1,720,974 research outputs found
Estudio de imanes moleculares de Co(ii): desde sistemas aislados hasta sistemas organizados y multifuncionales
No full textIn the discipline known as Molecular Magnetism, and following the disclosure of Mn12 in 1993, has awakened a great interest in molecules that shows slow relaxation effects of the magnetization below a blocking temperature (TB). These molecules are known as Single Molecule Magnets (SMMs). During the last decade, mononuclear complexes with a single slow-relaxing metal centre, the so-called Single Ion Magnets (SIMs), has been used as a new strategy to obtain metal complexes with larger energy barriers. These compounds have emerged as the simplest model systems for fundamental research on magnetic relaxation dynamics and tunnelling effects from both the experimental and theoretical points of view.
For all these reasons, in this Thesis, we have focused on the study and comprehension of CoII complexes that show slow magnetic relaxation behaviour, making the most of the inherent anisotropy of the high-spin CoII ion (S = 3/2). In addition, the CoII complexes have been designed with ligands that impose a high-distorted symmetry. Hence, these geometries can lead us to metal complexes with large axial-magnetic anisotropy values.
The results obtained in this work has been arranged in five chapters:
(i) The first chapter focus on the study of mononuclear CoII complexes with an octahedral symmetry. These compounds allow us to analyse how the electronic and structural changes affect the magnetic anisotropy parameters and, indirectly, the slow magnetic relaxation effects.
(ii) The second chapter deals with the analysis of CoII complexes with a lower coordination index, where there is a lesser effect of the ligand field. These geometrical parameters can be decisive in the axial-magnetic anisotropy. In this way, the magneto-structural study developed in the first chapter is concluded.
(iii) The third chapter expands a new strategy to synthesise materials that are able to act as chemical sensors or magnetic switches. To achieve this, we have used a couple of mononuclear complexes that modulate their coordination sphere by a reversible hydration/dehydration process, being only one of them capable of showing slow magnetic relaxation behaviour when the coordination environment is modulated.
(iv) In the fourth chapter, we have used the acquired knowledge of mononuclear compounds of chapter II to design polynuclear systems: dinuclear compounds with CoII weakly electronically connected by carboxylate ligands. The choice of the selected bridge, i.e. carboxylate-based ligand, exploit the possibility of keeping their SIM behaviour and being able to behave as a 2-qubits QG.
(v) The fifth and last chapter shows a new family of compounds that combines together two properties; the high porosity and the SIM behaviour, leading to a new SIM-MOFs family where the chemical nature of the hosted molecules in its pores modulates the SIM behaviour.Dentro de la disciplina conocida como Magnetismo Molecular, y a raíz del descubrimiento del Mn12 en 1993, ha surgido un gran interés por las moléculas que presentan una lenta relajación de la magnetización por debajo de una temperatura de bloqueo. A estas moléculas se les conoce como Imanes Unimoleculares (Single Molecule Magnets, SMMs). En la última década, la utilización de compuestos con un único centro metálico paramagnético (Single Ion Magnets, SIMs) se ha postulado como una de las nuevas estrategias utilizadas para la obtención de compuestos que presentan lenta relajación de la magnetización con barreras efectivas más elevadas. Además, desde un punto de vista experimental y teórico, los SIMs constituyen el modelo más simple para el estudio y comprensión de los fenómenos de lenta relajación de la magnetización y túnel cuántico.
Debido a todas estas razones, en esta Tesis Doctoral hemos focalizado nuestro interés en el estudio y la comprensión de, principalmente, compuestos mononucleares de CoII que presentan un comportamiento de molécula imán, aprovechando la inherente anisotropía magnética del ion CoII de espín alto (S = 3/2) y reforzándola con la utilización de ligandos que conduzcan a simetrías altamente distorsionadas. Así pues, estas geometrías podrán conducirnos a compuestos con grandes valores de la anisotropía magnética axial.
Los resultados obtenidos en este trabajo se han agrupado en cinco capítulos:
(i) Un primer capítulo que se centra en el estudio magneto-estructural de compuestos mononucleares de CoII en una simetría octaédrica. Estos compuestos nos permitirán analizar cómo afectan los cambios electrónicos y estructurales a los parámetros que determinan la anisotropía magnética e, indirectamente, al comportamiento de molécula imán.
(ii) Un segundo capítulo que se centra en el análisis de compuestos de CoII con un índice de coordinación más bajo, donde el efecto del campo de los ligandos es menor y puede ser determinante en la modulación de la anisotropía magnética axial. De ese modo completamos el estudio magneto-estructural desarrollado en el primer capítulo.
(iii) Un tercer capítulo donde se desarrolla una nueva estrategia de síntesis de materiales que pueden actuar como sensores químicos o “interruptores” magnéticos. En este capítulo se presenta una pareja de complejos de coordinación de CoII capaces de exhibir y modificar los efectos de lenta relajación de la magnetización al modularse su entorno de coordinación.
(iv) Un cuarto capítulo en el que, utilizando los conocimientos obtenidos en el estudio de los complejos mononucleares, se han diseñado y estudiado compuestos dinucleares de CoII débilmente conectados electrónicamente. Estos compuestos se encuentran unidos mediante un ligando puente de tipo carboxilato, manteniendo su comportamiento de SIM, y pudiendo comportarse como 2-qubits QG.
(v) Un quinto y último capítulo en el que se ha preparado y caracterizado una nueva familia de compuestos en la que se han conjugado dos propiedades; la alta porosidad y el comportamiento de SIM, dando lugar a una nueva familia de SIM-MOFs donde el comportamiento de SIM está modulado por la naturaleza del huésped alojado en sus poros
Materials for depollution based on the model of manganese dioxygenase
No full textLa utilización intensiva de pesticidas ha traído consigo la presencia de una gran cantidad de moléculas contaminantes en aguas y suelos. Los tratamientos para degradar estas moléculas son caros, por lo que es de gran interés la búsqueda de soluciones más baratas. En el caso de los catecoles, existen enzimas, las catecol-dioxigenasas, que abren el ciclo aromático por oxidación con dioxígeno. Las condiciones suaves y la gran disponibilidad de este oxidante podrían ofrecer una solución para esos problemas de polución.
El objetivo de este trabajo de tesis es mimetizar un tipo particular de dioxigenasa, la dioxigenasa de manganeso. Dado que tanto la primera esfera de coordinación como la segunda juegan un papel en la reacción enzimática, el modelo tiene que considerar estos dos aspectos.
El sitio activo se modeliza mediante un complejo de manganeso(II) que imita el entorno directo del metal en la enzima. La segunda esfera de coordinación se mimetiza mediante la incorporación de los complejos en sílices mesoporosas funcionalizadas. Éstas recrean el confinamiento presente en la enzima, mientras que los sustituyentes pueden estabilizar los intermedios de la misma manera que los amino ácidos enzimáticos.
Capitulo I
Este capitulo describe la síntesis y la caracterización de los complejos de manganeso(II). En primer lugar, se han diseñado y caracterizado los ligandos para que no haya interferencias entre la complejación con el ion metálico y el anclaje en la sílice. A continuación, se procedió a la síntesis de los complejos. Los dos primeros cristales obtenidos nos hicieron investigar en dos direcciones:
• La síntesis de complejos mononucleares, ya que éstos pueden ser mejores modelos del sitio activo que los dinucleares.
• La síntesis de otros complejos dinucleares con objeto de estudiar la interacción entre los dos iones manganeso(II) bajo diferentes ligandos puente o ligandos terminales.
En total, se han cristalizado cuatro complejos dinucleares, con cloruro, bromuro o azida. Los complejos con ligandos puente cloro presentan un comportamiento antiferromagnético a baja temperatura mientras que en el resto se observa acoplamiento ferromagnético. En el caso de los compuestos con ligando azida puente, se ha podido establecer, junto con otros valores del acoplamiento magnético J encontrados en la literatura, una relación entre este parámetro magnético y los parámetros estructurales.
El estudio EPR del solido y de una disolución de complejos dinucleares nos permitió concluir que los puentes se destruyen al disolver el complejo.
Capitulo II
En este capítulo se compara la síntesis de sílices mesoporosas LUS clásicas con las sintetizadas con microondas. Hemos optimizado la síntesis con microondas para obtener un material mesoporoso de calidad en un tiempo de síntesis más corto. El tiempo de síntesis ha sido reducido desde un día hasta un par de horas. Más aún, la caracterización de estos materiales ha demostrado que las propiedades mesoporosas se mantienen, es decir, que el material final posee poros de dimensión (3-4 nm) y volumen (0.7-0.9 cm3.g-1) comparables, así como superficies porosas similares (900-1000 m2.g-1). Se puede alcanzar una temperatura máxima de 190 °C en la síntesis con microondas, mientras que esta temperatura degrada el tensoactivo en la síntesis clásica. Además, el aumento de temperatura muestra diferentes comportamientos:
• En la síntesis clásica, los poros se agrandan con la temperatura, el volumen permanece constante y la superficie disminuye.
• En la síntesis con microondas, el volumen de poro y la superficie siguen la misma tendencia: una disminución entre 130 y 170 °C seguido por un aumento hasta 180 °C y estabilización a 190 °C. Por lo tanto, el aumento de temperatura no tiene el mismo efecto que para la síntesis clásica. Esa diferencia de comportamiento fue interpretada por efectos térmicos y por cambios en la homogeneidad de la calefacción.
Demostramos que la síntesis por microondas permite obtener sílices mesoporosas mucho más rápidamente y con una mejor homogeneidad del material. Los efectos de la agitación térmica son mucho menos importantes que los de la velocidad de condensación.
Capitulo III
En este último capítulo se detalla el diseño de materiales que soportarán los compuestos sintetizados en el primer capitulo. La sílice LUs mesoporosa sintetizada con microondas a 180 °C descrita en el capítulo II, se ha funcionalizado usando la estrategia de Molecular-Stencil Patterning (MSP). Esos materiales se dividen en dos familias: los que contienen una función de anclaje azida y una simple función espaciadora (trimetilsilano) y los que contienen una función de anclaje azida así como una función espaciadora coordinante (piridina). La difracción de rayos X en polvo nos permitió controlar la calidad de la estructura porosa.
La cicloadición de Huisgen catalizada con cobre(I) ha sido adaptada para el anclaje de los complejos en las sílices funcionalizadas. Se ha utilizado CuBr(PPh3)3 como catalizador para esta última etapa de anclaje, obteniéndose materiales con un contenido medio en manganeso del 2 % en peso. El estudio EPR ha mostrado que en el material final estaban presentes especies aisladas de manganeso(II) y la adsorción de N2 que los compuestos de manganeso(II) estaban dentro de los poros y que el volumen poroso restante es suficiente para permitir la catálisis dentro del sólido mesoporoso.
Las reacciones catalíticas se llevaron a cabo con complejos de manganeso(II) libres como catalizadores y con 4-ter-butilcatecol (4-TBC) así como 3,5-di-ter-butilcatecol (3,5-DTBC) como substratos. Los compuestos de manganeso(II) se comportan como oxidasas, al contrario que la actividad dioxigenasa del enzima: el substrato es oxidado desde alcohol hasta cetona sin incorporación de átomos de oxigeno del dioxígeno. Sin embargo, estos catalizadores son relativamente específicos comparados con otros encontrados en la literatura. La oxidación del 4-TBC con el complejo soportado produce la o-quinona sin otros subproductos como era el caso con los complejos libres.Intensive use of pesticides led to an excess of polluting molecules in water and soil. These molecules requires expensive treatments to be degraded, therefore cheaper alternative solutions are of interest. In the case of catechol derivatives, some enzymes called catechol dioxygenase perform an oxidative cleavage of the aromatic ring using O2 as oxidant. The mild conditions and the large availability of the oxidant make these enzymes a clever solution in such depollution issues.
This work aims at providing mimics of a particular kind of dioxygenase enzymes, the manganese-dependent dioxygenases. Given that both first and second coordination spheres plays an important role in enzymes, our model must consider these two aspects.
The active site is modelled by a manganese(II) complex that imitates the direct metal ion environment in the enzyme. The second coordination sphere is mimicked by incorporating these complexes inside a functionalized mesoporous silica. Then mesoporosity recreates the confinement present in the enzyme while side functions can play the role of the stabilizing enzymatic amino acids.
Chapter I
This chapter relates the synthesis and characterization of manganese(II) complexes. First, ligands were designed in such a way that complexation with metal ions and grafting on material would not interfere. Second, manganese(II) complexes were synthesized. The first two crystals we obtained with manganese(II) led us to investigate in two directions:
• The synthesis of mononuclear species, which would be closer to the actual enzymatic site.
• The synthesis of bridged dinuclear species, in order to study the evolution of the interaction between the two manganese(II) ions upon changes in the nature of the bridge or in the ligand.
In total, four dinuclear complexes were obtained with chloride, bromide or azide bridges. Chloride-bridged complex exhibited a weak antiferromagnetic behaviour at low temperature, whereas a ferromagnetic coupling was observed in the three others. Comparing the value of coupling J in the azido-bridged species with other similar species described in the literature allowed us to link this magnetic parameter with structural ones such as the Mn-X-Mn angle (where X is the bridging atom) and the Mn-Mn distance.
The EPR study in both solid and liquid states led us to conclude that although these complexes crystalize as a dinuclear unit, manganese centres do not interact anymore when dissolved.
Chapter II
This chapter compares two synthetic ways of LUS mesoporous silica with conventional and microwave ovens. Microwave synthesis is then optimized to provide mesoporous materials in a shorter time with a good porosity. The total synthesis time was reduced from one day to a couple of hours using the microwave irradiation. More important, characterizations of those materials showed that mesoporous properties are maintained through the process, i.e., the final materials possess similar pore sizes (between 3 and 4 nm), porous volumes (between 0.7 and 0.9 cm3.g-1) and specific surface areas (900 to 1000 m2.g-1).
A maximal temperature of 190 °C, impossible to reach without damaging the surfactant in classical syntheses, was reached with microwave oven. Furthermore, increasing the temperature in both protocols enlightened different behaviours:
• In the classical synthesis, the pores tend to enlarge upon such increase, the porous volume remains quasi-unchanged, and the specific surface area drops.
• In microwave synthesis, porous volume and surface area follow the same trend: a decrease between 130 °C and 170 °C followed by an increase until 180 °C and stabilization at 190 °C. Hence increasing temperature has not the same effect than for classical syntheses which was interpreted by both purely thermal and heating homogeneity effects.
We showed that microwave irradiation is a method that allows synthesis of mesoporous silica in much shorter time with a better homogeneity of the final material. The effects of thermal agitation become much less visible compared to the condensation speed ones.
Chapter III
This last chapter details the design of host materials to support the manganese(II) complexes synthesized in the first chapter. Mesoporous LUS silica synthesized at 180 °C using microwave irradiation described in chapter II were functionalized using the Molecular-Stencil Patterning (MSP) strategy. Host materials synthesized in this study were separated in two families, whether they contain a tethering azide function and a simple spacing function (trimethylsilyl) or a tethering azide function and a coordinating spacing function (pyridine). X-ray powder diffraction allowed controlling the integrity of the porous structure.
Huisgen copper-catalyzed cycloaddition was adapted to the grating of complexes into the pores of these silica hosts. Tris(triphenylphosphine) copper bromide was employed as catalyst for this ultimate grafting step, providing solids with an average manganese content of 2 wt%.. EPR study showed that isolated manganese(II) species were indeed present in the final material, and nitrogen sorption isotherm demonstrated that the complexes were inside the pores and that there was enough remaining porous volume to allow catalytic reactions inside the mesopores of the solid.
Catalytic reactions were then carried out using some free complexes as catalyst and 4-ter-butylcatechol (4-TBC) and 3,5-di-ter-butylcatechol (3,5-DTBC) as substrates. The complexes exhibit an oxidase activity, contrary to the dioxygenase one in the enzyme: the substrate is oxidized from alcohol to ketone without incorporation of oxygen atoms from dioxygen. However, these catalysts are thus relatively specific compared to other model catalysts described in literature. Oxidation of 4-TBC with supported complexes only provided the o-quinone. While the free catalysts also provided some by-products, this is no longer the case as supported complexes exclusively oxidize the catechol to its quinone form
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
- …
