L'intérêt de la recherche fondamentale pour les morceaux nanométriques de métaux nobles est principalement dû à la résonance localisée des plasmons de surface (LSPR) dans l'absorption optique. Différents aspects, liés à la compréhension théorique de la LSPR dans le cas de clusters de métaux nobles de taille dite intermédiaire, sont étudiés dans ce manuscrit. Afin d'avoir une vision plus large nous utilisons deux approches : l'approche électromagnétique classique et le formalisme ab initio en temps réel de la théorie de la fonctionnelle de la densité dépendant du temps (RT-TDDFT). Une comparaison systématique et détaillée de ces deux approches souligne et quantifie les limitations de l'approche électromagnétique lorsqu'elle est appliquée à des systèmes de taille quantique. Les différences entre les excitations plasmoniques collectives et celles impliquant les électrons d, ainsi que leurs interactions, sont étudiées grâce au comportement spatial des densités correspondantes. Ces densités sont obtenues en appliquant une transformée de Fourier dans l'espace à la densité obtenue par les simulations DFT utilisant une perturbation delta-kick. Dans ce manuscrit, des clusters de métaux nobles nus et protégés par des ligands sont étudiés. En particulier, motivé par de récents travaux sur les phénomènes d'émergence de plasmon, l'étude par TD-DFT de nano-alliages Au-Cu de taille tout juste inférieure à 2nm à fourni de subtiles connaissances sur les effets d'alliages sur la réponse optique de tels systèmes. / The fundamental research interest in nanometric pieces of noble metals is mainly due to the localized surface-plasmon resonance (LSPR) in the optical absorption. Different aspects related to the theoretical understanding of LSPRs in `intermediate-size' noble-metal clusters are studied in this thesis. To gain a broader perspective both the real-time \ai formalism of \td density-functional theory (RT-TDDFT) and the classical electromagnetics approach are employed. A systematic and detailed comparison of these two approaches highlights and quantifies the limitations of the electromagnetics approach when applied to quantum-sized systems. The differences between collective plasmonic excitations and the excitations involving $d$-electrons, as well as the interplay between them are explored in the spatial behaviour of the corresponding induced densities by performing the spatially resolved Fourier transform of the time-dependent induced density obtained from a RT-TDDFT simulation using a $\delta$-kick perturbation. In this thesis, both bare and ligand-protected noble-metal clusters were studied. In particular, motivated by recent experiments on plasmon emergence phenomena, the TDDFT study of Au-Cu nanoalloys in the size range just below 2~nm produced subtle insights into the general effects of alloying on the optical response of these systems.

abstract: Nanoscale semiconductors with their unique properties and potential applications have been a focus of extensive research in recent years. There are many ways in which semiconductors change the world with computers, cell phones, and solar panels, and nanoscale semiconductors having a promising potential to expand the efficiency, reduce the cost, and improve the flexibility and durability of their design. In this study, theoretical quantum mechanical simulations were performed on several different nanoscale semiconductor materials, including graphene/phosphorene nanoribbons and group III-V nanowires. First principles density functional theory (DFT) was used to study the electronic and structural properties of these nanomaterials in their fully relaxed and strained states. The electronic band gap, effective masses of charge carriers, electronic orbitals, and density of states were most commonly examined with strain, both from intrinsic and external sources. For example, armchair graphene nanoribbons (AGNR) were found to have unprecedented band gap-strain dependence. Phosphorene nanoribbons (PNRs) demonstrate a different behavior, including a chemical scissors effect, and studies revealed a strong relationship between passivation species and band gap tunability. Unlike the super mechanical flexibility of AGNRs and PNRs which can sustain incredible strain, modest yet large strain was applied to group III-V nanowires such as GaAs/InAs. The calculations showed that a direct and indirect band gap transition occurs at some critical strains and the origination of these gap transitions were explored in detail. In addition to the pure nanowires, GaAs/InAs core/shell heterostructure nanowires were also studied. Due to the lattice mismatch between GaAs and InAs, the intrinsic strain in the core/shell nanowires demonstrates an interesting behavior on tuning the electronic properties. This interesting behavior suggests a mechanical way to exert compressive strain on nanowires experimentally, and can create a finite quantum confinement effect on the core. / Dissertation/Thesis / Doctoral Dissertation Physics 2016

La Théorie de la Fonctionnelle de la Densité (DFT) combinée avec la méthode de la Symétrie Brisée (BS) est aujourd'hui très utilisée dans le domaine du magnétisme moléculaire pour le calcul des constantes d'échange. Cette méthode (DFT-BS) reste cependant semi-quantitative et elle souffre de défauts déjà discutés dans la littérature. Dans le but de mieux en comprendre l'origine, nous avons réexaminé les contributions physiques qui participent au mécanisme d'échange. Nous proposons alors plusieurs formules analytiques construites suivant deux approches complémentaires (orbitales moléculaires et liaison de valence). Au cours de notre analyse, nous avons soulevé un problème inédit relatif à l'état de symétrie brisée tel que livré par le calcul DFT. Nos modèles seront appliqués au cas des dimères de cuivre(II) et nous verrons comment quantifier les différents paramètres afin de reconstruire les constantes d'échange. Qui plus est, notre travail permet d'établir une correspondance quantitative originale entre les deux approches pré-citées. / Density Functional Theory (DFT) combined with the Broken Symmetry (BS) method is today widely used in the field of molecular magnetism for the computation of exchange coupling constants. But this method (DFT-BS) remains semi-quantitative as it suffers from a series of drawbacks already discussed in the literature. In order to better understand the origin of such problems, we reexamined the physical contributions acting in the exchange phenomenon. We then propose alternative analytical expressions built along two complementary approaches (molecular orbitals and valence bond). During our analysis, we found a new problem linked to the broken symmetry state as it comes out of a DFT calculation. Our models will be applied to copper(II) dimers and we will show how to quantify the different parameters involved in order to reconstruct the coupling constants. Moreover, our work allows for an original quantitative correspondence between the two above-mentioned approaches.

Le sujet de cette thèse porte sur l'étude des propriétés magnétiques de métaux de transition et leurs alliages sous haute pression au moyen de calculs ab initio. D'abord, les résultats de mesures de dichroïsme magnétique circulaire des rayons X (XMCD) au seuil K du nickel et du cobalt sont interprétés. Je montre que les données expérimentales doivent être comparées à celle de l'aimantation d'orbite projetée sur les états ``p''. Je mets en avant que la pression affecte différemment le spin et le moment orbitalaire. Dans le cas de l'alliage FeCo, la transition structurelle s'effectue sous une pression appliquée de l'ordre de 35 GPa. Je propose que l'émergence de l'antiferromagnétisme peut expliquer la disparition du signal XMCD au seuil K du fer et du cobalt. Ensuite, la transformation de phase dans FePd3, induite sous une pression de 12 GPa, est étudiée. Je démontre que le système est décrit par un modèle de Heisenberg étendu, contenant interactions d'échange biquadratiques forts. Selon nos résultats, FePd3 subit une transition de l'etat ferromagnétique à l'état triple-Q non-colinéaire, lorsqu'il est compressé. Enfin, une mise en oeuvre du tenseur des contraintes dans le code BigDFT est présentée. Il est montré qu'un traitement explicite des électrons de coeur permet de réduire considérablement les erreurs introduites par les pseudo-potentiels. Ainsi, les estimations des propriétés structurales peuvent être améliorées. / The subject of the present thesis is the investigation of magnetic properties of transition metals and their alloys under high pressure by means of first-principles calculations. First, the results of the K-edge x-ray magnetic circular dichroism (XMCD) experiments on Ni and Co are interpreted. It is shown that the experimental pressure evolution of the data should be compared with that of the p-projected orbital magnetization. I emphasize that the spin and orbital moments have different behavior upon compression. In the case of FeCo alloy the structural transition occurs under the pressure of 35 GPa. I propose that the emergence of antiferromagnetism can explain the disappearance of the XMCD signal at the Fe and Co K-edges. Then, the phase transformation in FePd3 , induced under pressure of 12 GPa, is investigated. I demonstrate that the system is described by an extended Heisenberg model, containing strong biquadratic exchange interactions. According to the results, FePd3 undergoes a transition from the ferromagnetic to the noncollinear triple-Q state when compressed. Finally, the implementation of the stress tensor in the BigDFT software package is presented. It is shown that an explicit treatment of core electrons can considerably reduce the errors introduced by the pseudopotentials. Thus the estimates of the structural properties can be improved.

Abstract The rare-earth elements (REEs) and bismuth, being classified as the ‘most critical raw materials’ (European Raw Materials Initiatives, 2017), have a high economic importance to the EU combined with a high relative supply risk. REEs are highly important for the evolving technologies such as clean-energy applications, high-technology components, rechargeable batteries, permanent magnets, electric and hybrid vehicles, and phosphors monitors. This scientific research work aims at building a fundamental knowledge base concerning the electronic/molecular structure and properties of rare-earth element (REE) and bismuth complexes with dithiocarbamate (DTC) and 1,10-phenanthroline (PHEN) by employing state-of-the-art experimental techniques such as nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction (XRD) techniques together with ab initio quantum mechanical computational methods. This combination of methods has played a vital role in analysing the direct and significant effect of the heavy metal ions on the structural and magnetic resonance properties of the complexes, thereby, providing a framework of structure elucidation. This is of special importance for REEs, which are known to exhibit similar chemical and physical properties. The objectives of the work involve i) a systematic investigation of series of REE(III) as well as bismuth(III) complexes to get a profound understanding of the structure-properties relationship and ii) to find an appropriate theoretical modelling and NMR calculation methods, especially, for heavy metal systems in molecular and/or solid-state. This information can later be used in surface interaction studies of REE/bismuth minerals with DTC as well as in design and development of novel ligands for extraction/separation of metal ions. The REE(III) and bismuth(III) complexes with DTC and PHEN ligands have all provided a unique NMR fingerprint of the metal centre both in liquid and solid phase. The solid-state ¹³C and ¹⁵N NMR spectra of the diamagnetic REE(III) and bismuth(III) complexes were in accord with their structural data obtained by single crystal XRD. The density functional theory (DFT) methods were used to get complementary and refined structural and NMR parameters information for all diamagnetic complexes in the solid-state. The relativistic contributions due to scalar and spin-orbit correlations for the calculated ¹H/¹³C/¹⁵N chemical shifts of REE complexes were analysed using two-component zeroth-order regular approximation (ZORA)/DFT while the ‘crystal-lattice’ effects on the NMR parameters were calculated by combining DFT calculations on molecular and periodic solid-state models. The paramagnetic REE complexes display huge differences in their ¹H and ¹³C NMR spectral patterns. The experimental paramagnetic NMR (pNMR) chemical shifts, as well as the sizable difference of the ¹H and ¹³C NMR shifts for these isoelectronic complexes, are well reproduced by the advanced calculations using ab initio/DFT approach. The accuracy of this approach is very promising for further applications to demanding pNMR problems involving paramagnetic f-block elements. The results presented in this thesis demonstrate that a multidisciplinary approach of combined experimental NMR and XRD techniques along with computational modelling and property calculations is highly efficient in studying molecular complexes and solids containing heavy metal systems, such as rare-earths and bismuth.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work we perform a theoretical study about fluorine adsorption in BC2N nanostructures. The fluorine interaction with the BC2N nanostructures was studied using only atomic fluorine on two nanotubos with different chiralities (zigzag(5,0) and armchair(3,3)) and a monolayer. We used first principles calculations based on the density functional theory (DFT) taking account the effects of the spin polarization. For the exchange and correlation term, we use the generalized gradient approximation (GGA) and to describe the electron-ion interaction we use the pseudopotential approximation. The charge density is obtained solving the selfconsistent Kohn-Sham equations and to represent the Kohn-Sham wave functions a linear combination of atomic orbital is used. Our results show that when a single fluorine atom is adsorbed, the most stable configuration occurs when the fluorine atom is adsorbed on the boron atom (FB). The configurations with the fluorine adsorbed on carbon atom (FCI and FCII) are less stable than the FB configuration and the FN reaction is unstable. When the fluorine atom is adsorbed in BC2N nanostructure the local configuration is modified. It is observed that the atom bonded to the fluorine atom moves outward from the surface of the nanotubes and the monolayer. The electronic properties present similar characteristics in both nanotubes and also in the monolayer. The FCII configuration introduce acceptors properties in a BC2N nanostructures and the FCI configuration introduce donor properties. The fluorine adsorption in the most stable configuration gives rise to electronic levels in the band gap. When we investigated the BC2N monolayer with different fluorine coverage, the most stable configuration is obtained when the fluorine atoms are adsorbed on the boron atoms, forming a line. Defects levels are observed near to the top of the valence band and the system exhibit a p-type semiconductor character to low fluorine density and metallic character to the high fluorine density. We observe that the fluorine adsorption induces spin polarization effects in BC2N nanostructures leaving the system to present a spin magnetic moment. The absolute value of the spin magnetic moment depends on the density of fluorine adsorbed. / Nesse trabalho realizamos o estudo teórico da adsorção de flúor em nanoestruturas de BC2N. A interação de flúor com as nanoestruturas de BC2N foi estudada a partir da adsorção de flúor atômico em nanotubos de diferentes quiralidades, zigzag (5,0) e armchair (3,3), além da monocamada. Usamos cálculos de primeiros princípios dentro do formalismo da teoria do funcional da densidade (DFT), levando em consideração os efeitos de polarização de spin. Para o termo de troca e correlação, utilizamos a aproximação do gradiente generalizado (GGA) e para descrever a interação elétron-caro¸co utilizamos a aproximação do pseudopotencial. A densidade de carga é obtida resolvendo-se as equações de Kohn-Sham de maneira autoconsistente, com as funções de onda de Kohn-Sham expandidas em uma combinação linear de orbitais atômicos. Nossos resultados mostram que para a adsorção de um átomo de flúor, a configuração mais estável ocorre quando o átomo de flúor é adsorvido sobre o átomo de boro (FB). As configurações com adsorção de flúor em carbono (FCI e FCII) são menos favoráveis do que a adsorção ao FB, enquanto que a reação FN é instável. Quando o átomo de flúor é adsorvido nas nanoestruturas de BC2N a configuração local é modificada. Observa-se que o átomo ligado ao átomo de flúor desloca-se para fora da superfície dos nanotubos e em relação à monocamada temos um deslocamento deste átomo para fora da sua superfície. As propriedades eletrônicas apresentam características semelhantes em ambos os nanotubos e também para a monocamada. A configuração FCII faz com que as nanoestruturas de BC2N apresentem características aceitadoras e a configuração FCI faz com que essa estrutura apresente características doadoras. Para a adsorção de flóuor na configuração mais estável temos níveis rasos presentes na estrutura de bandas. Quando é feita a variação da densidade de flúor em uma monocamada de BC2N, a configuração mais estável é obtida quando os átomos de flúor são adsorvidos sobre átomos de boro formando uma linha, e na região do topo da banda de valência níveis de defeito podem ser observados. Em relação às propriedades eletrônicas é possível observar que em todas as estruturas ocorre uma perturbação no fundo da banda de condução. Na região do topo da banda de valência níveis de defeitos podem ser observados. Onde podemos obsevar que essas estruturas apresentam características de semicondutor do tipo-p para baixas densidades de flúor e metálicas para altas densidades. Temos que a adsorção de flúor provoca uma quebra de degenerescência de spin o que faz com que o sistema apresente um momento magnético de spin. O valor absoluto desse momento magn´etico de spin depende da densidade de flúor adsorvido.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work we study simultaneously the structural, electronic and thermoelectric properties of PbSe and PbTe nanowires, analyzing the quantum confinement effects, the dependence with the planar stoichiometry and the spin-orbit interactions. We also study these nanowires in the presence of intrinsic defects (vacancies and antisites) and doped with group III (Al, Ga, In, and Tl) impurity. We use first principles calculations within the formalism of the density functional theory (DFT). We observed that the nanowires are more stable in the rock salt structure and aligned along the (001) direction. The electronic properties of nanowires are in uenced by three effects: the quantum confinement, spinorbit interactions and the planar stoichiometry. The quantum confinement increases the thermoelectric efficiency of the PbSe and PbTe nanowires when compared to the system in the bulk phase, reaching an increase up to two orders in the magnitude, leading the PbSe and PbTe nanowires with great potential to be used in thermoelectric devices. We studied the in uence of intrinsic defects and group III impurity doping in the main properties of PbSe and PbTe, we show that these defects give rise to different electronic properties in the nanowires as compared to the bulk one. Intrinsic defects and group III impurities, which modify the electronic density of states (DOS) near to the top of the valence band or near to the bottom of the conduction band increase the thermoelectric efficiency of the PbSe and PbTe nanowires. However, defects that introduce electronic levels in energy band gap are shown to cause and degradation in the thermoelectric efficiency. The increase (decrease) in thermoelectric efficiency is associated with a lower (higher) value of electronic part of the thermal conductivity. In summary, we show that PbSe and PbTe nanowires are very promising materials to be used in thermoelectric, electronic and optical devices. / Neste trabalho estudamos simultaneamente as propriedades estruturais, eletrônicas e termoelétricas de nanofios de PbSe e PbTe, analisando os efeitos do confinamento quântico, a dependência com a estequiometria planar e a interação spin-órbita. Estudamos também estes nanofios na presença de defeitos intrínsecos (vacâncias e antissítios) e impurezas do grupo III (Al, Ga, In e Tl). Utilizamos cálculos de primeiros pincípios dentro do formalismo da teoria do funcional da densidade (DFT). Observamos que os nanofios são mais estáveis na estrutura rock salt, e alinhados ao longo da direção (001). As propriedades eletrônicas desse nanofios são influenciadas por três efeitos: o confinamento quântico, a interação spin-órbita, e a estequiometria planar. O confinamento quântico aumenta a eficiência termoelétrica do PbSe e PbTe em comparação ao observado para o bulk, chegando a um aumento de até duas ordens de grandeza, fazendo com que os nanofios de PbSe e PbTe tenham um grande potencial para serem utilizados em dispositivos termoelétricos. Estudamos a influência de defeitos intrínsecos e da dopagem de impurezas do grupo III nas principais propriedades do PbSe e PbTe, onde mostramos que essa influência é diferente no bulk e no nanofio. Defeitos intrínsecos e impurezas do grupo III que alteram a densidade de estados eletrônicos (DOS) nas proximidades do topo da banda de valência ou do fundo da banda de condução, observamos um aumento da ficiência termoelétrica dos nanofios de PbSe e PbTe. Porém aqueles que introduzem níveis no gap de energia fazem com que a eficiencia termoelétrica diminua. O aumento (diminuição) da eficiência termoelétrica está associado(a) ao menor (maior) valor da comdutividade térmica eletrônica. De maneira geral, mostramos que nanofios de PbSe e PbTe são materiais muito promissores para a aplicação em dispositivos termoelétricos, eletrônicos, óticos, etc.

Orientador: Antonio Carlos Dias Angelo / Banca: Hamilton Brandão Varela de Albuquerque / Banca: Nelson Henrique Morgon / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Resumo: A utilização das fases intermetálicas ordenadas como eletrocatalisadores em células a combustível já pode ser considerada como uma solução iminente para os problemas que envolvem a eficiência e as questões econoômicas. Para assegurar que as propriedades geométricas e eletrônicas destes materiais sejam realmente as almejadas para atender a todas as exigências na eletrocatálise das reações de oxidação das moléculas é necessário um estudo aprofundado de caracterização das fases intermetálicas, comumente realizado por criteriosas técnicas experimentais. Entretanto, experimentalmente, a caracterização destes materiais não fornece informações precisas que permitam correlacionar as propriedades dos materiais com o seu desempenho frente a uma dada reação eletrostática. Desta forma, uma estratégia metodológica para se obter um conhecimento mais adequado no estudo das fases intermetálicas é a utilização de métodos computacionais, baseados na Teoria do Funcional da Densidade (DFT). A metodologia empregada neste trabalho aborda uma sistemática para a otimização das propriedades geométricas através da minimização da energia total do sistema, bem como uma avaliação da estrutura eletrônica para estes materiais por meio de projeções sobre os orbitais atômicos na densidade de estados e de mapas de densidade de carga. O processo de otimização é feito por cálculos de campo auto-consistente sucessivos que variam o parâmetro de rede até encontrar uma estrutura que possua energia mínima, este processo pode ser realizado de duas formas, manual e automaticamente pelo código computacional, os resultados obtidos mostram que ambas as formas possuem a mesma precisão, levando a valores quase idênticos e que permitem reproduzir bem os cristais para os materiais estudados. A análise comparativa entre os dados cristalográficos da literatura e os resultados... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The use of ordered intermetallic phases as electrocatalysts in fuel cells can now be regarded as an imminent solution for the problems concerning the efficiency of the device and for economic issue. To ensure that the geometric and electronic properties of these materials are actually suitable for the requirements in the electrocatalysis of melecules oxidation reactions need a meticulous characterization of the intermetallic phases, in general done by standard experimental techniques. However, the characterization of these materials performed solely experimentally does not provide accurate information to enable correlation of the properties of the materials with their performance against a given electrocatalytic reaction. Thus, a methodological strategy for obtaining a better knowledge in the study of ordered intermetallic phases is the use of computational methods, based on the Density Functional Theory. The methodology used in in thius research presents a sytematic optimization of the geometric properties by minimizing the total energy of the system and an evaluation of the electronic structure for these materials by means of the density of states projected onto atomic orbital and charge density maps. The optimization process is done by successively self-consistent field calculations that very the lattice parameter to find a structure that has a minimum energy, this process can be accomplished in two ways, manually and automatically by the computer code, the results show that both forms have the same precision, leading to almost identical identical values and allow to reproduce well the crystals of the studied materials. A comparative analysis of the crystallographic data from the literature and the results presented here show very small errors (in the order of 2-3% for most of the materials), which can be attributed exclusively to the various mathematical approaches applied... (Complete abstract click electronic access below) / Mestre

As reações de cicloadição 1,3-dipolar são uma poderosa ferramenta para a síntese de uma variedade de anéis heterocíclicos de cinco membros. A cicloadição de óxidos de nitrila à olefinas, em particular, é de considerável interesse para a obtenção de isoxazolinas, que são intermediários versáteis na síntese de produtos naturais e de materiais com potencial aplicação como cristais líquidos. A Teoria do Funcional da Densidade foi utilizada para estudar o mecanismo da reação cicloadição [3+2] 1,3-dipolar que ocorre, inicialmente, entre o óxido de benzonitrila e o ácido vinilacético. Para tal, foram empregados os funcionais PBE1PBE, B3LYP e CAM-B3LYP, no nível 6-311+G(2d,p). O efeito do solvente foi avaliado através dos modelos PCM e CPCM, com os solventes THF, acetonitrila e formamida. A análise dos Orbitais Moleculares de Fronteira e do recente modelo da distorção e interação do estado de transição (TS), foram utilizadas para explicar a regioquímica dos produtos obtidos e a formação do bisaduto 2:1, originário de duas sucessivas cicloadições envolvendo o óxido de benzonitrila. Na primeira etapa da reação, os cálculos evidenciaram a formação do produto 3,5-dissubstiuído. Embora existam diferenças quantitativas nas barreiras de ativação e reação calculadas com os três diferentes funcionais, a previsão dos produtos majoritários e estados de transição mais favoráveis é a mesma, independentemente do funcional utilizado. Contudo, a conformação dos estados de transição e dos produtos intermediário e final da reação sofre uma pequena alteração com a inclusão do efeito do solvente. A energia de ativação nas duas cicloadições aumenta com o incremento da polaridade do solvente, porém, a possibilidade de formação de uma ligação de hidrogênio no estado de transição é responsável por uma diminuição na energia total de ativação. Esse resultado está diretamente vinculado à polaridade do TS. Quando comparados os resultados obtidos com os dois modelos de solvente, observou-se que ΔE≠ e ΔEreação são essencialmente as mesmas, embora as energias eletrônicas calculadas com CPCM sejam levemente inferiores às calculadas com PCM. / The 1,3-dipolar cycloaddition reactions are a powerful tool for synthesizing a wide range of 5-membered heterocyclic rings. Particularly, the cycloaddition of nitrile oxides to olefins is considerably interesting to obtain isoxazolines, which are versatile intermediaries in the synthesis of natural products and materials with potential application such as liquid crystals. The Density Functional Theory has been used to study the [3+2] 1,3-dipolar cycloaddition reaction mechanism that initially occurs between benzonitrile oxide and vinylacetic acid. To do that, PBE1PBE, B3LYP and CAM-B3LYP functionals have been used at level 6- 311+G(2d,p). The solvent effect was evaluated through the PCM and CPCM models, with the THF, acetonitrile and formamide solvents. The analysis of the Frontier Molecular Orbitals and of the recent distortion and interaction model of transition state (TS) have been used to explain the regiochemistry of the products obtained and the formation of the bisadduct 2:1, which is originated from two successive cycloadditions involving benzonitrile oxide. In the first reaction step, the calculations showed the formation of the 3,5-dissubstituted product. Although there are quantitative differences in the activation and reaction barriers calculated with the three different functionals, the forecasting of more favorable majoritary products and transition states is the same, no matter the functional used. However, the conformation of the transition states and of the final and intermediary products of the reaction is slightly changed by the inclusion of the solvent effect. The activation energy of both cycloadditions increases with the polarity increment of the solvent, but the possibility of formation of a hydrogen bond in the transition state is responsible for a reduction of the total activation energy. That result is directly linked to the TS polarity. When we compare the results obtained with the two solvent models, we observe that ΔE≠ and ΔEreaction are essentially the same, although the electronic energies calculated with CPCM are slightly smaller than the ones calculated with PCM.

A constante emissão de CO2 na atmosfera devido a fontes antropogênica despertou uma preocupação crescente em função da sua atuação no efeito estufa. Um número crescente de metodologias para redução da concentração deste gás na atmosfera vem sendo proposta e uma alternativa atrativa é a da inserção do CO2 em anéis epóxidos para formação de carbonatos cíclicos. Apesar de já existirem inúmeros trabalhos a respeito destas reações, o mecanismo pelo qual elas ocorrem não está completamente esclarecido. Neste trabalho é apresentado o estudo mecanístico da cicloadição de CO2 em diferentes epóxidos catalisada por líquidos iônicos haletos de alquil-imidazólios, CnMIm X (n = 1, 2, 3, 4 e X = Cl, Br, I), através da Teoria do Funcional da Densidade empregando-se o funcional !B97X-D e o conjunto de base 6-31G(d,p) e LanL2DZ (somente para o I). Propuseram-se duas rotas distintas para o mecanismo, uma composta por três etapas e outra composta por somente duas etapas. Ambas as rotas tiveram a primeira etapa como sendo a mais energética e correspondendo à abertura do anel epóxido e mostraram ser competitivas entre si energeticamente. Esta etapa mostrou depender de ambos cátion e ânion do catalisador para ocorrer, onde uma ligação de hidrogênio não clássica com o H2 do imidazólio aparece no estado de transição. Verificou-se que no geral a energia de ativação da reação diminui com o aumento da cadeia alquílica do cátion imidazólio assim como diminui com o aumento do caráter nucleofílico do haleto (Cl > Br > I). O substituinte do anel epóxido também exerce influência sobre a energia de ativação da reação, porém não há uma tendência bem definida. Constatou-se que o sítio mais favorável para ataque nucleofílico é o carbono não substituído do anel epóxido tanto pela diferença de energia quanto por análise de índices de reatividade de Fukui e de interações não covalentes. Uma análise de 14 funcionais da densidade e do método perturbativo de segunda ordem MP2 em comparação ao método composto G4MP2 revelaram a forte dependência das energias de ativação com o método empregado. Através de cálculos de dinâmica molecular clássica foi possível estudar dinamicamente o sistema brometo de 1-butil-3-metil imidazólio, o óxido de estireno e o CO2 e notou-se a formação de duas fases com a presença de uma interface. Adicionalmente, observou-se que a probabilidade da reação ocorrer no bulk do líquido iônico é maior, pois a proporção catalisador/substrato é maior nesta região. Por meio da dinâmica molecular de Born-Oppenheimer constatou-se que o anel epóxido também pode ser ativado através de interações com os hidrogênios H4 e H5 do anel imidazólio. / The constant emission of CO2 into the atmosphere due to anthropogenic sources has generated a growing concern regarding the greenhouse effect. Many methodologies to reduce the atmospheric CO2 concentration have been proposed and an alternative is the insertion of CO2 into epoxides to form cyclic carbonates. Although there are a lot of studies in this area, the reaction mechanism by which they occur is still unclear. In this work the cycloaddition mechanism of CO2 into different epoxides catalyzed by alkyl-imidazolium halide ionic liquids, CnMIm X (n = 1, 2, 3, 4 e X = Cl, Br, I), is presented. Density Functional Theory in conjunction with the functional !B97X-D and 6-31G(d,p) and LanL2DZ (for I atoms) basis sets were employed. Two distinct routes were proposed for the mechanism: one composed of three steps and another composed by only two steps. Both routes showed that the first step regarding the epoxide ring opening is the determined one and they are energetically competitive with each other. This step depends on both cation and anion from the catalyst to proceed through a non-classical hydrogen bond in the transition state. It was found that the activation energy decreases with the chain length of the alkyl group from the imidazolium ring as well as with the nucleophilic character of the halide (Cl > Br > I). The epoxide ring substituent also exerts influence on the activation energy of this reaction, but there is no well defined behaviour. The most favourable site for nucleophilic attack is the non-substituted epoxide ring carbon as was shown by the reaction energy difference and through reactive Fukui index and non-covalent interaction (NCI) analysis. 14 exchange-correlation density functionals were investigated and compared to the well established second order perturbation theory (MP2) method and G4MP2 composite method. One found out that the activation energies strongly depends on the chosen method. Through classical molecular dynamics it was possible to study the system 1-butyl-3-methyl-imidazolium bromide together with styrene oxide and CO2 e the formation of two phases with the presence of an interface was observed. Additionally, it was shown that the probability of the reaction to occur in the ionic liquid bulk is bigger because the catalyst/substrate proportion is bigger in this region. Born-Oppenheimer molecular dynamics was used to prove that the H4 and H5 hydrogen atoms from the imidazolium ring may interact with the oxygen atom from the epoxide and activate the C–O bond for the reaction to proceed.