The development of Rhenium I photocatalysts has been pursued since Lehn first showed the excellent performance of the ReI bipyridine tricarbonyl catalyst. Since then, devel- opment has modified the organic ligand to demonstrate continued or improved activity with other α-diimine bidentate geometries. Geometry has been limited to bidentate motifs, with fac-(CO)3 and axial halides. This work will demonstrate the synthesis, characterization, and testing of a new terdentate, κ3(L3)−Re1(CO)2X (X = Cl, Br, CN, OTf) family of compounds for CO2 reduction, as well as computational investigations into the mechanism of the reduction of CO2 to CO and other species. Development of computational aides will be described as well.

Orientadores: Munir Salomão Skaf, Vitor Rafael Coluci / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-18T13:27:57Z (GMT). No. of bitstreams: 1 Faro_TatianaMellodaCosta_M.pdf: 2906176 bytes, checksum: c28c56bc08c5c23e3a6f8668b1148162 (MD5) Previous issue date: 2011 / Resumo: Zeólitos são aluminossilicatos cuja estrutura consiste em tetraedros TO4 (com T=Si,Al) que compartilham todos os seus vértices com os tetraedros vizinhos, formando uma rede tridimensional altamente porosa e de baixa densidade. A substituição de Si por Al na matriz zeolítica leva à formação de uma carga total negativa, contrabalanceada pela presença de cátions trocáveis no material. Os zeólitos são objetos de estudo importantes por serem muito usados na indústria como peneiras moleculares, trocadores iônicos e catalisadores. A localização dos cátions trocáveis nos sítios cristalográficos de um zeólito e a identidade desses cátions são fatores que governam a adsorção de gases e outras moléculas pequenas no zeólito e o comportamento catalítico dos zeólitos. Embora várias técnicas experimentais sejam usadas para caracterizar tais cátions, é comum ocorrerem situações nas quais alguns dos cátions trocáveis não conseguem ser localizados com exatidão pelos métodos experimentais; nesses casos, estudos computacionais por simulações de Dinâmica Molecular (DM) são bastante úteis para ajudar a elucidar o posicionamento dos cátions trocáveis na estrutura zeolítica e as suas respectivas mobilidades. Neste trabalho, realizamos simulações de Dinâmica Molecular dos zeólitos NaX e BaX na aproximação da matriz rígida e considerando a vibração dos átomos da matriz com o objetivo de analisar a influência da vibração da matriz zeolítica na determinação de diversas propriedades do sistema, como o posicionamento dos cátions trocáveis e o espectro de absorção desses cátions na região do infravermelho longínquo. O zeólito NaX foi escolhido por ser o precursor de quase todos os outros zeólitos do tipo X, havendo assim muitos estudos experimentais sobre ele na literatura; já o zeólito BaX é importante pois as faujasitas de bário são usadas na indústria em processos de separação de moléculas aromáticas, como os xilenos / Abstract: Zeolites are aluminosilicates whose structure consists of TO4 tetrahedra (with T=Si,Al) that share all its corners with the neighboring tetrahedra, thus forming a three-dimensional network highly porous and with a low density. The substitution of Si with Al in the zeolitic matrix leads to the generation of a net negative charge, balanced by the presence of exchangeable cations in the material. Zeolites are important objects of study because they are used industrially as molecular sieves, ion exchangers and catalysts. The location the exchangeable cations in the zeolite crystallographic sites and the identity of these cations are factors that govern the adsorption of gases and other small molecules in the zeolite and the catalytic behavior of the zeolite. Although many experimental techniques are used to characterize those cations, there are situations in which some of the exchangeable cations can not be located accurately by experimental methods; in these cases, computational studies by Molecular Dynamics (MD) simulations are very useful to help elucidate the location of the exchangeable cations in the zeolitic structure and its respective mobilities. In this work, we carried out molecular dynamics simulations of the zeolites NaX and BaX in the rigid matrix approximation and considering the vibration of the matrix atoms with the intention to analyze the influence of the vibration of the zeolitic matrix in the determination of several properties of the system, such as the location of the exchangeable cations and the absorption spectra of these cations in the far infrared region. Zeolite NaX was chosen because it is the precursor of almost all other type-X zeolites, so there are many experimental studies about it in the literature; zeolite BaX is important because barium faujasites are used in the industry in the separation process of aromatic molecules, such as xylenes / Mestrado / Físico-Química / Mestre em Química

The spin of an electron often misleadingly interpreted as the classical rotationof a particle. The quantum spin distinguishes itself from classicalrotation by possessing quantized states and can be detected by its magneticmoment. The properties of spin and its collective behavior with otherfundamental properties are fascinating in basic sciences. In many aspectsit offers scope for designing new materials by manipulating the ensemblesof spin. In recent years attention towards high density storage devices hasdriven the attention to the fundamental level were quantum physics rules.To understand better design of molecule based storage materials, studies onspin degrees of freedom and their coupling properties can not be neglected. To account for many body effect of two or more electrons consistent withrelativity, an approximation like the Breit Hamiltonian(BH) is used in modernquantum chemical calculations, which is successful in explaining the splitin the spectra and corresponding properties associated with it. Often differenttactics are involved for a specific level of computations. For example themulti-configurational practice is different from the functional based calculations,and it depends on the size of the system to choose between resourcesand accuracy. As the coupling terms offers extra burden of calculating theintegrals it is literally challenging. One can readily employ approximations as it suits best for the applicationoriented device computations. The possible methods available in the literatureare presented in chapter 2. The theoretical implementations of couplingfor the multi-reference and density functional method are discussed in detail.The multi-reference method precedes the density functional methodin terms of accuracy and generalizations, however it is inefficient in dealingvery large systems involving many transition elements, which is vital formolecule based magnets as they often possess open shell manifolds. On theother hand existing density functional method exercise perturbations techniqueswhich is extremely specialized for a specific system - highly coupledspins. The importance of spin-spin coupling(SSC) in organic radical-Oxyallyl(OXA)was systematically studied with different basis sets and compared with asimilar isoelectronic radical(TMM). The method of spin-spin coupling implementationsare also emphasized. Similar coupling studies were carriedivout for the species HCP and NCN along with spin-orbit coupling(SOC).The splitting of the triplet states are in good agreement with experiments / QC 20110210

Geometry optimization is a fundamental step in the numerical modelling of chemical reactions. Many thermodynamic and kinetic properties are closely related to the structure of the reactant, product, and the transition states connecting them. Different from the reaction and product, which are local minima on the potential energy surface, a transition state is the first-order saddle point with only one negative curvature. Over years, many methods have been devised to tackle the problem. Locating stable structures is relatively easy with a reliable algorithm and high accuracy. One can follow the gradient descent direction to pursuit the local minimum until convergence is reached. But for the transition state, the determination is more challenging as either the up-hill or down-hill direction is allowed in the process. Motivated by the difficulty, many well-designed optimization algorithms are elaborated specifically to stress the problem. The performance of geometry optimization is affected by various aspects: the initial guess structure, the coordinate system representing the molecule, the accuracy of the initial Hessian matrix, the Hessian update schemes, and the step-size control of each iteration. In this thesis, we propose a new geometry optimization algorithm considering all the important components. More specifically, in Chapter 2, a new set of robust dihedral and redundant internal coordinates is introduced to effectively represent the molecular structures, as well as a computational efficient transformation method to generate a guess structure. In Chapter 3 and 5, a sophisticated robust algorithm is presented and tested to solve intricate transition state optimization problems. In Chapter 4, a new algorithm to exploring reaction pathways based on redundant internal coordinates is illustrated with real chemical reactions. Last but not least, in Chapter 6, a systematic test to explore the optimal methods in each procedure is presented. A well-performed combination of optimization methods is drawn for generic optimization purposes. All the methods and algorithms introduced in this thesis is included in our forth-coming open-source Python package named GOpt. It's a general-purpose library that can work in conjunction with major quantum chemistry software including Gaussian. More features are under development and await to be released in the coming update. / Thesis / Doctor of Science (PhD)

The presented thesis is devoted to the field of organic light emitting diodes (OLEDs). Time-dependent Kohn-Sham density functional theory (TDDFT) is applied in order to eludicate optical properties such as fluorescence and phosphorescence for some of the most important materials. The accuracy of TDDFT is evaluated with respect to the calculated absorption and emission spectra for commonly used light emitting polymers. A continuation of this work is devoted to Polyfluorene as this polymer has proven to be very promising. In this study the chain length dependence of its singlet and triplet excited states is analyzed as well as the excited state structures. Understanding the phosphorescence mechanism of tris(2-phenylpyridine)Iridium is of importance in order to interpret the high efficiency of OLEDs containing these specimens. The mechanism is analyzed by calculating the electric transition dipole moments by means of TDDFT using quadratic response functions. As not only the optical properties are essential for effective devices, electron transfer properties are addressed. The electron transfer capability of the sulfur and nitrogen analogues of Oxadiazole is evaluated through their internal reorganization energy. / <p>QC 20101109</p>

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No description available.

Density Functional Theory is used in order to shed light on the reaction mechanisms and the origins of stereoselectivity in enantioselective organocatalytic reactions. The reactions investigated are the dipeptide-catalyzed aldol reaction, the cinchona thiourea-catalyzed nitroaldol reaction and the prolinol derivative-catalyzed hydrophosphination reaction. We can justify the stereoselectivity in the reactions from the energies arising from different interactions in the transition states. The major contributions to the energy differences are found to be hydrogen bond-type attractions and steric repulsions. This knowledge will be useful in the design of improved catalysts as well as general understanding of the basis of selection in other reactions. / QC 20101111

Thesis. 1978. M.S.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / Microfiche copy available in Archives and Science. / Bibliography: leaves 80-84. / M.S.

Over the past few decades, single-molecule magnets (SMMs) have been an area of significant interest due to their plethora of potential uses, including possible applications to quantum computing and compact data storage devices. Although theoretical chemistry calculations could aid our understanding of the magnetic couplings present in these types of systems, they are often multiconfigurational in nature, making them difficult to model with tradi- tional single-reference approaches. Methods to handle these types of strongly correlated systems have been developed but often have significant drawbacks, and so these molecules remain difficult to model computationally. In this work, we discuss the application of Fock-space CI approaches to large transition metal complexes. First, we introduce a novel formalism which combines the spin-flip (SF), ioniza- tion potential (IP), and electron affinity (EA) approaches. This redox spin-flip approach, the restricted active space spin-flip and ionization potential/electron affinity (RAS-SF-IP/EA) method, is applied to several molecules exhibiting double exchange behavior. Model Hamil- tonian parameters are extracted from energy gaps and found to be in qualitative agreement with experiment. Having shown the efficacy of this approach, we move on to optimization, using a diagrammatic approach to derive equations for several RAS-1SF-IP/EA schemes. These equations allow direct construction of the most expensive intermediates in the David- son algorithm and should provide significant speedup, allowing application of Fock-space CI approaches to larger systems than ever before. The derived equations are implemented in the LibRASSF package in Q-Chem, as well as in an open-source PyFockCI code, avail- able on GitHub. A Bloch effective Hamiltonian formalism is also utilized to extract model Hamiltonian parameters from RAS-1SF calculations, allowing more nuanced studies of the Heisenberg J couplings present in many molecules with magnetically coupled sites. Over- all, our work with Fock-space CI provides a way to study magnetic couplings in very large strongly correlated systems at relatively low computational cost. This work was supported by a grant from the U.S. Department of Energy: DE-SC0018326. / Doctor of Philosophy / Humans have been familiar with magnets for thousands of years, and we have found a variety of useful applications for them. Magnets are used in everything from navigational devices to credit cards to data storage. Most people are familiar with large, solid magnets, but in the 1990s, it was discovered that individual molecules, called single-molecule magnets (SMMs), could also exhibit magnetic behavior. This means that in the presence of some external magnetic field, like the field caused by the presence of another magnet, the electrons in a SMM will align themselves with the field, and the electrons will maintain that alignment for some period of time after the field is removed. These SMMs have been a significant area of interest to scientists because they have a variety of interesting applications, including applications to quantum computing. In cases such as these, theoretical chemistry can offer useful insight. Broadly, the purpose of theoretical chemistry is to describe chemical problems using mathematical equations. We can use computational models to obtain information about the behavior of electrons in a particular system (the so-called electronic structure) and consequently, we can model the magnetic couplings in a given molecule. However, SMMs are difficult to model using tra- ditional theoretical methods because they often contain multiple orbitals which have nearly the same energy. In these cases, it often becomes ambiguous which orbitals ought to be oc- cupied by electrons; the effect this has on the energy is called "strong correlation". Ideally, one ought to consider all possible fillings of the orbitals, but most methods do not account for this and assume only one configuration is important when solving for the shapes of the orbitals. In this work, we combine two previously-introduced approaches, the spin-flip (SF) and ioniza- tion potential/electron affinity (IP/EA) approaches, to handle strongly correlated systems. In the SF-IP/EA approach, one adds or removes electrons and flips their spins in order to remove all of the ambiguity in orbital occupations. Once we determine the shapes of the or- bitals for this unambiguous state, the electrons are added, removed, or spin-flipped in order to obtain the desired strongly correlated state. We then solve for the energy of the system while considering all possible configurations within the set of ambiguously-occupied orbitals, allowing us to treat them on equal footing. We also study the effect of adding additional configurations to account for contributions from other orbitals, which provides more accu- rate results, albeit at a higher computational cost. Our method is less expensive than many other wavefunction-based methods used for these systems, and it yields qualitatively correct results, allowing theoreticians to study magnetic couplings in SMMs in a straightforward and inexpensive way. We also discuss optimization of our code, as well as an extension of our code that allows us to obtain coupling information for systems containing multiple magnetic sites. It is our hope that these developments will provide useful insights into the electronic structure of these SMM systems.