Cette thèse comporte deux volets distincts. Le premier, qui fait l'objet du chapitre 2, porte sur les modèles mathématiques en calcul de structures électroniques, et consiste plus particulièrement en l'étude des modèles de type Kohn-Sham avec fonctionnelles d'échange-corrélation LDA et GGA. Nous prouvons, pour un système moléculaire neutre ou chargé positivement, que le modèle Kohn-Sham LDA étendu admet un minimiseur, et que le modèle Kohn-Sham GGA pour un système contenant deux électrons admet un minimiseur. Le second volet de la thèse traite de problématiques diverses en homogénéisation. Dans les chapitres 3 et 4, nous nous intéressons à un modèle de matériau aléatoire dans lequel un matériau périodique est perturbé de manière stochastique. Nous proposons plusieurs approches, certaines rigoureuses et d'autres heuristiques, pour calculer au second ordre en la perturbation le comportement homogénéisé de ce matériau de manière purement déterministe. Les tests numériques effectués montrent que ces approches sont plus efficaces que l'approche stochastique directe. Le chapitre 5 est consacré aux couches limites en homogénéisation périodique, et vise notamment, dans le cadre parabolique, à comprendre comment prendre en compte les conditions aux limites et initiale, et comment corriger en conséquence le développement à deux échelles sur lequel repose classiquement l'homogénéisation, pour obtenir des estimations d'erreur dans des espaces fonctionnels adéquats / This thesis is divided into two parts. The first part, that coincides with Chapter 2, deals with mathematical models in quantum chemistry, and specifically focuses on Kohn-Sham models with LDA and GGA exchange-correlation functionals. We prove, for a neutral or positively charged system, that the extended Kohn-Sham LDA model admits a minimizer, and that the Kohn-Sham GGA model for a two-electron system admits a minimizer. The second part is concerned with various issues in homogenization. In Chapters 3 and 4, we introduce and study a model in which the material of interest consists of a random perturbation of a periodic material. We propose different approaches, either rigorous or formal, to compute the homogenized behavior of this material up to the second order in the size of the perturbation, in an entirely deterministic way. Numerical experiments show the efficiency of these approaches as compared to the direct stochastic homogenization process. Chapter 5 is devoted to boundary layers in periodic homogenization, in particular in the parabolic setting. It aims at giving a better understanding of how to take into account boundary and initial conditions, and how to correct the two-scale expansion on which homogenization is classically grounded, to obtain fine error estimates

Cette thèse a pour objectif l’étude des propriétés optiques non linéaires (ONL) de second ordre de molécules de référence et de composés organiques photochromes, via des calculs de chimie quantique et des expériences de diffusion Hyper-Rayleigh (HRS). L’étude des réponses optiques de second harmonique d’une série de solvants organiques met tout d’abord en évidence la nécessité d’utiliser des méthodes théoriques intégrant les effets de la corrélation électronique, les effets d’environnement, ainsi que des bases très étendues pour évaluer quantitativement les premières hyperpolarisabilités moléculaires. La décomposition du tenseur de première hyperpolarisabilité en contributions dipolaires et octupolaires via le formalisme des harmoniques sphériques permet également d’illustrer l’effet de la symétrie des composés sur leurs réponses ONL. La seconde étude porte sur la caractérisation des effets de solvant sur les propriétés ONL d’un dérivé anil présentant un équilibre entre une forme énol et une forme cétone. Il est démontré que l’équilibre énol/cétone peut être déplacé vers la forme cétone en augmentant la proportion d’éthanol dans le mélange binaire de solvants éthanol/cyclohexane, conduisant à une augmentation de l’intensité HRS du mélange. La dernière partie de la thèse propose une étude théorique systématique des effets de substitution sur les propriétés ONL de sels de flavylium, dans le but d’optimiser le contraste des réponses ONL entre les différentes formes des composés. Il est démontré qu’une combinaison optimale de substituants permet d’obtenir de forts contrastes, démontrant le potentiel de ces composés comme systèmes multi-états pour des applications ONL. / This PhD addresses the nonlinear optical (NLO) properties of the reference organic molecules and molecular switches, by means of quantum chemical calculations and Hyper-Rayleigh Scattering (HRS) measurements. The thorough investigation of the NLO responses of a series of organic solvents first evidences the need of using theoretical methods including environment and electron correlation effects, as well as highly extended basis sets to quantitatively evaluate the molecular first hyperpolarizabilities. Decomposing the first hyperpolarizability tensor into dipolar and octupolar contributions via the spherical harmonics formalism also highlights the impact of the molecular symmetry on the NLO responses. The second part addresses the solvent effects on the NLO properties of an anil derivative, which exhibits an equilibrium between a enol and a keto form. It is shown that the enol/keto equilibrium is shifted towards the keto form when increasing the ethanol ratio in ethanol/cyclohexane solvent mixtures, leading to an increase of the HRS intensity. The last part addresses the effects of the chemical substituents on the NLO properties of flavylium salts, with the aim of optimizing the NLO contrasts between the various forms of the compounds. It is found that optimal combinations of chemical substituents lead to high NLO contrats, which demonstrates the potential of these compounds as multi-states systems for NLO applications

Les interrupteurs moléculaires sont des composés capables de commuterréversiblement entre deux ou plusieurs états stables en réponse à un stimulusextérieur. Lʼobjectif de la thèse est la conception dʼinterrupteurs moléculairesprésentant des contrastes optiques non-linéaires (ONL) et la mise évidence desparamètres structuraux et électroniques menant à dʼimportants contrastes depremière hyperpolarisabilité (β) via une approche multidisciplinaire qui combine lasynthèse de nouveaux composés, la caractérisation de leurs réponses optiqueslinéaires et non-linéaires par spectroscopie dʼabsorption UV-visible et par diffusionhyper-Rayleigh et lʼutilisation des méthodes de la chimie théorique afin de prédire etdʼinterpréter les propriétés moléculaires. Ces phénomènes de commutationsréversibles et les changements de propriétés qui les accompagnent présentent denombreux intérêts, tant technologiques comme lʼélaboration dʼordinateursmoléculaires quʼau niveau des organismes vivants où de nombreuses fonctionsbiologiques sont basées sur un phénomène de commutation. Les principaux résultatsde nos travaux se situent au niveau de lʼinterprétation des réponses ONL et de leurscontrastes en fonction de la nature, de la position et du caractère donneur/accepteurdes substituants présents sur le squelette des interrupteurs moléculaires. / Molecular switches are compounds presenting the ability to commutereversibly between two or more states in response to external stimuli. The goal of thework is the design of molecular switches exhibiting contrasts of their second-ordernonlinear optical (NLO) properties and the highlight of the structural and electronicparameters leading to large contrasts of first hyperpolarizability (β) via amultidisciplinary approach combining the synthesis of new compounds, thecharacterization of their linear (by UV-Visible absorption spectroscopy) and nonlinearoptical properties (by hyper Rayleigh scattering), and the theoretical simulations inorder to predict and interpret molecular properties. These reversible switchingprocesses and the resulting variations of molecular properties have many interests intechnological area such as the development of molecular computers or in lifesciences since many biological functions are based on commutation mechanisms.The major results of our investigations are the interpretation of the NLO responsesand contrasts as a function of the nature, the positioning, and the donor/acceptorcharacter of the substituents.

Cette thèse concerne l'ouverture de cycle des spiropyranes (SP) et leur utilisation comme interrupteurs contrôlés par impulsions laser. Nous avons élaboré un modèle théorique pour étudier cette réaction photochimique et développer des stratégies de contrôle.Les SP présentent des effets non adiabatiques impliquant un traitement quantique pour les électrons et les noyaux. L'approche multiconfigurationnelle dépendante du temps (MCTDH) est idéale car elle peut traiter des dynamiques quantiques impliquant plusieurs états électroniques. MCTDH a été appliqué avec succès aux calculs de spectres électroniques de systèmes montrant de forts effets non adiabatiques. Cependant, cette approche requiert au préalable les surfaces d'énergie potentielle (PES). Ces applications sont basées sur un modèle de couplage vibronique local de la PES à proximité du point de Franck-Condon (FC). Contrairement aux calculs de spectres de photoabsorbtion impliquant souvent de courtes échelles de temps et de faibles déformations autour du point de FC, la simulation de réactions photochimiques requiert une représentation adéquate des mouvements de large amplitude. Ce modèle doit donc être rendu plus flexible. Les modes normaux, utilisés dans le modèle standard, n'étant pas adaptés aux grandes déformations, nous avons choisi d'utiliser la méthode MCTDH en coordonnées curvilignes avec une reformulation systématique du modèle en coordonnées polysphériques afin d'obtenir une énergie cinétique nucléaire séparable.Même si le processus n'implique que l'état fondamental et le premier état excité, leurs configurations électroniques dominantes peuvent changer fortement le long de mouvements de grande amplitude. Nous avons donc développé une approche générale basée sur une étude systématique de quelques données ab initio pour générer le meilleur jeu d'états diabatiques d'un problème donné.De premières applications au benzopyrane (chromophore des SP) ont montré un bon accord avec l'expérience. / The focus of this thesis is the ring opening of spiropyrans (SP), and how these molecules can be used as photoswitches controlled with laser pulses. We have built a theoretical model to study this photochemical reaction and develop strategies for control.SP exhibit nonadiabatic effects, and therefore, their modelling requires a quantum description for both the electrons and the nuclei. The multiconfiguration time-dependent Hartree (MCTDH) approach is ideal in this case because it can treat quantum dynamics involving several electronic states. MCTDH was successfully applied to electronic spectra calculations of systems showing strong nonadiabatic effects. However, the corresponding potential energy surfaces (PES) for this approach are required first. These applications are based on a local vibronic coupling model of the PES in the neighbourhood of the Franck-Condon (FC) point.As opposed to calculating photoabsorption spectra that often involves short timescales and small deformations around the FC geometry, simulating photochemical reactions requires an adequate representation of large-amplitude motions. Thus, this model must be made more flexible.Normal modes, usually used for the standard model, are not adapted to large-amplitude deformations. We thus chose to run MCTDH in curvilinear coordinates and recast systematically the model in terms of polyspherical coordinates to produce a separable form for the nuclear kinetic energy.Even if only the ground and the first excited electronic states are involved in the process, their dominant electronic configurations may change significantly along large-amplitude motions. We have developed a general approach based on a systematic analysis of a few ab initio data to generate the best set of diabatic states for a given problem.Preliminary results applied to benzopyran (the chromophore of the SP) showed good agreement with the experiments.

Este trabalho teve como foco principal a caracterização espectroscópica dos haletos de berílio, BeCl, BeBr e BeI. O conhecimento acerca dessas espécies foi ampliado significativamente através do cálculo de parâmetros de difícil caracterização experimental. Como os poucos trabalhos experimentais realizados para esses haletos apresentam resultados inconclusivos para a transição C 2Σ+ - X 2Σ+, através do cálculo acurado de probabilidades de transição foi possível oferecer uma nova interpretação para os dados existentes, apresentando inclusive resultados para as outras transições eletrônicas até o presente desconhecidas. Cálculos utilizando o método de Interação de Configurações Multirreferencial (MRCI) com um conjunto de funções de base consistentes na correlação de qualidade quintupla-zeta foram utilizados para se obter as curvas de energia potencial associadas ao primeiro e segundo canais de dissociação para os estados dubleto e quarteto dessas espécies. Parâmetros espectroscópicos como as constantes vibracionais we, wexe, a constante rotacional Be e a distância de equilíbrio foram determinados para os estados de mais baixa energia. Além disso, foram calculadas a energia de excitação (Te) e a energia de dissociação (De) com e sem a inclusão de efeitos spin-órbita. Os resultados obtidos expandiram significativamente nosso conhecimento sobre os estados eletrônicos dessa espécies. Para os estados já investigados experimentalmente, houve boa coerência entre os parâmetros calculados e experimentais. As diferenças de energia entre os dois canais no limite de dissociação para as moléculas BeCl, BeBr e BeI obtidas neste trabalho foram, respectivamente: 21835, 21889 e 21998 cm-1, em concordância com o respectivo resultado experimental das três espécies (21980 cm-1). A determinação teórica da energia de dissociação D0 foi bastante satisfatória. Obtivemos 92,24; 72,77 e 51,75 kcal·mol-1, respectivamente, para as moléculas BeCl, BeBr e BeI incluindo os efeitos spin-órbita, comparados a 91,78, 71 e 57 kcal·mol-1. Para uma análise mais completa das curvas de energia potencial, elas também foram obtidas considerando-se os efeitos relativísticos. A constante de acoplamento spin-órbita calculada na região de equilíbrio de cada uma das moléculas BeCl, BeBr e BeI foi, respectivamente: 41, 207 e 324 cm-1, em boa concordância com os resultados experimentais: 52,8, 198 e 361,1 cm-1, respectivamente. A separação spin-órbita no limite de dissociação calculada foi de 823, 3446 e 6975 cm-1 (BeCl, BeBr e BeI), também coerentes com resultados experimentais: 882, 3685,24 e 7603,15 cm-1, respectivamente. Foi ainda realizada uma análise detalhada sobre os canais de dissociação das moléculas HBeP e BePH. A construção de um diagrama de energias relativas permitiu obter um conjunto de dados importantes a respeito de todos os possíveis canais de dissociação. Em particular é mostrado a estabilidade relativa e a energia no limite de dissociação. Para a energia no limite de dissociação obtivemos resultados consistentes com os valores experimentais. Nossos cálculos para os três primeiros canais de dissociação HBeP = H + Be resultaram nos seguintes valores: H (2Sg) + Be (X 4Σ+), 0 cm-1; H (2Sg) + Be (a 2π), 426 cm-1 (exp. 502 cm-1); H (2Sg) + Be (b 2Σ-), 1896 cm-1 (exp. 1976 cm-1). Por fim, pela primeira vez na literatura, o calor de formação e de atomização foram calculados para estas espécies. Para os isômeros HBeP e BePH, a energia de atomização a 298,15 K calculada foi de 119,02 kcal·mol-1 e 107,40 kcal·mol-1, respectivamente; para o calor de formação a 298,15 K, obtivemos 86,14 e 97,76 kcal·mol-1 para as espécies HBeP e BePH, respectivamente. / This work has mainly focused on the spectroscopic characterization of the beryllium halides, BeCl, BeBr and BeI. Knowledge about these species was substantially increased through the calculation of parameters that are difficult to access in an experimental characterization. As the few experimental works carried out for these halides presented inconclusive results for the C 2Σ+ - X2Σ+ transition, we carried out accurate calculations of transition probabilities, thus offering a new interpretation about the existing data, including also results for other electronic transitions as yet unknown. Calculations using the method Multireference Configuration Interaction (MRCI) along with correlation-consistent basis set functions of quintuple-zeta quality were used to obtain the potential energy curves associated with the first and second dissociation channels for the doublet and quartet states of these species. Spectroscopic parameters such as vibrational constants We, WeXe, the rotational constant Be and the equilibrium distance were determined for the low-lying states. Furthermore, we also calculated the excitation energy (Te) and the dissociation energy (De) with and without the inclusion of spin-orbit effects. The obtained results significantly expanded our knowledge about the electronic states of this species. For states already investigated experimentally, there was good consistency between the calculated and the experimental parameters. The energy differences between the two channels in the dissociation limit for BeCI, BeBr and Bel molecules obtained in this work were: 21835, 21889 and 21998 cm-1, in agreement with the corresponding experimental results of the three species (21 980 cm-1). The theoretical determination of the dissociation energy D0 was very satisfactory. We obtained 92.24, 72.77, and 51.75 kcal·mol-1, respectively, for the BeCl, BeBr and BeI molecules including spin-orbit effects, compared to 91.78, 71 and 57 kcal . mol-1. For a more complete analysis of the potential energy curves, they also were obtained considering relativistic effects. The calculated spin-orbit coupling constants in the equilibrium region of BeCl, BeBr and BeI molecules were respectively 41, 207 and 324 cm-1, in good agreement with the experimental results: 52.8, 198, and 361.1 cm-1, respectively. The calculated spin-orbit splitting in the dissociation limit was 823, 3446 and 6975 cm-1 (BeCl, BeBr and BeI) also consistent with the experimental results: 882, 3685.24 and 7603.15 cm-1, respectively. Additionally, a detailed analysis of the dissociation channels of HBeP and BePH molecules was performed. The construction of a relative energies diagram allowed us to derive a set of important data for all the possible dissociation channels. In particular, it is shown the relative stability and the energy in the dissociation limit. For the energy in the dissociation limit, we obtained results consistent with the experimental values. Our calculations for the first three dissociation channels HBeP = H + Be resulted in the following values: H (2Sg) + Be (X 4Σ+), 0 cm-1; H (2Sg) + Be (a 2π), 426 cm-1 (exp. 502 cm-1); H (2Sg) + Be (b 2Σ-), 1896 cm-1 (exp. 1976 cm-1). Finally, for the first time in the literature, the heat of formation and the atomization energy were calculated for these species. For the HBeP and BePH isomers, the atomization energy calculated at 298.15 K was 119.02 and 107.40 kcal . mol-1, respectively; for the heat of formation at 298.15 K, we obtained 86.14 and 97.76 kcal· mol-1 for HBeP and BePH species, respectively.

Novas espécies moleculares HSeI e HISe presentes na superfície de energia potencial (SEP) singleto 1[H, Se, I], como também a molécula diatômica monoiodeto de selênio (SeI), foram investigadas teoricamente em alto nível de teoria, CCSD(T)/CBS e SACASSCF/ MRCI, respectivamente, pela primeira vez na literatura. Todos estados eletrônicos dupletos e quartetos (Λ+S) que se correlacionam com os três primeiros canais de dissociação da molécula SeI, assim como os estados Ω associados, fornecem resultados conáveis para ajudar a entender a falta de dados experimentais de transições eletrônicas e no planejamento experimental para a determinação de parâmetros espectroscópicos. Ainda sobre este sistema, as probabilidades de transições foram calculadas para as transições X2 - X1, A1 - X1, A2 - X1, e A2 - X2, originadas pela transição proibida por spin 14Σ- X 2II, e os correspondentes tempos de vida radiativa obtidos. Os aspectos energéticos, estruturais e espectroscópicos associados aos estados estacion ários na SEP 1[H, Se, I] também foram caracterizados, assim como determinados os calores de formação dos isômeros. Os cálculos foram realizados com uma série de conjuntos de funções de base atômicas consistentes na correlação (aug-cc-pVnZ-PP, n = D, T, Q e 5) e os resultados extrapolados no limite de base completa (CBS). Contribuições de efeitos de correlação caroço-valência na função de onda e de anarmonicidade nas frequências vibracionais também foram explorados, fornecendo uma fonte de dados bastante confiáveis para estas espécies químicas ainda desconhecidas. O isômero HSeI é mais estável em relação ao HISe em 42; 04 kcal mol-1. Estes dois isômeros estão separados por uma barreira (ΔG#) de 52; 35 kcal mol-1. Correções devido aos efeitos de acoplamento spin- órbita e de relativísticos escalares também foram considerados na obtenção das energias de atomizações. Para o SeI, estimamos valores de calores de formação (ΔfH) de 36; 87 e 35; 16 kcal mol-1 a 0 K e a 298; 15 K; para o HSeI, obtivemos 18; 25 e 16; 72 kcal mol-1, respectivamente. De modo geral, esperamos que a conabilidade dos resultados presentes neste trabalho possa servir como um excelente guia para espectroscopistas na busca e caracterização dessas espécies químicas ainda desconhecidas experimentalmente. / New molecular species HSeI and HISe lying on the 1[H, Se, I] potencial energy surface (PES) as well as the diatomic molecule selenium monoiodide (SeI) were investigated theoretically for the rst time at a high level of theory, CCSD(T)/CBS and SACASSCF/MRCI, respectively. The overall picture of all doublet and quartet (Λ+S) states correlating with the three lowest dissociation channels of the SeI and the associated states provide reliable results to help understand the lack of experimental data on its transitions and to plan the investigation and determination of spectroscopic parameters. Transition probabilities were computed for the transitions X2 - X1, A1 - X1, A2 - X1, and A2 - X2, originated from the spin-forbidden 14Σ- - X 2II system, and the orresponding radiative lifetimes evaluated. The energetic, structural and spectroscopic aspects associated with the stationary points and transition state in the PES 1[H, Se, I] were also characterized, as well as the heat of formation of the isomers. Computations were carried out with the series correlation consistent basis sets (aug-cc-pVnZ-PP) and the results extrapolated to the complete basis set limit. Accounting for core-valence correlation into the wavefunction, and of anharmonic eects on the vibrational frequencies were also explored, making the results of the structural, energetic, and vibrational properties a very reliable source of data for these yet unknown species. The isomer HSeI turned out to be more stable by 42:04 kcal mol-1 than HSeI. These two isomers are separated by a barrier (ΔG#) of 52:35 kcal mol-1. Corrections arising from spin-orbit and scalar relativistic eects were also considered in the evaluation of atomization energies. For SeI, we estimate ΔfH values of 36:87 and 35:16 kcal mol-1 at 0 K and 298:15 K; for HSeI, we had 18:25 and 16:72 kcal mol-1, respectively. In general, we hope that the reliability of the results present in this work can serve as an excellent guide for spectroscopists in the search and characterization of these chemical species still unknown experimentally.

Uma das propriedades fundamentais de soluções de micelas aquosas é sua capacidade de solubililizar uma ampla faixa de solutos orgânicos com diferentes graus de polaridade e hidrofobicidade. Recentemente Quina et. aI., estudaram as interações químicas específicas que governam a solubilidade para um ampla faixa de solutos orgânicos em micelas usando relações lineares de energia livre de solvatação (LSERs), baseado nos parâmetros empíricos do soluto desenvolvido por Abraham. O presente trabalho trata a solubilização de solutos orgânicos em micelas baseado em parâmetros obtidos unicamente da estrutura molecular do soluto através de cálculos de química quântica. Nossos resultados mostram que valores experimentais de Ks para solubilização de solutos orgânicos em detergentes aniônicos (micelas de SDS, dodecil sulfato de sódio e agregados de SDS com PEG-polietilenoglicol) podem ser reproduzidos usando parâmetros teóricos. A relação linear teórica de energia livre de solvatação resultante é quimicamente condizente com LSERs baseados em parâmetros empíricos do soluto. No caso de detergentes catiônicos e não iônicos, a metodologia apresenta limitações, por que o método de cálculo de cargas formais usando a definição de análise populacional de Mulliken não descreve de maneira apropriada o caráter ácido e básico das pontes de hidrogênio entre os solutos e a água. Alternativas para resolver o problema da limitação imposta pelos parâmetros teóricos, mantendo a aproximação linear de energia livre, são discutidas. / One of the most fundamental properties of aqueous micelar solutions is their ability to solubilize a wide variety of organic solutes with quite distinct polarities and degree of hydrophobicity. Recently Quina et al., studied the specific chemical interactions governing the solubilities of a wide range of solutes in micelles using linear solvation energy relationships (LSERs) based on empirical solute parameters developed by Abraham. This work reports an investigation of the solubilization of organic solutes in micelles based on the use of theoretical solute parameters derived from quantum chemical calculations Our results show that the experimental Ks values for solubilization of organic solutes in anionic detergents (micelles of SDS-sodium dodecyl sulfate and aggregate formed between SDS and PEG-poly(ethylene glycol)) can be reproduced using theoretical parameters. The resultant theoretical linear solvation energy relationships are chemically consistent with LSERs based on empirical solute parameters. In the case of cationic and nonionic detergents, the theoretical parameters used to describe hydrogen bonding fail because methods of calculating formal charges using the concept of Mulliken populational analysis cannot describe hydrogen bonding donor and acceptor character in an appropriate way. Possible strategies to solve the theoretical descriptor problem in the context of the linear free energy approximation are discussed.

Novas espécies moleculares HSeI e HISe presentes na superfície de energia potencial (SEP) singleto 1[H, Se, I], como também a molécula diatômica monoiodeto de selênio (SeI), foram investigadas teoricamente em alto nível de teoria, CCSD(T)/CBS e SACASSCF/ MRCI, respectivamente, pela primeira vez na literatura. Todos estados eletrônicos dupletos e quartetos (Λ+S) que se correlacionam com os três primeiros canais de dissociação da molécula SeI, assim como os estados Ω associados, fornecem resultados conáveis para ajudar a entender a falta de dados experimentais de transições eletrônicas e no planejamento experimental para a determinação de parâmetros espectroscópicos. Ainda sobre este sistema, as probabilidades de transições foram calculadas para as transições X2 - X1, A1 - X1, A2 - X1, e A2 - X2, originadas pela transição proibida por spin 14Σ- X 2II, e os correspondentes tempos de vida radiativa obtidos. Os aspectos energéticos, estruturais e espectroscópicos associados aos estados estacion ários na SEP 1[H, Se, I] também foram caracterizados, assim como determinados os calores de formação dos isômeros. Os cálculos foram realizados com uma série de conjuntos de funções de base atômicas consistentes na correlação (aug-cc-pVnZ-PP, n = D, T, Q e 5) e os resultados extrapolados no limite de base completa (CBS). Contribuições de efeitos de correlação caroço-valência na função de onda e de anarmonicidade nas frequências vibracionais também foram explorados, fornecendo uma fonte de dados bastante confiáveis para estas espécies químicas ainda desconhecidas. O isômero HSeI é mais estável em relação ao HISe em 42; 04 kcal mol-1. Estes dois isômeros estão separados por uma barreira (ΔG#) de 52; 35 kcal mol-1. Correções devido aos efeitos de acoplamento spin- órbita e de relativísticos escalares também foram considerados na obtenção das energias de atomizações. Para o SeI, estimamos valores de calores de formação (ΔfH) de 36; 87 e 35; 16 kcal mol-1 a 0 K e a 298; 15 K; para o HSeI, obtivemos 18; 25 e 16; 72 kcal mol-1, respectivamente. De modo geral, esperamos que a conabilidade dos resultados presentes neste trabalho possa servir como um excelente guia para espectroscopistas na busca e caracterização dessas espécies químicas ainda desconhecidas experimentalmente. / New molecular species HSeI and HISe lying on the 1[H, Se, I] potencial energy surface (PES) as well as the diatomic molecule selenium monoiodide (SeI) were investigated theoretically for the rst time at a high level of theory, CCSD(T)/CBS and SACASSCF/MRCI, respectively. The overall picture of all doublet and quartet (Λ+S) states correlating with the three lowest dissociation channels of the SeI and the associated states provide reliable results to help understand the lack of experimental data on its transitions and to plan the investigation and determination of spectroscopic parameters. Transition probabilities were computed for the transitions X2 - X1, A1 - X1, A2 - X1, and A2 - X2, originated from the spin-forbidden 14Σ- - X 2II system, and the orresponding radiative lifetimes evaluated. The energetic, structural and spectroscopic aspects associated with the stationary points and transition state in the PES 1[H, Se, I] were also characterized, as well as the heat of formation of the isomers. Computations were carried out with the series correlation consistent basis sets (aug-cc-pVnZ-PP) and the results extrapolated to the complete basis set limit. Accounting for core-valence correlation into the wavefunction, and of anharmonic eects on the vibrational frequencies were also explored, making the results of the structural, energetic, and vibrational properties a very reliable source of data for these yet unknown species. The isomer HSeI turned out to be more stable by 42:04 kcal mol-1 than HSeI. These two isomers are separated by a barrier (ΔG#) of 52:35 kcal mol-1. Corrections arising from spin-orbit and scalar relativistic eects were also considered in the evaluation of atomization energies. For SeI, we estimate ΔfH values of 36:87 and 35:16 kcal mol-1 at 0 K and 298:15 K; for HSeI, we had 18:25 and 16:72 kcal mol-1, respectively. In general, we hope that the reliability of the results present in this work can serve as an excellent guide for spectroscopists in the search and characterization of these chemical species still unknown experimentally.

Quantum Monte Carlo (QMC) methods are a class of stochastic techniques that can be used to compute the properties of electronic systems accurately from first principles. This thesis is mainly concerned with the development of trial wave functions for QMC. An extension of the backflow transformation to inhomogeneous electronic systems is presented and applied to atoms, molecules and extended systems. The backflow transformation I have developed typically retrieves an additional 50% of the remaining correlation energy at the variational Monte Carlo level, and 30% at the diffusion Monte Carlo level; the number of parameters required to achieve a given fraction of the correlation energy does not appear to increase with system size. The expense incurred by the use of backflow transformations is investigated, and it is found to scale favourably with system size. Additionally, I propose a single wave function form for studying the electron-hole system which includes pairing effects and is capable of describing all of the relevant phases of this system. The effectiveness of this general wave function is demonstrated by applying it to a particular transition between two phases of the symmetric electron-hole bilayer, and it is found that using a single wave function form gives a more accurate physical description of the system than using a different wave function to describe each phase. Both of these developments are new, and they provide a powerful set of tools for designing accurate wave functions. Backflow transformations are particularly important for systems with repulsive interactions, while pairing wave functions are important for attractive interactions. It is possible to combine backflow and pairing to further increase the accuracy of the wave function. The wave function technology that I have developed should therefore be useful across a very wide range of problems.

Nesta tese foi analisada a evolução das propriedades eletrônicas e estruturais de oligômeros do fluoreno e de dois derivados, relacionada com a inserção gradativa de unidades monoméricas, utilizando-se os métodos de química quântica semi-empríricos, ab initio Hartree-Fock e Teoria do Funcional da Densidade (DFT). Para tal finalidade, foram realizadas análises metodológicas assaz detalhadas com os monômeros e dímeros (fluoreno, FF e TFFF), cujos resultados mostraram que os métodos semi-empíricos AM1 e PM3 são inadequados para descrever as distribuições das cargas derivadas do potencial eletrostático e, por conseguinte, as propriedades que dependem das mesmas. Além disso, as análises conformacionais realizadas para as três espécies de dímeros mostraram que ambas as estruturas estáveis do TFFF apresentaram o mesmo momento de dipolo total, provavelmente devido à influência dos átomos de flúor sobre o sistema. Ademais, com as investigações concernentes ao crescimento gradual das cadeias oligoméricas foi constatado que os espectros de UV e as energias dos orbitais fronteira calculados para os oligômeros seguiram a mesma tendência exibida por seus polímeros correspondentes, segundo os resultados de voltametria cíclica e de UV divulgados na literatura. Enquanto que os ângulos diedros adotados pelos oligômeros indicaram que suas conformações não são planares. Quanto à energia total, verificou-se que o decaimento linear relativo aos oligômeros do TFFF sucedeu aproximadamente três vezes mais rápido que o observado para os do fluoreno. Portanto, as aplicações dos fundamentos da Química Quântica e a correlação entre esses resultados teóricos e medidas experimentais reportadas na literatura, evidenciaram que com estas informações é factível estimar algumas propriedades de polímeros orgânicos ?-conjugados a partir de seus oligômeros, utilizando-se métodos de química quântica apropriados. / In this thesis, the evolution of electronic and structural properties of fluorene oligomers and two derivatives were accomplished relating to the gradual insertion of monomeric units using the quantum chemical methods semiempirical, ab initio Hartree-Fock and Density Functional Theory (DFT). For this purpose, detailed methodological analyses were performed with their monomers and dimers (fluorene, FF and TFFF). These results showed that the semiempirical methods AM1 and PM3 are inadequate to describe the atomic charges derived from electrostatic potential distributions and, hence, for the properties depending of them. In addition, the conformational analysis achieved for these three dimers showed that both stable structures found for the TFFF dimer possess the same total dipole moment, this is probably due to the influence of the fluorine atoms on the system. Moreover, with investigations regarding to the gradual increase in the oligomeric chain, it was verified that the UV spectra and the frontier orbital energies calculated for the oligomers provided the same tendency bespoken by their relative polymers from cyclic voltametry and UV measurements shown in the literature. Besides, the dihedral angles found in the oligomers indicated that their conformations are not planar. Furthermore, the total energy linear decay of the TFFF oligomers behaves approximately three times faster than that exhibited by the fluorene oligomers. In this way, the applications of the Quantum Chemistry principles and the correlation between these theoretical results and experimental data reported in the literature evidenced that using these information is feasible to estimate some properties of polymers from their oligomers by means of appropriate methods of quantum chemistry.