may be an advantage in the prevention of its metabolic conversion to homocysteine thiolactone, the major culprit in HCYSH pathogenesis. This dissertation has thus revealed new potential therapeutic way for the modulation of HCYSH related cardiovascular diseases.

The effect of stoichiometry on the pyrolytic decomposition, oxidation and ignition behaviour of synthetic violarite and pentlandite has been established. These minerals, of general formula (Fe,Ni)(subscript)3S(subscript)4 and (Fe,Ni)(subscript)9S(subscript)8 respectively, may vary considerably in Fe:Ni ratio. Pentlandite can also show some variation in metal:sulfur ratio. A series of samples, ranging in stoichiometry from Fe(subscript)0.96Ni(subscript)1.97S(subscript)4 to Fe(subscript)0.20Ni(subscript)2.72S(subscript)4 and Fe(subscript)5.80Ni(subscript)3.15S(subscript)8 to Fe(subscript)3.40Ni(subscript)5.55S(subscript)8, were synthesised and characterised using wet chemical analysis, electron probe micro-analysis (EPMA), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer, Emmett and Teller (BET) surface area analysis.The thermal behaviour of these sulfides was examined using simultaneous Thermogravimetry-Differential Thermal Analysis (TG-DTA) at different heating rates and in different atmospheres. Partially reacted samples were collected at various temperatures and analysed using XRD, EPMA, SEM, optical microscopy (OM), and Fourier transform infrared (FTIR) spectroscopy. The endmembers of the violarite and pentlandite series were examined in detail to determine the effect of stoichiometry on the reaction mechanism. In this study the reaction mechanism refers to the sequence of reactions occurring during pyrolytic decomposition or oxidation of the sulfide minerals. Samples were sieved into four particle size fractions, 125-90, 90-63, 63-45 and 45-20 gm, to determine the effect of particle size on the reaction mechanism.When violarite was heated in an inert atmosphere at 10 degrees celsius min(subscript)-1, it initially decomposed to a monosulfide solid solution (mss), (Fe,Ni)(subscript)1-xS, and vaesite, (Ni,Fe)S(subscript)2, indicated by a ++ / sharp endothermic peak in the DTA trace. The decomposition temperature was found to be linearly dependent on the iron:nickel ratio, decreasing from 495 degrees celsius to 450 degrees celsius as the iron:nickel ratio decreased from 0.49 to 0.07. This was followed by a broader endothermic peak coinciding with a rapid mass loss, which was associated with the decomposition of vaesite to mss with the loss of sulfur. Between 615-805 degrees celsius the mss was converted to a high temperature form of heazlewoodite, (Fe,Ni)(subscript)3+/-S(subscript)2 melted incongruently at 835 degrees celsius and 805 degrees celsius for Fe(subscript)0.96Ni(subscript)1.97S(subscript)4 and Fe(subscript)0.20Ni(subscript)2.72S(subscript)4 respectively, with further loss of sulfur vapour forming a central sulfide liquid of general formula (Fe,Ni)(subscript)1+xS.Under similar experimental conditions, pentlandite pyrolytically decomposed forming mss and heazlewoodite with no associated loss of sulfur. The decomposition temperature decreased as the iron:nickel ratio deviated from the ideal value of 1:1. A maximum decomposition temperature of 610 degrees celsius was found at an iron:nickel ratio of 1.00, decreasing to 580 degrees celsius at a ratio of 1.84 and 0.61. Sulfur was evolved slowly at temperatures in excess of 760 degrees celsius as mss was converted to heazlewoodite, indicated by a gradual mass loss. The heazlewoodite then melted incongruently in excess of 840 degrees celsius indicated by a sharp endothermic peak, resulting in a further loss of sulfur.The oxidation of violarite and pentlandite was investigated at a heating rate of 10 degrees celsius min(subscript)-1 in an air atmosphere. The oxidation of violarite was initiated by decomposition to mss resulting in a rapid mass loss associated with the evolution of sulfur vapour, and an exothermic peak due to the gas phase oxidation of ++ / the sulfur. The iron sulfide component of the mss was then preferentially oxidised to iron(II) sulfate between 485-575 degrees celsius, upon which the sulfate decomposed and the remaining iron sulfide was preferentially oxidised to hematite. The mss core was then converted to (Fe,Ni)(subscript)3+/-xS(subscript)2 between 635-715 degrees celsius, resulting in the loss of further sulfur which was oxidised. The sulfide core, which consisted of predominantly Ni(subscript)3+/-xS(subscript)2 with a minor amount of iron still remaining in solid solution, incongruently melted at a constant temperature of 795 degrees celsius regardless of the initial stoichiometry of the violarite sample. This was followed by the rapid oxidation of the liquid sulfide resulting in a sharp exothermic peak in the DTA trace.For pentlandite, the TG-DTA curve exhibited an initial mass gain commencing at approximately 400 degrees celsius, which was attributed to the preferential oxidation of iron. Evidence from SEM indicated that iron migrated towards the oxygen interface, where it was oxidised to hematite. During this process the metal: sulfur ratio decreased and pentlandite was converted to mss. The iron sulfide component of the mss phase was then preferentially oxidised to hematite as indicated by a major exotherm, which occurred in the temperature range 575-665 degrees celsius, forming an oxide product layer around a nickel sulfide core. The oxidation of the remaining nickel sulfide followed the same reaction sequence to that of violarite.By increasing the heating rate to 40 degrees celsius min(subscript)-1, and carrying out the oxidation in pure oxygen, the tendency of the sulfides to ignite was established. Ignition was characterised by a highly exothermic reaction which coincided with a rapid mass loss over a short time period. Overheating of the samples above the programmed furnace ++ / temperature was also observed. Violarite exhibited ignition behaviour while pentlandite did not.Both sulfides were subjected to shock heating conditions (heating rate = 1500 - 5000 degrees celsius min(subscript)-1, oxygen atmosphere) using isothermal thermogravimetry (TG). This method produces heating rates analogous to those which are experienced in the reaction shaft of an industrial flash smelter. The effect of stoichiometry on ignition temperature and extent of oxidation for the entire series of synthetic violarites and pentlandites was determined. Partially ignited and ignited products were collected from isothermal TG experiments and were examined by OM, SEM and EPMA to establish the ignition mechanism.Both violarite and pentlandite ignited using the isothermal TG technique. A clear relationship was found between the stoichiometry of violarite and pentlandite and the ignition temperature, with an increase in the iron:nickel ratio causing a decrease in the ignition temperature. The ignition temperature also decreased as the size of the particles decreased.The extent of oxidation increased as the iron:nickel ratio increased, and also increased as the particle size decreased.

Understanding relationships between biodiversity and ecosystem function is a critical challenge, particularly in freshwater ecosystems where species losses are occurring at unprecedented rates. There is a particular need to examine these relationships in natural settings at large spatial scales. Ongoing, disease-driven amphibian declines may influence the structure and function of stream ecosystems, but little is known of the potential roles of stream-dwelling tadpoles in consumer-resource dynamics, ecosystem functions such as decomposition, and ecosystem-level biogeochemical cycling. Tadpoles in tropical streams likely regulate flows and ratios of nitrogen (N), phosphorus (P), and carbon (C), influencing ecosystems by altering nutrient supplies to other animals and their food resources. I used ecological stoichiometry as a framework to assess how the sudden loss of consumer biodiversity in neotropical headwater streams affected ecosystem function. I quantified N and P excretion and C:N:P ratios of tadpoles, macroinvertebrates, and food resources in healthy sites (pre-decline) and sites where disease-driven amphibian declines had occurred (post-decline). I tested the hypothesis of consumer homeostasis (i.e., that organisms maintain consistent body nutrient ratios by altering excretion chemistry) over a range of taxa and size classes. I also used mesocosms in a natural stream setting to quantify the effects of grazing tadpoles, shredding macroinvertebrates and a combination of the two on leaf decomposition and associated microbial activity. Finally, I examined macroinvertebrate community structure and quantified biomass and nutrient storage in tadpoles, macroinvertebrates, and basal resources in pre-decline and post-decline sites. I also measured excretion rates, volumetric excretion, and nutrient turnover for both tadpoles and macroinvertebrates. Patterns of consumer-resource stoichiometry varied with the presence or absence of tadpoles. There were higher concentrations of C, N, and P in basal resources in pre-decline sites compared to post-decline sites, but little variation in elemental ratios among sites. Elemental composition and molar ratios in grazers and shredders varied, with pronounced differences in %N for gatherers and filterers across sites. Macroinvertebrate grazer elemental composition was higher for all elements and had lower C:N, N:P, and C:P molar ratios in pre-decline sites compare to grazers in post-decline sites, while shredders showed the opposite pattern. There were differences in both taxon-specific allometric and stoichiometric relationships in tadpoles and macroinvertebrates between pre- and post-decline sites. Body P content was a good predictor of tadpole P excretion and tadpoles in pre-decline sites excreted more P per unit body P than those in post decline sites. Individuals deviated from strict homeostasis, and the degree of deviation varied among taxa. Tadpoles also affected leaf decomposition by influencing microbial communities and altering shredding macroinvertebrate feeding. Higher respiration rates of leaf discs in chambers with tadpoles suggested that tadpoles enhanced microbial activity by excreting nutrients through feeding and excretion. Shredders alone had little effect on respiration rates, indicating that tadpoles play an important and unique role in enhancing microbial activity and litter decomposition. Leaf area loss was greatest when tadpoles and macroinvertebrates were together, indicating facilitation. Macroinvertebrates are important nutrient recyclers in neotropical headwater streams, but their role is greatly decreased in the absence of larval amphibians. I measured ~80% lower standing stocks and storage of C, N, and P in basal resources in post-decline compared to pre-decline sites. Storage of C, N, and P in both tadpoles and macroinvertebrates also decreased in post-decline sites. I also observed 98% decreases in tadpole nutrient excretion and egestion rates, and an additional decrease in macroinvertebrate excretion rates (~80%) for both N and P in post-decline versus pre-decline sites. These decreases led to >8,000% increase in the distance that it took tadpoles to turn over the ambient N pools in post-decline sites, and a 130% increase for macroinvertebrates. Similar patterns were evident for P turnover, with turnover distance increasing by 6,000% and 400% in post-decline sites for tadpoles and macroinvertebrates, respectively. My results indicate that N and P excretion by both tadpoles and macroinvertebrates constitute significant nutrient fluxes in these headwater streams. Both tadpole and macroinvertebrate communities were excreting nutrients at similar rates in pre-decline sites, suggesting that they were playing equally significant roles in their contribution to ecosystem demand. My results demonstrate that tadpoles are important consumers in Neotropical headwater streams and their loss significantly alters stream food webs and ecosystem functions.

abstract: This work is concerned with the study and numerical solution of large reaction diffusion systems with applications to the simulation of degradation effects in solar cells. A discussion of the basics of solar cells including the function of solar cells, the degradation of energy efficiency that happens over time, defects that affect solar cell efficiency and specific defects that can be modeled with a reaction diffusion system are included. Also included is a simple model equation of a solar cell. The basics of stoichiometry theory, how it applies to kinetic reaction systems, and some conservation properties are introduced. A model that considers the migration of defects in addition to the reaction processes is considered. A discussion of asymptotics and how it relates to the numerical simulation of the lifetime of solar cells is included. A reduced solution is considered and a presentation of a numerical comparison of the reduced solution with the full solution on a simple test problem is included. Operator splitting techniques are introduced and discussed. Asymptotically preserving schemes combine asymptotics and operator splitting to use reasonable time steps. A presentation of a realistic example of this study applied to solar cells follows. / Dissertation/Thesis / Doctoral Dissertation Applied Mathematics 2020

Proportional reasoning refers not only to the ability to manipulate the proportions, but also to detect, express, analyze, explain, and provide evidence in support of assertions about proportional relationships. Students’ understanding of the proportional relationships that they encounter in science can be improved through well-designed instruction. In other words, teacher practice is key to the successful learning of both proportional reasoning and science. Stoichiometry is a basic topic in chemistry that focuses on the proportional relationship between the amount of reactants and/or that of products in a chemical reaction. This study explored 10 chemistry teachers’ knowledge and beliefs on integrating proportional reasoning in teaching stoichiometry mainly through interview, survey, and lesson materials. The framework of pedagogical content knowledge was used to examine key dimensions of teacher knowledge that were triggered as they teach stoichiometry. Moreover, teachers’ problem-solving strategies were sorted by using the proportional reasoning strategies framework. Three representative case studies allowed a deep analysis and the relation of each component of pedagogical content knowledge with implications for teacher education and professional development design.