The results of kinetic isotope determination and computational studies on Noyori-type catalytic systems for the hydrogenation of ketones are presented. The catalysts examined include RuH2(NHCMe2CMe2NH2)(R-binap) and RuH(NHCMe2CMe2NH2)(PPh3)2. These complexes are active catalysts for ketone hydrogenation in benzene without addition of an external base. The kinetic isotope effect (KIE) for catalysis by RuH2(NHCMe2CMe2NH2)(R-binap) was determined to be 2.0 ± (0.1). The calculated KIE for the model system RuH(NHCH2CH2NH2)(PH3)2 was 1.3, which is smaller than the experimentally observed value but does not include tunneling effects. The complex OsH(NHCMe2CMe2NH2)(PPh3)2 is known to display autocatalytic behaviour when it catalyzes the hydrogenation of acetophenone in benzene. Pseudo first-order reaction conditions are obtained via addition of the product alcohol at the beginning of each kinetic experiment. The KIE determined using various combinations of deuterium-labeled gas, alcohol and ketone was found to be 1.1 ± (0.2). DFT calculations were used to explore the effect of the alcohol and the KIE. An induction period is observed at the start of the hydrogenation that is attributed to the formation of an alkoxide complex. A novel, diamine-orchestrated hydrogen-bonding network is proposed based on DFT calculations to explain how the alkoxide is converted back to the active catalyst. The tetradentate complexes trans-RuHCl[PPh2(ortho-C6H4)CH2NHCH2)]2 and RuHCl[PPh2(ortho-C6H4)CH2NHCMe2)]2 are known to be catalysts for the hydrogenation of acetophenone and benzonitrile in toluene when activated by KOtBu/KH. DFT studies were performed and a mechanism is proposed. The calculated rate limiting step for acetone hydrogenation was found to be heterolytic splitting of dihydrogen, which agrees well with experiment. The novel outer-sphere sequential hydrogenation of a CN triple bond and then a C=N double bond is proposed. A mechanism is proposed, which is supported by DFT studies, to explain the selectivity observed in the nucleophilic attack of amines or aziridines on palladium -prenyl phosphines complexes. Calculations on based on a palladium complex with two phosphorus donor ligands indicated that the observed selectivity would not be produced. Using two new model intermediates with either THF or aziridine substituted for a phosphine ligand trans to the unhindered side of the prenyl ligand did predict the experimentally observed selectivity.

Energetic materials comprise a set of systems of tremendous technological importance. Besides helping shape landscapes to establish communications, they have been used to reach fuel reservoirs, deploy safety bags and prevent heart strokes. Understanding its behavior can help in attaining strategic and tactical superiority, and importantly, preserve lives of people who handle these materials. The large discrepancy in length and time scales at which characteristic processes of energetic materials are of relevance pose a major challenge for current simulation techniques. We present a systematic study of crystalline energetic materials of different sensitivity and analyze their properties at different theoretical levels. Equilibrium structures, vibrational frequencies, conformational rearrangement and mechanical properties can be calculated within the density functional theory and molecular dynamics at finite temperatures. We have found marked differences in the calculated properties in systems with ranging sensitivities. Reactions at elevated temperatures have been studied using ab initio molecular dynamics method for crystals of nitroethane. Furthermore, while presenting the state of the art of energetic materials modeling, the limitations of each methodology are also discussed. Prospective systems and an elasticity driven approach that can be applied to other type of materials is also presented.

La metátesis de olefinas es el intercambio de sustituyentes entre dos unidades de alqueno. La importancia de esta reacción radica en su aplicación directa en la síntesis de una gran cantidad de sustancias, desde polímeros hasta moléculas con actividad biológica. Entre todos los procesos derivados de la metátesis de olefinas, la metátesis intramolecular de eninos y dienos son dos de las reacciones más usadas en síntesis orgánica ya que llevan a la formación de productos cíclicos que son habituales en los productos naturales. En particular, la reacción intramolecular de eninos genera un dieno cíclico y el acoplamiento intramolecular de dos fragmentos alqueno forma un cicloalqueno. La metátesis de olefinas procede solo en presencia de un catalizador. Entre todos los compuestos activos en metátesis, los compuestos tipo Grubbs-Hoveyda representan una de las familias más activas. En realidad, estos sistemas no representan el catalizador propiamente dicho sino un precatalizador que necesita ser activado para llevar a cabo la reacción deseada. Así pues, se pueden distinguir tres etapas en una reacción de metátesis catalizada por estos compuestos: activación del precatalizador, ciclo catalítico y la eventual recuperación del precursor. Esta tesis se dedica al estudio del proceso global de metátesis (activación, recuperación y selectividad del ciclo catalítico) de las reacciones intramoleculares de eninos y dienos. En concreto, la tesis se centra en: i) analizar la viabilidad de los mecanismos propuestos experimentalmente para la activación del precatalizador; ii) determinar si el proceso conocido como efecto boomerang (metátesis cruzada de la especie activa con el estireno del precursor) puede racionalizar las cantidades de catalizador que se recuperan; y iii) establecer los factores que influyen en la obtención del producto exo y endo durante el ciclo catalítico. Los resultados sugieren que, independientemente de la naturaleza del precursor y de la olefina, la etapa determinante de la velocidad global del proceso de activación del precatalizador es la disociación del estireno al final de la metátesis cruzada. Igualmente, se ha encontrado que, al nivel de cálculo empleado, el mecanismo disociativo es ligeramente más favorable que el mecanismo de intercambio y el asociativo. Sin embargo, los dos primeros pueden ser competitivos. Además, se observa que el efecto boomerang es posible termodinámica y cinéticamente y, además, debería ser un proceso fácil. Esto hace pensar que otros procesos deben tener lugar para justificar las cantidades de catalizador recuperado en la mayoría de experimentos. Finalmente, los cálculos indican que la presencia de sustituyentes en el enino influye de manera significativa en el producto final obtenido en el ciclo catalítico y que en especial los eninos con un fragmento alquino interno y un grupo alqueno disubstituido deberían favorecer la formación del compuesto endo. Las conclusiones obtenidas en esta tesis pueden ayudar en el futuro al diseño de nuevos catalizadores y en la optimización de procesos hacia un producto determinado. / The olefin metathesis reaction is the substituent exchange between two alkene molecules. This reaction is important because of its application in the synthesis of several compounds, from polymers to biologically active molecules. Among all derivative processes, the ring closing enyne and diene metathesis reactions (RCEYM and RCDEM respectively) are the most currently used reactions in organic synthesis, since they lead to the formation of cyclic products as found in many natural products. The RCEYM reaction generates a cyclic diene product, while the RCDEM one forms a cycloalkene and ethene. These processes require an appropriate catalyst to take place, the Grubbs-Hoveyda-type family being one of the most active in metathesis. Actually, these systems are not the catalytic species but precursors that need to be activated to carry out the reaction. Therefore, it can be distinguished three stages of the global metathesis process catalyzed by these compounds: precursor activation, the catalytic cycle and the eventual precursor recovery. This PhD thesis is devoted to the study of the global metathesis reaction (activation, catalytic cycle and recovery) of the ring closing enyne and diene metathesis reactions. In particular, this thesis focuses on: i) the analysis of the viability of the three experimentally proposed activation mechanisms: ii) determining whether the boomerang effect (cross metathesis between the active species and the styrene) can rationalize the amounts of experimentally recovered precursor and iii) evaluating the influence of reactant substituents in controlling the nature of the final product (exo or endo). Results suggest that the rate-determining step of the activation process is the styrene decoordination at the end of the cross metathesis process, independently of the precursor and olefin nature. Moreover, the dissociative mechanism seems to be the most favorable over the interchange and associative ones. However, the two first mechanisms tend to be competitive. On the other hand, it was found that the precursor recovery by the boomerang effect is possible both kinetically and thermodynamically and, also, it must be an easy process. This fact leads us to think that other processes must to take place in order to justify the amounts of experimentally recovered precursor. Finally, it was found that the presence of substituents in the enyne skeleton significantly influences the product obtained during the catalytic cycle of the ring closing enyne metathesis reaction. In particular, the reacting enynes with internal alkynes and disubstituted alkenes seem to favor the obtaining of the endo product in larger proportions. All these results can be used to rationalize the further design of new catalytic systems and the optimization of appropriate processes to obtain as specific product.

The [2+2+2] cycloaddition reaction involves the formation of three carbon-carbon bonds in one single step using alkynes, alkenes, nitriles, carbonyls and other unsaturated reagents as reactants. This is one of the most elegant methods for the construction of polycyclic aromatic compounds and heteroaromatic, which have important academic and industrial uses. The thesis is divided into ten chapters including six related publications. The first study based on the Wilkinson’s catalyst, RhCl(PPh3)3, compares the reaction mechanism of the [2+2+2] cycloaddition process of acetylene with the cycloaddition obtained for the model of the complex, RhCl(PH3)3. In an attempt to reduce computational costs in DFT studies, this research project aimed to substitute PPh3 ligands for PH3, despite the electronic and steric effects produced by PPh3 ligands being significantly different to those created by PH3 ones. In this first study, detailed theoretical calculations were performed to determine the reaction mechanism of the two complexes. Despite some differences being detected, it was found that modelling PPh3 by PH3 in the catalyst helps to reduce the computational cost significantly while at the same time providing qualitatively acceptable results. Taking into account the results obtained in this earlier study, the model of the Wilkinson’s catalyst, RhCl(PH3)3, was applied to study different [2+2+2] cycloaddition reactions with unsaturated systems conducted in the laboratory. Our research group found that in the case of totally closed systems, specifically 15- and 25-membered azamacrocycles can afford benzenic compounds, except in the case of 20-membered azamacrocycle (20-MAA) which was inactive with the Wilkinson’s catalyst. In this study, theoretical calculations allowed to determine the origin of the different reactivity of the 20-MAA, where it was found that the activation barrier of the oxidative addition of two alkynes is higher than those obtained for the 15- and 25-membered macrocycles. This barrier was attributed primarily to the interaction energy, which corresponds to the energy that is released when the two deformed reagents interact in the transition state. The main factor that helped to provide an explanation to the different reactivity observed was that the 20-MAA had a more stable and delocalized HOMO orbital in the oxidative addition step. Moreover, we observed that the formation of a strained ten-membered ring during the cycloaddition of 20-MAA presents significant steric hindrance. Furthermore, in Chapter 5, an electrochemical study is presented in collaboration with Prof. Anny Jutand from Paris. This work allowed studying the main steps of the catalytic cycle of the [2+2+2] cycloaddition reaction between diynes with a monoalkyne. First kinetic data were obtained of the [2+2+2] cycloaddition process catalyzed by the Wilkinson’s catalyst, where it was observed that the rate-determining step of the reaction can change depending on the structure of the starting reagents. In the case of the [2+2+2] cycloaddition reaction involving two alkynes and one alkene in the same molecule (enediynes), it is well known that the oxidative coupling may occur between two alkynes giving the corresponding metallacyclopentadiene, or between one alkyne and the alkene affording the metallacyclopentene complex. Wilkinson’s model was used in DFT calculations to analyze the different factors that may influence in the reaction mechanism. Here it was observed that the cyclic enediynes always prefer the oxidative coupling between two alkynes moieties, while the acyclic cases have different preferences depending on the linker and the substituents used in the alkynes. Moreover, the Wilkinson’s model was used to explain the experimental results achieved in Chapter 7 where the [2+2+2] cycloaddition reaction of enediynes is studied varying the position of the double bond in the starting reagent. It was observed that enediynes type yne-ene-yne preferred the standard [2+2+2] cycloaddition reaction, while enediynes type yne-yne-ene suffered β-hydride elimination followed a reductive elimination of Wilkinson’s catalyst giving cyclohexadiene compounds, which are isomers from those that would be obtained through standard [2+2+2] cycloaddition reactions. Finally, the last chapter of this thesis is based on the use of DFT calculations to determine the reaction mechanism when the macrocycles are treated with transition metals that are inactive to the [2+2+2] cycloaddition reaction, but which are thermally active leading to new polycyclic compounds. Thus, a domino process was described combining an ene reaction and a Diels-Alder cycloaddition. / La reacció de cicloaddició consisteix en la formació de tres enllaços carboni-carboni en un únic pas de reacció on poden estar involucrats alquins, alquens, nitrils, carbonils i altres compostos insaturats. És un dels mètodes més elegants per a la construcció de compostos aromàtics i heteroaromatics policíclics amb importants usos acadèmics i industrials. La tesi es divideix en deu capítols que contenen sis publicacions relacionades. El primer estudi es basa en el catalitzador de Wilkinson, RhCl(PPh3)3, on es compara el mecanisme de reacció del procés de cicloaddició d’acetilè pel que s’obté amb el model del complex, RhCl(PH3)3. Aquest projecte de recerca va ser iniciat per estudiar la substitució de lligands PPh3 per PH3 en els estudis de DFT, que s’aplica habitualment per reduir el cost computacional, tot i que els efectes electrònics i estèrics produïts pels lligands PPh3 són molt diferents dels creats per PH3. Malgrat algunes diferències observades, es va constatar que la substitució de PPh3 per PH3 en el catalitzador pot ser utilitzada per reduir el cost computacional de manera significativa i a l’hora obtenir resultats qualitativament acceptables. Un cop obtinguts els resultats anteriors, es va utilitzar el model del catalitzador de Wilkinson, RhCl(PH3)3, per l’estudi teòric de diferents reaccions de cicloaddició amb sistemes insaturats duts a terme al laboratori. En el grup de recerca es va trobar que en el cas de sistemes totalment tancats, concretament els macrocicles de 15 i 25 baules, poden donar sistemes benzènics policíclics excepte en el cas del macrocicle de 20 baules, que va resultar inactiu vers el catalitzador de Wilkinson. En aquest estudi, la realització de càlculs teòrics va permetre determinar l’origen de la diferent reactivitat del macrocicle de 20 baules, on es va trobar que la barrera d’activació de l’addició oxidativa entre dos alquins és molt més alta que les que es van obtenir pel macrocicle de 15 i 25 baules. Aquesta barrera es va atribuir bàsicament a l’energia d’interacció, la qual correspon a l’energia que s’allibera quan els dos reactius deformats interaccionen en l’estat de transició. Concretament el principal factor que hi contribueix és que el macrocicle de 20 baules presenta més estabilitat i més deslocalització de l’orbital HOMO en el pas d’addició oxidativa. A més a més, es va observar que la formació d’anells de 10 baules durant la cicloaddició del macrocicle de 20 baules presenta impediments estèrics importants. Per altra banda, en el Capítol 5 es presenten estudis electroquímics realitzats en col•laboració amb la Prof. Anny Jutand de París, que van permetre estudiar el cicle catalític de la reacció de cicloadició entre un dií i un monoalquí. Es van obtenir així les primeres dades cinètiques dels dos principals passos del cicle catalític amb el complex de Wilkinson, on es va observar que el pas determinant de la reacció pot variar en funció de l’estructura dels reactius de partida. En el cas en què en la reacció de cicloaddició participin dos triples i un doble enllaç en la mateixa molècula (endiins), és conegut que l’addició oxidativa pot donar-se entre dos triples enllaços o un triple i un doble enllaç. El model del catalitzador de Wilkinson va ser utilitzat mitjançant càlculs DFT per analitzar els diferents factors que poden influir en el mecanisme de reacció. Aquí es va observar que els endiins cíclics sempre prefereixen l’addició oxidativa entre els dos triples enllaços, mentre que els acíclics tenen diferent preferència en funció del linker i dels substituents presents en els triples enllaços. A més a més, el mateix model de Wilkinson es va utilitzar per explicar els resultats experimentals realitzats en el Capítol 7 on s’estudia la reacció de cicloaddició d’endiins variant la posició del doble enllaç en el reactiu de partida. Es va observar que els sistemes in-en-in preferien la cicloaddició convencional donant el producte ciclohexadienic esperat, mentre que els sistemes in-in-en patien una β-eliminació seguida d’una eliminació reductiva del catalitzador de Wilkinson i donant, finalment, productes ciclohexadiènics els quals són isòmers dels que s’obtindrien mitjançant una cicloaddició convencional. Finalment, l’últim capítol d’aquesta tesi es basa en l’ús de càlculs DFT per determinar el mecanisme de reacció quan els macrocicles són tractats amb metalls de transició inactius per donar la reacció de cicloaddició, però reaccionen tèrmicament obtenint nous compostos policíclics. Així es va descriure un procés dòmino on es combina una reacció ene seguida d’una cicloaddició de Diels-Alder.

La Ingeniería molecular se basa en el conocimiento de las características estereoelectrónicas que definen el reconocimiento molecular, que es el resultado de una complementariedad, tanto geométrica como electrónica, entre diferentes entidades moleculares. La importancia de las diferentes contribuciones electrostáticas nos permite realizar un estudio teórico de predicción de la reactividad y otras propiedades moleculares a partir de cálculos de potencial electrostático y de polarización moleculares. El presente trabajo se basa en el estudio de la utilización de los mapas de potencial electrostático y de potencial de polarización como descriptores moleculares. En primer lugar se realiza un estudio del efecto de la base y de la metodología empleada en el cálculo de propiedades eléctricas de primer y segundo orden. El análisis se lleva a cabo con las moléculas de cianuro de hidrógeno, formaldehído y urea. Las bases utilizadas son del tipo doble zeta estándar, a las cuales se han añadido funciones de polarización y difusas. En particular, se han utilizado la base doble zeta 6-31G(d), les bases doble zeta aumentadas con uno o dos conjuntos de funciones de polarización : 6-31G(d,p) , 6-31G(2d,2p) y también se ha utilizado la base 6-311G++(2d,2p), que incluye funciones difusas. Los diferentes niveles de cálculo utilizan metodologías Hartree-Fock, MÆller-Plesset de segundo y cuarto orden y teoría del funcional de la densidad (DFT) : SCF, MP2, MP4, BLYP i B3LYP. Se analiza el efecto de los diferentes conjuntos de base a la contribución de la polarización a la energía de interacción, calculando para cada sistema propiedades de primer orden, como son los momentos dipolares y los momentos cuadrupolares, y propiedades de segundo orden, como la polarizabilidad y hiperpolarizabilidad moleculares. Seguidamente se evalúa el efecto de la base y el método de cálculo en la obtención de potenciales electrostáticos y de polarización moleculares. Se realiza un estudio comparativo de los mapas calculados con diferentes bases y metodologías, en concreto un estudio de la distribución espacial y un análisis de correlación entre las diferentes bases y metodologías. Un análisis de los mapas de polarización molecular a partir del cálculo de las diferencias de polarización relativas y las desviaciones estándar correspondientes nos permite un estudio comparativo de las diferentes metodologías y bases utilizadas. En particular se realiza un análisis comparativo entre diferentes métodos de cálculo con la base 6-311G++(2d,2p), tomando como referencia el cálculo MP4. Finalmente, se utilizan los mapas de potencial electrostático, de polarización y de interacción para el análisis de las características de reconocimiento molecular de un conjunto de compuestos bioactivos, a fin de analizar la importancia de la contribución de la polarización. Por este motivo, se elige para el estudio un conjunto de moléculas con una alta polarizabilidad, y en concreto, dos familias de compuestos con abundantes átomos de cloro y con una actividad tóxica definida, que forman parte de los grupos de dioxinas y furanos. Para ello se realiza el estudio de la inclusión de la polarización molecular como descriptor en la predicción de la actividad biológica de dioxinas y furanos, realizando el cálculo de potenciales electrostáticos y de polarización, un análisis de los mapas de potencial, y definiendo las principales zonas de interacción electrostática y de polarización molecular a partir de cálculos de componentes principales (PCA), así como la predicción de la actividad biológica en base a un estudio realizado mediante cálculos de mínimos cuadrados parciales (PLS). / Molecular engineering is based on the knowledge of the stereoelectronic features that define the molecular recognition, which is the result of the complementarity of geometric and electronic features between two different molecular entities. The importance of different electrostatic contributions allows us to make a theoretical prediction of molecular properties, from calculations of electrostatic potential and molecular polarization. This work is based on the study of the use of maps of electrostatic potential and polarization potential as molecular descriptors. First there is a study of the effect of the basis set and the methodology used in the calculation of first and second order electrical properties. The analysis was carried out with the molecules of hydrogen cyanide, formaldehyde and urea. The base sets used are the standard double-zeta, to which were added polarization and diffuse functions. In particular, we have used the double zeta basis 6-31G (d), double zeta basis augmented with one or two sets of polarization functions: 6-31G (d, p), 6-31G (2d, 2p) and also used the base 6-311G + + (2d, 2p), which includes diffuse functions. The different levels of calculation methodologies used Hartree-Fock, Moller-Plesset second and fourth order and the theory of density functional (DFT): SCF, MP2, MP4, BLYP B3LYP i. We analyze the effect of different sets based on the contribution of polarization to the interaction energy, calculated for each system of first order properties, such as dipolar moments and quadrupolar moments, and second order properties such as polarizability and molecular hyperpolarizability. Further, the effect of the basis and method on the calculation of the electrostatic potential and molecular polarization potentials is evaluated. For this purpose we performed a comparative study of the maps calculated with different basis sets and methodologies, in particular a study of the spatial distribution and correlation analysis between the different data bases and methodologies was done. An analysis of molecular polarization maps by calculating the differences in polarization and relative standard deviation allows for a comparative study of different methodologies and bases used. Specifically, a comparative analysis between different methods of calculating the base 6-311G + + (2d, 2p), calculated by reference to the MP4 was done. We use maps of electrostatic, polarization and interaction potentials for the analysis of molecular recognition features of a set of bioactive compounds, to discuss the importance of the contribution of polarization. For this reason, we choose to study a set of molecules with high polarizability, specifically, two families of compounds with heavy atoms of chlorine and defined toxic activity, which are part of groups of dioxins and furans. A study of the polarization maps as indicators of biological activity of dioxins and furans, based on the best methodology is done. The inclusion of polarization as a molecular descriptor for predicting biological activity of dioxins and furans was studied from the calculation of electrostatic potentials and polarization, an analysis of the potential maps, and defining the main areas of interaction molecular electrostatic and polarization interaction from calculations of principal components (PCA) and the prediction of biological activity based on a study by calculation of partial least squares (PLS).

Nous avons étudié par calcul ab initio la structure et la stabilité de différentes phases cristallographiques du trioxyde de tungstène en volume et ensurface, ainsi que l'effet du dopage par le potassium. L'étude a été motivéepar des travaux expérimentaux sur la croissance de nano-bâtonnets WO3 surun substrat de mica ; ces nanostructures adoptent la phase hexagonale, mé-tastable dans le WO3 massif. La première partie du manuscrit est consacrée àl'étude du volume. Les structures atomiques des phases hexagonales et monocliniquessont déterminées et comparées aux différents modèles proposés dansla littérature expérimentale. La stabilité relative de ces deux phases est calculée en fonction du taux de potassium permettant d'obtenir un diagrammede phase approximatif de KxWO3 montrant que la phase hexagonale est stablepour x compris entre 3 et 35%. Par ailleurs, les distorsions disparaissentpour des concentrations de K suffisamment élevées. Ces résultats expliquentla stabilisation de la phase hexagonale dans les nano-bâtonnets par l'insertiond'atomes de K de la surface du mica. Dans la seconde partie, une étudesystématique des surfaces, en phase monoclinique et hexagonale, est présentée. Pour chaque phase, les trois orientations de surface de plus faibles indicesde Miller sont considérées. Toutes les terminaisons non-reconstruitesainsi que plusieurs types de reconstructions ont été analysées en terme depolarité, stabilité et structure atomique. Pour la phase monoclinique, l'orientation(010) avec reconstruction c(2x2) est la surface la plus stable, enaccord avec des données expérimentales. Quant à la phase hexagonale, c'estla surface c(1x1)O2(1120) qui a la plus petite énergie de surface (0.02 eV/Å2).Nous trouvons que la stabilité des différentes orientations est très sensible audopage par le potassium. En effet, pour KxWO3 avec x = 13 , c'est l'orientation (0001) qui est la plus stable, avec une terminaison KO3 et une énergiede surface de 0.05 eV/Å2.

The purpose of this diploma project has been to investigate the elastic properties of hexagonal close-packed Fe-Ni-Mg alloys at high pressure. Recent research has suggested that iron and magnesium can form an alloy under high pressure because of the great compressibility of the magnesium atoms. This also makes it possible for magnesium alloying with nickel atoms which are very similar to iron so that we get Fe-Ni-Mg alloys. Learning more about the elastic properties of iron alloys at high pressure will give us a better understanding of the inner core of our planet, which is believed to be composed primarily of iron. The calculations are based on a ab-inito method supported on the Density Functional Theory. The calculations were performed with a simulation package based on the Exact Muffin-Tin Orbitals theory, in conjuction with the Coherent Potential Approximation. The effects that small impurities can have on iron are remarkable.

Currently, there are two main methodologies for the calculation of the electronic structure and properties of crystalline solids. Known as the Hartree-Fock Method (HF) and the Density Functional Theory (DFT) methods, they are based on two different theories for the numerical solution of the many electron Schrödinger equation. Unfortunately, in highly correlated electron systems like transition metal complexes, both the HF and DFT methods have severe shortcomings. In some cases they fail to provide the correct description of the electronic structure. <p>In general, the HF method overestimates the energy band gap due to the neglect of electron correlation effects and the incorrect description of electron interactions in the unoccupied orbitals. In contrast, even though electron correlation effects are implicitly included in the density functional, DFT often underestimates the band gap due to the improper treatment of the electron self-interaction. <p> To amend these problems, two approaches have been proposed. The deficiency in the HF scheme can be corrected using a hybrid method which adds exchange correlation energy borrowed from DFT to help reduce the band gap energy and bring the predictions in better agreement with experiment. To improve DFT, the LDA+U approach, which uses a model Hubbard-like Hamiltonian including an on-site repulsion parameter U, can be employed. This method is a convenient semi-quantitative way to efficiently calculate the band gap of insulators and semiconductors.<p> In this thesis, the electronic structure of YTiO₃ under pressure is investigated using the aforementioned approaches. The performance and reliability of these methods will be examined, compared and discussed.

This work tries to show the significant competence and functionality of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) as theoretical approaches, supporting experimental measurements in various fields of physics from astrophysics to surface science, to study the electronic structure and spectroscopic properties of molecules and clusters: Silicon nanocrystals: Due to their optical properties, silicon nanocrystals have attracted considerable attention in astrophysics. In this work, the optical properties of H-passivated silicon nanocrystals are determined by the energetics of the frontier orbitals and their dependence on the deformation in the relaxed excited state, using DFT and TD-DFT. The Jahn-Teller effect in the lowest excited state results in a distortion toward tetragonal symmetry, contributing significantly to the red shift of the photoluminescence (PL) spectra. Therefore, the deformation in the relaxed excited state consists of a symmetry conserving part and of a symmetry-breaking distortion from Td toward D2d. For nanocrystals up to a diameter of 1.5 nm, we project the deformations at the minima of the excited state potential surface onto the different symmetries, allowing for a discrimination of the respective contributions to the total Stokes shift. The results show a quantitative agreement between the TD-DFT calculations of PL gap and the observed PL energies better than 0.2 eV. It is also seen that the large PL linewidth is the fundamental property of each cluster, not of ensemble average over clusters of different size. 2,3-Benzofluorene: We have presented new theoretical results on the absorption spectroscopy of 2,3-benzofluorene (Bzf) for the transition from the ground state, S0, to the first electronically excited singlet state, S1, to support the measurements of S1(1A´) <- S0(1A´) absorption spectrum of jet-cooled Bzf. The potential energy surfaces (PESs) of the S(n=0,1,2) states of Bzf have been investigated with calculations based on DFT and TD-DFT. At the B3LYP/TZ level of theory, TD-DFT does not deliver a realistic difference between the excited S1 and S2 potential energy surfaces, a problem which can be avoided by introducing a reference geometry (q*) where this difference coincides with the observation. In this geometry, an expression for the Herzberg-Teller corrected intensities of the vibronic bands is proposed, allowing a straightforward assignment of the observed a′ modes below 900 cm−1, including realistic calculated intensities. In spite of the difficulties caused by the small energy difference between the S1 and S2 states, we have obtained a reasonable theoretical absorption spectrum based on a TD-DFT approach applied to the slightly modified molecular geometry. Although the agreement between the theoretical and observed spectra is very good only for vibrational modes with frequencies lower than 900 cm−1, we consider our calculations to be the best approach to an ab initio study realized for Bzf until now since only parametrized force fields had been used before. (Ni–, Pd–, Pt–) Phthalocyanine: We studied the HOMO–LUMO gap shrinking in order to investigate the tip-induced polarization in scanning tunneling spectroscopy (STS) of d8 (Ni, Pd, and Pt) phthalocyanines. By means of DFT, the electronic structure and vibronic properties of single neutral NiPc, PdPc, and PtPc and their singly and doubly ionized cations and anions have been calculated. Interestingly, the position of the HOMO decreases systematically with increasing the atomic number of the central metal atom. The first ionization energies of neutral molecules increase by changing the central metal atom, while the electron affinities remain constant. This causes an increase in the HOMO–LUMO gap. These results show a clear correlation to experimental observations. Furthermore, partitioning of the reorganization energy, corresponding to the photoelectron spectra of the first and second ionizations of studied molecules, into normal mode contributions shows that the major contributions are due to several vibrational modes with a1g symmetry and energies lower than 1600 cm−1. The results reveal that the reorganization energy due to the singly positive ionization in the studied molecules is about one order of magnitude less than other reorganization energies, which makes these metal–phthalocyanines more attractive as electron donor for intramolecular electron transfer in electron acceptor–donor systems.

The formation of base pair radical anions and cations is closely related to many fascinating research fields in biology and chemistry such as genetic mutation, radiation-induced DNA damage and dynamics of charge transfer in DNA. However, the relevant knowledge so far mainly comes from studies on isolated base pair radical anions and cations, and their behavior in the DNA environment is less understood. In this study, we focus on how the nucleobase sequence affects the properties of the guanine¡Vcytosine (G:C) base pair radical anion and cation. The energetic barrier and reaction energy for the proton transfer along the N1(G)¡VH¡E¡E¡EN3(C) hydrogen bond and the stability of (G:C)¡E (i.e., electron affinity and ionization potential of G:C) embedded in different sequences of base-pair trimer were evaluated using density functional theory and two-layer ONIOM method. The computational results demonstrated that the presence of neighboring base pairs has an important influence on the behavior of (G:C)¡E in the gas phase. The excess electron and positive hole were found to be localized on the embedded G:C and the charge leakage to neighboring base pairs was very minor in all of the investigated sequences. Accordingly, the sequence behavior of the proton transfer reaction and the stability of (G:C)¡E is chiefly governed by electrostatic interactions with adjacent base pairs. However, the effect of base stacking, due to its electrostatic nature, is severely screened upon hydration, and thus, the sequence dependence of the properties of (G:C)¡E in aqueous environment becomes relatively weak and less than that observed in the gas phase. The effect of geometry relaxation associated with neighboring base pairs as well as the possibility of proton transfer along the N2(G)¡VH¡E¡E¡EO2(C) channel have also been investigated. The implications of the present findings to the electron transport and radiation damage of DNA are discussed.