La préparation de y-lactols et méthylène-y-lactols a été réalisée via un processus monotope, alternative efficace à la réaction de Ueno-Stork, qui implique la cyclisation radicalaire d’acétal aluminiques alpha-halogéné à basse température. La réaction d’oxydation in situ des espèces aluminiques obtenues après cyclisation de type 5-exo-dig permet de conduire à des buténolides énantio-enrichis polysubstitués dans des conditions très douces. Cette approche a été comparée avec des conditions de cyclisation classiques d’esters alphahalogénés à plus haute température. Des expériences de marquage au deutérium ont permis de mettre en évidence le transfert 1-5 d’atome d’hydrogène (1,5- HAT) qui se produisent à haute température et les conséquences de celle-ci dans l’épimérisation de centres stéréogènes. Une étude mécanistique combinant des analyses RMN 13C-Inept DOSY à basse température et des calculs DFT nous a permis de proposer une structure pour les espèces aluminiques impliquées et de réaliser une étude de la diastéréosélectivité des réactions de cyclisation 5-exo-trig et 5-exo-dig. Cette étude in silico nous ont permis d’envisager un contrôle du centre C3 lors de la cyclisation grâce à un transfert de chiralité du centre acétalique, formé de façon stéréosélective par réduction d’esters alpha-halogénés énantiomériquement enrichis. Ces prédictions ont été validées à travers la synthèse de diols 1-4 et dans l’élaboration de motifs tétrahydrofuranes tétrasubstitués optiquement enrichis. / The preparation of y-lactols and methylene-y-lactols was achieved via a one-pot procedure, which represents a useful alternative to the classical Ueno-Stork reaction and involves the radical cyclisation of alphahaloaluminium acetals at low temperature. The oxidation in situ of the cyclic aluminium acetal species resulting from a 5-exo-dig process led to enantioenriched butenolides under mild reaction conditions. This approach compared favorably with the classical reaction conditions for the cyclisation of alphabromoesters at high temperature. Deuterium-labelling experiments allowed us to highlight 1,5-hydrogen atom transfers that occur at thigh temperature and are responsible for epimerization reactions. A mechanistic study combining 13C-Inept DOSY NMR analyses at low temperature and DFT calculations allowed us to propose a structure for the aluminium acetal species involved and to conduct a systematic study of the diastereoselectivity of the 5-exo-trig and 5- exo-dig radical cyclisation. Thanks to this study in silico, the control of stereocentre at C3 could be envisaged via a chirality transfer from the aluminium acetal centre, obtained in a diastereoselective reduction of enantioenriched bromoesters. This prediction proved to be correct and was illustrated through the stereoselective synthesis of diols 1-4 and tetrasubstituted tetrahydrofurans.

No description available.

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The topic of this thesis is the hat problem. In this problem, a team of n players enters a room, and a blue or red hat is randomly placed on the head of each player. Every player can see the hats of all of the other players but not his own. Then each player must simultaneously guess the color of his own hat or pass. The team wins if at least one player guesses his hat color correctly and no one guesses his hat color wrong, otherwise the team loses. The aim is to maximize the probability of winning. This thesis is based on publications, which form the second chapter. In the first chapter we give an overview of the published results. In Section 1.1 we introduce to the hat problem and the hat problem on a graph, where vertices correspond to players, and a player can see the adjacent players. To the hat problem on a graph we devote the next few sections. First, we give some fundamental theorems about the problem. Then we solve the hat problem on trees, cycles, and unicyclic graphs. Next we consider the hat problem on graphs with a universal vertex. We also investigate the problem on graphs with a neighborhood-dominated vertex. In addition, we consider the hat problem on disconnected graphs. Next we investigate the problem on graphs such that the only known information are degrees of vertices. We also present Nordhaus-Gaddum type inequalities for the hat problem on a graph. In Section 1.6 we investigate the hat problem on directed graphs. The topic of Section 1.7 is the generalized hat problem with q >= 2 colors. A modified hat problem is considered in Section 1.8. In this problem there are n >= 3 players and two colors. The players do not have to guess their hat colors simultaneously and we modify the way of making a guess. We give an optimal strategy for this problem which guarantees the win. Applications of the hat problem and its connections to different areas of science are presented in Section 1.9. We also give there a comprehensive list of variations of the hat problem considered in the literature.

Evaporative gas turbine cycles (EvGT) show an exceptionalexhaust heat recovery potential, which makes them a strongcompetitor to other advanced gas turbine cycles, especiallyfrom small to intermediate sizes. Evaporative gas turbines aredistinguished by humidifying the working fluid beforecombustion at temperatures below the boiling point of water;and the heat required for evaporation of water is partly takenout of the exhaust gas. Thus, humidification is a key operationin these cycles. This thesis investigates, both theoreticallyand experimentally, two alternative approaches tohumidification: the packed-bed humidification tower and thetubular humidifier. Both these equipments involvecountercurrent contact between water and the working fluid.Humidifier design criteria are developed and criticalparameters such as flooding, wetting rate and entrainment arediscussed. The experimental parts were carried out on thepacked-bed tower in the EvGT pilot plant, and on a tubularhumidifier test rig especially erected for this purpose. Thetheoretical models were confirmed by the experiments. The height of a transfer unit, necessary for designingpacked beds, was calculated for the packing employed in theEvGT pilot plant. It was found that the data provided by themanufacturer may be used with minor corrections. The tubular test rig operated satisfactorily delivering hothumid air. The theoretical models coincided well with theexperimental results, verifying the design criteria developedhere. The heat transfer calculations indicated that mostresistance to heat transfer is on the exhaust gas side. Thus, asurface extended tube (Sunrod) was used in the test rig. Itcould be concluded that the tubular humidifier is a strongalternative to the packed-bedtower, especially in smallhigh-pressure gas turbines. Furthermore, the importance of the non-ideality of theair-water vapor mixture in modeling evaporative cycles wasfirst highlighted in this work. Through applying realthermodynamic properties of air-water vapor mixtures in cyclecalculations, it was found that the compressed air contains ahigher amount of moisture than indicated by the ideal gasmixture model. This affects the design of the heat recoverysystem and cannot be neglected. <b>Key words:</b>evaporative gas turbine, indirect-fired gasturbine, humidification, packed bed, tubular humidifier,evaporator, saturator.

<p>Evaporative gas turbine cycles (EvGT) show an exceptionalexhaust heat recovery potential, which makes them a strongcompetitor to other advanced gas turbine cycles, especiallyfrom small to intermediate sizes. Evaporative gas turbines aredistinguished by humidifying the working fluid beforecombustion at temperatures below the boiling point of water;and the heat required for evaporation of water is partly takenout of the exhaust gas. Thus, humidification is a key operationin these cycles. This thesis investigates, both theoreticallyand experimentally, two alternative approaches tohumidification: the packed-bed humidification tower and thetubular humidifier. Both these equipments involvecountercurrent contact between water and the working fluid.Humidifier design criteria are developed and criticalparameters such as flooding, wetting rate and entrainment arediscussed. The experimental parts were carried out on thepacked-bed tower in the EvGT pilot plant, and on a tubularhumidifier test rig especially erected for this purpose. Thetheoretical models were confirmed by the experiments.</p><p>The height of a transfer unit, necessary for designingpacked beds, was calculated for the packing employed in theEvGT pilot plant. It was found that the data provided by themanufacturer may be used with minor corrections.</p><p>The tubular test rig operated satisfactorily delivering hothumid air. The theoretical models coincided well with theexperimental results, verifying the design criteria developedhere. The heat transfer calculations indicated that mostresistance to heat transfer is on the exhaust gas side. Thus, asurface extended tube (Sunrod) was used in the test rig. Itcould be concluded that the tubular humidifier is a strongalternative to the packed-bedtower, especially in smallhigh-pressure gas turbines.</p><p>Furthermore, the importance of the non-ideality of theair-water vapor mixture in modeling evaporative cycles wasfirst highlighted in this work. Through applying realthermodynamic properties of air-water vapor mixtures in cyclecalculations, it was found that the compressed air contains ahigher amount of moisture than indicated by the ideal gasmixture model. This affects the design of the heat recoverysystem and cannot be neglected.</p><p><b>Key words:</b>evaporative gas turbine, indirect-fired gasturbine, humidification, packed bed, tubular humidifier,evaporator, saturator.</p>

No description available.

Numerous small linear moraines occur in Upper Hat Creek valley, British Columbia. The ridges lie transverse to the axis of the valley. They are, upon average, 1000 feet (305 m.) long, 8 feet (2.4 m.) high, and spaced 130 feet (40 m.) apart. Their profile is asymmetrical, with a steeper proximal slope. The alignment of a-axes of pebbles in the till is transverse to the moraine crest, regardless of the orientation of the crest with respect to the direction of regional ice movement. The dip of the pebbles is in the direction of the surface slope, however, it is less than the slope on the proximal side and greater than the slope on the distal side. The Hat Creek moraines form an extensive system of push moraines, deposited during the retreat of the last ice sheet. During the winter, a slight readvance of the glacier pushed the ablation moraine of the previous summer into a ridge along the ice front. The ridge was not overridden by the ice. Therefore, the asymmetry of a simple push moraine was maintained. During the subsequent ablation season the moraine was isolated from the retreating ice margin. The next winter's readvance rarely extended as far as the previous ridge, thereby preserving the moraines. / Arts, Faculty of / Geography, Department of / Graduate

This dissertation represents a study on Ceratocystis spp. with hat-shaped ascospores. Previously only six species of Ceratocystis with this spore form were known. These include C. fimbriata, C. pirilliformis, C. albofundus, C. moniliformis, C. moniliformopsis and C. acericola. In this study, we have discovered and tentatively described three new species with hat-shaped ascospores. One of these group with the larger C. fimbriata clade, while the other two reside within the larger C. coerulescens clade. Chapter one provides a concise summary of the literature pertaining to the genus Ceratocystis. The intention of the chapter is to introduce readers to this important genus of plant pathogens and to provide a background regarding their taxonomy, ecology, biochemistry and variation in species. Emphasis is placed on the distinction of Ceratocystis from Ophiostoma, as ell as on those Ceratocystis spp. with hat-shaped ascospores that negatively impact upon plantation forestry species. This chapter shows how Ceratocystis spp. associated with hardwood species in commercial forestry plantations have increased in number and it provides the background for research presented in the following five chapters. In the second chapter of this dissertation, a new species of Ceratocystis was discovered amongst isolates from the Himalayan mountain range of Bhutan. This fungus, in association with the bark beetle Ips schmutzenhoferi, is responsible for large-scale deaths of Himalayan spruce trees in Bhutan. The fungus is morphologically very similar to C. moniliformis and C. moniliformopsis, but differences in culture morphology, survival at different incubation temperatures and DNA sequence data based on three different gene regions supported the fact that this is a unique species. The fungus has thus been tentatively described as Ceratocystis bhutanensis prov. nom. In chapter 3, I consider isolates of a Ceratocystis sp. recently discovered associated with dying clove trees in Northern Sulawesi, Indonesia. The fungus was found at a very high level of incidence, but was at first identified as C. fimbriata, based on morphological characteristics. Differences were observed in cultures of this fungus when they were compared with C. fimbriata especially in terms of colony colour and growth at different temperatures. Morphological differences were also observed when the clove fungus was compared with C. fimbriata isolates. When three different DNA gene regions were sequenced and compared, it was clear that this fungus represents a new species. The fungus is, therefore, tentatively described as Ceratocystis polychrome prov. nom. in this dissertation. Ceratocystis polychrome prov. nom. Isolates obtained from cloves in Sulawesi displayed three distinctly different culture morphologies. In Chapter 4 of this dissertation we used DNA sequence data and microsatellite markers to consider whether these differences could be observed at the molecular level. Comparisons of sequence data for the ITS region gave no distinction between any of the morphological groups. A total of 50 isolates were studies using microsatellites markers developed for C. fimbriata. No distinction could be obtained between isolates representing the three different culture morphological groups. The 50 isolates were subsequently treated as one population in further analyses. With the aid of the microsatellite markers, it was shown that this population probably originated from Sulawesi and that it benefits form sexual outcrossing. In chapter five of this dissertation, a study was undertaken to consider the taxonomic status of C. moniliformis. Consideratble variation has been noted in different descriptions of this species. It also has a very wide host and geographic distribution raising speculation that C. moniliformis represents a species complex rather than a single taxon. Based on morphological and DNA sequence data from three gene regions, isolated from Sumatra were described as a new species, which we have tentatively named C. tribiliformis prov. nom. The other C. moniliformis isolates were all the same, despite the fact that they originated from a wide range of hosts and areas. The fungus correctly bearing the name C. moniliformis, C. moniliformis sensu stricto, therefore does not seem to tepresent a species complex. Species such as the closely related C. tribiliformis prov. nom., C bhutanensis prov. nom., C. omanensis prov. nom. and C. moniliformopsis all belong to the larger C. moniliformis sensu lato group, and all have hat-shaped ascospores, conical spines on the ascomatal bases, disc-shaped bases to the ascomatal necks and are phylogenetically closely related to C. moniliformis. Studies presented in this dissertation provide considerable new knowledge regarding various Ceratocystis spp. with hat-shaped ascospores. Three new species are described and I have also been able to show that C. moniliformis sensu stricto is monophyletic. Two of the species (C. tribiliformis prov. nom. and C. bhutanensis nom. prov.) group within the larger C. coerulescens clade while C. polychrome prov. nom. groups within the larger C. fimbriata clade. Studies in this dissertation have also improved our knowledge of the identity of several species previously incorrectly identified as either C. moniliformis or C. fimbriata. What has clearly emerged from this dissertation is the need for a monograph of Ceratocystis to include all new species and to thoroughly consider the population biology and ecology of all species. / Dissertation (MSc)--University of Pretoria, 2008. / Microbiology and Plant Pathology / MSc / Unrestricted

The field of cybersecurity is relatively uncharted by rhetoricians and sociologists but nevertheless laden with terminological assumptions, violent metaphors, and ethical conflicts. This study explores the discourse surrounding the morally contentious practice of hackers selling software vulnerabilities to third parties instead of disclosing them to the affected technology companies. Drawing on grounded theory, I utilize a combination of quantitative word-level analysis and qualitative coding to assess how notions of right and wrong on this topic are framed by three groups: 1) the hackers themselves, 2) technology companies, and 3) reporters. The results show that the most commonly constructed argument was based on a "greater good" ethic, in which rhetors argue for reducing risk to "us all" or to innocent computer users. Additionally, the technology companies and hackers assiduously build their ethos to increase their trustworthiness in the public mind. Ultimately, studying this unexplored area of "gray hat hacking" has important implications for policymakers creating new cybersecurity legislation, reporters attempting to accurately frame the debate, and information technology professionals whose livelihoods are affected by evolving social norms. / Master of Arts