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This works examines the question whether a vapour-liquid phase transition exists in systems of particles with purely dipolar interactions, a topic which has been the subject of a longstanding debate. Monte Carlo simulation results for two modi operandi to tackle this issue are presented. One approach examines the phase behaviour of fluids of charged hard dumbbells (CHD), each made up of two oppositely charged hard spheres with diameters σ and separation d. In the limit d/σ → 0, and with the temperature scaled accordingly, the system corresponds to dipolar hard spheres (DHS) while for larger values of d ionic interactions are dominant. The crossover between ionic and dipolar regimes is examined and a linear variation of the critical temperature T*c in dipolar reduced units as a function of d is observed, giving rise to an extrapolated T*cDHS ≈ 0:15. The second approach focuses on the dipolar Yukawa hard sphere (DYHS)fluid, which is given by a dipolar hard sphere and an attractive isotropic interaction Y of the Yukawa tail form. In this case, the DHS limit is obtained for Y → 0. It is found that T*c depends linearly on the isotropic interaction strength Y over a wide range, coinciding with the results for the CHD model and extrapolating to a similar value of T*c;DHS. However, with the use of specially adapted biased Monte Carlo techniques which are highly efficient, it is shown that the linear variation of T*c is violated for very small values of the Yukawa interaction strength, almost two orders of magnitude smaller than the characteristic dipolar interaction energy. It is found that phase separation is not observable beyond a critical value of the Yukawa energy parameter, even though in thermodynamic and structural terms, the DYHS and DHS systems are very similar. It is suggested that either some very subtle physics distinguishes the DYHS and DHS systems, or the observation of a phase transition in DHSs is precluded by finite-size effects. In the context of phase separation in highly correlated fluids, new flat-histogram Monte Carlo simulation techniques based on the Wang-Landau algorithm are evaluated and shown to be useful tools. This work presents a general and unifying framework for deriving Monte Carlo acceptance rules which facilitate flat histogram sampling. The framework yields uniform sampling rules for thermodynamic states given either by the mechanically extensive variables appearing in the Hamiltonian or, equivalently, uniformly sample the thermodynamic fields which are conjugate to these mechanical variables.

Realizamos a caracterização elétrica de alexandrita (BeAl2O4:Cr3+), formas sintética e natural, através de medidas de Corrente de Despolarização Termicamente Estimulada (CDTE). Obtivemos evidências conclusivas do fenômeno de relaxação dipolar em ambos os tipos de amostra, e que as cwas experimentais devem ser ajustadas por urna distribuição contínua dos parârnetros de relaxação. Para a amostra sintética a banda de CDTE está centralizada em tomo de 179K e para as naturais em 187 a 195K. Utilizando o método de Havriliak-Negarni são necessárias duas distribuições continuas de Ea e &#9640 para ajustar as curvas experimentais, sendo que uma delas, em torno de 177 K, com Ea &#8773 0,56 e &#9640 &#8773 1,2x10-14s sente em ambos os tipos de amostras. As bandas de CDTE são atribuídas a dipolos do tipo impureza-vacância de oxigênio ou a deformação local da estrutura causada pela diferença de raio iônico entre os íons Cr3+ (0,615 &#197) e A13+(0,535 &#197). Também realizamos medidas de CDTE fotoinduzidas, onde as amostras são irradiadas com um laser sintonizado em comprimentos de onda entre 3373 e 676,4nm. Verificamos que as bandas de CDTE podem ser \"destruídas\" ou \"criadas\" com a incidência de luz com diferentes condições iniciais de polarização. Para ajudar a interpretação dos resultados de CDTE nós usamos outras técnicas de caracterização, tais como Absorção Óptica, Luminescência, Difração de Raios X e micro análises de EDX e WDX. Todas estas técnicas foram também aplicadas às amostras naturais após tratamentos térmicos consecutivos / We have done electrical characterization of natural and synthet ic alexandrite (BeAl2O4:Cr3+), usimg the thermally stimulated depolarization current (TSDC) technique. We have obtained conclusive evidences of dipole relaxation in both kinds of samples. Besides, the experimental data must be fitted by a continuous distribution of relaxation parameters. For the synthetic sample, TSDC band has a peak about 179K and for natural samples, TSDC bands have peaks about 187K at 195K. Using Havriliak-Negami method, we need two continuous distributions of activation energy (Ea) and relaxation time constant (&#9640) to fit experimental data. One of these two curves, centered at 177K, is present for both kinds of sarnples and has Ea &#8773 0,56 e &#9640 &#8773 1,2x10-14s. T SDC bands are attributed to impurity-oxygen vacancy dipoles or local structure deforrnation caused by the dserence between ionic radius of Cr3+ (0,615 &#197) and A13+(0,535 &#197) ions. We have also carried out photo-induced TSDC, where sarnples are irradiated with a tunable laser with wavelength fiom 337.5nm to 676.5nm. We have observed that TSDC bands rnay be destroyed or created with illumination fiom daerent polarization conditions. To help the interpretation of TSDC results we have used other techniques of characterization such as optical absorption, luminescence, X-ray difliaction, besides EDX and WDX rnicroanalyses. All of these techniques were also applied to natural samples afier consecutive annealing

Realizamos a caracterização elétrica de alexandrita (BeAl2O4:Cr3+), formas sintética e natural, através de medidas de Corrente de Despolarização Termicamente Estimulada (CDTE). Obtivemos evidências conclusivas do fenômeno de relaxação dipolar em ambos os tipos de amostra, e que as cwas experimentais devem ser ajustadas por urna distribuição contínua dos parârnetros de relaxação. Para a amostra sintética a banda de CDTE está centralizada em tomo de 179K e para as naturais em 187 a 195K. Utilizando o método de Havriliak-Negarni são necessárias duas distribuições continuas de Ea e &#9640 para ajustar as curvas experimentais, sendo que uma delas, em torno de 177 K, com Ea &#8773 0,56 e &#9640 &#8773 1,2x10-14s sente em ambos os tipos de amostras. As bandas de CDTE são atribuídas a dipolos do tipo impureza-vacância de oxigênio ou a deformação local da estrutura causada pela diferença de raio iônico entre os íons Cr3+ (0,615 &#197) e A13+(0,535 &#197). Também realizamos medidas de CDTE fotoinduzidas, onde as amostras são irradiadas com um laser sintonizado em comprimentos de onda entre 3373 e 676,4nm. Verificamos que as bandas de CDTE podem ser \"destruídas\" ou \"criadas\" com a incidência de luz com diferentes condições iniciais de polarização. Para ajudar a interpretação dos resultados de CDTE nós usamos outras técnicas de caracterização, tais como Absorção Óptica, Luminescência, Difração de Raios X e micro análises de EDX e WDX. Todas estas técnicas foram também aplicadas às amostras naturais após tratamentos térmicos consecutivos / We have done electrical characterization of natural and synthet ic alexandrite (BeAl2O4:Cr3+), usimg the thermally stimulated depolarization current (TSDC) technique. We have obtained conclusive evidences of dipole relaxation in both kinds of samples. Besides, the experimental data must be fitted by a continuous distribution of relaxation parameters. For the synthetic sample, TSDC band has a peak about 179K and for natural samples, TSDC bands have peaks about 187K at 195K. Using Havriliak-Negami method, we need two continuous distributions of activation energy (Ea) and relaxation time constant (&#9640) to fit experimental data. One of these two curves, centered at 177K, is present for both kinds of sarnples and has Ea &#8773 0,56 e &#9640 &#8773 1,2x10-14s. T SDC bands are attributed to impurity-oxygen vacancy dipoles or local structure deforrnation caused by the dserence between ionic radius of Cr3+ (0,615 &#197) and A13+(0,535 &#197) ions. We have also carried out photo-induced TSDC, where sarnples are irradiated with a tunable laser with wavelength fiom 337.5nm to 676.5nm. We have observed that TSDC bands rnay be destroyed or created with illumination fiom daerent polarization conditions. To help the interpretation of TSDC results we have used other techniques of characterization such as optical absorption, luminescence, X-ray difliaction, besides EDX and WDX rnicroanalyses. All of these techniques were also applied to natural samples afier consecutive annealing

No description available.

No description available.

This dissertation describes the development of a novel ring fragmentation reaction in which cyclic γ-silyloxy-β-hydroxy-α-diazoesters undergo efficient rupture of the Cβ−Cγ bond when treated with tin tetrachloride to provide tethered aldehyde ynoate products with varying tether length, in high yield. Tethered aldehyde ynoates are versatile intermediates and this functional group combination is unique to this fragmentation. The synthetic utility of tethered aldehyde ynoates is highlighted by their predisposition to undergo facile intramolecular [1,3]- dipolar cycloaddition reactions in presence of bistrimethylsylil-α-amino acids to provide 2,5-dihydropyroles. The ability to quickly assemble such structural motifs encouraged studies on the synthesis of steroidal alkaloid solanidine. A new method for the N-amination of oxazolidiones with NH2Cl was investigated and provides efficient access to acyl hydrazines. However, the full scope of the reaction has not been yet fully explored. The study of isopropyl radical additions to N-acylhydrazones and comparison of several stereocontrol elements on the oxazolidinone moiety revealed that all are effective.

Grâce à ses propriétés uniques, le groupement pentafluorosulfanyle (SF5) a récemment retenu beaucoup d'attention. Ce substituant encombrant, hautement lipophile et électro-attracteur est souvent considéré comme un "super groupement trifluorométhyl". Aujourd’hui, la chimie de groupement SF5 est un domaine de la chimie du fluor qui se développe rapidement, et les méthodologies de synthèse des nouveaux « building-blocks » pentafluorosulfanylés sont constamment développées. Néanmoins, les méthodes permettant l'introduction du groupement SF5 dans les hétérocycles restent encore assez limitées. L’objectif de cette thèse était de synthétiser des nouveaux « building-blocks » pentafluorosulfanylés, d’une part, et de développer des nouvelles approches synthétiques, conduisant à des hétérocycles azotés originaux, substitués par un groupement SF5, d’autre part. Dans la première partie de notre travail, nous avons développé une synthèse en 4 étapes, conduisant à de nouveaux esters et amides acryliques substitués par un groupement SF5, à partir des dérivés commerciaux de l’acetate allylique. En utilisant la même stratégie, quelques nouveaux allylsilanes pentafluorosulfanylés ont été synthétisés. Nous avons aussi développé une réaction d’allylation des aldéhydes, catalysée par indium, en utilisant le bromure SF5-allylique, conduisant aux alcools alpha- et gamma-homoallyliques correspondants.Dans la deuxième partie, deux stratégies différentes pour la synthèse de nouveaux hétérocycles SF5-substitués, ont été explorées. La première était la fluoration directe des thiols ou disulfures hétérocycliques, suivant la procédure de Umemoto. La deuxième approche était basée sur les réactions de cycloaddition de nos "building-blocks" pentafluorosulfanylés. Ainsi, nous avons développé avec succès la synthèse des premières pyrrolidines et isoxazolidines pentafluorosulfanylées. Enfin, des post-fonctionnalisations des SF5-pyrrolidines ont été effectuées, conduisant aux plusieurs "plateformes" pentafluorosulfanylées qui pourraient être facilement incorporées dans les molécules d’intérêt biologique. / Due to its unique properties, the pentafluorosulfanyl (SF5) group has recently reached much attention. This bulky, highly lipophilic and electron-withdrawing substituent is often called as a “super-trifluoromethyl group”. Currently, the SF5-chemistry is becoming a fast growing field of the fluorine chemistry and the new pentafluorosulfanylated building-blocks are under continuous development. Nevertheless, the methods for the introduction of the SF5-group into heterocycles remain still quite limited.The aim of this thesis was to synthesize new pentafluorosulfanylated building-blocks, on the one hand, and to develop new synthetic approaches leading to original pentafluorosulfanylated nitrogen-containing heterocyles, on the other hand.In the first part of our work, we have developed an efficient four-step synthesis of new SF5-acrylic esters and amides, starting from commercially available allyl acetate derivatives. Using the same strategy, several new pentafluorosulfanylated allylsilanes were synthesized. We have also developed an In-mediated allylation reaction of aldehydes, using the SF5-allyl bromide, leading to the corresponding pentafluorosulfanylated alpha- and gamma-homoallylic alcohols.In the second part, two different strategies for the synthesis of new SF5-substituted heterocycles were explored. The first one was the direct fluorination of heterocyclic thiols or disulfides, according to the Umemoto’s procedure. The second approach was based on the cycloaddition reactions of our pentafluorosulfanylated building-blocks. As a result, we have successfully developed the synthesis of the first SF5-substituted pyrrolidines and isoxazolidines. Finally, post-functionalizations of SF5-pyrrolidines were performed, leading to several pentafluorosulfanylated scaffolds which could be easily introduce into molecules of biological interest.

A new synthetic method toward 5-substituted isoxazolidines by [3 plus 2] cycloaddition of nitrones generated from nitrosobenzene and styrene was discovered. The formation of nitrones from nitrosobenzene and mono-substituted aromatic styrenes was demonstrated. The cycloaddition reactions between styrenes and nitrosobenzenes work well when a moderate excess of styrenes was employed. The labeling studies support that cleavage of the styrene double bond occurred and accounted for all the carbons in the starting materials and products. A [3 plus 2] dipolar cycloaddition is implicated by the available mechanistic data and allows for the rapid assembly of various substituted isoxazolidines directly from nitrosobenzenes, electron deficient alkenes, and styrene.

This article reviews the properties and applications of a series of liquid crystal solvents for NMR spectroscopy of, particularly, biological compounds with emphasis placed on n-alkyl-poly(ethylene glycol)/n-alkyl alcohol, the most versatile liquid crystal system hitherto discovered. The applications of this solvent to mixtures, polymers and proteins are also mentioned.