Many-body perturbation theory and coupled-cluster theory, combined with carefully constructed basis sets, can be used to accurately compute the properties of small molecules. We applied a series of methods and basis sets aimed at reaching the ab initio limit to determine the barrier to planarity for ethylene cation. For potential energy surfaces corresponding to bond dissociation, a single Slater determinant is no longer an appropriate reference, and the single-reference hierarchy breaks down. We computed full configuration interaction benchmark data for calibrating new and existing quantum chemical methods for the accurate description of potential energy surfaces. We used the data to calibrate single-reference configuration interaction, perturbation theory, and coupled-cluster theory and multi-reference configuration interaction and perturbation theory, using various types of molecular orbitals, for breaking single and multiple bonds on ground-state and excited-state surfaces. We developed a determinant-based method which generalizes the formulation of many-body wave functions and energy expectation values. We used the method to calibrate single-reference and multi-reference configuration interaction and coupled-cluster theories, using different types of molecular orbitals, for the symmetric dissociation of water. We extended the determinant-based method to work with general configuration lists, enabling us to study, for the first time, arbitrarily truncated coupled-cluster wave functions. We used this new capability to study the importance of configurations in configuration interaction and coupled-cluster wave functions at different regions of a potential energy surface.

This thesis presents Quantum Chemical calculations on the Photochemistry of Halogenated benzenes, Phytochromobilin, Ruthenium Polypyridyl complexes and 6-4 photoadducts in DNA. The work is focused on improving the understanding of a number of experimentally observed photochemical processes in these systems. New results regarding the mechanism of photodissociation of halogenated arenes, photointerconversion of phytochromobilin are presented, as well as of the photoprocesses of Ruthenium Polypyridyl complexes and new mechanistic insights in the repair of 6-4 photoadducts in DNA.

This thesis is dedicated to the theoretical study of the methyl cation CH3+ electronic states and, globally, falls within the study of the interstellar clouds molecular synthesis and the chemistry of the hydrocarbons which are present in high-energy plasmas such as in the experimental nuclear fusion reactor ITER. Among the different possible formation reactions, we chose two reactions involved in these fields: the ionization of the methyl radical CH3 ground state and the reactive collisions between simple carbonated or hydrocarbonated (C, CH+ and CH2+) and hydrogen species (H3+, H2 and H, respectively).<p><p>As this cation is characterized by 8 electrons and 4 nuclei, this allowed us to perform high-level ab initio calculations using the CASSCF/MRCI method with the Dunning aug-cc-p(C)VXZ basis sets. These calculations were completed by a study of different methodological effects such as the core-valence electronic correlation, the complete basis set extrapolation and the basis set superposition error.<p><p>We calculated equilibrium geometries (precision within 10^-5 angtröm and 10^-2° and their energies for the methyl radical and cation, studied the potential energy surfaces involved by the Jahn-Teller effect targeting the methyl cation E' states and achieved frequencies calculations. From these values were derived ionization potentials (IP) from the methyl radical ground state towards the methyl cation lowest-lying states (precision within 10^-2 eV). Vibrational corrections and nuclear relaxation effects were taken into account.<p><p>The calculated IPs and frequencies should facilitate the analysis of methyl radical threshold photoelectron spectra leading to the methyl cation lowest-lying states, recorded at the synchrotron facility Soleil by the Dr. Alcaraz group from the Université Paris-Sud.<p><p>Through reaction paths calculations using a quadratic steepest-descent method, we have proposed new reactional pathways enabling the connections between the different dissociation channels correlated to the cation lowest-lying triplet states. The absence of potential barriers in the energy profiles allows us to propose these reactions as sources, in interstellar clouds, of hydrocarbonated molecules whose stability increases with growing size according to the series C -> CH+ -> CH2+ -> CH3+.<p><p>The groups of Pr. Urbain from UCL and Dr. Savin from Columbia University studied the collisions in copropagating beams of C and H3+ leading to the formation of these hydrocarbonated species. The proposed reactional mechanisms are in good agreement with the experimental observations, what permits a better understanding of the chemistry behind these collisional processes of astrochemical interest. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The accuracy of a quantum chemical calculation inherently depends on the ability to account for the completeness of the one- and n-particle spaces. The size of the basis set used can be systematically increased until it reaches the complete one-particle basis set limit (CBS) while the n-particle space approaches its exact full configuration interaction (FCI) limit by following a hierarchy of electron correlation methods developed over the last seventy years. If extremely high accuracy is desired, properly correcting for very small effects such as those resulting the Born-Oppenheimer approximation and the neglect of relativistic effects becomes indispensable. For a series of chemically interesting and challenging systems, we identify the limits of conventional approaches and use state-of-the-art quantum chemical methods along with large basis sets to get the “right answer for the right reasons.” First, we quantify the importance of small effects that are ignored in conventional quantum chemical calculations and manage to achieve spectroscopic accuracy (agreement of 1 cm−1 or less with experimental harmonic vibrational frequencies) for BH, CH+ and NH. We then definitively resolve the global minimum structure for Li₆ , Li₆⁺ , and Li₆- using high accuracy calculations of the binding energies, ionization potentials, electron affinities and vertical excitation spectra for the competing isomers. The same rigorous approach is used to study a series of hydrogen transfer reactions and validate the necessary parameters for the hydrogen abstraction and donation steps in the mechanosynthesis of diamondoids. Finally, in an effort to overcome the steep computational scaling of most high-level methods, a new hybrid methodology which scales as O(N⁵) but performs comparably to O(N⁶) methods is benchmarked for its performance in the equilibrium and dissociation regimes.

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

Firstly, we have developed a Tensor Contraction Engine-based implementation of the BW-MRCCSD approach. The scalability tests have been performed across thousand of cores. We have further developed a novel two-level parallel algorithm for Hilbert-space MRCC methods which uses the processor groups. In this approach, references are distributed among processor groups (reference-level parallelism) and tasks of each reference are distributed inside of a given processor group (task-level parallelism). We have shown that our implementation scales across 24000 cores. The usability of our code was demonstrated on larger systems (dodecane, polycarbenes and naphthyne isomers). Finally, we present novel universal state- selective (USS) corrections to the state-specific MRCC methods. The USS-corrected MRCC results were compared with the full configuration interaction (FCI) results.

Resumo: Esta pesquisa é fruto do desenvolvimento de uma sequência didática sobre o comportamento das entidades físicas: partícula, onde e partícula-onda no Experimento da Supla Fenda, numa versão adaptada de Feynman e colaboradores e foi desenvolvida com estudantes de terceira série do ensino superior do curso de Química, disciplina Química Quântica, de uma universidade pública estadual. O objetivo geral do estudo foi discutir os fundamentos e práticas de experimentos mentais, tendo como parâmetro a utilização de diversas alternativas didáticas e seus desdobramentos para o ensino nas disciplinas científicas. A pesquisa de cunho qualitativo ancora-se nos fundamentos dos experimentos mentais e tem por base as teorias que visam promover a comunicação verbal e escrita dos conhecimentos científicos. O problema de pesquisa originou-se das dificuldades que os estudantes demonstravam em sala de aula para explicar e debater suas idéias, de maneira significativa, inteligível e dinâmica, quando estas exigiam deles o domínio da verbalização de conhecimentos científicos sobre o experimento da dupla fenda. Para interpretação e análise dos dados, utilizou-se a vertente interpretativa por meio de instrumentos e técnicas que permitem, principalmente, a descrição e interpretação de fatos e fenômenos, a recuperação de sentidos, tendo como base a investigação interpretativa do conjunto de materiais empíricos que se transformaram em textos. Os resultados revelam que os procedimentos didáticos utilizados são relevantes por possibilitarem a problematização, a discussão, o uso da imaginação e da linguagem verbal e escrita, pouco usuais em contextos de aulas de Química Quântica. Com relação aos textos, tanto aqueles transcritos das discussões e das falas dos estudantes quanto os que foram produzidos durante o desenvolvimento da pesquisa, por solicitação do professor... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: This research is the result of the development of a didactic sequence on the behavior of physical entities: particle, wave and wave-particle in the Double-Slit Experiment, in an adapted version from Feynman and assistants, which was developed with university students from the third grade of the Chemistry course, in the discipline of Quantum Chemistry, at a Brazilian public state university. The study aimed to discuss the fundaments and practices of thought experiments by using several didactic alternatives and its unfoldings to the teaching of scientific disciplines. The research, of qualitative basis, is anchored to the fundaments of thought experiments grounded on theories aiming to promote verbal and written communication of scientific knowledge. The research problem focuses on the difficulties that the students show in the classroom in order to explain and discuss their ideas when theses require that they master verbalization of scientific knowledge on the double-slit experiment in a meaningful, significant and dynamic manner. To the purpose of data interpretation and analysis, we used the interpretative perspective by means of the methodological and theoretical. Results revealed that the didactic procedures used are relevant since they allow problematization, discussion, the use of imagination and of verbal and written language, quite rare in classroom contexts of Quantum Chemistry. Regarding the texts, both the ones transcribed during the discussions and the students' speeches and those produced during the development of the research, required by the teacher, deserve attention. The text productions were found considerably reduced, fragmented, lacking progression and continuity of ideas, and also lacking the interconnected sequence of the parts that constitute an articulated whole. By means of discourse analysis, we detected loss of the line of unity that assures its interpretability... (Complete abstract click electronic access below) / Orientador: Aguinaldo Robinson de Souza / Coorientador: Ourides Santin Filho / Banca: Luiz Carlos de Menezes / Banca: Luiz Henrique Ferreira / Banca: João José Caluzi / Banca: Eduardo Adolfo Terrazzan / Doutor

A química do selênio é um assunto de crescente interesse devido a sua presença em diversos ambientes químicos, em particular, na atmosfera terrestre. A ausência de estudos sobre espécies relativamente simples contendo 2-4 átomos motivou este projeto, que se concentrou na investigação teórica rigorosa de uma serie de espécies moleculares: SeF, SeCl, SeBr, HSeF, HFSe, HSeCl, HClSe, HSeBr, HBrSe e de vários isômeros na superfície de energia potencial 1[H, S, Se, Cl]. Propriedades espectroscópicas de um conjunto de estados eletrônicos e o calor de formação das moléculas SeF, SeCl e SeBr foram determinados. Juntamente com os novos resultados desta investigação, sugerimos uma revisão e correção de alguns dados teóricos e experimentais da literatura. Aspectos energéticos, estruturais e espectroscópicos associados aos pontos estacionários nas superfícies de energia potencial singleto [H, Se, X], X = F, Cl e Br, e [H, S, Se, Cl] também foram caracterizados, assim como determinados os calores de formação dos isômeros mais estáveis. Barreiras energéticas para os vários processos de isomerização foram estimadas bem como o gasto energético envolvido nas diferentes possibilidades de dissociação dos isômeros mais estáveis. No caso dos sistemas triatômicos ainda estimamos as constantes de velocidade para as reações de isomerizações direta e reversa. Nesse contexto, esperamos que este trabalho possa servir como uma referência para estudos teóricos e experimentais futuros desses sistemas e/ou de outros de complexidade idêntica. / The chemistry of selenium is a subject of increasing interest due to its presence in many chemical enviroments, specially in the Earth\'s atmosphere. The lack of studies of relatively simple species containing 2-4 atoms has motivated this project which was focused on a rigorous theoretical investigation of a series of molecular especies: SeF, SeCl, SeBr, HSeF, HFSe, HSeCl, HClSe, HSeBr, HBrSe, and the isomers on the 1[H, S, Se, Cl] potential energy surface. Spectroscopic properties of a set of electronic states and the heat of formation of SeF, SeCl, and SeBr were determined. Along with the new results from this investigation, we showed that some theoretical and experimental data reported in the literature be revised and corrected. Energetic, structural, and spectroscopic aspects associated with the stationary points on the singlet potential energy surfaces [H, Se, X], X = F, Cl e Br, and [H, S, Se, Cl] were also characterized, and the heats of formation of the most stable isomers evaluated. Energetic barriers for the various processes of isomerization were estimated, as well as the energy involved in the dierent possibilities of dissociation of the most stable isomers. In the case of triatomic systems, we still estimated the rate constants for the direct and reverse reactions. In this context, we expect that this work should serve as reference in future theoretical and experimental studies on these systems and/or others of similar complexity

Parâmetros teóricos obtidos pela otimização de geometria da estrutura molecular em fase gasosa com o método quântico semi-empírico PM3, foram usados para correlacionar constantes de partição (log Poct; log Pc16 e log Pcic) para uma série de 700 solutos, considerando como referência as LSER obtidas com os parâmetros experimentais dos solutos: acidez (&#931;&#945;2), basicidade (&#931;&#946;2), e dipolaridade/polarizabilidade (&#960;2) de ligação de hidrogênio; refração molar de excesso (R2) e volume característico (Vx). Os parâmetros avaliados foram: as energias HOMO e LUMO; o momento de dipolo elétrico e as cargas atômicas parciais do hidrogênio mais positivo e do átomo mais negativo (classe IV - modelo CM1 ); e o índice de polarizabilidade como proposto por Famini e col.. Estes parâmetros teóricos, juntamente com Vx, são adequadas para serem utilizados em LSER na correlação dos valores de log P. Para a maioria dos solutos considerados, as quantidades eletrostáticas q+ , &#921;q-&#921; e &#181; descrevem adequadamente os termos de acidez, basicidade e dipolaridade de ligação de hidrogênio, o que não ocorre para solutos com mais de um polo positivo ou negativo relevante, como hidroquinona, nem para solutos como 2-nitrofenol com capacidade de formar ligação de hidrogênio intramolecular. As LSER para log Poct; log Pc16 e log Pcic com estes parâmetros teóricos são quimicamente consistentes. Cargas atômicas parciais, largamente usadas qualitativamente para descrever a reatividade de compostos e de grupos funcionais, podem ser usadas como descritor quantitativo de interações moleculares em estudos de solubilização, o primeiro estágio, que governa muitas reações químicas. / Semiempirical quantum theoretical parameters, obtained from PM3 geometry optimization method in gas-phase for a wide range of 700 solutes, were used to correlate experimental partition coeficients (log Poct; log Pc16 and log Pcyc), bearing as reference the LSER with experimentally derived parameters: Hydrogen bond acidity (&#931;&#945;2), basicity (&#931;&#946;2) , and dipolarity/polarizability (&#960;2); excess molar refraction and characteristic volume (Vx). Theoretical quantities tested includes: HOMO and LUMO energies; class IV, CM1 model atomic partial charges (q+ and &#921;q-&#921;) and dipole moment (&#181;); Famini\'s polarizability index (&#960;F); and Vx. Those quantum derived quantities can successfully be used, jointly with Vx, to correlate log P values in the LSER approach. For most of the considered solutes, electrostatic quantities q+, &#921;q-&#921; and &#181; describes fairly well Hydrogen bond acidity, basicity and dipolarity terms, but do not describe hydrogen bond terms for solutes with more than one relevant positive or negative centers, like hydroquinone, nor solutes like 2-nitrophenol wich forms intramolecular hydrogen bond when in condensed phases. LSER with those theoretical parameters for log Poct, log Pc16 and log Pcic are chemically consistent. Atomic partial charges, widelly used to describe compounds and functional group reactivity in a qualitative way, can be used as a quantitative descriptor of molecular interactions in solubility, the first step wich drive a lot of chemical reactions.

Em um curso tradicional de Química Quântica, os modelos estudados para ilustrar algumas das ferramentas da Mecânica Quântica relevantes para a compreensão da estrutura da matéria no nível atômico e molecular são apresentados no que se convencionou chamar, numa apresentação mais formal, de representação da posição. Nesta representação, o estado do sistema é descrito por uma função de onda dependente das posições das partículas que o constituem. Isso leva o estudante de química a uma concepção distorcida de que na natureza os estados dos sistemas devem ser obrigatoriamente descritos em termos das posições de suas partículas. Aqui mostramos que essa não é a única forma de abordar quanticamente a descrição de um sistema físico. Uma outra forma é servir-se da representação do momento, onde a função de estado depende do momento de cada uma das partículas. Existem dois caminhos para se obter as funções de estado na representação do momento. Uma delas é fazer-se a transformada de Fourier das funções de estado na representação da posição, e a outra é buscar resolver a equação de Schrödinger diretamente na representação do momento. Neste trabalho, foram discutidas essas duas abordagens para os modelos mais comuns estudados num curso de Química Quântica, sendo eles: a partícula na caixa, o oscilador harmônico, o átomo de hidrogênio, o átomo de hélio, o íon-molécula de hidrogênio (H2 +) e a molécula de hidrogênio (H2). Buscou-se mostrar uma perspectiva diferente na descrição desses sistemas bem como uma abordagem matemática distinta da usual e, também, as dificuldades, principalmente matemáticas, de sua aplicação e ensino num curso de Química Quântica. / In a conventional course in Quantum Chemistry, the models usually presented to illustrate the use of some quantum mechanical tools that are relevant for a comprehension of the structure of matter at the atomic and molecular levels are approached in a way that has been termed, in a more formal presentation, as position representation. In this representation, the state of a system is described by a wavefunction that is dependent on the positions of all particles that define the system. As a consequence of this presentation, chemistry students assimilate a distorted conception that in nature the state of a system must necessarily be described in terms of particles positions. Here we show that this is not the only way to approach quantum mechanically the description of a physical system. In an alternative way, known as momentum representation, the state function is expressed in a way that it is explicitly dependent on the momentum of each particle. There are two ways to obtain wavefunctions in the momentum representation. In of them, use is made of a Fourier transform of the wavefunctions in the position representation, and in the other one, an attempt is made to solve Schroedinger´s equation directly in the momentum representation. In this work, we have discussed these two approaches by examining the most common models studied in a Quantum Chemistry course, namely: the particle in a box, the harmonic oscillator, the hydrogen atom, the helium atom, the hydrogen molecular ion, and the hydrogen molecule. We have tried to show a different physical perspective in the description of these systems as well as a distinct mathematical approach than the usual one, and also the difficulties, mainly mathematical, of applying and teaching this representation in a Quantum Chemistry course.