Teses / dissertações sobre o tema "DFRT"

De nombreuses applications actuelles dans le secteur de la microélectronique reposent sur l’utilisation de matériaux piézoélectriques et ferroélectriques sous forme de films minces. Par exemple, des dispositifs MEMS incluant les capteurs, actionneurs et convertisseurs d'énergie tirent profit des propriétés piézoélectriques des matériaux. Par ailleurs, l'émergence de nombreux sujets de recherche a fait suite à la découverte du potentiel de certains de ces matériaux pour les dispositifs microélectroniques. C’est par exemple le cas de l'utilisation du HfO2 pour ses propriétés ferroélectriques dans les mémoires non volatiles FeRAM et FeFET. Dans ce contexte, diverses techniques de dépôt de films minces de matériaux piézoélectriques et ferroélectriques sont actuellement en cours d'optimisation. Les méthodes de caractérisation spécifiques à ces matériaux sont essentielles pour évaluer la qualité de leur élaboration et pour approfondir la compréhension des phénomènes physiques sous-jacents, ce qui est crucial pour leur intégration dans des dispositifs microélectroniques avancés.C’est dans ce cadre que s’inscrit cette thèse, dont l’objectif est de développer des techniques de caractérisation des propriétés piézoélectriques et ferroélectriques de dispositifs élaborés sous forme de films minces, à l’échelle nanométrique. La microscopie à force piézoélectrique permet de telles analyses, mais est sensible à de nombreux artefacts, tels que les effets électrostatiques, qui peuvent significativement influencer les résultats. L'objectif de cette thèse est de développer, mettre en œuvre et coupler de nouvelles techniques avancées basées sur la microscopie à force piézoélectrique pour minimiser les artefacts de mesure en dissociant leur contribution de celle des effets piézoélectriques et ferroélectriques, et pour caractériser un éventail plus large de propriétés des matériaux. C’est par exemple le cas du mode couplé de microscopie à force piézoélectrique en spectroscopie de commutation avec suivi de résonance à double fréquence, développé pour la première fois durant cette thèse, permettant de maximiser le rapport signal sur bruit tout en réduisant et mesurant l'influence de divers artefacts. De plus, il permet de mesurer la variation des propriétés du matériau sous l'influence d'un champ électrique, reflétant ainsi des conditions d'utilisation plus réalistes. Ce mode a été étendu pour des mesures de cartographie permettant également de déterminer la variation des propriétés mesurées sur la surface du matériau à l’échelle nanométrique. Le développement d'un ensemble de programmes informatiques a joué un rôle clé dans l'élaboration de ces nouveaux modes, depuis le pilotage des équipements jusqu'à l'analyse avancée des mesures
Many current applications in the microelectronics sector rely on the use of piezoelectric and ferroelectric materials in the form of thin films. For instance, MEMS devices, including sensors, actuators, and energy harvesters, take advantage of the piezoelectric properties of materials. Moreover, the discovery of the potential of certain materials for microelectronic devices has spurred significant research, as exemplified by the use of HfO2 for its ferroelectric properties in non-volatile memories like FeRAM and FeFET. In this context, various deposition techniques for piezoelectric and ferroelectric thin films are currently under optimization. Specific characterization methods for these materials are essential to evaluate the quality of their fabrication and to enhance the understanding of the underlying physical phenomena, which is critical for their integration into advanced microelectronic devices.This thesis aims to develop characterization techniques for the piezoelectric and ferroelectric properties of devices fabricated as thin films at the nanoscale. Piezoelectric force microscopy enables such analyses but is susceptible to numerous artifacts, such as electrostatic effects, which can significantly impact the results. The objective of this thesis is to develop, implement, and combine new advanced techniques based on PFM to minimize measurement artifacts by separating their contributions from those of piezoelectric and ferroelectric effects, and to characterize a wider range of material properties. For instance, the coupled PFM mode in switching spectroscopy with dual-frequency resonance tracking, developed for the first time in this thesis, maximizes the signal-to-noise ratio while reducing and measuring the influence of various artifacts. Furthermore, this mode allows the measurement of material property variations under the influence of an electric field, thus reflecting more realistic operating conditions. This mode has been extended for mapping measurements, allowing the determination of variations in the measured properties across the material surface at the nanoscale. The development of a suite of software programs played a key role in the creation of these new modes, from equipment control to advanced data analysis

Du fait d’une faible conductivité thermique, les composés de la série homologue n(PbTe)-m(Bi2Te3) ont été reportés dans la littérature comme de bons thermoélectriques. Dans ces chalcogénures présentant une structure cristalline en couches, deux types d’empilement, nommés A et B, ont été mis en évidence pour le composé Pb2Bi2Te5. De façon à comprendre les différences existantes entre les propriétés de ces empilements, nous avons déterminé les propriétés électroniques et thermoélectriques du composé Pb2Bi2Te5 dans chacun des empilements en réalisant des calculs et analysé la densité électronique de ces structures en utilisant la théorie quantique des atomes dans les molécules. Les modules élastiques, les constantes diélectriques, les charges effectives de Born et la dispersion des phonons ont été calculés et à partir de ces résultats, la conductivité thermique a été déterminée en résolvant l’équation de transport de Boltzmann. De façon à obtenir un ensemble complet de propriétés thermoélectriques et expliquer la faible conductivité thermique observée dans ces composés, une étude théorique complète des structures électroniques, des propriétés de transport et des propriétés dynamiques de réseau de quatre composés (PbTe, Bi2Te3, PbBi2Te4 and PbBi4Te7) a été réalisée. La conductivité thermique κ_l a été évaluée en calculant les constantes de forces interatomiques du second et du troisième ordre. L’ingénierie de bandes a été mise sur différents composés de la série (Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5), en appliquant des contraintes biaxiales en compression et en tension, de façon à améliorer leurs propriétés thermoélectriques
Owing to their low lattice thermal conductivity, compounds of the n(PbTe)-m(Bi2Te3) homologous series have been reported in the literature with good thermoelectric properties. Among these layered chalcogenides, the Pb2Bi2Te5 compound has been evidenced with two stacking sequences termed A and B. In order to understand the differences in their properties, we have determined the electronic and the thermoelectric properties of the Pb2Bi2Te5 compound with the two different stacking sequences from a series of first principles calculations using density functional theory and analysed the electronic density of these compounds by using the quantum theory of atoms in molecules. The elastic moduli, dielectric constants, Born effective charges, and phonon dispersion within the quasi-harmonic approximation have also been calculated and based on these calculations results, the thermal conductivity has been determined by solving the Boltzmann transport equation. In order to get a comprehensive set of thermoelectric properties and explain the low lattice thermal conductivity observed in these compounds, a full theoretical study of the electronic structures, transport behaviour, and lattice dynamic properties of four chalcogenides compounds (PbTe, Bi2Te3, PbBi2Te4 and PbBi4Te7) has been performed. The lattice thermal conductivity κ_l has been evaluated by calculating the second- and third-order interatomic force constants. The band engineering approach has then been implemented by applying biaxial tensile and compressive strains on various compounds of this series, namely Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5 in order to improve their TE properties

In der vorliegenden Arbeit wird die Korrektur weitreichender Dispersionswechselwirkungen fuer Dichtefunktionaltheorie fuer Rechnungen unter Anwendung periodischer Randbedingungen erweitert. Am Beispiel des Graphit wird der Einfluss der Dispersionskorrektur auf Strukturparameter und Energien gezeigt. Die berechneten Werte fuer Schichtabstand und Wechselwirkungsenergie stimmen sehr gut mit experimentell bestimmten Daten ueberein. Anhand von Clusterstudien wird gezeigt, dass die Dispersionskorrektur nur sehr langsam mit der Systemgroesse konvergiert. Die genaue Beschreibung der Dispersionswechselwirkungen zwischen Graphitschichten mit der PBE+D-Methode ist nur bei Anwendung periodischer Randbedingungen oder durch eingebettete Clustermodelle moeglich. Der Vergleich der PBE+D- mit der genauen, aber sehr aufwendigen [MP2:PBE + delta-CCSD(T)]-Methode zeigt, dass die strukturellen Unterschiede zwischen beiden Methoden gering sind. Die berechneten Reaktionsenergien unterscheiden sich hingegen deutlich. Die neu entwickelte, effiziente [PBE+D + delta-MP2 + delta-CCSD(T)]-Methode ergaenzt die PBE+D-Energie um zwei Korrekturterme. Der erste Term, die delta-MP2-Korrektur, behebt die Ueberstabilisierung polarer Strukturen (PBE) mit einer MP2-Rechnung am Basissatzlimit. Der zweite Term ueberprueft die delta-MP2-Korrektur durch eine CCSD(T)-Rechnung fuer einen kleinen Cluster. Die [PBE+D + delta-MP2 + delta-CCSD(T)]-Methode wird fuer die Reaktion von C4H8-Kohlenwasserstoffen mit H-Ferrierit angewendet. In der Zeolithpore wurden pi-Komplexe, Butylkationen und Oberflaechenalkoxide als Intermediate identifiziert. Die Isomerisierung von Butenen in der H-Ferrierit-Pore wird mit der Umlagerung linearer Butylkationen in der Gasphase verglichen. Der geschwindigkeitsbestimmende Schritt ist in beiden Faellen die Bildung des tert-Butylkations aus einem methylverbrueckten Butylkation. Die CCSD(T)-Methode ist zur Bestimmung genauer Energieprofile erforderlich.
In this work, the long-range dispersion correction for density functional theory is extended to periodic boundary conditions. The influence of the dispersion correction on energy and structural parameters is shown for graphite. The calculated values of the interlayer distance and the interaction energy are in good agreement with experimental ones. By a series of cluster calculations it is shown, that the dispersion correction converges very slowly with respect to the system size. The accurate description of the dispersion interaction between graphite layers requires the usage of PBE+D method applying periodic boundary conditions or embedded cluster models. For structural parameters, the PBE+D methods compares well with the accurate but computationally very demanding [MP2:PBE+CCSD(T)] method. However, the calculated reaction energies differ remarkably. The newly developed, efficient [PBE+D + MP2 + CCSD(T)] method extends the PBE+D energy by two correction terms. The first one, the MP2 correction, rectifies the over stabilization of polar structures (PBE) by a MP2 calculation at the basis set limit. The second term verifies the MP2 correction by a CCSD(T) calculation for a small cluster model. The [PBE+D + MP2 + CCSD(T)] method is applied for the reaction of C4H8 hydro carbons witr the zeolite Ferrierite. Within the pore of a zeolite, pi complexes, butyl cations and surface alkoxides are identified as minima on the potential energy surface. The isomerization of butenes is compared to the rearrangement of linear butyl cations in the gas phase. In both cases, the rate determining step is the formation of the tertial butyl cation from a methyl bridged cation. The CCSD(T) method is for the determination of accurate energy profiles required.

Abstract Titanium dioxide nanoparticles are used in an enormous amount of applications. Their properties are different from bulk TiO₂ and are affected by adsorbates that are unavoidably present on the surface. In this thesis, the effect of OH and SO₄ groups (the adsorbants present on the surface during manufacturing) on the properties of anatase-structured TiO₂ nanoparticles is studied. It was found that the above mentioned groups change both the geometric and electronic structure of nanoparticles, resulting in changes in the photoabsorption spectrum. Bader charges are calculated using electron density from Density Functional Theory calculations. They can be used for determination of the oxidation state of the atom. The relation between computed partial charges and oxidation states for binary oxides using data from open materials database has been demonstrated in this work using a linear regression. The applicability of the oxidation state determination by Bader charges for mixed valence compounds and surfaces is considered
Tiivistelmä Titaanidioksidinanopartikkeleita käytetään lukuisissa sovelluksissa. Niiden ominaisuudet poikkeavat kiinteän TiO₂:n ominaisuuksista, ja niihin vaikuttavat pinnalle väistämättä absorboituvat aineet. Tässä työssä on tutkittu OH- ja SO₄-ryhmien vaikutusta anataasirakenteisten TiO₂-nanopartikkelien ominaisuuksiin. Tällaisia ryhmiä esiintyy yleisesti nanopartikkelien pinnalla valmistusprosessien aikana. Työssä havaittiin, että nämä ryhmät muuttavat nanopartikkelien rakenteellisia ja sähköisiä ominaisuuksia, ja siten vaikuttavat myös fotoabsorptiospektriin. Baderin varaukset voidaan laskea käyttäen tiheysfunktionaaliteoriaan perustuvista laskuista saatavaa elektronitiheyttä. Niitä voidaan käyttää atomin hapetustilan laskemiseen. Tässä työssä on osoitettu, että binääristen oksidien tapauksessa laskettujen osittaisvarauksien ja hapetustilan välillä on yhteys. Tämä yhteys voitiin osoittaa käyttämällä lineaarista regressiota. Työssä tarkastellaan myös menetelmän soveltuvuutta hapetustilojen määrittämiseen sekavalenssiyhdisteille ja pinnoille
Original papers Original publications are not included in the electronic version of the dissertation. Miroshnichenko O., Auvinen S., & Alatalo M. (2015). A DFT study of the effect of OH groups on the optical, electronic, and structural properties of TiO₂ nanoparticles. Phys. Chem. Chem. Phys., 17, 5321–5327. https://doi.org/10.1039/c4cp02789b Miroshnichenko O., Posysaev S., & Alatalo M. (2016). A DFT study of the effect of SO4 groups on the properties of TiO₂ nanoparticles. Phys. Chem. Chem. Phys., 18, 33068–33076. https://doi.org/10.1039/c6cp05681d http://jultika.oulu.fi/Record/nbnfi-fe201707037608 Posysaev S., Miroshnichenko O., Alatalo M., Le D., & Rahman T.S. (2019). Oxidation states of binary oxides from data analytics of the electronic structure. Comput. Mater. Sci., 161, 403–414. https://doi.org/10.1016/j.commatsci.2019.01.046

Oscillator strengths and transition energies are calculated for several mono-substitutes of benzene and naphthalene molecules. The substituents investigated are chlorine, bromine and iodine. Calculations for these molecules are presented, at the Hartree-Fock and DFT level of theory. The functional used in DFT is CAM-B3LYP.

Back injuries and other illnesses for Tranz Rails Locomotive Engineers, is claimed to be attributed to vibration of locomotive cabs. The purpose of this research was to conduct a study in order to determine the complex motion of a DFT locomotive cab and to objectively and quantifiably measure the vibration present in the locomotive. This thesis examines the ability of a six Degrees Of Freedom rigid body mathematical model of a DFT locomotive cab to determine the natural frequencies present. The goal was to use this model to provide information on different mount alternatives, in order to reduce vibration. The results of this model were inadequate for the selection of another elastomeric mounting alternative. While experimental test of the locomotive cab showed that the rigid body assumption was valid it was found that there are other continuous vibrations and non-linear effects that are also very important in determining cab vibration. Auto-correlation results from acceleration measurements at the base of the mounts were overlaid with the cross-correlation of the acceleration measurements of the cab side of the mounts. These results showed that the current mounts have 100% transmissibility. In some cases and directions the mount was shown to actually amplify the vibrational input from the locomotive. The amplitude of the steady-state vibration of the locomotive cab that was being transmitted was so great that it rendered the dynamic characteristics of the cab insignificant. From the experimental results, the elastomeric mounts were found to be incapable of attenuating the vibration. In addition, the vibration levels due to locomotive running are found to be well above comfort levels of various publications.

In the drug discovery process, there are multiple factors that make a successful candidate other than whether it antagonises a chosen active site, or performs allosteric regulation. Each test candidate is profiled by its absorption into the bloodstream, distribution throughout the organism, its products of metabolism, method of excretion, and overall toxicity; summarised as ADMET. There are currently methods to calculate and predict such properties, but the majority of these involve rule-based, empirical approaches that run the risk of lacking accuracy as one's search of chemical space ventures into the more novel. The lack of experimental data on organometallic systems also means that some of these methods refuse to predict properties on them outright, losing the opportunity to exploit this relatively untapped area that holds promise for new antibacterial and antineoplastic pharmaceutical compounds. Using the more transferable and definitive quantum mechanical (QM) approach to drug discovery is desirable, but the computational cost of conventional Hartree-Fock (HF) and Density Functional Theory (DFT) calculations are too high. Using the linear-scaling DFT program, onetep, we aim to exploit the benefits of DFT in calculations with much larger fragments of, and in some cases entire biomolecules, in order to demonstrate calculations which could ultimately be used in developing more accurate methods of profiling drug candidates, with a computational cost that albeit still high, is now feasible with the provision of modern supercomputers. In this thesis, we first use linear-scaling DFT methods to address the lack of electron polarisation and charge transfer effects in energy calculations using a molecular mechanics forcefield. Multiple DFT calculations are performed on molecular dynamics(MD) snapshots of small molecules in a waterbox, with the aim of computing a MM!QM correction term, which can be applied to a forcefield binding free energy approach (such as thermodynamic integration) which will process a far greater number of MD snapshots. As a result, one will obtain the precision from processing very large numbers of MD snapshots of biomolecular systems, but the accuracy of QM. To improve efficiency of the QM phase of the overall method, we use electrostatic embedding to model the regions of the waterbox that are far from the solute, yet are still important to include. As this is a relatively new module in onetep, we present validation data prior to its use in the main work. Secondly, we validate different methods of calculating the pKa of a wide variety of molecules: from small, organic compounds, to the organometallic cisplatin, with the ultimate goal being of such calculations to eventually address questions such as, assuming oral intake, where in the gastrointestinal tract will a drug molecule be absorbed into the bloodstream, and how much of the original dose will be absorbed. These calculations are then scaled up significantly to examine the potential of using linear-scaling DFT to calculate the pKa of specific residues in proteins. This is performed with a 305-atom tryptophan cage, the 814-atom Ovomucoid Silver Pheasant Third Domain(OMSVP3) and a 2346-atom section of the T99A/M102Q T4-lysozyme mutant. We also highlight the challenges in calculating protein pKa. Finally, we study the hydrogen-abstraction reaction between cyclohexene and cytochrome P450cam, through onetep single point energy calculations of a 10-snapshot adiabaticreaction profile generated by the Mulholland Group(University of Bristol). Following this, the LST and QST methods of determining the transition state (available through onetep) are used, with the aims of determining the importance of the protein surrounding the active site in regards to the activation energy and structural geometry of the calculated transition state. The LST and QST methods are also validated, through modelling of the SN2 reaction between fluoride and chloromethane. The aim of this part of our work is to eventually assist in developing a metabolism (and toxicity) model of the different isoforms of cytochrome P450. Overall, this thesis aims to highlight not only the capability of linear-scaling DFT in becoming an important part of biomolecular simulation, but also the challenges that one will face upon scaling up calculations that were previously simple to perform, based on the small size of the system being modelled.

Gold catalysed hydrochlorination of acetylene is a convenient alternative to the use of toxic mercury derivatives in large scale production of vinyl chloride. The catalytic performance of supported gold nanoparticles is highly dependent on a series of factors. Amongst them the catalyst support plays a fundamental role. Acid wash of carbonaceous supports has proven to influence the catalytic performances of gold nanoparticles. Atomic force microscopy revealed that after acid treatments the morphology of the graphite support presents spherical features. X-ray photoemission spectra detected the presence of hydroxyl and carbonyl groups which were formed at the surface sites after acid treatments. DFT calculations have been used to give an insight on the mechanisms of formation these features, functional groups and, to support the experimental data. In good agreement with the experiment, it has been found that when water molecules are intercalated within the interlayer space, the graphitic surface presents a curvature and the interlayer distance is largely increased. While pristine graphite is hydrophobic and inert to water, defective surfaces presenting under coordinated carbons show increased reactivity and are able to dissociate water also at room temperature. Pristine graphite surface has a poor particle adhesion but unsaturated carbons of defects or edges can chemisorb atoms and molecules to form strong bonds. Finally, in good agreement with the experiment, DFT results show that the interactions between gold nanoparticles and the hydroxyl groups are stronger than those between gold and carboxyl groups suggesting that the hydroxyls are better anchoring sites for gold nanoparticles than carbonyls.

During my Ph.D. I focused my attention on the study of properties of the halogen bonding from a computational point of view. Due to the growing attention towards this kind of interaction, it is important to have some computational and theoretical models able to explain and reproduce its features. Halogen bonding, indeed, has been demonstrated to be a powerful tool due to the large number of applications in different fields, ranging from biological macromolecules to supramolecular chemistry (such as assemblies with nonlinear optical properties), from nanomaterials and crystal engineering (like the self-assembly control of host-guest solids, with applications in liquid-crystalline, porous, magnetic and organic phosphorescent materials) to materials science (such as the development of solid-state materials with peculiar electronic properties). Though a plethora of theoretical studies have been carried out on this interaction, there are still some open questions to be solved. I have investigated halogen bonding by using mainly two computational approaches, that is the Density Functional Theory (DFT) and the Valence Bond Spin-Coupled Theory. The first one is implemented in a number of widely used computational chemistry software programs, while the second one has been implemented by a restricted number of research groups, according to different strategies. In this thesis, after an introduction on the main properties of halogen bonding, I will briefly describe the Spin-Coupled Theory formulated on the Slater determinants, adopted for the present studies. The related software, developed by my supervisor Prof. M. Sironi, was able to treat up to 11 valence electrons. During my Ph.D. thesis, I worked in order to overcome this limitation: the software now runs on systems with up to 14 valence electrons. Finally, I will show the issues tackled and discuss the results obtained.

Les travaux présentés dans cette thèse relèvent principalement de la réactivité des surfaces et des interactions gaz-surface. Les champs d'application de ce travail sont variés et s'inscrivent principalement dans le domaine de la fusion nucléaire et du projet ITER.Dans ce cadre, la modélisation à l'échelle atomique est un outil important pour comprendre et interpréter les résultats expérimentaux. Notre domaine de compétences est celui du calcul de structures électroniques et des propriétés chimiques. Ces calculs sont principalement conduits dans le cadre de la Théorie de la Fonctionnelle de la Densité (DFT) et de la thermodynamique statistique.Bien que composé de six chapitres, ce manuscrit comporte trois parties principales. La première est dédiée à la présentation des méthodes de calculs utilisées tout au long de cette thèse. La deuxième partie est consacrée à l'étude de la formation du carbure de béryllium à partir d'un dépôt de béryllium sur une surface de graphite. Le degré de fiabilité des résultats DFT a été évalué et les principales étapes de la formation de carbure de béryllium ont été déterminées. La troisième partie développée sur deux chapitres est consacrée à l'étude de l'interaction entre l'hydrogène et le tungstène métallique. La dissolution, la diffusion ainsi que le piégeage de l'hydrogène dans le tungstène ont été étudiés. Un excellent accord a été obtenu entre les valeurs calculées et les résultats expérimentaux de référence
The work herein presented deals with the reactivity of surfaces and the gas–surface interaction. This work is connected to different fields of applied science and more specifically to the field of nuclear materials for fusion devices like the International Thermonuclear Experimental Reactor (ITER).Numerical simulations at the atomic scale can provide an in depth understanding of the mechanisms at the origin of experimental observations. More specifically, our skills are about electronic structure calculations and chemical properties modelling; most of the work we produced has been conducted within the framework of the Density Functional Theory (DFT) and statistical thermodynamics. While made of six chapters, the manuscript can be cast in three main parts. The first one is dedicated to the methods used throughout this thesis. The second is devoted to the formation of beryllium carbide from deposited beryllium atoms on graphite surfaces; the reliability of the DFT results was benchmarked and the main steps of the beryllium carbide formation were determined. The third part explores the interaction between hydrogen and metallic tungsten. The formation of vacancies in the material, its impact on the solubility and diffusion of hydrogen in tungsten were investigated, and the results were compared with experiment; an excellent agreement was found

Les travaux présentés dans cette thèse relèvent principalement de la réactivité des surfaces et des interactions gaz-surface. Les champs d'application de ce travail sont variés et s'inscrivent principalement dans le domaine de la fusion nucléaire et du projet ITER.Dans ce cadre, la modélisation à l'échelle atomique est un outil important pour comprendre et interpréter les résultats expérimentaux. Notre domaine de compétences est celui du calcul de structures électroniques et des propriétés chimiques. Ces calculs sont principalement conduits dans le cadre de la Théorie de la Fonctionnelle de la Densité (DFT) et de la thermodynamique statistique.Bien que composé de six chapitres, ce manuscrit comporte trois parties principales. La première est dédiée à la présentation des méthodes de calculs utilisées tout au long de cette thèse. La deuxième partie est consacrée à l'étude de la formation du carbure de béryllium à partir d'un dépôt de béryllium sur une surface de graphite. Le degré de fiabilité des résultats DFT a été évalué et les principales étapes de la formation de carbure de béryllium ont été déterminées. La troisième partie développée sur deux chapitres est consacrée à l'étude de l'interaction entre l'hydrogène et le tungstène métallique. La dissolution, la diffusion ainsi que le piégeage de l'hydrogène dans le tungstène ont été étudiés. Un excellent accord a été obtenu entre les valeurs calculées et les résultats expérimentaux de référence
The work herein presented deals with the reactivity of surfaces and the gas–surface interaction. This work is connected to different fields of applied science and more specifically to the field of nuclear materials for fusion devices like the International Thermonuclear Experimental Reactor (ITER).Numerical simulations at the atomic scale can provide an in depth understanding of the mechanisms at the origin of experimental observations. More specifically, our skills are about electronic structure calculations and chemical properties modelling; most of the work we produced has been conducted within the framework of the Density Functional Theory (DFT) and statistical thermodynamics. While made of six chapters, the manuscript can be cast in three main parts. The first one is dedicated to the methods used throughout this thesis. The second is devoted to the formation of beryllium carbide from deposited beryllium atoms on graphite surfaces; the reliability of the DFT results was benchmarked and the main steps of the beryllium carbide formation were determined. The third part explores the interaction between hydrogen and metallic tungsten. The formation of vacancies in the material, its impact on the solubility and diffusion of hydrogen in tungsten were investigated, and the results were compared with experiment; an excellent agreement was found

In the history of Chemistry, materials chemists have developed their ideas mainly by doing experiments in laboratories. The underlying motivation for this laboratory work has generally been pure curiosity or the ambition to find a solution to a specific problem. Minor changes in the composition or structure of a material can cause major changes in its properties. The development of powerful computers has now opened up the possibility to calculate properties of new materials using quantum mechanical methods.

The Chemistry of different boron-related materials has been evaluated in this thesis by Density Functional Theory (DFT). Cubic boron nitride (c-BN) is a most interesting material for the microelectronics and tool industry. During thin film deposition of c-BN, several problems arise which most often result in unwanted BN isomorphs. Chemical processes at the (110) and (111) surface of c-BN have been investigated in order to shed light upon some of these complex processes. Typically adsorption energies and surface reconstruction were found to differ significantly between the two surfaces.

Other materials investigated are layered transition-metal diborides (MeB₂). Incorporation of transition-metal atoms into elemental boron in its most fundamental structure, ά-boron, has also been investigated. The calculations on MeB₂ focused on the stability of the planar compared to the puckered structure of MeB₂. Stability was investigated by calculating Density of States (DOS) and bond populations. Deviations in the cell parameters from their ideal values were also considered.

A separate project concerned reactivity of the TiB₂(001) surface. Molecular and dissociated adsorption energies and adsorption geometries were calculated for H₂, H₂O and O₂. It was concluded that the titanium surface was more reactive than the boron surface and that the adsorption energies were comparable to or stronger than other well known surface-active compounds like TiO₂.

The complexes Ru(CO)3(dpae) and Ru(CO)2(dpae)(PPh3) have been found experimentally to undergo various reactions with para-hydrogen and substrates. Reactions with para-hydrogen and diphenylacetylene led to the detection of hydrogenation products, confirming the complexes as hydrogenation catalysts. The catalytic behaviour was identified to be different to that of the equivalent phosphine containing complex. High level DFT investigations have revealed significant insight into the mechanism of reaction. The experimentally detected dihydride complex Ru(H)2(CO)(dpae) was calculated to be a viable reaction product, with various pathways modelled for rearrangement. In contrast, the rearrangements for the complex Ru(H)2(CO)dpae)(PPh3) were found to compete with the reductive elimination of dihydrogen. The routes of reaction by initiation method was examined, with the high energy 14-electron intermediates only accessible photochemically. Routes for the hydrogenation of diphenylacetylene were identified, alongside the mechanism of cis-trans scrambling of stilbene and formation of 1,2-diphenylethane. The formation of 1,2,3,4-tetraphenylbutadiene was also rationalised. The reaction of hydrogen with W(N2)2(dppe-κ2P)2 was shown theoretically to involve an intra-molecular ortho-metallation reaction from the reactive 14-electron intermediate W(dppe-κ2P)2. Low barriers were obtained from the 16-electron intermediate W(H)2(dppe-κ2P)2. This rationalised the formation of the experimentally proposed complex W(H)3(dppe-κ2P)(PPh(C6H4CH2CH2Ph2P)-κ2P). The 14-electron intermediate W(dppe-κ2P)2 was calculated to adopt a butterfly geometry in a singlet state, which than rearranges upon reaction to form 16-electron intermediates. The observation of PHIP in the end products confirms the involvement of an electronic singlet state. Limited solvation was predicted from THF despite its ability to coordinate to metal centres. In summary, the combination of high level DFT models and the use of para-hydrogen reactions is demonstrated to be a powerful tool for probing chemical processes and pathways, and contributes to achieving a greater understanding of reactivity in these metal complex systems.

Discrete Fourier Transform (DFT) encoding over the real (or complex) field has been proposed as a means to reconstruct samples lost in multimedia transmissions over packet-based networks. A collection of simple sample reconstruction (and error detection) algorithms makes DFT codes an interesting candidate. A common problem with DFT code sample reconstruction algorithms is that the quantization associated with practical implementations results in reconstruction errors that are particularly large when lost samples occur in bursts (bursty erasures).
Following a survey of DFT decoding algorithms, we present herein the Tandem Filterbank/DFT Code (TFBD). The TFBD code consists of a tandem arrangement of a filterbank and DFT encoder that effectively creates DFT codes along the rows (temporal codevectors) and columns (subband codevectors) of the frame under analysis. The tandem arrangement ensures that subband codevectors (the frame columns) will be DFT codes, and we show how the temporal codevectors (frame rows) can also be interpreted as DFT codes. All the subband and temporal codevectors can be used to reconstruct samples entirely independently of each other. An erasure burst along a particular codevector can then be broken up by reconstructing some lost samples along the remaining orientation; these samples can then be used as received samples in reconstructing the original codevector, a technique that we refer to as pivoting. Expressions related to the performance of the Tandem Filterbank/DFT (TFBD) code, including an expression for the temporal code reconstruction error and for temporal-to-subband pivoting operations, are derived and verified through simulations. The expressions also prove useful in the selection of the many parameters specifying a TFBD encoder. The design process is illustrated for two sample TFBD codes that are then compared to a benchmark DFT code at the same rate. The results show that the TFBD encoder is capable of reconstruction error improvements that are more than four orders of magnitude better than that of the benchmark DFT code.

Distributed source coding, separate encoding (compression) and joint decoding of statistically dependent sources, arises in an increasing number of applications like sensor networks and multiview video coding. Many of those applications are highly interactive, requiring the development of low-delay, energy-limited communication and computing schemes. Currently, this compression is performed by using capacity-approaching binary channel codes. As a natural extension, distributed lossy source coding is realized by cascading a quantizer and Slepian-Wolf coding in the binary domain. Despite big strides in practical distributed source coding techniques, this problem is still demanding in terms of processing power, bandwidth, and delay.In this dissertation, we develop a new framework for distributed lossy source coding, in which we use real-number codes for binning. Specifically, we use a class of Bose-Chaudhuri-Hocquenghem (BCH) codes in the real/complex field known as the discrete Fourier transform (DFT) codes. Contrary to the conventional scheme, we first compress the continuous-valued sources and then quantize them. The new scheme exploits the correlation between continuous-valued sources, rather than quantized ones, which is more accurate. Also, by using short BCH-DFT codes, it reduces the complexity and delay and offers the potential to avoid the problems of the conventional quantization and binning approach, with relatively simple encoder/decoder.We propose both syndrome- and parity-based schemes, and we extend the parity-based scheme to distributed joint source-channel coding based on a single DFT code. Further, to adapt to uncertainty in the degree of statistical dependence between the sources, we construct rate-adaptive BCH-DFT codes. This allows the encoder to switch flexibly between encoding sample rates, if the degree of statistical dependence varies. The construction of rate-adaptive codes is based on transmission of additional syndrome samples and a simple extension of the subspace-based decoding.Another major contribution of this dissertation is to generalize the encoding/decoding of BCH-DFT codes. We prove that the parity frequencies of a BCH-DFT code, or equivalently the zeros of codewords in the frequency domain, are not required to be adjacent; we provide the decoding algorithm as well. This offers flexibility in constructing BCH-DFT codes and further improvement in the decoding which can be exploited in channel coding as well.
Le codage de sources distribué, c'est-á-dire l'encodage séparé et le décodage conjoint de sources statistiquement dépendantes, survien dans un grand nombre d'applications telles que les réseaux de capteurs et le codage de vidéos multi-vues. Plusieurs de ces applications sont hautement interactives, ce qui demande le développement de communications et deschémas de calcul á faibles délais et limités énergétiquement. Actuellement, cette compression est effectuée en utilisant des codes binaires pour approcher la capacité de canal. En tant qu'extension naturelle, le codage de sources distribué avec pertes est réalisé en mettant en cascade un quantificateur et un codage Slepian-Wolf dans le domaine binaire. Malgré les grands progrés effectués dans les techniques pratiques de codage de sources distribué, ce probléme reste exigeant en termes de puissance de traitement, de bande passante et de délai.Dans cette dissertation, nous développons un nouvel axe d'étude pour le codage de sources distribué avec pertes, dans lequel nous utilisons des codes á nombre réels pour l'encapsulage. Plus spécifiquement, nous utilisons une classe de codes Bose-Chaudhuri-Hocquenghem (BCH) dans le domaine réel, plus connus sous le nom de codes de la transformée de Fourier discréte (TFD). Contrairement au schéma conventionnel, nous comprimons d'abord les sources valeurs continues et nous les quantifions ensuite. Le nouveau schéma exploite la corrélation entre les sources valeurs continues plutˆ ot que celles quantifiées, ce qui offre plus de précision. De plus, l'utilisation de codes BCH-TFD de petite taille, réduit la complexité et le délai et permet d'éviter les problmes liés á la quantification conventionnelle et á l'approche d'encapsulage, avec un encodeur/décodeur relativement simple.Nous proposons deux schémas: un basé sur le syndrome et l'autre sur la parité, et nous étendons le schéma basé sur la parité au codage source-canal conjoint distribué á partir d'un seul code TFD. Par la suite, pour s'adapter á l'incertitude du degré de dependence statistique entre les sources, nous construisons des codes BCH-TFD avec taux adapté. Cela permet á l'encodeur de permuter facilement entre les taux de code, si le degré de dependence statistique varie. La construction de codes á taux adapté est basée sur la transmission d'échantillons-syndrome supplémentaires et sur une simple extension du décodage de type sous-espace.Une autre contribution majeure de cette dissertation est la généralisation du processus encodage/décodage des codes BCH-TFD. Nous prouvons que les fréquences de parité d'un code BCH-TFD, équivalents aux zéros des mots de code dans le domaine fréquentiel, n'ont pas besoin d'ˆ etre adjacents; nous fournissons l'algorithme de décodage également. Nous apportons ainsi de la flexibilité lors de la construction des codes BCH-TFD et d'autres améliorations au niveau du décodage, lesquelles peuvent également ˆ etre exploitées au niveau du codage de canal.

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CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Neste trabalho e apresentado um conjunto de resultados obtidos via simulação computacional para o módulo de Young de nanotubos de carbono do tipo armchair. Os casos tratados são os CNTs de coordenadas quirais (6; 6) perfeitos; com defeitos estruturais de vacâncias; com defeitos do tipo Stone-Wales; e por fim o caso no qual o tubo (6; 6) perfeito e interno a outro de coordenadas (11; 11), também perfeito, em con figura ção de parede dupla. O objetivo final e fornecer uma comparação, em relacão a precisão e ao custo computacional, entre a utilização de dois diferentes pacotes capazes de simular o comportamento dos nanotubos: o programa SIESTA, baseado na Teoria do Funcional da Densidade (DFT), e o programa DFTB+, baserado na aproximação Tight-Binding com DFT. Ao longo do trabalho são trazidas também algumas explicações sobre o interesse nos nanotubos de carbono, a m de validar seu estudo, e a caracterização dos diferentes tipos conhecidos. E também efetuada uma descrição geral das simulações computacionais e das teorias nas quais se baseiam os programas utilizados.
This work presents a set of results, obtained by computer simulation, for the Young modulus of (6; 6) armchair type carbon nanotubes. Cases covered are the (6; 6) perfect nanotube; with vacancy structural defects; with Stone-Wales defects; and nally a multiwall case with (6; 6) and (11; 11) perfect tubes. The ultimate goal is to provide a comparison, regarding accuracy and computational cost, between the use of two di erent packages capable of simulating the behavior of these nanotubes: the SIESTA code, based on Density Functional Theory (DFT), and the DFTB+ code, based on Density Functional Tight-Binding scheme (DFTB). Throughout the work are brought some explanations about the interest in carbon nanotubes, in order to validate the study, and the characterization of di erent types known. It is also given an overview of computer simulations and theories in which are based the programs used.

La química computacional s'ha establert com una eina clau per entendre la reactivitat química y està dirigint la catàlisi cap a un disseny més racional. El desenvolupament constant i l'increment de la sofisticació en el camp experimental ha implicat diversos reptes pels químics computacionals, que busquen mètodes per lidiar amb aquesta classe de reaccions complexes. En aquest context, els sistemes situats a la frontera de la química homogènia i heterogènia estan guanyant importància, ja que permeten la combinació de les millors característiques de cada camp. Des d'un punt de vista teòric, les reaccions homogènies i heterogènies es simulen de maneres diferents. Hi ha una creixent necessitat d'investigar la millor manera per calcular aquesta classe de sistemes. L'objectiu d'aquesta tesi és explorar fins a quin punt els mètodes de la química homogènia es poden aplicar en sistemes situats en el límit entre l'homogènia i l'heterogènia. Especialment, la nostra atenció s'ha dirigit cap a les reaccions mecanoquímiques i les reaccions en les quals participen reductors sòlids. Amb aquest objectiu, cada capítol s'ha dirigit a l'estudi d'una o vàries reaccions en aquestes categories. Els nostres resultats demostren que els mètodes que s'utilitzen en catàlisi homogènia computacional es poden aplicar per entendre reaccions induïdes a través del molinet de boles o reductors sòlids. A més a més, hem demostrat l'importància dels models cinètics per entendre aquestes transformacions.
La química computacional se ha establecido como una herramienta crucial para entender la reactividad química y está dirigiendo la catálisis hacia un diseño más racional. El desarrollo constante y el incremento de la sofisticación en el campo experimental ha implicado diversos retos para los químicos computacionales, que buscan métodos para lidiar con estas reacciones complejas. En este contexto, los sistemas situados en la frontera de la química homogénea y heterogénea están ganando importancia, ya que permiten la combinación de las mejores características de cada campo. Des de un punto de vista teórico, las reacciones homogéneas y heterogéneas se simulan de formas distintas. Hay una creciente necesidad de investigar la mejor manera para calcular este tipo de sistemas. La meta de esta tesis es explorar hasta que punto los métodos de química homogénea se pueden aplicar en sistemas situados en el “limbo” entre la homogénea y la heterogénea. Especialmente, nuestra atención se ha dirigido hacia las reacciones mecanoquímicas y las reacciones en las que participan reductores sólidos. Con este objetivo, cada capítulo se ha dirigido al estudio de una o varias reacciones en estas categorías. Nuestros resultados demuestran que los métodos que se utilizan en catálisis homogénea computacional se pueden aplicar para entender reacciones inducidas a través de molinillo de bolas o reductores sólidos. Además, hemos demostrado la importancia de los modelos cinéticos para comprender estas transformaciones.
Computational chemistry has been established as a crucial tool for the understanding of chemical reactivity and is driving catalysis towards a more rational design approach. The constant development and the increasing sophistication of experiments has raised numerous challenges for the computational chemists, who seek methods to deal with such complex transformations. In this context, systems located on the frontier of homogeneous and heterogeneous worlds are gaining importance, as they permit the combination of the best features of each area. From a theoretical perspective, homogeneous and heterogeneous reactions are modelled through substantially different approaches. There is thus an increasing need to investigate the most suitable manner to model these types of systems. The goal of this thesis is to explore to what extend methods commonly employed for the study of homogeneous reactions can be applied to systems located in the “limbo” between homogeneous and heterogeneous fields. Specifically, our attention has been directed towards mechanochemical reactions and homogeneous reactions with participation of solid reductants. To this end, each chapter has been devoted to the study of one or several transformation(s) within these categories. Our results demonstrate that methods emerging from computational homogeneous catalysis can be indeed applied to rationalize transformations induced through ball-milling techniques and reactions involving solid reductants. Moreover, we have demonstrated the importance of microkinetic modelling to provide a full understanding of these transformations.

With ever shrinking geometries, growing metal density and increasing clock rate on chips, delay testing is becoming a necessity in industry to maintain test quality for speed-related failures. The purpose of delay testing is to verify that the circuit operates correctly at the rated speed. However, functional tests for delay defects are usually unacceptable for large scale designs due to the prohibitive cost of functional test patterns and the difficulty in achieving very high fault coverage. Scan-based delay testing, which could ensure a high delay fault coverage at reasonable development cost, provides a good alternative to the at-speed functional test. This dissertation addresses several key challenges in scan-based delay testing and develops efficient Automatic Test Pattern Generation (ATPG) and Design-for-testability (DFT) algorithms for delay testing. In the dissertation, two algorithms are first proposed for computing and applying transition test patterns using stuck-at test vectors, thus avoiding the need for a transition fault test generator. The experimental results show that we can improve both test data volume and test application time by 46.5% over a commercial transition ATPG tool. Secondly, we propose a hybrid scan-based delay testing technique for compact and high fault coverage test set, which combines the advantages of both the skewed-load and broadside test application methods. On an average, about 4.5% improvement in fault coverage is obtained by the hybrid approach over the broad-side approach, with very little hardware overhead. Thirdly, we propose and develop a constrained ATPG algorithm for scan-based delay testing, which addresses the overtesting problem due to the possible detection of functionally untestable faults in scan-based testing. The experimental results show that our method efficiently generates a test set for functionally testable transition faults and reduces the yield loss due to overtesting of functionally untestable transition faults. Finally, a new approach on identifying functionally untestable transition faults in non-scan sequential circuits is presented. We formulate a new dominance relationship for transition faults and use it to help identify more untestable transition faults on top of a fault-independent method based on static implications. The experimental results for ISCAS89 sequential benchmark circuits show that our approach can identify many more functionally untestable transition faults than previously reported.
Ph. D.

La síntesis y el estudio de complejos polinucleares de metales de transición constituyen un área de gran interés en la Química Inorgánica. La presencia de varios centros metálicos en una misma molécula puede modificar la reactividad que cabría esperar en compuestos metálicos. Por otra parte, estos sistemas polinucleares sirven como modelos a escala molecular para el estudio de ciertos procesos catalíticos como los que tienen lugar sobre superficies metálicas u óxidos metálicos.

El estudio teórico de dos familias de compuestos metálicos bien diferenciadas constituye el núcleo de esta tesis. En ambos casos el objetivo principal es ayudar a racionalizar las características químicas de las especies analizadas. El tratamiento se basará en cálculos teóricos mediante la teoría del funcional de la densidad, que han demostrado ser apropiados a la hora de estudiar agregados metálicos.

Una primera parte, consta del estudio de la nueva familia de compuestos cubánicos de titanio que han sido recientemente sintetizados por el grupo del profesor Mena de la universidad de Alcalá de Henares. Un estudio teórico de dichos compuestos en colaboración conjunta con el trabajo en el laboratorio se ha venido realizando. Fruto de esta colaboración, nuevas pautas de investigación se han tratado de proponer por ambas partes. Como ejemplo, el análisis teórico para la obtención de espectros de resonancia magnética nuclear por métodos ab initio, que se expone en el segundo capitulo de la tesis, es fruto de un esfuerzo por parte de nuestro grupo por entender y racionalizar los datos de RMN obtenidos en el laboratorio para la caracterización de los compuestos. Además, fruto de los excelentes resultados obtenidos en este estudio, los conocimientos han sido aplicados a otros campos de interés dentro del grupo. Un estudio de la viabilidad para el análisis teórico del espectro de los núcleos de 183W en compuestos de Keggin forma un apartado importante en esta memoria.

Fruto de una estancia de seis meses en el grupo del doctor McGrady de la universidad de York surge un segundo apartado basado en el estudio de las importantes capacidades catalizadoras de los compuestos binucleares de molibdeno con azufres puente. La gran importancia industrial de procesos de hidrogenación catalizados por este tipo de compuestos ha despertado un gran interés, no obstante algunos aspectos del mecanismo de estas reacciones permanecen sin aclarar. El estudio teórico de esta reacciones puede proporcionar un poco de luz a estos aspectos.

Dentro del estudio de los compuestos cubánicos de titanio, los resultados principales se refieren a los asuntos siguientes:

- Descripción de las características estructurales y electrónicas de las especies implicadas. La disposición de los ligandos tridentados, (precubanos) de fórmula general [{TiCp*(-O)}3(3-CR)] y [{TiCp*}(-NH)}3(3-N)], para actuar como ligandos macrocíclicos donores de seis-electrones proporciona una ruta eficaz para la obtención de estructuras cúbicas con esqueleto [MTi3(3-Y)(-X)3].



Reacción del ligando con el fragmento metálico


Es de esperar que fragmentos de casi todos los grupos metálicos sean capaces de interactuar con estos ligandos. Así se ha estudiado la formación de estructuras cubánicas a partir de fragmentos metálicos de los grupos 1,2, 4-6, 9, 13 y 14. Cambios en la naturaleza de la interacción, en el comportamiento del ligando, estructura molecular han sido analizados. Los resultados muestran las versatilidad de estos ligandos para actuar no sólo como donores de electrones si no también como posibles aceptores a través de los orbitales vacíos de los metales. Una mención especial en este apartado es necesaria para los isomeros acetilénicos y vilidénicos encontrados a partir de la deprotonación del compuesto [{TiCp*(-O)}3(3-CMe)] por la interacción con fragmentos metálicos del grupo 2. En este caso, el núcleo formado por los átomos de titanio sirve como base para la estabilización de estas especies.

- Estudio de la estabilidad de los compuestos y de los procesos energéticos en que están implicados (energías de la reacción, procesos de la isomerización). Un estudio de las condiciones experimentales que conducen a la obtención de estructuras cúbicas será analizado. El desplazamiento de ligandos lábiles en fragmentos metálicos por parte de los ligandos tridentados para formar los correspondientes cubanos ha sido probado, o también la activación de los enlaces nitrógeno-protón del ligando por interacción con el fragmento metálico. Estos procesos vienen caracterizados por unas condiciones energéticas favorables, estas condiciones son analizadas y estudiadas por su utilidad a la hora de proponer nuevas vías de síntesis en el laboratorio. Además, algunos compuestos presentarán evidencias del tener más que un isómero, o de experimentar un comportamiento dinámico en solución. Estas situaciones también se han descrito. Como resultado ha sido descrito el comportamiento de estas estructuras como ligandos mono- di- y tridentados, y adicionalmente como ligandos neutros o aniónicos. A su vez, respecto de la interacción con el fragmento metálico, un comportamiento como ligandos simples, que solo estabilizan el metal, o dobles, estabilizando metal y su respectivo contraión, han sido descritos.

- La influencia del solvente también se ha evaluado debido a la importancia que tiene su presencia en la obtención o no de ciertos compuestos. La termoquímica relacionada con la presencia del disolvente ha sido analizada desde el punto de vista teórico, debido a la imposibilidad de explicar ciertos procesos a partir de estudios en fase gas. Un modelo basado en un medio continuo de constante dieléctrica conocida se utiliza para demostrar los efectos del solvente. Para una misma familia de compuestos, solamente en algunos casos la interacción con los ligandos precubánicos conducen a la formación de los correspondientes cubanos, o en condiciones muy específicas. Por ejemplo, ha sido estudiado que los efectos del solvente son un factor decisivo en la obtención de los cubanos a partir de las sales de metales alcalinos y alcalino térreos. Este efecto permite explicar como la energía de red de las sales es fuertemente debilitada por la presencia del solvente, como factor principal en la obtención de las especies, por encima de la energía de interacción con el ligando.

- Descripción de algunas características químicas importantes. La contribución de los espectros de resonancia magnética nuclear resulta decisiva para la caracterización de los compuestos. Una descripción en detalle de las particularidades en el cálculo de los desplazamientos químicos para espectros de RMN es revisado en un apartado puramente teórico, especialmente para los métodos DFT. La activación del enlace C-H del carbono apical en el ligando [{TiCp*(-O)}3(3-CH)] durante la formación de estructuras diméricas fue seguida mediante espectroscopia RMN de 13C. Los grandes cambios experimentados en los desplazamientos químicos entre las especies protonadas y no protonadas han sido descritos y racionalizados perfectamente mediante cálculos teóricos. Los resultados obtenidos corroboran las suposiciones experimentales y proporcionan una visión muy clara de los procesos electrónicos que conducen a estos resultados. El carácter predictivo de estos estudios para con otras especies es claro, no obstante cuantitativamente hablando se encuentran dificultades a la hora de atribuir todos los desplazamientos.

Fruto de los excelentes resultados obtenidos en la descripción de los valores de RMN para los compuestos cubánicos, estos estudios se han ampliado a otros campos de interés para nuestro grupo de trabajo. Una sección, en parte diferente a la estructura global de la tesis, se ha dedicado ha mostrar estos primeros pasos a la hora de examinar las capacidades de los métodos DFT para computar los desplazamientos en núcleos de 183W en compuestos de Keggin. Nos encontramos ante unas mayores dificultades a la hora de reproducir los espectros experimentales, y gran parte de nuestros esfuerzos se han centrado en tratar de buscar la forma de trabajo más adecuada en términos de precisión y de coste computacional. La investigación y el interés continúan abiertos en un campo que necesita nuevas revisiones, sobre todo a la hora de racionalizar las posibles fuentes de error.

Las importantes características catalíticas de los sulfuros de molibdeno son objeto de estudio en la segunda parte de la tesis. El estudio teórico intentará aclarar ciertos aspectos confusos en la mecanistica asociada con las reacciones de estos compuestos. Estos complejos dinucleares de molibdeno con ligandos puente de azufre son una de las familias que posee una gran flexibilidad estructural y electrónica. Parte de los posibles isómeros del compuesto tipo [(CpMo)-S2]2 han sido estudiados, prestando gran interés al intercambio electrónico metal ligando con el cambio de estructura.



















Ejemplos de estructuras cerradas (a-b) y abiertas (c-d) para el complejo


La capacidad de los complejos del sulfuro de activar el hidrógeno molecular y de catalizar la reducción del substrato por el hidrógeno es particularmente relevante en la química. En especial los procesos relacionados con la activación de hidrógeno y posterior reducción de un sustrato ligado a los azufre puente son analizados; y en particular aquellos relacionados con la estructura electrónica del catalizador y del substrato. En estos estudios se pone de manifiesto la importancia catalizadora del metal, sirviendo como puente para protones en la reducción. A su vez se muestra como la naturaleza del sustrato resulta decisiva a la hora de observar una eliminación de hidrógeno molecular o una obtención del substrato reducido.


Tarragona, 2004 J. Gracia
The overall framework of this report aims to summarize the work we have done over the past four years. Conceptually, this thesis is divided into three parts. The nucleus is based on a joint project with prof. Mena, of the University of Alcalá, whose group is involved in the synthesis and characterization of new aza- and oxo- titanium cubanes. These families of compounds are basically obtained from preorganized titanium tridentate ligands which can incorporate almost all the metals of the periodic table and yield the corresponding heterometallic cubanes. In this respect, the theoretical study is based on and adapted from the progress in the laboratory work. Practically every section in this part starts by describing the experimental data available for the specific system studied when we began the analysis. Then we go on to describe the theoretical study and attempt to involve the reader in the problems discussed. As a consequence of the advance in the experimental work, NMR spectra are used to characterize the new compounds. This leads us to initiate studies in the computational simulation of NMR chemical shifts. The most theoretical part of this report is an assimilation of these studies. The excellent results obtained for the cubane compounds enabled us to extend the knowledge acquired to other interesting fields and our group has started a study of the NMR shielding tensors in polyoxometalate compounds. The third part of the thesis focuses on some molybdenum-sulphide clusters and their activity as catalysts.

Chapters 3 and 4 focuse on the electronic structure of the new titanium cubane compounds recently synthesized by Mena et al. We work parallel to an experimental group to compare and contrast the theoretical and experimental data, and so establish the base chemistry of these new compounds. In this sense, the various reaction energies were studied, and the compounds and their possible isomers were characterised by means of DFT methods. Particular emphasis is paid to the properties of incomplete Ti3 core cubane tridentate ligands (precubanes) as bases for obtaining cubanes. The electronic structure, bonding energies, electron transfer processes and reactivity in the titanium Ti3 core of the metal clusters are studied.

The reaction of the azatitanium compound with some titanium derivatives leads to nitrocubane formation (see Scheme 1.3). This reaction forms a cubane compound by adding the fourth vertex to a structure that becomes a precubane.


Scheme 1.3. Reaction of with


Chapter 5 summarizes the work carried out during a six-month stay in Dr. John E. McGrady's group, which uses density functional theory to explore problems of structure, bonding and reactivity in inorganic compounds. The main focus of the research is metal-metal bonding in cluster compounds. Transition metals have a prominent position in catalysis, both in industrial and biological contexts, and the ultimate goal is to understand the part played by the metal and its ligand environment in facilitating a particular chemical transformation. Chapter 5 studies the electronic structure of the core catalysts and the mechanism of their, unresolved, catalytic activity. The various possible isomers with the generic formula were studied, and the energy of the electron transfer between the metallic centres and the sulphide ligands, so important in the catalytic process, was investigated in depth. Subsequently, the mechanism of the hydrogen activation reaction promoted by these bi-molybdenum compounds (Equation 1.1) was studied, and the nature of the different reaction pathways was elucidated.


[(CpMo(-S))2(S2CH2)] promotes the hydrogenation of alkynes to cis alkenes, allenes to alkenes, and ketenes to aldehydes and the hydrogenolysis of carbon disulphide to hydrogen sulphide and thioformaldehide. The mechanisms of hydrogen interaction with these complexes have not been established. It is apparent that the sulphide ligands play a role in hydrogen coordination, and this ligand reactivity was a dominant characteristic throughout the study of these dinuclear molybdenum systems.



Equation 1.1

In the framework of this thesis, a wide-ranging study of azaborinine derivatives was conducted, with a particular interest towards 9,10-B,N-phenanthrenes, holding an isosteric B-N unit in the place of a C=C bond and incorporating a B-C chiral axis. For this purpose, a preliminary theoretical study for four compounds of this class was carried out: conformational analysis and rotational energy barriers, UV-Vis absorption and fluorescence emission as well as theoretical ECD spectra of the atropisomeric structures were calculated by means of DFT and TD-DFT. An experimental attempt to synthesize and characterize another derivative of this class followed. The main synthetic concept evolved around the obtention of anti-aromatic 9-borafluorene precursors by boron-tin exchange from the reaction of a 9-stannafluorene derivative with boron reagents, followed by an aromaticity-driven ring-opening reaction of these borafluorene precursors with an organic azide. Several approaches based on similar previous works were reproduced to the closest possible version, as no absolutely inert conditions could be achieved, but the expected products were not formed; in one case, an open structure was obtained instead, for the formation of which a mechanism was proposed.

La catálisis es un campo de la ciencia que explora soluciones a los problemas ambientales como la contaminación, la eliminación de los residuos generados en el proceso de síntesis de materiales o la regeneración de los recursos naturales. En la presente Tesis, hemos reportado un estudio de cálculos DFT para la σ activación del enlace NH de amoníaco considerando las especies μ3-alquilidinos de titanio utilizando el complejo modelo [{Ti(η5-C5H5)(μ-O)}3(μ3-CH)]. Posteriormente, con el fin de analizar la hidroformilación asimétrica de estireno catalizada por complejos Rh-Binaphos, se han combinando estudios basados en la aproximación de la determinación del estado de transición y un análisis cualitativo a través de un descriptor molecular recién definido (volumen de distancia ponderada, VW). Usando nuestro conocimiento mecanicista anterior, hemos presentado un estudio QSPR para predecir la actividad y la enantioselectividad de la hidroformilación de estireno catalizada por complejos Rh-difosfinas. También, hemos desarrollado una nueva metodología 3D-QSSR para predecir la enantioselectividad basada en la cuantificación de la representación de diagramas por cuadrantes y aplicándola en el ciclopropanación asimétrica de alquenos catalizadas por complejos de cobre.
Catalysis is a field of science that explores solutions to environmental problems such as pollution, elimination of waste generated in the process of materials synthesis or regeneration of natural resources. In the present Thesis, we have reported a DFT study on the N-H σ-bond activation of ammonia by the µ3-alkylidyne titanium species using the [{Ti(η5-C5H5)(µ-O)}3(µ3-CH)] model complex. Afterwards, we have combined the TS-based approach and qualitative analysis through a newly defined molecular descriptor (distance-weighted volume, VW), in order to analyze the asymmetric hydroformylation of styrene catalyzed by Rh-binaphos complexes. Using our previous mechanistic knowledge, we have presented a QSPR study to predict the activity and the enantioselectivity in the hydroformylation of styrene catalyzed by Rh-diphosphane complexes. We have also developed a new methodology to predict enantioselectivity based on the quantitative quadrant-diagram representation of the catalysts and 3D-QSSR modeling; and we have applied it in the asymmetric cyclopropanation of alkenes catalyzed by copper complexes.

Density functional theory (DFT) is currently the most widely applied electronic structure theory in Chemistry. It is favoured for its computational efficiency, coupled with good accuracy. Although formally exact, approximations are required when practically applied. In the Kohn-Sham formalism, these approximations are contained within the exchange-correlation functional. Well established exchange-correlation functionals, such as the ubiquitous B3LYP, provide reasonable accuracy, but their continued use is increasingly based on the collective experience with the functional that has been accumulated, rather than the results that can be achieved. This thesis considers the circumstances under which conventional functionals fail and how a recent modification— coulomb attenuation—can resolve such issues. An outline of basic electronic structure theory is provided in chapter 1, particularly the formulation of the Hartree-Fock approach. This is extended to more sophisticated wavefunction based methods. Chapter 2 provides a formal proof for the validity of DFT as well as a framework for its implementation. A recently developed exchange-correlation functional (CAM-B3LYP) based on a varying quantity of exact exchange is outlined. Also discussed is the time-dependent DFT (TDDFT) approach to the determination of excitation energies, its failures and how such failures can be predicted and eliminated. The subsequent chapters consider the application of CAM-B3LYP to the description of weak interactions and excited states. Chapter 3 considers some key problems facing modern DFT—dispersion, fractional spins and fractional charges—in terms of the force and the Feynman electron density distortion, in addition to the conventional viewpoint of the energy. Two model systems, H2 and H+2 are employed to illustrate how increasing quantities of exact exchange can increase the fractional spin error while decreasing the fractional charge error, respectively. This is reflected in the improved description offered by CAM-B3LYP for H2 and the corresponding poor performance for H+2 . Chapter 4 takes a more detailed look at the dispersion interaction. C6 dispersion coefficients are calculated using a range of functionals—CAM-B3LYP showing a clear improvement over GGA and hybrid functionals. Dispersion corrected potential energy surfaces and interaction energies are determined with CAM-B3LYP providing comparable accuracy to other, existing long-range corrected functionals. Chapters 5 and 6 consider the application of CAM-B3LYP to the excited states of large systems of chemical and biological importance, respectively. In the former, the difficulty of comparing theoretically determined excitation energies with experimentally observed absorption spectra is of particular focus. In the latter, the failure of conventional functionals to correctly predict the energy and character of charge-transfer excitations is highlighted. For both cases, it is shown that CAM-B3LYP can provide a significant improvement over conventional functionals, all but eradicating the charge- transfer issue in the latter case. Chapter 7 further investigates the charge-transfer issue experienced by conventional functionals and illustrates how the error can manifest itself as an inaccuracy in the character of an excited state rather than the energy. CAM-B3LYP provides an accurate description of both aspects. Triplet excitation energies are determined from TDDFT and the ∆SCF approach—the latter providing improved results for conventional functionals.

The donor-acceptor couple CuPc/C60 has been the subject of recent studies in organic solar cells due to its combined abilities in light absorption (CuPc) and charge transport (C60). By better understanding the electronic and geometric nature of the system it is possible to shed light on how the molecules act under different conditions. In this study the geometric properties for three different configurations have been studied by means of Density Functional Theory (DFT). By comparing the molecular structure of pristine CuPc with the structure of CuPc in the presence of C60, a slight elongation of the bonds is observed when the fullerene is present. This is especially true for the Cu-N bonds. By further including van der Waals interactions, no change in bond lengths is observed. This, in turn, suggests that, most likely, the interaction between the two molecules is relatively weak and the C60 will not have a major influence on the electronic structure of CuPc. The N1s X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) calculations confirm these conclusions, as only very small changes in peak positions are observed when comparing pristine CuPc with CuPc/C60.
Tack vare sina egenskaper inom absorption och laddningsöverföring har CuPc och fullerenen C60 varit föremål för omfattande studier bland forskare inom organiska solceller. Genom att få större förståelse för den geometriska såväl som den elektroniska konfigurationen inom och mellan paret kan man förutse hur dessa kommer att bete sig i olika sammansättningar. I denna studie har de geometriska förutsättningarna studerats där olika konfigurationer beräknats genom täthetsfunktionalteori (DFT). Genom att mäta bindningslängderna mellan koppar, kol och de olika typer av kväve i CuPc i de olika systemen, kan det inses att bindningarna förlängs då C60 läggs till. Då van der Waals-interaktioner inkluderades observerades ingen större förändring i bindingslängderna i jämförelse med fallet utan van der Waals-interaktioner. Detta tyder på att växelverkan mellan de två molekylerna är relativt svag och att C60-fullerenen ej har någon större påverkan på elektronkonfigurationen i CuPc. Beräkningarna av N1s X-ray Photoelectron Spectroscopy (XPS) och Near Edge X-ray Absorption Fine Structure (NEXAFS) stödjer denna slutsats då endast små skiftningar i topparna observerades vid jämförelse mellan rent CuPc och CuPc/C60.

This diploma thesis is focused on calculation of both structure and electronic properties of the graphene after the adsorption of atomic and molecular oxygen and urea using the Density Functional Theory (DFT). The influence of van der Waals interactions on the structure and adsorption energy is studied, as well as influence of the thermal corrections, the charge density spatial distribution and the electronic doping of graphene after the adsorption of the adsorbant on the graphene.

In the electronics low-frequency noise is often used for reliability estimation of semiconductor devices. Spectral power density of this noise proportional to 1/ f, where γ is the spectral exponent. In [1] shown, that for spectral analysis of non-white noise exists optimum resolution bandwidth. Total mean square error of the estimate is minimal when analysis filter bandwidth is optimum. In [1] it is shown that DFT could be represented as result of processing by filters with different bandwidths by means of frequency averaging.

Le dopage non covalent de nanostructures carbonées par transfert de charge depuis/vers des molécules donneuses ou acceptrices (EDA) ou bien par des molécules d’acide sulfurique H2SO4, est considéré comme potentiellement intéressant pour de nombreuses applications. Parmi celles-ci on peut citer: capteur chimique, transistor à effet de champ, et d’autre l'électronique. Cependant, d'un point de vue théorique, on en sait peu au sujet de ces processus de transfert de charge par électrons ou par trous.Dans un premier temps, nous nous sommes intéressés à l’interaction entre des molécules d’acide sulfurique et des nanostructures modèles, car elles sont capables de doper des nanotubes, de s’intercaler dans le graphite et même d’aligner les tubes dans une phase nématique, ce qui pourrait mener à la création de matériaux composites à forte valeur ajoutée.Bien que certaines études théoriques DFT ont été menées récemment, leurs résultats restent source de confusion. Par exemple, même s’il est rapporté un transfert de charge entre une molécule de H2SO4 et un plan de graphène, tous nos efforts pour reproduire ces calculs ont été infructueux. Nous proposons dans ce travail de thèse, un mécanisme de réaction qui expliquent la "protonation" des parois du tube, tel que proposé dans la littérature. Enfin nous proposons un scénario possible pour une meilleure compréhension de la structuration à grande échelle des molécules d'acide autour de points d'ancrage, telles que des défauts, de la structure carbonée
Non-covalent doping of carbon nanostructures by charge transfer from/to donor/acceptor molecules (EDA) or by H2SO4 molecules, be it with holes or electrons, is usually thought as potentially interesting for many applications of carbon based nano-devices. However, from a theoretical point of view, little is known about such “charge transfer” processes.Employing first-principles method based on Density Functional Theory (DFT), we have studied in details, and proposed a model to rationalize, the interaction between a prototypical donor molecule the tetrathiafulvalene (TTF), a standard acceptor organic molecule, tetracyanoethylene (TCNE) and carbon nanostructures: graphene layer and SWNTs with various chiral indices. Main results concern structural and thermodynamic aspects including dispersion forces effects, and evidently electronic structure modifications of the nanostructures. Various adsorption modes and concentration effects have been investigated. At very low coverage values, we have reported a charge transfer between graphene and TCNE or TTF. Moreover, we have shown that the charge transfer can be enhanced by increasing the concentration of those two EDA molecules, as it has been demonstrated experimentally. Those results are beneficial for comprehending the nonchemical doping mechanism in graphene structure by means of charge transfers. Considering the interaction between these prototypical molecules and carbon nanotubes, we have found that charge transfers tend to decrease while the curvature of nanotube is increasing. Besides, a strong influence of the metallic/semi-conductor character of the SWNTs can be observed and be explained by the change of polarisability of the curved carboneous substrates. Additionally, we have studied the adsorption properties of sulfuric acid molecules, in its non-hydrated form, on carboneous nanostructures. Against the common believe, no charge transfer is observed in the H2SO4@graphene or H2SO4@CNTs cases, even at very high concentrations. Instead, in order to elucidate the origin of p-doping observed experimentally, we have proposed that molecule is responsible of the reversible doping. Besides we have shown that a proton transfer could cause the experimental phenomenon of crystallization of H2SO4 molecules on SWNT’s surface. Finally in such process, defects like vacancy are of first importance, since they could provide anchorage points for hydrogen atoms. The results of the present work will certainly help to understand the charge transfer and doping mechanism of carbon nanostructures by means of non-covalent functionalization, which is a promising method for their future applications

L'objectif de ce travail consistait à caractériser l'interface entre semi-conducteur et colorant, siège d'un transfert électronique responsable de la conversion de la lumière en électricité dans les cellules photovoltaïques de dernière génération. Deux systèmes dont les propriétés diffèrent nettement d'un point de vue expérimental ont été envisagés~: N3/TiO2 et éosine-Y/ZnO. La caractérisation a été effectuée de façon progressive en étudiant chacun des éléments pris séparément puis, dans un deuxième temps, les systèmes en interaction ont été pris en compte. Les spectres UV-Visibles de divers colorants, sous différents états de protonation et de chimies variées, ont tout d'abord été simulés à l'aide d'une approche Théorie de la Fonctionnelle de la Densité Dépendant du Temps (TDDFT) et d'une fonctionnelle hybride PBE0. Outre l'excellent accord avec les données expérimentales, cette étude nous a permis de proposer un nouveau complexe de coordination à base de sélénium dont les propriétés pour ce genre d'applications sont légèrement meilleures que celles du complexe le plus efficace à l'heure actuelle. Les systèmes solides ont été décrits à l'aide d'une approche périodique basée sur des fonctions locales en Théorie de la Fonctionnelle de la Densité (DFT) hybride. L'influence de divers paramètres calculatoires a été dégagée. Ce travail préliminaire a permis d'envisager l'interface entre semi-conducteur et colorant suivant une approche supercellule avec un seul point k (Gamma) et les propriétés structurales et électroniques des systèmes optimisés, ainsi que la faisabilité du transfert électronique à l'interface ont été étudiées. Nos résultats nous ont permis de proposer une hypothèse visant à expliquer les différences de rendement de conversion entre les systèmes N3/TiO2 et éosine-Y/ZnO.

Cette thèse présente l'implémentation des gradients analytiques pour une méthode mixte courte-portée DFT/ longue-portée WFT. Ces gradients ont été développés et implémentés dans le programme de calcul de chimie théorique MOLPRO, puis appliqués à différents systèmes d'intérêts chimiques/biologiques : les polymères conjugués polyacétylène, polyméthinéimine, les composés des bases de données de complexes faiblement liés DI6/04, HB6/04, WI9/04, CT7/04 et NHTBH38/04 ainsi qu'au composé à valence mixte triarylamine
This thesis presents the implementation of the analytical gradients for a range-separated short-range DFT / long-range WFT scheme. These gradients were developed and implemented in the ab initio program for molecular electronic structure calculation MOLPRO, and applied to systems of chemical/biological interest: conjugated oligomers (polyacetylene, polymethineimine), valence mixed coumponds, and to the weakly bond system databases HB6/04, CT7/04, DI6/04, WI9/04 and NHTBH38/04

In the vast realm of inorganic materials, the Cu2+-containing cuprates form one of the richest classes. Due to the combined effect of crystal-field, covalency and strong correlations, all undoped cuprates are magnetic insulators with well-localized spins S=1/2, whereas the charge and orbital degrees of freedom are frozen out. The combination of the spin-only nature of their magnetism with the unique structural diversity renders cuprates as excellent model systems. The experimental studies, boosted by the discovery of high-temperature superconductivity in doped La2CuO4, revealed a fascinating variety of magnetic behaviors observed in cuprates. A digest of prominent examples should include the spin-Peierls transition in CuGeO3, the Bose-Einstein condensation of magnons in BaCuSi2O6, and the quantum critical behavior of Li2ZrCuO4. The magnetism of cuprates originates from short-range (typically, well below 1 nm) exchange interactions between pairs of spins Si and Sj, localized on Cu atoms i and j. Especially in low-dimensional compounds, these interactions are strongly anisotropic: even for similar interatomic distances |Rij|, the respective magnetic couplings Jij can vary by several orders of magnitude. On the other hand, there is an empirical evidence for the isotropic nature of this interaction in the spin space: different components of Si are coupled equally strong. Thus, the magnetism of cuprates is mostly described by a Heisenberg model, comprised of Jij(Si*Sj) terms. Although the applicability of this approach to cuprates is settled, the model parameters Jij are specific to a certain material, or more precisely, to a particular arrangement of the constituent atoms, i.e. the crystal structure. Typically, among the infinite number of Jij terms, only several are physically relevant. These leading exchange couplings constitute the (minimal) microscopic magnetic model. Already at the early stages of real material studies, it became gradually evident that the assignment of model parameters is a highly nontrivial task. In general, the problem can be solved experimentally, using elaborate measurements, such as inelastic neutron scattering on large single crystals, yielding the magnetic excitation spectrum. The measured dispersion is fitted using theoretical models, and in this way, the model parameters are refined. Despite excellent accuracy of this method, the measurements require high-quality samples and can be carried out only at special large-scale facilities. Therefore, less demanding (especially, regarding the sample requirements), yet reliable and accurate procedures are desirable. An alternative way to conjecture a magnetic model is the empirical approach, which typically relies on the Goodenough-Kanamori rules. This approach links the magnetic exchange couplings to the relevant structural parameters, such as bond angles. Despite the unbeatable performance of this approach, it is not universally applicable. Moreover, in certain cases the resulting tentative models are erroneous. The recent developments of computational facilities and techniques, especially for strongly correlated systems, turned density-functional theory (DFT) band structure calculations into an appealing alternative, complementary to the experiment. At present, the state-of-the-art computational methods yield accurate numerical estimates for the leading microscopic exchange couplings Jij (error bars typically do not exceed 10-15%). Although this computational approach is often regarded as ab initio, the actual procedure is not parameter-free. Moreover, the numerical results are dependent on the parameterization of the exchange and correlation potential, the type of the double-counting correction, the Hubbard repulsion U etc., thus an accurate choice of these crucial parameters is a prerequisite. In this work, the optimal parameters for cuprates are carefully evaluated based on extensive band structure calculations and subsequent model simulations. Considering the diversity of crystal structures, and consequently, magnetic behaviors, the evaluation of a microscopic model should be carried out in a systematic way. To this end, a multi-step computational approach is developed. The starting point of this procedure is a consideration of the experimental structural data, used as an input for DFT calculations. Next, a minimal DFT-based microscopic magnetic model is evaluated. This part of the study comprises band structure calculations, the analysis of the relevant bands, supercell calculations, and finally, the evaluation of a microscopic magnetic model. The ground state and the magnetic excitation spectrum of the evaluated model are analyzed using various simulation techniques, such as quantum Monte Carlo, exact diagonalization and density-matrix renormalization groups, while the choice of a particular technique is governed by the dimensionality of the model, and the presence or absence of magnetic frustration. To illustrate the performance of the approach and tune the free parameters, the computational scheme is applied to cuprates featuring rather simple, yet diverse magnetic behaviors: spin chains in CuSe2O5, [NO]Cu(NO3)3, and CaCu2(SeO3)2Cl2; quasi-two-dimensional lattices with dimer-like couplings in alpha-Cu2P2O7 and CdCu2(BO3)2, as well as the 3D magnetic model with pronounced 1D correlations in Cu6Si6O18*6H2O. Finally, the approach is applied to spin liquid candidates --- intricate materials featuring kagome-lattice arrangement of the constituent spins. Based on the DFT calculations, microscopic magnetic models are evaluated for herbertsmithite Cu3(Zn0.85Cu0.15)(OH)6Cl2, kapellasite Cu3Zn(OH)6Cl2 and haydeeite Cu3Mg(OH)6Cl2, as well as for volborthite Cu3[V2O7](OH)2*2H2O. The results of the DFT calculations and model simulations are compared to and challenged with the available experimental data. The advantages of the developed approach should be briefly discussed. First, it allows to distinguish between different microscopic models that yield similar macroscopic behavior. One of the most remarkable example is volborthite Cu3[V2O7](OH)2*2H2O, initially described as an anisotropic kagome lattice. The DFT calculations reveal that this compound features strongly coupled frustrated spin chains, thus a completely different type of magnetic frustration is realized. Second, the developed approach is capable of providing accurate estimates for the leading magnetic couplings, and consequently, reliably parameterize the microscopic Hamiltonian. Dioptase Cu6Si6O18*6H2O is an instructive example showing that the microscopic theoretical approach eliminates possible ambiguity and reliably yields the correct parameterization. Third, DFT calculations yield even better accuracy for the ratios of magnetic exchange couplings. This holds also for small interchain or interplane couplings that can be substantially smaller than the leading exchange. Hence, band structure calculations provide a unique possibility to address the interchain or interplane coupling regime, essential for the magnetic ground state, but hardly perceptible in the experiment due to the different energy scales. Finally, an important advantage specific to magnetically frustrated systems should be mentioned. Numerous theoretical and numerical studies evidence that low-dimensionality and frustration effects are typically entwined, and their disentanglement in the experiment is at best challenging. In contrast, the computational procedure allows to distinguish between these two effects, as demonstrated by studying the long-range magnetic ordering transition in quasi-1D spin chain systems. The computational approach presented in the thesis is a powerful tool that can be directly applied to numerous S=1/2 Heisenberg materials. Moreover, with minor modifications, it can be largely extended to other metallates with higher value of spin. Besides the excellent performance of the computational approach, its relevance should be underscored: for all the systems investigated in this work, the DFT-based studies not only reproduced the experimental data, but instead delivered new valuable information on the magnetic properties for each particular compound. Beyond any doubt, further computational studies will yield new surprising results for known as well as for new, yet unexplored compounds. Such "surprising" outcomes can involve the ferromagnetic nature of the couplings that were previously considered antiferromagnetic, unexpected long-range couplings, or the subtle balance of antiferromagnetic and ferromagnetic contributions that "switches off" the respective magnetic exchange. In this way, dozens of potentially interesting systems can acquire quantitative microscopic magnetic models. The results of this work evidence that elaborate experimental methods and the DFT-based modeling are of comparable reliability and complement each other. In this way, the advantageous combination of theory and experiment can largely advance the research in the field of low-dimensional quantum magnetism. For practical applications, the excellent predictive power of the computational approach can largely alleviate designing materials with specific properties.

In recent years, multicarrier modulation techniques have stirred great interest among engineers and researchers working in the field of telecommunications. Multicarrier systems are characterized by the fact that constellation symbols are modulated in parallel onto several distinct subcarriers. One specific form of multicarrier modulation, referred to as OFDM (orthogonal frequency division multiplexing), has been deployed in many applications, such as in wireless LAN (local area network) routers, in high-definition television tuners, and in DSL (digital subscriber line) modems. Multicarrier and OFDM systems have proved to be much more robust against impairments such as impulse noise and sudden channel fadings than their single carrier counterparts. Moreover, channel equalization in OFDM systems can be performed at a very low computational cost by a set of per-subcarrier one-tap equalizers.
Despite their many advantages, OFDM systems have a few, but important, drawbacks. In particular, OFDM relies on the inverse FFT for modulation purposes, which leads to a very poor spectral containment and a high susceptibility to narrowband noise. To mitigate this problem, we propose in this thesis to perform multicarrier modulation using a perfect reconstruction (PR) DFT filter bank instead of employing the inverse FFT. The design of such filter banks is addressed using a novel method that guarantees the PR property to be satisfied while the spectral containment is being maximized. Equalization in the proposed DFT filter bank transceiver takes advantage of the fact that the filter banks do not contribute to any distortion due to its PR nature. Two equalization schemes are presented. The first one is based on a zero-padded block linear equalization approach, and the second one utilizes a one-tap per subcarrier configuration. The estimation of the channel coefficients in the proposed transceiver is also addressed. A blind estimation method that exploits the inherent cyclostationarity of the transmitted signal is derived. Computer experiments presented in this thesis indicate that the spectral containment of the proposed PR DFT filter bank transceiver is indeed superior to that of the OFDM system. Moreover, simulations conducted in a DSL-like environment contaminated by a narrowband noise show that the achievable bit rate of the proposed transceiver is much higher than that of a conventional OFDM system.

The enantioselective hydrogenation of a-ketoesters over a platinum surface modified by cinchona alkaloids has been studied using accurate computational modelling. A series of small prototype compounds were used to interact with the metal surface, leading to the study of acetone adsorption on a platinum surface. It was found that the enol and enolate isomers of acetone may play a crucial role in the surface chemistry. The enol form of acetone was significantly more stable than the keto-form, and the enolate form was also more stable. Vibrational frequency analysis suggests that the enolate form may prove to be a better fit to some of the experimental data than the commonly-accepted ketone form. The interaction of an a-ketoester, methyl pyruvate, with a platinum surface was then studied using the same techniques. The enol form was again the most stable surface species compared to the enolate and ketone forms. Additionally, the cis form of methyl pyruvate was generally more stable than the trans form. A full vibrational analysis was performed, allowing comparison with future experimental work. In order to investigate the conversion between the different types of isomer, an eigenvector following scheme is under development to find reaction barriers and transition states. To date, this has been applied to simpler systems with success and is in the process of being extended to more complicated applications. Concurrently, a technique to study the whole enantioselective reaction system was also being developed. This involves a modified simple QM/MM scheme containing a model for the surface, the substrate and modifier. This was successfully applied to a simple system (methyl pyruvate interacting with the surface and modifier) and yielded predicted ees in agreement with experimental observation. This was extended to a more complex system under study at Cardiff University and reproduced the experimental enantioselectivity this was then used to suggest possible improvements to enhance the ee of this system.

This dissertation focuses on computational study of the geometry and energetics small molecules and nanoclusters involving transition metals (TM). These clusters may be used for various industrial applications including catalysis and photonics. Specifically, in this work we have studied hydrides and carbides of 3d-transition metal systems (Sc through Cu), small nickel and gold clusters. Qualitatively correct description of the bond dissociation is ensured by allowing the spatial and spin symmetry to break. We have tested applicability of new exchange-correlation functional and alternative theoretical descriptions (spin-contamination correction in broken symmetry DFT and ensemble Kohn-Sham (EKS)) as well. We studies TM hydrides and carbides systems to understand the importance of underlying phenomenon of bond breaking in catalytic processes. We have tested several exchange-correlation functionals including explicit dependence on kinetic energy density for the description of hydrides (both neutral and cationic) and carbides formed by 3d-transition metals. We find M05-2x and BMK dissociation energies are in better agreement with experiment (where available) than those obtained with high level wavefunction theory methods, published previously. This agreement with experiment deteriorates quickly for other functionals when the fraction of the Hartree-Fock exchange in DFT functional is decreased. Higher fraction of HF exchange is also essential in EKS formalism, but it does not help when spin-adapted unrestricted approach is employed. We analyze the electron spin densities using Natural Bond Orbital population analysis and find that simple description of 3d electrons as non-bonding in character is rarely correct. Unrestricted formalism results in appreciable spin-contamination for some of the systems at equilibrium, which motivated us to investigate it further in details. In order to correct the spin contamination effect on the energies, we propose a new scheme to correct for spin contamination arising in broken-symmetry DFT approach. Unlike conventional schemes, our spin correction is introduced for each spin-polarized electron pair individually and therefore is expected to yield more accurate energy values. We derive an expression to extract the energy of the pure singlet state from the energy of the broken-symmetry DFT description of the low spin state and the energies of the high spin states (pentuplet and two spin-contaminated triplets in the case of two spin-polarized electron pairs). We validate our spin-contamination correction approach by a simple example of H2 and applied to more complex MnH system. Ensemble KS formalism is also applied to investigate the dissociation of C2 molecule. We find that high fraction of HF exchange is essential to reproduce the results of EKS treatment with exact exchange-correlation functional. We analyze the geometry and energetics of small nickel clusters (Ni2-Ni5) for several lowest energy isomers. We also study all possible spin states of small nickel cluster isomers and report observed trends in energetics. Finally we determine the geometry and energetics of ten lowest energy isomers of four small gold clusters (Au2, Au4, Au6, and Au8). We have also investigated the influence of cluster geometry, ligation, solvation and relativistic effects on electronic structure of these gold clusters. The effect of one-by-one ligand attachment in vacuum and solvent environment is also studied. Performance of five DFT functionals are tested as well; Local Spin Density Approximation (SVWN5), Generalized Gradient Approximation (PBE), kinetic energy density-dependent functional (TPSS), hybrid DFT (B3LYP), and CAM-B3LYP which accounts for long-range exchange effects believed to be important in the analysis of metal bonding in gold complexes and clusters. Our results exhibit the ligand induced stability enhancement of otherwise less stable isomers of Au4, Au6 and Au8. Ligands are found to play a crucial role in determining the 2D to 3D transition realized in small gold clusters. In order to select an appropriate theory level to use in this study, we investigate the effect of attachment of four different ligands (NH3, NMe3, PH3, PMe3) on cluster geometry and energetics of Au2 and Au4 in vacuum and in solution. Our results benchmark the applicability of DFT functional model and polarization functions in the basis set for calculations of ligated gold cluster systems. We employ five different basis sets with increasing amount of polarization and diffuse functions; LANL2DZ, LANL2DZ-P, def2-SVP, def2-TZVP, and def2-QZVP. We obtain NMe3 = NH3 < PH3 < PMe3 order of ligand binding energies and observe shallow potential energy surfaces in all molecules. Our results suggest appropriate quantum-chemical methodologies to model small noble metal clusters in realistic ligand environment to provide reliable theoretical analysis in order to complement experiments.
Ph.D.
Department of Chemistry
Sciences
Chemistry PhD

DFT methods were used to study the mechanism of methane oxidation using H2O2 over a TiO2 supported Au and Fe-ZSM-5 catalyst to produce CH3OH. DFT+U was employed to improve the model of the oxide surface, framework and framework metal centre in CHA and MFI structures. The catalysts were modelled in VASP. Au10 clusters were used to produce a preliminary mechanism, which was tested on the different design catalysts. The Au10 clusters were supported on TiO2 to model small clusters and extended Au, Pd and PdO surfaces were used to model larger clusters. The mechanism is tested on [Fe2O2]2+ which is in the zeolite extra-framework in both the CHA structure and MFI structure with two Al ions in the framework as counter ions. H2O2 is found to spontaneously break the HO – OH bond in the presence of Au10 clusters, Au(111) surface and Pd(111) surface. CH3OOH is produced via OOH formed from H2O2 and a radical methyl on the Au(111) and Pd(111) surfaces. Fe2O2 as an extra-framework species is modified with water to produce an active site. The active site was then further modified with H2O2 to produce a Fe4+ ═ O which is used to break a C – H bond in methane. The mechanism on the metal/oxide was shown to differ from the mechanism in the zeolite. The formation of the C – O bond in CH3OOH is the most important step on the metal/oxide whereas this is low energy or spontaneous in the zeolite. CH4 directly producing CH3OOH has a barrier of 50 kJ mol-1 on the modified Fe2O2 in the Al-MFI structure.

In this thesis, the kinetic issues in heterogeneous catalysis was widely investigated and developed except chapter 3 as an independent project. The micro-kinetics simulation was employed to investigate the activity as well as the selectivity in heterogeneous catalysis. Then we noticed the adsorbate-adsorbate interaction which could impose significant effect on the kinetic behaviour and was focused to building a bridge to connect the adsorbate-adsorbate interaction with the catalytic activity. In this situation, we first brought out a new micro-kinetics model to include the adsorbateadsorbate interaction on surface and em:ploy it in the reaction of I -butene to 2-butene in chapter 5. Moreover, we have frn1her studied the general trend of coverage effect on the BEP relation as well as the volcano curve which are considered as the key building blocks in theoretical calculation in heterogeneous catalysis. In the last chapter, we created a novel Kinetic Monte Carlo Simulation method that can address the lateral interaction to study the mysterious 'synergic effect' for bimetallic catalyst. It is admitted that coverage effect is a complex and widely existed issue while our method is qualitatively rather than quantitatively to address the impact. However, with our results, the understanding of the general trend as well as the nature behind the coverage effect is more clear and easy to understand.

The insatiable global demand for energy cannot be sustained by fossil fuels without irreparable damage to the environment. Various alternative energy sources are being investigated to provide renewable clean energy. One promising technology is the hydrogen fuel cell, which uses hydrogen and oxygen to produce electricity. However, the currently used catalyst support material, carbon black, corrodes in the low pH and oxidative environment. Therefore, new catalyst support materials are being sought. A new class of material, called MAX phases, shows potential because some possess a combination of properties of metals and ceramics. Three of them, Ti2AlC, Ti3AlC2, and Ti3SiC2, show good electrical conductivity and oxidation resistance. These MAX phases have been investigated using density functional theory (DFT) in this thesis to determine their properties. The density of states show that they are electrically conductive, with a continuous band over the Fermi level primarily from the Ti d orbital. Calculating the Boltzmann transport properties, yielded electrical resistivity values of 0.460 µΩ m for Ti2AlC, 0.370 µΩ m for Ti3AlC2, and 0.268 µΩ m for Ti3SiC2 at 300 K. Therefore, Ti3SiC2 should be the most electrically conductive of the three. The vacancy formation energy of an A group atom was investigated using a 2 x 2 x 2 supercell. The vacancy formation energies were calculated to be 2.882 eV for Ti2AlC, 2.812 eV for Ti3AlC2, and 2.167 eV for Ti3SiC2. The formation of a vacancy increases the electrical resistivity of the bulk MAX phases. As a catalyst support material, a MAX phase particle will have surfaces present. Due to the layered structure of the MAX phases, multiple terminations of (0 0 0 1) surfaces could be possible, which were investigated. It was shown that terminations where the Ti-C cage structure remained intact produced the lowest cleavage energies. For Ti2AlC, the two low cleavage energy surfaces are Al(Ti) and Ti(C), for Ti3AlC2, Al(Ti2) and Ti2(C), and for Ti3SiC2, Si(Ti2) and Ti2(C). The surfaces with the lowest cleavage energy should be more stable than other surfaces and would therefore be expected to be present on a MAX phase particle. Vacancies were also formed in the surface systems. The surfaces with the vacancy in the surface layer had the lowest vacancy formation energy, with that of Si(Ti2) being positive. The surface slabs generally showed a higher electrical resistivity than the bulk systems, while the formation of a vacancy generally increased the resistivity, in agreement with the bulk vacancy trend. These MAX phases are electrically conductive, however a quantifiable oxidation resistance was not able to be calculated. They do however show signs of being good electrocatalyst support materials.

Au cours de cette thèse, nous avons défini un protocole de calcul quantique permettant des simulations qualitativement et quantitativement précises des spectres d'absorptions et d'émission de colorants appartenant aux familles des aza-BODIPY, BODIPY, boranils et dioxaborines, Ce protocole utilise la TD-DFT pour déterminer avec efficacité les paramètres structuraux et vibrationnels des états fondamentaux et excités, et l'approche SOS-CIS(D) pour calculer précisément les énergies d'absorption et de fluorescence à l'aide de calcul verticaux. Cette approche "hybide" permet de traiter un grand nombre de composés en un temps de calcul relativement faible, d'atteindre une précision de l'ordre de 0. 1 eV ainsi qu'une cohérence avec les données expérimentales impressionnante (R2 > 0. 95). Plus précisément, nous avons montré que la fonctionnelle d'échange-corrélation M06-2X est la plus performante pour déterminer les énergies 0-0 et les topologies des bandes de ces molécules. En étudiant les effets de la base atomique, nous avons pu montrer que les structures et signatures vibrationnelles peuvent être calculées précisément avec une base compacte, 6-31G(d), tandis que les énergies totales requièrent l'utilisation d'une base plus étendue, 6-311+G (2d,p). Lors de cette thèse, plusieurs modèles de prise en compte des effets de solvant (LR, cLR et SS-PCM) ont été testés. Il en découle que SS-PCM est le modèle le plus adéquate pour les aza-BODIPY et les BODIPY tandis que les approches LR et cLR sont plus adaptées pour les boranils et les dioxaborines. Les modèles utilisés dans cette thèse ont été testés sur un grand nombre de composés en vue d'analyser les effets auxochromes, solvatochromes, acidochromes. . . En lien avec des données expérimentales récentes
During this thesis, we have defined an efficient protocol to simulate qualitatively and quantitatively the absorption and emission spectra of aza-BODIPY, BODIPY, boranils and dioxaborines dyes. In this protocol, we have used the TD-DFT level to determine the structural parameters and vibrational effects for both ground and excited states, and we have calculated the vertical absorption and emission transition energies at the SOS-CIS(D) level. This "hybride" approach can be used for a large number of compounds in a relatively low computing time to achieve an accuracy of about 0. 1 eV and an impressive consistency with the experimental data (R2 > 0. 95). Specifically, we have shown that the M06-2X exchange-correlation functional is the most efficient to determine the 0-0 energies and the topologies of the bands of these molecules. By studying the atomic basis set effects, we have found that the structures and vibrational signatures can be calculated precisely with a compact atomic basis set, 6-31G(d), while the total energies require the use of a larger atomic basis set, 6-311+G(2d,p). To Take into account the solvent effect, several PCM models (LR, cLR and SS-PCM) have been tested. SS-PCM approach is the most appropriate model for the aza-BODIPY and BODIPY while LR and cLR approaches are more suitable for boranils and dioxaborines. The models used in this thesis have been tested on a large number of compounds to analyze the auxochromic, solvatochromic, acidochromic effects. . . In connection with recent experimental data

The chemistry of actinides in solution is a very important aspect of the nuclear fuel cycle, especially as the energy needs of the world continue to increase. However, the radio-active nature of the actinides makes experimentation very difficult and dedicated expensive instruments are required. In addition, the disposal of radio-active waste materials requires a proper understanding of their chemistry at a molecular level. To tackle the problem, and to underpin the experimental studies, in this thesis we have studied the redox chemistry and disproportionation mechanism of actinyl complexes in solution using state-of-the art computational methods. Reduction potentials of actinyl complexes in solution have been estimated in solution using density functional theory (DFT) approaches. Solvation effects were included in the quantum chemistry calculations with the conductor like polarisable continuum model (CPCM) solvation method. First of all, we have validated our computational method by studying a variety of solute cavity definitions within the CPCM solvation model and assessed the performance of a range of DFT functionals to suitable to accurately describe the actinide chemistry in solution. Penta-valent uranyl(V) ions are unstable and readily disproportionate; in this study we have explored outer-sphere electron transfer and disproportionation mechanisms to determine the stability of these ions in solution. We have found that the process of outer-sphere disproportionation is unlikely to occur in non-aqueous solutions, such as DMSO, DMF, DCM, acetonitrile and pyridine, when the uranyl(V) ion is bound with a multi-dentate organic ligand. However, our computational results hypothesise that the presence of a trace of water in the experimental conditions can promote a disproportionation reaction by protonating the uranyl(V) ‘yl’ oxygen atoms and then the electron transfer process would proceed through either inner or outer sphere mechanism. In addition, the effect of alkali metal cations on the outer-sphere disproportionation mechanisms was also studied. Overall it has been shown that DFT can be used to accurately predict the redox properties of actinyl complexes in solution and thus contributing for an effective and efficient design of nuclear material separations, proper as well as safer radioactive waste disposal.