Chapter 1 contains a brief background into subjects such as Robin-Day classes, binary code, logic gates and electrochemistry in order to aid understanding of the rest of the chapter. The unique paradigm of Molecular Quantum Cellular Automata (MQCA) is presented along with the advantages it offers to traditional silicon based electronics. A summary of the existing modelled and synthesised MQCA systems is included along with an explanation of the characteristics required for materials to be suitable for MQCA. The subject of chapter 2 is cyclopentadiene cobalt cyclobutadiene complexes for the application of MQCA. The introduction examines the mechanism for the formation of cyclopentadiene cobalt cyclobutadiene complexes and the bonding in these compounds. A range of acetylenes were prepared for the formation of cyclopentadiene cobalt cyclobutadiene complexes were examined and characterised. Metal fragments including {Ru(dppe)2Cl} and AuPPh3Cl were attached to a cyclopentadiene cobalt cyclobutadiene core and these materials were characterised. The subject of chapter 3 is benzene based materials for the application of MQCA. 1,2,4,5-tetrakis(ferrocenylethynyl)benzene was prepared, characterised and the electrochemistry was examined for electronic communication between the ferrocene sites. A range of two metal centre compounds were examined for solubility and electrochemical stability with the view of preparing four metal centre compounds with a benzene core. The subject of chapter 4 is porphyrin based materials. This was the first area of work for this thesis and was discontinued. A brief summary of the synthetic work carried out is described, along with some literature work that was published whilst this work was being carried. Chapter 5 contains the experimental information for chapters 2-4.

The formation of new phases close to itinerant electron quantum critical points has been observed experimentally in many compounds. We present a unified analytical model that explains the emergence of new types of phases around itinerant ferromagnetic quantum critical points. The central idea of our analysis is that certain deformations of the Fermi surface enhance the phase-space available for low-energy quantum fluctuations and so self-consistently lower the free energy. Using this quantum order-by-disorder mechanism, we find instabilities towards the formation of a spiral ferromagnet and spin-nematic phase close to an itinerant ferromagnetic quantum critical point. Further, we employ the quantum order-by-disorder mechanism to describe the partially ordered phase of MnSi. Using the simplest model of a Stoner-like helimagnetic transition, we show that quantum fluctuations naturally lead to the formation of an unusual phase near to the putative quantum critical point that shares many of the observed features of the partially ordered phase in MnSi. In particular, we predict an angular dependence of neutron scattering that is in good agreement with neutron-scattering data.

- / -

Les mécanismes chimiques détaillés de combustion des carburants à basse température font intervenir un nombre important d'espèces et de réactions. Les logiciels de génération automatique permettent de faire face à cette complexité. Ces logiciels font appel à des corrélations permettant de prédire les propriétés thermodynamiques (enthalpies de formation, entropies et capacités calorifiques) et cinétiques (constantes de vitesse) associées aux espèces et aux réactions. Ces corrélations reposent cependant, généralement, sur un nombre limité de données de référence. Dans ce travail de thèse, nous avons utilisé une méthode faisant appel à la chimie quantique pour développer de nouvelles corrélations pour le logiciel de génération automatique EXGAS. En ce qui concerne les données thermodynamiques, des groupes de Benson associés aux fonctions hydroxyle et hydroperoxyle ont été évalués, ainsi que des énergies de liaison C-H, et O-H de fonctions hydroxyle et hydroperoxyle. En ce qui concerne les données cinétiques, nous nous sommes plus particulièrement intéressés aux réactions d'isomérisation des radicaux alkylperoxyles et hydroxyalkylperoxyles, ainsi qu'aux réactions de formation d'éthers cycliques. Nous avons également étudié l'impact de certaines règles de globalisation et introduit de nouvelles réactions pour rendre compte de la formation de produits de combustion jusqu'ici négligés ou sommairement prédits par les modèles. Les résultats obtenus ont été introduits dans des mécanismes chimiques détaillés générés par EXGAS. Ces mécanismes ont servi de base pour réaliser des simulations de la combustion du propane et du n-butane à basse température. Les résultats de ces simulations ont été comparés à des résultats expérimentaux récents afin de valider les calculs effectués. Les résultats des simulations ont montré que la prise en compte des nouvelles corrélations ainsi que la modification des règles de globalisation dans les mécanismes permettaient d'améliorer la prédiction de certains polluants mineurs pouvant avoir un impact sanitaire et environnemental majeur / Detailed chemical kinetic models for the low-temperature combustion of fuels involve a large number of species and reactions. Automatic generation of kinetic mechanisms is a powerful tool to deal with this complexity. These softwares are based on correlations that predict thermodynamic (enthalpies of formation, entropies and heat capacities) and kinetic (rate constants) properties associated with species and reactions. However, these correlations are generally based on a limited number of reference data. In this work, we used a method involving quantum chemistry to develop new correlations for EXGAS, a software for the automatic generation of kinetic mechanisms. For thermodynamic data, new Benson groups associated with hydroxyl and hydroperoxyl functions were evaluated. Bond dissociation energies of C-H bonds and O-H bonds of these groups were also evaluated. For the kinetic data, we focused our study on the isomerization reactions of alkylperoxyl and hydroxyalkylperoxyl radicals, and on the reactions of formation of cyclic ethers. We also studied the impact of some rules of globalization and introduced new reactions to account for the formation of minor combustion products usualy neglected or imperfectly predicted by the current models. The results obtained have been introduced into detailed chemical mechanisms generated by EXGAS. These mechanisms were used to simulate the combustion of propane and n-butane at low temperatures. The results of these simulations were compared with recent experimental results to validate the calculations. The simulations showed that taking into account the new correlations and the modifications of the rules of globalization in the mechanisms improved the prediction of some minor pollutants that can have major health and environmental impacts

Στην παρούσα εργασία μελετώνται οι μη γραμμικές ιδιότητες παραγώγων φουλλερενίων καθώς και η επίδραση της μεταφοράς φορτίου στην ενίσχυση της μη γραμμικότητας. Για τις πειραματικές μετρήσεις χρησιμοποιήθηκε η τεχνική ΟΚΕ ενώ έγιναν και θεωρητικοί υπολογισμοί , κβαντικής χημείας, προκειμένου να γίνει η απαραίτητη σύγκριση μεταξύ πειράματος και θεωρίας. / Ιn this project the nonlinear properties of some fullerene derivatives is studied. Particularly we examine the role of charge transfer in the nonlinear enhancement. The method that it was used for the experimental part was OKE technique. Moreover in the second part of this study theoretical calculations were held, in order to do a satisfying comparison between theory and experiment.

Ce travail de thèse a porté sur l'étude des réactions unimoléculaires de décomposition des alcènes à basse température. L'oxydation de ces molécules est primordiale dans la compréhension des mécanismes de combustion, car elles peuvent être présentes initialement dans des carburants usuels ou formées en tant qu'intermédiaires dans le processus d'oxydation. L’étude théorique menée peut se décomposer en deux parties : - L’étude des réactions de décomposition des radicaux hydroxyalkylperoxyles (HOROO•) obtenus à partir de l’addition d’un radical •OH sur l’alcène, suivi par une addition du radical •ROH sur l’oxygène moléculaire. Cette étude a été conduite de façon systématique sur un ensemble de radicaux représentatifs d'alcènes afin de mettre en évidence l'influence de l'environnement intramoléculaire sur les constantes de vitesse. Les résultats obtenus ont montré l’influence très nette de certains paramètres comme la présence d'un groupement hydroxyle en position β par rapport à l'atome de carbone réactif, la nature des atomes de carbone portant les hydrogènes transférés, la taille des cycles de transition ou encore la présence d'une liaison hydrogène intramoléculaire. - L’étude des réactions de décomposition unimoléculaires des radicaux allylperoxyles obtenus à partir de l’addition des radicaux allyliques sur l’oxygène. La grande complexité de ces systèmes a été mise en avant au cours de ce travail. Il a ainsi pu être mis en évidence que la formation d'un composé de type 1-alcenyloxirane est favorisée à basse température ce qui montre l'importance de ces réactions, souvent négligées dans les modèles / This work focused on the study of the unimolecular decomposition of the low-temperature reactions of alkenes. The oxidation of these molecules is essential in the understanding of the mechanisms of combustion, because they are initially present in conventional fuels or formed as intermediates in the oxidation process. The theoretical study can be divided in two parts: - The study of the decomposition of hydroxyalkylperoxyl radicals (HOROO•) obtained from the addition of an •OH radical on the alkene, followed by a subsequent addition of the formed radical •ROH on molecular oxygen. This study was conducted in a systematic way on a set of representative radicals of alkenes in order to highlight the influence of the intramolecular environment on the rate constants. The results clearly showed the major influence of parameters such as the presence of a hydroxyl group in β position with respect to the reactive carbon atom, the nature of the carbon atoms bearing the transferred hydrogen, the size of the transition state ring and the presence of intramolecular hydrogen bonds. - The study of the unimolecular decomposition of the allylperoxy radicals obtained from the addition of a resonance-stabilized radical on molecular oxygen. The complexity of these systems, specifically induced by the mesomeric structures of heavy allyl radicals and the role of cis and trans conformations alkenes parents, have been highlighted in this work. It has been shown that the formation of a compound 1-alkenyloxirane is favored at low-temperatures, which shows the importance of these reactions, usually neglected in the kinetic models for the combustion of alkenes

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)

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)

Les recherches menées dans le cadre de cette thèse avaient un double objectif. Premièrement, le développement d’une nouvelle méthode de chimie quantique à croissance linéaire basée sur le concept d’Orbitales Moléculaires Extrêmement Localisées (ELMOs) et adaptée à l’étude de très gros systèmes moléculaires. Deuxièmement, il s’agit d’évaluer le potentiel des méthodes de calcul utilisant de fonctions d’ondes contraintes et leur capacité à reproduire des données de diffraction aux rayons-X. En ce qui concerne le premier objectif, notre approche se base sur le principe de transférabilité, à savoir l’observation que les systèmes moléculaires sont composés par des unités fonctionnelles récurrentes qui conservent leurs caractéristiques lorsqu’elles se trouvent dans un même environnement chimique. Malheureusement, les orbitales moléculaires traditionnellement employées en chimie théorique dans des modèles de particule indépendante (Hartree-Fock, Kohn-Sham) sont complètement délocalisées sur le système étudié et, par conséquent, ne peuvent pas être transférées d’une molécule à une autre. Ce problème peut être résolu en ayant recours à des orbitales moléculaires déterminées de manière variationnelle sous la contrainte d’être exprimées à partir des fonctions de base centrées sur des atomes de fragments présélectionnés : les ELMOs. En fait, puisqu’elles sont strictement localisées, ces orbitales sont en principe transférables d’une molécule à une autre. L’objectif à terme est d’exploiter cette transférabilité en construisant une base de données d’ELMOs permettant de calculer quasiment instantanément, de manière approximative, des fonctions d’ondes et des densités électroniques de macromolécules. Dans la première partie de cette thèse, nous avons évalué le degré de transférabilité des orbitales moléculaires extrêmement localisées et nous avons proposé une approximation appropriée pour les molécules modèles servant à la détermination des ELMOs qui seront stockées dans la future base de données. Nous avons également comparé la transférabilité des ELMOs avec celle de densités électroniques atomiques asphériques (pseudo-atomes) qui sont largement répandues en cristallographie pour le raffinement de structure cristallographique de grands systèmes. La seconde partie de la thèse se focalise sur les méthodes quantiques utilisant des fonctions d’ondes contraintes. Dans ces méthodes, on cherche à déterminer des fonctions d’ondes qui minimisent l’énergie électronique des systèmes étudiés, mais qui en même temps doivent reproduire un jeu d’amplitudes de facteurs de structure expérimentaux. Cette technique, initialement proposée par Jayatilaka, a récemment été étendue à la théorie des orbitales moléculaires extrêmement localisées. Dans ce contexte, nous avons tout d’abord étudié les effets d’une localisation stricte sur la structure électronique dans des calculs de la fonction d’onde contrainte. Puis, nous avons déterminé si la fonction d’onde contrainte (et la densité associée) est capable de capturer des effets de la corrélation électronique. Enfin, en utilisant une nouvelle technique dite Valence Bond "expérimentale", basée sur les ELMOs, nous avons effectué une étude théorique sur le syn-1,6:8,13- Biscarbonyl[14] annulène (BCA) pour expliquer la rupture partielle de son aromaticité à haute pression observée expérimentalement. Cette dernière étude illustre positivement la potentialité du concept d’orbitale moléculaire strictement localisée en chimie quantique, qui ouvre des perspectives très larges notamment pour l’étude statique ou dynamique de systèmes moléculaires complexes. / The goal of the present work was dual. At first, this thesis aimed at proposing new lin- ear scaling quantum chemistry methods based on Extremely Localized Molecular Orbitals (ELMOs) and, secondly, it focused on the assessment of the capabilities of the X-ray con- strained wave function approaches. Concerning the first target, our approach is based on the transferability principle, namely the observation that molecular systems are composed by recurrent functional units that generally keep their features when they are in a similar chemical environment. In this context, it is possible to take advantage of the intrinsic trans- ferability of molecular orbitals strictly localized on small molecular subunits to recover wave functions and electron densities of large systems. Unfortunately, the molecular or- bitals traditionally used in quantum chemistry are completely delocalized on the system in exam and, therefore, are not transferable from a molecule to another. This problem can be solved only considering molecular orbitals variationally determined under the constraint of expanding them on local basis sets associated with pre-determined molecular fragments: the ELMOs. In fact, since they are strictly localized, these orbitals are in principle transfer- able from molecule to molecule and our final goal is to construct databanks of ELMOs that will enable to recover almost instantaneously approximate wave functions and electron densities of macromolecules at a very low computational cost. In the first part of this the- sis, we have evaluated the transferability of the Extremely Localized Molecular Orbitals and we have defined a suitable model molecule approximation for the computation of the ELMOs to be stored in the future databases. We have also compared the transferability of the ELMOs to the one of the aspherical atomic electron densities (pseudoatoms), which are largely used in crystallography to refine crystallographic structures of large systems. The second part of this work focuses on the X-ray constrained wave function approach. This method consists in determining wave functions that not only minimize the electronic energy of the systems under exam, but that also reproduce sets of experimental structure factor amplitudes within a desired accuracy. The technique, initially proposed by Jayatilaka has been recently extended to the theory of the Extremely Localized Molecular Orbitals. In this context, we have first studied the effects of introducing a strict a priori localization on the electronic structure in X-ray constrained wave function calculations. Then, we have determined if the X-ray constrained wave function is intrinsically able to capture the elec- tron correlation effects on the electron densities. Finally, also exploiting a novel X-ray con- strained ELMO-based Valence Bond technique, we have reported theoretical studies on the syn-1,6:8,13-Biscarbonyl[14] annulene (BCA) to explain the partial rupture of the aromatic character of the molecule occurring at high-pressure

Ce travail est destiné au développement de méthodes approchées de chimie quantique capables de traiter des systèmes biologiques de grande taille. En particulier, nous réalisons des simulations de dynamique moléculaire dans l'approximation de Born-Oppenheimer, permettant une description quantique de l'Hamiltonien électronique du système dans son entier : SEBOMD (SemiEmpirical Born-Oppenheimer Molecular Dynamics). Notre approche se base sur un Hamiltonien électronique semiempirique (SE). L'une des principales difficultés rencontrées lors d'une simulation SEBOMD de la phase condensée est représentée par le choix de la méthode SE. La plupart des méthodes courantes ne permettant pas une bonne description de certaines interactions fondamentales, nous avons développé une nouvelle approche. Cette méthode, dénommée PM3-PIF3, a été appliquée à l'étude par dynamique moléculaire de molécules organiques dans l'eau. Les résultats obtenus montrent que notre méthode est appropriée pour le traitement de molécules comportant des groupements hydrophobes et/ou hydrophiles en milieu aqueux. L'analyse des propriétés électroniques et vibrationnelles de ces molécules en présence du solvant valide également nos résultats vis-À-Vis d'autres études expérimentales et théoriques présentes dans la littérature. Finalement, nous nous sommes intéressés au processus d'autoprotolyse de l'eau en milieux confinés. Après avoir discuté du choix de l'Hamiltonien SE à utiliser pour cette étude, nous avons caractérisé le transfert de proton dans un agrégat d'eau. Nous avons établi une corrélation entre l'énergie libre associée à la première étape de ce transfert et certaines propriétés physiques collectives / The present work is devoted to the development of approximate quantum chemistry methods that are suitable to treat biological systems of large size. In particular, we run molecular dynamics under the Born-Oppenheimer approximation, allowing a quantum mechanical description of the electronic Hamiltonian of the full system: SEBOMD (SemiEmpirical Born-Oppenheimer Molecular Dynamics). Our method is based on a semiempirical (SE) electronic Hamiltonian. One of the key issues arising in a condensed phase SEBOMD simulation is represented by the choice of the SE method. Since most of the currently available approaches fail in describing some relevant intermolecular interactions, we developed a new correction of SE Hamiltonians. This method, which we named PM3-PIF3, was applied to study the molecular dynamics of organic molecules in water. The results that we obtained showed that our technique is suitable to treat molecules having hydrophobic and/or hydrophilic groups in an aqueous medium. The analysis of the electronic and vibrational properties of these molecules in the presence of the solvent validates our results with respect to experimental and theoretical studies in the literature. Finally, we investigated the water self-Dissociation process in confined environments. After discussing the choice of the SE Hamiltonian to be used for this purpose, we characterized the proton transfer in a water cluster. We established a correlation between the free energy of the first step of this process and some collective physical properties