Síntese de cristais líquidos termotrópicos derivados da fenilpiperzina

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas. Programa de Pós-Graduação em QuímicaOs cristais líquidos termotrópicos são de grande importância tecnológica. Diversas pesquisas estão voltadas para o desenvolvimento de compostos estáveis e de temperaturas de transição próxima da temperatura ambiente. Neste trabalho tem-se interesse na síntese de novas unidades precursoras de cristais líquidos termotrópicos que contenham as unidades fenilpiperazinas e/ou fenilacetilenos. Vários homólogos de uma série de bases de Schiff e de uma série de eninos foram sintetizados permitindo a comparação entre as suas propriedades mesogênicas e termodinâmicas. As bases de Schiff apresentaram comportamento mesomórfico esmetogênico, A e 1, e os eninos sintetizados, um dimorfismo esmético e/ou nemático. A metodologia sintética utilizada mostrou-se eficiente para a síntese dos intermediários desejados, p-[(N-carboxietilpiperazinil)fenil]propargil aldeído e p-[(N-decilpiperazinil)fenil]propargil aldeído. Pode-se constatar que os precursores são adequados para o desenvolvimento de mesomorfismo. Trabalhos futuros visam a utilização destes precursores na síntese de cristais líquidos termotrópicos que possuam temperaturas de fusão e de transição mais baixas para adequá-los à aplicações tecnológicas

As mulheres na Escola de Educação Física da UFRGS : visibilidades conquistadas

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Synthesis, structure, magnetic properties and kinetics of formation of a cluster containing a {Cu₃(μ₃-OH)} core supported by a triazole-based ligand

<p>The trinuclear copper complex, [Cu₃(μ₃-OH)(CTMB)₃(NO₃)₂(CH₃CN)₂]·5CH₃CN·H₂O (<b>1</b>) {CTMB = cyclohexotriazole-3-(4-methoxybenzamide)}, has been prepared by mixing Cu(NO₃)₂·2.5H₂O and CHMBH {CHMBH = N,N′-cyclohexane-1,2-diylidene-bis(4-methoxybenzoylhydrazide)} in acetonitrile under ambient conditions. Compound <b>1</b> was characterized by IR and UV–visible spectroscopies as well as elemental analyses. X-ray crystallography shows that the cluster contains a {Cu₃(μ₃-OH)} core supported by three triazole-based Schiff base ligands. Each Cu is bound to the 2-N of one triazole ring and the 1-N of another. However, the coordination sphere of each Cu is different, one is five-coordinate and the other two are six-coordinate and bridged by a NO₃ group. The six-coordinate sites are different, one has a terminal NO₃ and the other a MeCN ligand. Magnetic measurements revealed the presence of isotropic and antisymmetric exchange between the copper(II) centers. The data were analyzed using the Hamiltonian containing isotropic exchange for an isosceles triangle together with antisymmetric exchange: <i>H</i> = –<i>J</i>₁(<i>S</i>₁<i>S</i>₂ + <i>S</i>₂<i>S</i>₃)−<i>J</i>₂<i>S</i>₁<i>S</i>₃ + <i>G</i>[<i>S</i>₁ × <i>S</i>₂ + <i>S</i>₂ × <i>S</i>₃ + <i>S</i>₃ × <i>S</i>₁]. Compound <b>1</b> exhibits strong antiferromagnetic coupling with <i>J</i>₁ = −180 and <i>J</i>₂ = −118 cm⁻¹ and antisymmetric exchange with <i>G</i>_z = 15 cm⁻¹. Stopped flow spectrophotometric studies show that the formation of <b>1</b> occurs in three distinct phases and the kinetics of each phase has been determined.</p

Sub-micron level investigation reveals the inaccessibility of stabilized carbon in soil microaggregates

Abstract Direct evidence-based approaches are vital to evaluating newly proposed theories on the persistence of soil organic carbon and establishing the contributions of abiotic and biotic controls. Our primary goal was to directly identify the mechanisms of organic carbon stabilization in native-state, free soil microaggregates without disrupting the aggregate microstructure using scanning transmission x-ray microscopy coupled with near edge x-ray absorption fine structure spectroscopy (STXM-NEXAFS). The influence of soil management practices on microaggregate associated-carbon was also assessed. Free, stable soil microaggregates were collected from a tropical agro-ecosystem in Cruz Alta, Brazil. The long-term experimental plots (>25 years) comparing two tillage systems: no-till and till with a complex crop rotation. Based on simultaneously collected multi-elemental associations and speciation, STXM-NEXAFS successfully provided submicron level information on organo-mineral associations. Simple organic carbon sources were found preserved within microaggregates; some still possessing original morphology, suggesting that their stabilization was not entirely governed by the substrate chemistry. Bulk analysis showed higher and younger organic carbon in microaggregates from no-till systems than tilled systems. These results provide direct submicron level evidence that the surrounding environment is involved in stabilizing organic carbon, thus favoring newly proposed concepts on the persistence of soil organic carbon