The scientific objective of this research program was to determine the feasibility of manufacturing an ionic liquid-based supercapacitor that could operate at temperatures up to 220 °C. A secondary objective was to determine the compatibility of ionic liquids with other cell components (e.g. current collectors) at high temperature and, if required, consider means of mitigating any problems. The industrial motivation for the present work was to develop a supercapacitor capable of working in the harsh environment of deep offshore boreholes. If successful, this technology would allow down-hole telemetry under conditions of mechanical vibration and high temperature. The obstacles, however, were many. All supercapacitor components had to be stable against thermal decomposition up to T ≥ 220 °C. Volatile components had to be eliminated. If possible, the finished device should be able to withstand voltages greater than 4 V, in order to maximise the amount of stored energy. The internal resistance should be as low as possible. Side reactions, particularly faradaic reactions, should be eliminated or suppressed. All liquid components should be gelled to minimise leakage in the event of cell damage. Finally, any emergent problems should be identified.

Magnetic ordering of dopants in diamond holds the prospect of exploiting diamond’s unique properties in the emerging field of spintronics. Several transition metal defects have been reported to order ferromagnetically in various semiconductors, however, low Curie temperatures and lack of other fundamental material properties have hindered practical implementation in room temperature spintronic applications. In this Thesis, we consider the energetic stability of 3d transition metal doped-diamond and its magnetic ordering properties at various lattice sites and charge states using ab initio Density Functional Theory methods. We find the majority of 3d transition metal impurities in diamond at any charge state to be energetically most stable at the divacancy site compared to substitutional or interstitial lattice sites, with the interstitial site being highly unstable (by ~8 - 10 eV compared to the divacancy site). At each lattice site and charge state, we find the formation energies of transition metals in the middle of the 3d series (Cr, Mn, Fe, Co, Ni) to be considerably lower compared to those early or late in the series. The energetic stability of transition metal impurities across the 3d series is shown to be strongly dependent on the position of the Fermi level in the diamond band gap, with the formation energies at any lattice site being lower in p-type or ntype diamond compared to intrinsic diamond. Further, we show that incorporation of isolated transition metal impurities into diamond introduces spin polarised impurity bands into the diamond band gap, while maintaining its semiconducting nature, with band gaps in both the spin-up and spin-down channels. These impurity bands are shown to originate mainly from s, p-d hybridization between carbon sp 3 orbitals with the 3d orbitals of the transition metal. In addition, the 4p orbitals contribute significantly to hybridization for transition metal atoms at the substitutional site, but not at the divacancy site. In both cases, the spin polarisation and magnetic stabilization energies are critically dependent on the lattice site and charge state of the transition metal impurity. By allowing magnetic interactions between transition metal atoms, we find that ferromagnetic ordering is likely to be achieved in divacancy Cr+2, Mn+2, Mn+1 and Co0 as well as in substitutional Fe+2 and Fe+1, indicating that transition metal-doped diamond is likely to form a diluted magnetic semiconductor which may successfully be considered for room temperature spintronic applications. In addition, these charge states correspond to p-type diamond, except for divacancy Co0, suggesting that co-doping with shallow acceptors such as B ( will result in an increase of charge concentration, which is likely to enhance mediation of ferromagnetic spin coupling. The highest magnetic stabilization energy occurs in substitutional Fe+1 (33.3 meV), which, also exhibits half metallic ferromagnetic ordering at the Fermi level, with an induced magnetic moment of 1.0 μB per ion, thus suggesting that 100 % spin polarisation may be achieved in Fe-doped diamond. / Physics / D. Litt. et Phil. (Physics)

Desde o início do uso de ferramentas computacionais na química, pesquisadores notaram a necessidade de um adequado tratamento da correlação eletrônica para a acurada descrição da estrutura eletrônica. Algum tempo depois, perceberam que, além da correlação, haviam ainda deficiências nos resultados obtidos, as quais estavam relacionadas com efeitos relativísticos, principalmente quando tentavam tratar moléculas com átomos pesados. Um terceiro aspecto a ser levado em consideração nestes estudos diz respeito ao uso de um conjunto acurado de funções de base. No caso específico de cálculos relativísticos, se possível, tais conjuntos ainda precisam garantir a eliminação do prolapso variacional, um problema que está relacionado com uma descrição deficiente da região mais próxima dos núcleos. Com esse propósito, o conjunto RPF-4Z foi desenvolvido recentemente. Sendo assim, este projeto foi voltado para a análise comparativa da qualidade do conjunto RPF-4Z. Desta forma, foram avaliadas propriedades fundamentais das moléculas como, por exemplo, geometrias, momentos de dipolo e constantes espectroscópicas. Simultaneamente, uma propriedade desafiadora, o gradiente de campo elétrico na posição de núcleos de xenônio, também foi considerada. Os resultados obtidos até agora reforçam a eficiência do conjunto RPF-4Z, que é capaz de fornecer valores comparáveis àqueles dados por conjuntos de base de mesma qualidade, mas com menor demanda por recursos computacionais. / Early in the application of computational framework in chemistry, researchers noticed the need for an adequate electronic correlation treatment in order to accurately describe electronic structures. Some years later they realized that, besides correlation, there were also deficiencies in the results, which were related to relativistic effects, especially when dealing with molecules containing heavy atoms. A third aspect to be taken into account is related to the use of an accurate basis set. Specifically for relativistic calculations, these basis sets must also ensure the elimination of variational prolapse if possible, which is a problem related to a poor description of the innermost region close to the nuclei. Thus, the RPF-4Z basis set was recently developed for this aim. This project is focused on a comparative analysis regarding the RPF-4Z basis set\'s quality. Thereby, fundamental properties of molecules were evaluated, such as geometries, dipole moments and spectroscopic constants. At the same time, a challenging property, the electric field gradient at the xenon nuclei, was considered as well. The results obtained so far reinforce the efficiency of the RPF-4Z basis set, which is capable of providing comparable values to those obtained by same quality basis sets but with a lower demand for computational resources.

Neste trabalho, a reação O(3P) + HBr → OH + Br, que se insere no contexto da química atmosférica, mais especificamente nos ciclos de destruição catalítica do ozônio, foi estudada empregando superfícies de energia potencial ab initio de alta qualidade. Construímos superfícies para os estados eletrônicos 3A\" e 3A\', baseadas na expansão de muitos corpos, com potenciais de dois corpos ajustados por funções \"switched-MLJ\" e potenciais de três corpos interpolados utilizando o método RKHS. Os pontos ab initio utilizados foram obtidos no nível de teoria MRCISD+Q/CBS+SO. A superfície final para o estado eletrônico 3A\" foi corrigida por um fator multiplicativo de modo que reproduzisse cálculos benchmark para a altura de barreira, em que foram considerados efeitos relativísticos (escalares e spin-órbita), correlação caroço-valência e correlação de ordem alta (excitações triplas e quádruplas). As principais características da SEP 3A\" são dois mínimos de van der Walls no canal de entrada e saída e um estado de transição com uma barreira energética de 5,01 kcal mol-1. A SEP 3A\' tem um estado de transição linear em 6,45 kcal mol-1. O valor obtido para a entalpia de reação, a 0 K, foi de -15,7 kcal mol-1, muito próximo do valor experimental de -15,14 kcal mol-1. Calculamos constantes de velocidade, no intervalo de 200 a 1000 K, utilizando a teoria do estado de transição variacional com contribuições de tunelamento multidimensional (ICVT/µOMT) e com uma aproximação de espalhamento reativo quântico (QM/JS). Estes valores estão em ótima concordância com os dados experimentais da literatura, em todo intervalo de temperatura em que estão disponíveis: de 221 a 554 K para reação O + HBr e de 295 a 419 K para reação O + DBr. As constantes de velocidade, a 298 K e em cm3 molécula-1 s-1, obtidas para a reação O + HBr são: 3,62·10-14 (ICVT/µOMT) e 3,35·10-14 (QM/JS), enquanto que o valor experimental é 3,78·10-14. A qualidade destes resultados reforça nossa confiança nos procedimentos e aproximações utilizadas, abrindo caminho para a caracterização, em alto nível, de uma grande variedade de reações em fase gasosa. / In this work, the O(3P) + HBr → OH + Br reaction, which is relevant to atmospheric chemistry, specially for the catalytic ozone depletion, was studied using high-level ab initio potential energy surfaces. We constructed surfaces for the 3A\" and 3A\' electronic states, based on the many-body expansion, with the two-body potentials adjusted by the switched-MLJ function and the three-body potentials interpolated using the RKHS method. The ab initio points were calculated at the MRCISD+Q/CBS+SO level of theory. The main features of the 3A\" are the presence of two van der Waals minima, at the entrance and exit channels, and a transition state with a barrier height of 5.01 kcal mol-1. The 3A\' PES has a linear transition state at 6.45 kcal mol-1. We obtained the enthalpy of reaction, at 0 K, of -15.7 kcal mol-1, in close agreement with the experimental value of -15.14 kcal mol-1. Rate constants, in the temperature range from 200 to 1000 K, were calculated using the variational transition state theory with contributions of multidimensional tunneling (ICVT/µOMT) and also a quantum reactive scattering approach (QM/JS). Their values are in fair agreement with the experimental data in the literature in the whole temperature range available: from 221 to 554 K for the O + HBr reaction, and from 295 to 419 K for the O + DBr reaction. At 298 K, the calculated rate constants, in cm3 molecule-1 s-1, for the O + HBr reaction are 3.62·10-14 (ICVT/µOMT) and 3.35·10-14 (QM/JS); and the experimental value is 3.78·10-14. The quality of these results reinforces our confidence in the procedures and approximations used, leading to the possibility of high-level characterization of a variety of gas phase reactions.

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.

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.

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.

No description available.

Les agrégats de molécules d'eau constituent une classe d'espèces chimiques importante du fait de leur rôle central dans de nombreux processus physico-chimiques et biologiques terrestres, en particulier, les processus atmosphériques. Leurs propriétés physiques et chimiques sont particulièrement sensibles aux effets de taille et aux effets de température, ce qui les rend particulièrement difficile à caractériser expérimentalement. Ainsi, mes travaux de thèse ont porté sur l'étude théorique des propriétés structurales, dynamiques et thermodynamiques ainsi que sur la réactivité de divers agrégats de molécules d'eau avec pour objectif de mettre en place des outils de modélisation adaptés, permettant une description plus fine de ces systèmes. Pour cela, nous avons utilisé des approches de dynamique moléculaire de type "parallel-tempering" qui ont été couplées avec des calculs d'énergie et de gradient réalisés par la méthode Self-Consistent-Charge Density-Functional based Tight-Binding (SCC-DFTB). Trois grands volets ont été abordés au cours de ces travaux. Dans la première partie, l'analyse détaillée des structures des agrégats d'eau (H2O)nSO24- et (H2O)nH2SO4 avec n=1-20 est présentée. Nous avons mis en évidence l'influence de la nature de l'impureté soufrée sur le réseau de liaisons hydrogène de ces agrégats. La deuxième partie de cette thèse porte sur l'étude de la transition de phase "solide-liquide" dans divers agrégats de molécules d'eau. En plus des agrégats soufrés évoqués ci-dessus, nous avons également étudié des agrégats d'eau protonés contenant de 19 à 23 molécules d'eau. Pour mieux comprendre le mécanisme de transition de phase, nous avons considéré divers changements structuraux associés à la transition tels que l'évolution des distributions d'angles intermoléculaires et l'évolution du nombre de cycles de molécules dans l'agrégat. Nous avons également caractérisé la transition de phase au travers d'indicateurs dynamiques tels que la fréquence de transfert du proton en excès. La dernière partie de cette thèse est consacrée à l'étude de l'influence de petits agrégats d'eau (allant de 1 à 10 molécules d'eau) sur la réaction de recombinaison entre l'atome H et la molécule CO. Cette réaction est la première étape de formation des molécules organiques oxygénées simples dans le milieu interstellaire. Elle présente donc un intérêt particulier. Grâce à l'analyse de dynamiques collisionnelles entre H et CO ainsi qu'au calcul de sections efficaces de réaction, nous avons montré que la présence des molécules d'eau joue un rôle important dans le processus de formation du radical HCO. / Water clusters constitute an important class of chemical species due to their central role in many physico-chemical and biological processes, in particular, atmospheric processes. Their physical and chemical properties are particularly sensitive to size and finite-temperature effects, which makes them particularly difficult to characterize experimentally. This thesis focused on the theoretical investigation of the structural, dynamical and thermodynamical properties as well as on the reactivity of various water clusters with the aim to implement appropriate modeling tools to enable a more detailed description of these systems. To do so, we used the paralleltempering molecular dynamics approach that was coupled with calculations of energies and gradients carried out by the Self-Consistent-Charge Density-Functional based Tight-Binding (SCC-DFTB) method.Three main areas were addressed during the work. In the first part, a detailed analysis of the structure of water clusters (H2O)nSO24- and (H2O)nH2SO4 with n=1-20 is performed. This study highlights the influence of the nature of the sulfur impurity on the hydrogen bond network of these species.The second part of this thesis focuses on the study of the "solid-liquid" phase transition in various water clusters. In addition to the sulfur-containing water clusters mentioned above, we also investigated protonated water clusters containing from 19 to 23 water molecules. To better understand the phase transition mechanism, we considered various structural changes associated with the transition, such as the evolution of the distributions of intermolecular angles and the evolution of the number of molecular rings in the cluster. We also characterized the phase transition through dynamical indicators such as the crossover frequency of the excess proton. The last part of this thesis is devoted to the study of the influence of small water clusters (from 1 to 10 water molecules) on the recombination reaction between the H atom and the CO molecule. This reaction is the first step in the formation of simple oxygenated organic molecules in the interstellar medium. It is therefore of particular interest. Due to the analysis of collisional dynamics between H and CO and the calculation of effective reaction cross sections we showed that the presence of water molecules plays an important role in the HCO radical formation.

Les carburants dérivés du pétrole constituent la première source mondiale énergétique et leur approvisionnement constitue un défi actuel majeur impliquant des enjeux économiques et environnementaux cruciaux. Une des voies les plus efficaces pour peser simultanément sur ces deux enjeux passe par la diminution de la consommation en carburant. La simulation numérique constitue dès lors un outil précieux pour améliorer et optimiser les moteurs et les carburants. Les modèles chimiques détaillés sont nécessaires pour comprendre les phénomènes d’auto-inflammation et caractériser la nature et les quantités de polluants émis. Ces modèles mettent en jeu un nombre très important d’espèces et de réactions élémentaires, pour une espèce donnée et pour lesquelles la détermination des données thermodynamiques et cinétiques est un problème crucial. La chimie quantique constitue un outil précieux permettant d’une part de déterminer de façon précise les données thermocinétiques pour bon nombre de systèmes chimiques et d’autre part de mieux comprendre la réactivité de ces systèmes. Dans ce travail, les réactions unimoléculaires de décomposition d’hydrocarbures monocycliques et polycycliques (amorçages, réactions moléculaires, ß-scissions, formations d’éthers cycliques) ont été étudiées à l’aide des méthodes de la chimie quantique. Un mécanisme détaillé de pyrolyse d’un alcane polycyclique a été développé à partir des données thermodynamiques et cinétiques et des corrélations entre structure et réactivité déterminées pour les cyclanes à partir des calculs quantiques. Les simulations effectuées à partir de ce modèle sont en très bon accord avec les résultats expérimentaux de la littérature / Petroleum fuels are the world’s most important primary energy source and the need to maintain their supply is a major actual challenge involving both economical and environmental features. Decreasing fuels consumption is one of the more efficient ways to reconcile the goals of energy price and environmental protection. Numerical simulations become therefore a very important tool to optimize fuels and motors. Detailed chemical kinetic models are required to reproduce the reactivity of fuels and to characterize the amount of emitted pollutants. Such models imply a very large number of chemical species and elementary reactions, for a given species, and the determination of thermodynamic and kinetic data is a critical problem. Nowadays, quantum chemistry methods are able to calculate accurately thermodynamic data for a large number of chemical systems and to elucidate the reactivity of these systems. In this work we have used quantum chemistry to study the unimolecular reactions (initiation, molecular reactions, ß-scissions, cyclic ethers formations) involved in the decomposition of monocyclic and polycyclic hydrocarbons. From the results of quantum chemical calculations, a detailed chemical kinetic mechanism of the pyrolysis of a polycyclic alkane has been developed and validated against experimental data