A standard methodology was developed for performing avoidance-preference tests, using Rainbow Trout (Salmo gairdneri) as the test organism. Experiments were conducted in a hydraulic channel, 9.15 m long by 0.30 m wide, partly divided along its length, and at a flow depth of 0.30 m. The design combined steep and shallow gradient characteristics. The toxicants investigated included Cu(II), Cr(III) and Cr(VI). The lowest avoidance threshold values were established at 2.1 $ mu$g/l for Cu(II) and 0.0026 mg/l and 0.026 mg/l for Cr(III) and Cr(VI) respectively while avoidance reactions increased with levels of toxicant in the channel. Similar experiments were performed with rainbow trout which were pre-exposed at sublethal levels to the toxicant, in order to assess the influence of toxic pre-exposure to the subsequent fish avoidance response. The length of pre-exposure varied between 7-20 weeks. Avoidance threshold values were correlated with safe levels of toxicant exposure. / Pre-exposed fish exhibited decreasing avoidance reactions compared to non-exposed populations. Increased tolerance to the toxicant, was suggested by the increase in avoidance threshold values with pre-exposure levels. Fish exposed to test concentrations matching their pre-exposure levels, clearly preferred this same concentration over the adjacent lower or higher test concentration. / A two mechanism avoidance model was proposed independent of toxicant used or level of pre-exposure. The toxicant concentration where the second mechanism begins to dominate was referred to as avoidance breakpoint, and was correlated to a MATC level for the toxicant in question. Olfactory responses were proposed to be associated with fish avoidance responses below the avoidance breakpoint, while hypoxic stress along with osmo- and iono regulatory stress appeared to be responsible for driving fish avoidance reactions beyond the avoidance breakpoint. / A clearance period of 7 days was sufficient to allow fish to recover normal avoidance behaviour following pre-exposure to Cr(VI) below the avoidance breakpoint.

High carbon ferrochromium serves as the main chromium source for almost all chromium containing steel alloys. The traditional method for the production of high carbon ferrochromium via the reduction of chromite using coke in electric arc furnaces, draws its considerable energy requirement from electrical power. The escalation in cost of electric power in South Africa has motivated research into alternative, fuel fired, reduction processes. One such process involves the partial solid state reduction of chromite in coal fired rotary kilns at temperatures between 1200 and 140rrc, prior to electric smelting. such processes are currently operated on a commercial scale and result in considerable savings in electrical energy, despite slow reduction kinetics and low reaction extents. A large amount of research conducted in the past, . aimed at establishing the fundamentals of the reduction process, has not provided satisfactory answers to questions regarding the mechanism of reduction. It was therefore necessary to conduct further test work on the process to establish the mechanism and factors limiting the rate and extent of reduction. Thermodynamic analysis of the reaction system indicates that at temperatures above 1050C reduction of the ore will proceed, and should reach an extent of approximately 90% reduction at 1200C. Complete reduction should be achievable at approximately 125ifc. However experimental results indicate the persistence of a stable magnesiochromite spinel under normal reducing conditions even at 140ifc. This limits the degree of chromium reduction to approximately 65%. Kinetic data from thermobalance studies and electron microscope examination of the reduction product showed independent reduction of iron and chromium. The rate of iron reduction was found to be relatively rapid and to go to completion, compared to that of chromium where the formation of a relatively inert picrochromite- spinel solid solution (MgO(Cr,AI)203) at the surface of the grain liinited the rate and extent of reduction to approximately 65% in the case of LG6 chromite. These findings suggested that the only way in which the kinetics of the process might be improved was through the addition of a component capable of disrupting the spinel layer at the surface of the chromite grain. In this study, fluoride containing mixtures such as CaF2 - NaF and fluorspar- feldspar- silica were successfully used to accelerate the reaction. Such mixtures are commercially interesting and highly effective even at low additions (4- 10%) . The mechanism whereby such mixtures operate was shown to involve the dissolution of all the spinel components in the liquid flux phase. Following dissolution, rapid i i recrystallization of spinel (Mg . Al2 ~) occurs , simultaneous to the transport of Fe 2+ and cr 3+ ions through the liquid to a site where reduction can take place. The main effect of this is to increase the rate and extent of chromium reduction to the point where virtual total reduction can be achieved in less than 90 min at temperatures as low as 1200C. Although the reduction kinetics in the presence of such solvent flux phases are still largely limited by the rate of solid state diffusion, the disruption of the surface enables faster overall diffusion rates to be achieved. Ultimately as the particle size and separation between oxide and reductant is increased, the rate of dissolution and transport through the flux phase become rate limiting. / Thesis (Ph.D.)-University of Natal, Durban, 1989.

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There has been a growing demand for specific linear alpha olefins in the polyethylene industry in order to control polymer rheology. This growing demand thereby increases the need for highly active and selective ethylene oligomerization catalysts. Chromium-based catalysts continue to be of high interest for this application due to this metal’s versatility in both selective and non selective ethylene oligomerization. Ligand design is an important consideration in oligomerization chemistry: the ability of the ligand to stabilize low valent chromium and to support a two-electron redox process will allow the catalytic systems to follow the selective ring expansion mechanism for oligomerization. Chelating aminophosphane based ligands, previously studied by our group, have been shown to support both tri- and tetramerization of ethylene. We have explored modifications of one of the NP arms by replacing with a different coordinating group in an attempt to further stabilize the monovalent state of chromium and increase selectivity. Other ligands explored in this work are pyrrole based ligands, which have shown high activity and selectivity towards ethylene oligomerization. One example of this is the commercial Chevron-Phillips system. Recently, the co-polymerization of CO2 with epoxides has been studied as an environmentally friendly route to convert CO2 into biodegradable polymers. The first successful catalytic system to achieve these results consisted of a diethyl-zinc complex. More recently, aluminum, chromium, cadmium and cobalt have been studied as polycarbonate catalysts. To date, the only reported chromium catalysts for CO2-epoxide copolymerization are Cr-salen and Cr-porphyrin complexes, studied by Darrensbourg and Holmes, respectively. We were particularly interested in finding new chromium-based complexes able to catalyze epoxide/CO2 copolymerization by using molecules with the nitrogen donor motif embedded in different functions such as neutral pyridines with anionic pendants, pyrroles with either imine or amine pendants, or a combination of these.

This thesis describes the application of (arene)Cr(CO)3 methodology to the Stereoselective and enantioselective synthesis of substituted arenes. Chapter one reviews the main methods of preparation and decomplexation of (arene)Cr(CO)₃ complexes and the electronic and steric influences of the Cr(CO)₃ unit on the arene. Chapter two demonstrates that the benzylic oxygen directing effect in complexation reactions operates via a direct oxygen bond to the incoming metal unit. Attachment of bulky ft-acceptor groups, such as t-butyldimethylsilyl, to the benzylic oxygen overrides this directing effect. Chapter three describes the regioselective Cl functionalisation of the cryptopine skeleton. Complexation of dihydrocrytopine gives only a single product, the relative configuration of the product being determined using an X-ray crystal structure analysis. Subsequent alkylation of the O-methyl derivative gives Cl alkylated products. Chapter four describes the regioselective ortho functionalisation of ephedrine and pseudoephedrine derivatives. Treatment of (1S,2R)-(N,O-dimethylpseudoephedrine) Cr(CO)₃ with n-butyllithium leads to exclusive removal of the pro-(R) ortho proton. The observed stereoselectivity arises via</em deprotonation from cyclic bidentate five-membered chelates. Chapter five describes the regioselective C4 and C5 functionalisation of (hydrocotarnine)Cr(CO)₃. Complexation of 1-methylhydrocotarnine occurs to give exclusively the exo-1-methyl derivative. Further functionalisation to give the 1,5- and 4,5-dimethyl products is also described. Chapter six describes the synthesis of ortho substituted (benzaldehyde)Cr(CO)₃ complexes. Chiral material is available via preferential kinetic hydrolysis of, or classical separation of, the L-valinol derived imines. Chapter seven describes the Stereoselective addition of nucleophiles to (o-anisaldehyde) Cr(CO)₃ and (o-trialkylsilylbenzaldehyde)Cr(CO)3. With (o-anisaldehyde)- Cr(CO)₃ the additions are completely stereoselective giving the (RR,SS) diastereoisomer. With (o-trialkylsilylbenzaldehyde)Cr(CO)₃ the ratio of products is influenced by the nature of Lewis acidic species present. Chapter eight describes the Stereoselective benzylic elaboration of (o-methoxybenzyl methyl ether)Cr(CO)₃ achieved via selective removal of the exo benzylic proton from transition states with the methoxy groups anti to each other.

Thesis (Ph. D.)--University of Kentucky, 2007. / Title from document title page (viewed on March 25, 2008). Document formatted into pages; contains: xii, 216 p. : ill. (some col.). Includes abstract and vita. Includes bibliographical references (p. 171-210).

Thesis (Ph. D.)--University of Oregon, 2004. / On t.p. "m̲" and "n̲" are subscript. Typescript. Includes vita and abstract. Includes bibliographical references (leaves 147-153). Also available for download via the World Wide Web; free to University of Oregon users.