A escolha do catalisador no processo de reforma a vapor de etanol é de suma importância na determinação da eficiência de produção de hidrogênio, utilizado em células combustíveis. Superfícies de metais de transição (MT) são normalmente utilizadas como catalisadores, porém resultados experimentais têm apresentado que clusters MT suportados em óxidos são mais eficientes e mais seletivos que seus análogos macroscópicos. Assim, para se compreender o mecanismo de interação de etanol e água, e também do radical hidroxila (simples caracterização da ligação O-H), com clusters mágicos MT13 (MT = Ni, Cu, Pd, Ag, Pt e Au), foram utilizados cálculos ab initio baseados na teoria do funcional da densidade com o funcional proposto por Perdew-Burke-Ernzerhof (PBE) e a aproximação de Tkatchenko-Scheffler para as correções de van der Waals (vdW). Os orbitais de Kohn-Sham foram expandidos através de orbitais numéricos centrados nos átomos (NAOs), que estão implementados no pacote Fritz Haber Institute - ab initio molecular simulations (FHI-aims). Foi encontrado que clusters 3d e 4d apresentam configurações mais compactas, enquanto os sistemas de Pt13 e Au13 adotam geometrias mais abertas. Em particular, Au13 possui uma estrutura planar como configuração mais estável, enquanto o seu primeiro isômero mais energético possui uma estrutura 3D. Para os cálculos de adsorção foram selecionados os clusters de menor energia (LOW), e os clusters icosaédricos (ICO). Observou-se que a hidroxila (OH) não possui sítio preferencial ou dependência geométrica durante adsorção, enquanto água e etanol se ligam preferencialmente através do oxigênio no sítio top. Para o caso do etanol também é possível se encontrar isômeros com energias mais altas, na ordem de kT, que se ligam via hidrogênio do grupo CH e que são prováveis a temperatura ambiente. OH possui um elétron desemparelhado, o que favorece a formação de ligações químicas entre a molécula e MT13, de forma que a energia de adsorção varia entre -4.11 e -2.94 eV, condizente com a transferência de carga do cluster para a molécula. Quando se compara a adsorção de água (etanol) sobre superfícies MT(111) e MT13, nota-se que a energia de adsorção, que segue a ordem 3d > 4d > 5d para MT(111), é maior em relação aos clusters e varia entre -255 meV (-317 meV) < -Ead < -670 meV (-837 meV); porém a magnitude da interação ocorre na ordem 3d > 4d, mas 4d < 5d. A quebra da tendência pode ser explicada através dos efeitos de tamanho, que potencializam a atividade catalítica dos clusters de platina e ouro. É possível se observar ainda que as moléculas interagem mais fortemente com os MT13 dos metais com orbitais d semi-preenchidos, uma vez que esses proporcionam um maior rearranjo da densidade eletrônica. Ocorre uma pequena transferência de carga das moléculas de água e etanol para os clusters, apontando para a fraca interação. Cálculos vibracionais suportam as modificações estruturais e energéticas que ocorrem no sistema, onde νO-H aumenta para a hidroxila e diminui para água e etanol. / The choice of the catalyst in steam reforming of ethanol process has a central hole in determining the hydrogen production efficiency, used in fuel cells. Transition-metal (TM) surfaces are commonly used as catalysts, but experimental results have shown that TM clusters supported on oxides are more efficient and more selective than their macroscopic analogues. Therefore, to understand the mechanism of interaction between ethanol and water, and also the hydroxyl radical (simple characterization for the O-H bonding) with TM13 magic clusters (TM = Ni, Cu, Pd, Ag, Pt and Au), we performed ab initio calculations based on density functional theory within the functional proposed by Perdew-Burke-Ernzerhof (PBE) and Tkatchenko-Scheffler approach for van der Waals corrections (vdW). The Kohn-Sham orbitals were expanded by numerical atom-centered orbitals (NAOs), which are implemented at the Fritz Haber Institute - ab initio molecular simulations package (FHI-AIMS). We found that clusters 3d and 4d prefer more compact configurations, while Pt13 and Au13 systems adopt more open geometries. In particular case of Au13, our lowest energy configuration is a planar structure, while the first high energy isomer is a 3D configuration. For adsorption calculations, we selected the lowest energy configurations (LOW), and the icosahedral clusters (ICO). We observed that the hydroxyl (OH) has no adsorption site preference and also it does not show a geometric trend, while water and ethanol preferentially bind via oxygen at the top site. In the case of ethanol, it is also possible to find isomers with higher energies, which bind via hydrogen from CH group, and they are probable at room temperature. OH has an electron unpaired, which favors the formation of chemical bonds between the molecule and TM13, so that the adsorption energy varies from -4.11 to -2.94 eV, consistent with charge transfer from cluster to the molecule. When we compare the water and ethanol adsorption on surfaces TM(111) and on TM13, we note that the adsorption energy, that follows the order 3d > 4d > 5d for MT(111), is higher compared to the clusters energy. For water (ethanol), this property varies between -255 meV (-317 meV) < -Ead < -670 meV (-837 meV); however, the magnitude of the interaction occurs in the order 3d > 4d, but also 4d < 5d. The breaking of the trend can be explained by size effects, that enhances the catalytic activity of platinum and gold clusters. It is also observed that the stronger interaction between molecules and TM13 with d-orbitais partly occupied, since these metals provide a higher rearrangement of the electron density. There is a small charge transference from water and ethanol molecules to the clusters, pointing to the weak interaction. Vibrational calculations support the structural and energetic changes that occur in the system, where νO-H increases for the hydroxyl and decreases to water and ethanol.

Estudos pioneiros na década de 1970 com cálculos ab initio inauguraram o campo da química quântica relativística computacional. Desde então, também devido ao crescimento exponencial da capacidade computacional, a aplicação de Hamiltonianos relativísticos em cálculos de química teórica tem se tornado cada vez mais frequente. No entanto, o desenvolvimento de conjuntos de funções de base para estes cálculos não seguiu no mesmo ritmo. Após décadas, as melhores opções disponíveis ainda apresentam certas limitações técnicas, como sinais de prolapso variacional. Desta forma, o objetivo deste trabalho foi a geração de eficientes conjuntos relativísticos de funções de base Gaussianas para aplicação em cálculos ab initio correlacionados. Usou-se então de uma versão polinomial do método da coordenada geradora Dirac-Fock para elaboração de um conjunto primitivo de funções otimizadas variacionalmente livres de prolapso. Funções de correlação/polarização e funções difusas foram adicionadas em acordo com uma incrementação do tipo quadruple-ζ. Estas funções foram escolhidas através do tratamento multi-referencial de interação de configurações com o Hamiltoniano Dirac-Coulomb. Cálculos moleculares de propriedades fundamentais atestaram a qualidade destes conjuntos, denominados RPF-4Z, em comparação com outras bases relativísticas bastante utilizadas na literatura. Adicionalmente, momentos de quadrupolo nuclear de isótopos de bismuto e potássio e uma curva de energia potencial do dímero de rádio foram obtidos com estes conjuntos. Ao prover funções de base que retornam resultados comparáveis a outros conjuntos relativísticos disponíveis de qualidade quadruple-ζ, mas com uma significativa redução na demanda computacional, espera-se que as bases RPF-4Z se traduzam em um grande avanço na área. / Pioneering studies with ab initio calculations from the 1970s gave rise to the computational relativistic quantum chemistry field. Since then, also due to the exponential growth in computational resources, the application of relativistic Hamiltonians in theoretical chemistry calculations has become more common. However, the development of basis sets to be used in such calculations have presented its struggles. After decades, technical limitations, such as the variational prolapse issue still persist. Hence, the aim of the present thesis was the development of efficient relativistic Gaussian basis sets to be applied in correlated ab initio calculations. In order to do that, a polynomial version of the generator coordinate Dirac-Fock method has been applied to obtain variationally optimized primitive sets that are free of prolapse. Correlating/polarization functions, as well as diffuse ones, were added to these sets in a quadruple-ζ type of increment. Such functions were chosen by means of the Dirac-Coulomb multi-reference configuration interaction treatment. Molecular calculations of fundamental properties have attested the high quality of the designated RPF-4Z sets in comparison with other well-known relativistic sets. Furthermore, nuclear electric quadrupole moments of bismuth and potassium isotopes and a potential energy curve of the radium dimer were also analyzed. Since these generated sets are as accurate as commonly used relativistic quadruple-ζ sets already available, but with the advantage of a reduced computational demand, the RPF-4Z sets are expected to represent a great advance in the field.

Lau Kai-Chung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references. / Abstracts in English and Chinese. / Abstract / Acknowledgements / Table of Contents / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- The Gaussian-2 Method / Chapter 1.2 --- The G2 Method with Reduced M φller-Plesset Order and Basis Set / Chapter 1.3 --- The Gaussian-3 Method / Chapter 1.4 --- Our Modified G2 Methods / Chapter 1.5 --- Calculation of Thermodynamical Data / Chapter 1.6 --- Scope of the Thesis / Chapter 1.7 --- Remark on the Location of Transition Structures / Chapter 1.8 --- References / Chapter Chapter 2 --- A Gaussian-2 Study of the [C2H5O-] Potential Energy Surface and the Fragmentation Pathways of the Ethoxide Anion --- p.7 / Chapter 2.1 --- Introduction / Chapter 2.2 --- Theoretical Methods / Chapter 2.3 --- Results and Discussion / Chapter 2.3.1 --- Stable Isomers and TSs for the [C2H50-] Anions / Chapter 2.3.2 --- Fragmentation Pathways of the Ethoxide Anion (1-) / Chapter 2.4 --- Conclusions / Chapter 2.5 --- Publication Note / Chapter 2.6 --- References / Chapter Chapter 3 --- A Gaussian-2 Study of Isomeric C2H2N and C2H2N+ --- p.20 / Chapter 3.1 --- Introduction / Chapter 3.2 --- Theoretical Methods / Chapter 3.3 --- Results and Discussion / Chapter 3.3.1 --- The C2H2N and C2H2N+ Isomers / Chapter 3.3.2 --- The Cyanomethyl Radical (1) and Cation (1+) / Chapter 3.3.3 --- The Isocyanomethyl Radical (2) and Cation (2+) / Chapter 3.3.4 --- The lH-Azirinyl Radical (5) and Cation (5+) / Chapter 3.3.5 --- Other C2H2N and C2H2N+ Isomers / Chapter 3.3.6 --- The Unidentified C2H2N Isomer Formed in N + CH=CH2 / Chapter 3.4 --- Conclusions / Chapter 3.5 --- Publication Note / Chapter 3.6 --- References / Chapter Chapter 4 --- A Gaussian-2 Study of the Photoionization and Dissociative Photoionization Channels of Ethylene Oxide --- p.35 / Chapter 4.1 --- Introduction / Chapter 4.2 --- Theoretical Methods / Chapter 4.3 --- Results and Discussion / Chapter 4.3.1 --- Bond Cleavage Reactions / Chapter 4.3.2 --- Dissociation Channels Involving Transition Structure(s) / Chapter 4.4 --- Conclusions / Chapter 4.5 --- Publication Note / Chapter 4.6 --- References / Chapter Chapter 5 --- A Gaussian-2 Study of the Photoionization and Dissociative Photoionization Channels of Propylene Oxide --- p.47 / Chapter 5.1 --- Introduction / Chapter 5.2 --- Theoretical Methods / Chapter 5.3 --- Results and Discussion / Chapter 5.3.1 --- Bond Cleavage Reactions / Chapter 5.3.2 --- Dissociation Channels Involving Transition Structure(s) / Chapter 5.4 --- Conclusions / Chapter 5.5 --- Publication Note / Chapter 5.6 --- References / Chapter Chapter 6 --- A Gaussian-3 Study of the Thermochemistry of Chlorofluoromethanes (CFnCl4-n) and Their Cations (CFnCl4_n+)， n =0-4 --- p.61 / Chapter 6.1 --- Introduction / Chapter 6.2 --- Theoretical Methods / Chapter 6.3 --- Results and Discussions / Chapter 6.3.1 --- Heats of Formation of Chlorofluoromethanes and Their Cations / Chapter 6.3.2 --- Ionization Energies of Chlorofluoromethanes / Chapter 6.3.3 --- Proton Affinities of Chlorofluoromethanes / Chapter 6.4 --- Conclusions / Chapter 6.5 --- Publication Note / Chapter 6.6 --- References / Chapter Chapter 7 --- "A Gaussian-2 and Gaussian-3 Study of the Energetics and Structures of Cl2On and Cl2On+, n = 1，4, 6，and7" --- p.69 / Chapter 7.1 --- Introduction / Chapter 7.2 --- Theoretical Methods / Chapter 7.3 --- Results and Discussion / Chapter 7.3.1 --- Cl20 and Cl20+ / Chapter 7.3.2 --- Cl204 and Cl204+ / Chapter 7.3.3 --- Cl206 and Cl206+ / Chapter 7.3.4 --- Cl207 and Cl207+ / Chapter 7.4 --- Conclusions / Chapter 7.5 --- Publication Note / Chapter 7.6 --- References / Chapter Chapter 8 --- Conclusions --- p.81 / Chapter Appendix A --- The Gaussian-n (n = 1 - 3) Theoretical Models --- p.82 / Chapter A.1 --- The G1 and G2 theories / Chapter A.2 --- The G2(MP2) theory / Chapter A.3 --- "The G2(MP2,SVP) theory" / Chapter A.4 --- The G3 theory / Chapter Appendix B --- "Calculation of Enthalpy at 298 K, H298" --- p.86

Law Chi-Kin. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references. / Abstracts in English and Chinese. / Abstract --- p.i / Acknowledgements --- p.iii / Table of Contents --- p.iv / List of Tables --- p.vi / List of Figures --- p.viii / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- The Gaussian´ؤ3 Method --- p.1 / Chapter 1.2 --- The G3 Method with Reduced Mφller-Plesset Order and Basis Set --- p.2 / Chapter 1.3 --- The Gaussian-3X Method --- p.2 / Chapter 1.4 --- Calculation of Thermodynamical Data --- p.3 / Chapter 1.5 --- Remark on the Location of Equilibrium and Transition Structures --- p.3 / Chapter 1.6 --- Natural Bond Orbital (NBO) Analysis --- p.4 / Chapter 1.7 --- Scope of the Thesis --- p.4 / Chapter 1.8 --- References --- p.5 / Chapter Chapter 2 --- Gaussian´ؤ3 Heats of Formation for (CH)6 Isomers --- p.6 / Chapter 2.1 --- Introduction --- p.6 / Chapter 2.2 --- Methods of Calculation and Results --- p.7 / Chapter 2.3 --- Discussion --- p.9 / Chapter 2.4 --- Conclusion --- p.11 / Chapter 2.5 --- Publication Note --- p.12 / Chapter 2.6 --- References --- p.12 / Chapter Chapter 3 --- A Gaussian-3 Investigation of N7 Isomers --- p.14 / Chapter 3.1 --- Introduction --- p.14 / Chapter 3.2 --- Computational Method and Results --- p.16 / Chapter 3.3 --- Discussion --- p.16 / Chapter 3.4 --- Conclusion --- p.24 / Chapter 3.5 --- Publication Note --- p.24 / Chapter 3.6 --- References --- p.24 / Chapter Chapter 4 --- A Gaussian-3 Study of N7+ and N7- Isomers --- p.27 / Chapter 4.1 --- Introduction --- p.27 / Chapter 4.2 --- Computational Method and Results --- p.29 / Chapter 4.3 --- Discussion --- p.30 / Chapter 4.3.1 --- The N7+ isomers --- p.30 / Chapter 4.3.2 --- The N7- isomers --- p.37 / Chapter 4.4 --- Conclusion --- p.41 / Chapter 4.5 --- Publication Note --- p.41 / Chapter 4.6 --- References --- p.41 / Chapter Chapter 5 --- "Thermochemistry of Chlorine Fluorides ClFn, n = 1-7, and Their Singly Charged Cations and Anions: A Gaussian-3 and Gaussian-3X Study" --- p.45 / Chapter 5.1 --- Introduction --- p.45 / Chapter 5.2 --- Methods of Calculations --- p.47 / Chapter 5.3 --- Results and Discussion --- p.48 / Chapter 5.3.1 --- Comparison of the G3 and G3X methods --- p.48 / Chapter 5.3.2 --- Assessments of the experimental results --- p.54 / Chapter 5.3.3 --- "Bond dissociation energies of ClFn, ClFn+, and ClFn-" --- p.57 / Chapter 5.3.4 --- Summary of the thermochemical data --- p.58 / Chapter 5.4 --- Conclusion --- p.59 / Chapter 5.5 --- Publication Note --- p.60 / Chapter 5.6 --- References --- p.60 / Chapter Chapter 6 --- A Gaussian-3 Study of the Photoionization and Dissociative Photoionization Channels of Dimethyl Sulfide --- p.63 / Chapter 6.1 --- Introduction --- p.63 / Chapter 6.2 --- Methods of Calculations --- p.64 / Chapter 6.3 --- Results and Discussion --- p.64 / Chapter 6.3.1 --- Bond cleavage reactions --- p.67 / Chapter 6.3.2 --- Dissociation channels involving transition structures --- p.68 / Chapter 6.4 --- Conclusion --- p.70 / Chapter 6.5 --- Publication Note --- p.70 / Chapter 6.6 --- References --- p.70 / Chapter Chapter 7 --- "Theoretical Study of the Electronic Structures of Carbon and Silicon Nanotubes, Carbon and Silicon Nanowires" --- p.72 / Chapter 7.1 --- Introduction --- p.72 / Chapter 7.2 --- Models and Computational Methods --- p.74 / Chapter 7.3 --- Results and Discussion --- p.75 / Chapter 7.4 --- Conclusion --- p.87 / Chapter 7.5 --- Publication Note --- p.87 / Chapter 7.6 --- References --- p.87 / Chapter Chapter 8 --- Conclusion --- p.90 / Appendix A --- p.91 / Appendix B

Cheng Mei-Fun. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references. / Abstracts in English and Chinese. / Abstract --- p.i / Acknowledgements --- p.iii / Table of Contents --- p.iv / List of Tables --- p.vi / List of Figures --- p.viii / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- The Gaussian-3 Method --- p.1 / Chapter 1.2 --- The G3 Method with Reduced Mφller-Plesset Order and Basis Set --- p.2 / Chapter 1.3 --- Density Functional Theory (DFT) --- p.3 / Chapter 1.4 --- Calculation of Thermodynamical Data --- p.3 / Chapter 1.5 --- Remark on the Location of Transition Structures --- p.3 / Chapter 1.6 --- Natural Bond Orbital (NBO) Analysis --- p.4 / Chapter 1.7 --- Scope of the Thesis --- p.4 / Chapter 1.8 --- References --- p.5 / Chapter Chapter 2 --- Heats of Formation for the Azine Series: A Gaussian-3 Study --- p.7 / Chapter 2.1 --- Introduction --- p.7 / Chapter 2.2 --- Methods of Calculation and Results --- p.8 / Chapter 2.3 --- Discussion --- p.8 / Chapter 2.4 --- Conclusion --- p.9 / Chapter 2.5 --- Publication Note --- p.10 / Chapter 2.6 --- References --- p.10 / Chapter Chapter 3 --- Heats of Formation for Some Boron Hydrides: A Gaussian-3 Study --- p.16 / Chapter 3.1 --- Introduction --- p.16 / Chapter 3.2 --- Methods of Calculation and Results --- p.18 / Chapter 3.3 --- Discussion --- p.19 / Chapter 3.4 --- Conclusion --- p.21 / Chapter 3.5 --- Publication Note --- p.21 / Chapter 3.6 --- References --- p.21 / Chapter Chapter 4 --- Structural and Energetics Studies of Tri- and Tetra-tert-butylmethane --- p.30 / Chapter 4.1 --- Introduction --- p.30 / Chapter 4.2 --- Methods of Calculation and Results --- p.32 / Chapter 4.3 --- Discussion --- p.34 / Chapter 4.3.1 --- Mono-tert-butylmethane --- p.34 / Chapter 4.3.2 --- Di-tert-butylmethane --- p.35 / Chapter 4.3.3 --- Tri-tert-butylmethane --- p.37 / Chapter 4.3.4 --- Tetra-tert-butylmethane --- p.38 / Chapter 4.4 --- Conclusion --- p.39 / Chapter 4.5 --- Publication Note --- p.40 / Chapter 4.6 --- References --- p.40 / Chapter Chapter 5 --- A Computational Study of the Diels-Alder Reactions Involving Acenes: Reactivity and Aromaticity --- p.49 / Chapter 5.1 --- Introduction --- p.49 / Chapter 5.2 --- Methods of Calculation and Results --- p.50 / Chapter 5.3 --- Discussion --- p.51 / Chapter 5.4 --- Conclusion --- p.53 / Chapter 5.5 --- Publication Note --- p.53 / Chapter 5.6 --- References --- p.53 / Chapter Chapter 6 --- A Computational Study of the Charge- Delocalized and Charge-Localized Forms of the Croconate and Rhodizonate Dianions --- p.65 / Chapter 6.1 --- Introduction --- p.65 / Chapter 6.2 --- Methods of Calculation and Results --- p.67 / Chapter 6.3 --- Discussion --- p.68 / Chapter 6.3.1 --- Charge-Localized Forms of C5052- (C2v) and C6O6 2-(C2v) --- p.68 / Chapter 6.3.2 --- Charge-Delocalized Forms of C5052- (D5h) and C6062- (D6h) --- p.71 / Chapter 6.4 --- Conclusion --- p.72 / Chapter 6.5 --- Publication Note --- p.73 / Chapter 6.6 --- References --- p.74 / Chapter Chapter 7 --- Conclusion --- p.89 / Appendix A --- p.90 / Appendix B --- p.92

No description available.

O emprego da química computacional fornece subsídios para a interpretação dos estudos experimentais e permite fazer previsões sobre as propriedades de sistemas ainda não sintetizados. Devido aos avanços computacionais, métodos de química quântica com diferentes níveis de tratamento de estrutura eletrônica são mais facilmente encontrados. Entretanto, certas aproximações ainda são necessárias devido à grande demanda por recursos computacionais dos métodos mais avançados. Desta forma, os resultados obtidos podem não representar adequadamente as propriedades dos sistemas estudados. Para compostos contendo elementos pesados, como é o caso de muitos catalisadores, seriam necessários estudos que levem em conta os efeitos relativísticos. Desta forma, foi realizado um estudo cinético de quatro tipos de reações envolvendo sistemas contendo átomos pesados. Na determinação das geometrias otimizadas verificou-se que os efeitos relativísticos escalares são geralmente predominantes e que não há diferença significativa entre os resultados do método de dois componentes ZORA em relação ao tradicional tratamento RECP. No entanto, cálculos DKH2 não conseguiram descrever os parâmetros geométricos de compostos de platina com a mesma eficácia. Por outro lado, foi possível observar que os efeitos relativísticos são muito mais importantes para uma determinação confiável de outras propriedades relacionadas à cinética química, como as energias relativas ao longo do mecanismo de reação. Neste caso, além dos efeitos relativísticos escalares, o acoplamento spin-órbita também se torna crucial para uma descrição acurada das barreiras de ativação em compostos com elementos do sexto período em diante. Assim, recomenda-se o uso de um tratamento combinado em duas etapas para uma correta descrição dos sistemas. Em geral, a etapa de otimização de geometrias pode ser realizada em nível RECP. Entretanto, para a obtenção de valores acurados de energia por métodos relativísticos, sugere-se a utilização dos tratamentos de quatro componentes, como Dirac-Coulomb (DC), e dos formalismos exatos de dois componentes, como X2C-MMF. / The use of computational chemistry provides support for the interpretation of experimental studies and allows making predictions about the properties of systems not yet synthesized. Quantum chemistry methods with different levels of electronic structure treatments are more easily found due to computational advances. However, some approximations are still required because of the significant demand for computational resources associated to more advanced methods. Thus, the results obtained may not adequately represent the properties of the systems studied. In the case of compounds containing heavy elements, such as many catalysts, studies that take into account the relativistic effects would be need. Thus, a kinetic study of four reaction categories involving systems containing heavy atoms was conducted. It was noticed that the scalar relativistic effects are generally predominant and that there is not significant differences between the results from the ZORA method in relation to the traditional RECP treatment during the determination of optimized geometries. Nevertheless, the DKH2 calculations failed to describe the geometric parameters of platinum compounds with the same efficiency. On the other hand, it was possible to observe that relativistic effects are very important for a reliable determination of other properties related to chemical kinetics, such as the relative energies along the reaction mechanism. In this case, in addition to scalar relativistic effects, the spin-orbit coupling also becomes crucial for an accurate description of the activation barriers in compounds with elements from the sixth period and beyond. Therefore, a two-step combined treatment is recommended for a correct description of the systems. In general, the geometry optimization step can be performed at the RECP level. However, in order to obtain accurate energy values by relativistic methods, we suggest the use of four-component treatments such as Dirac-Coulomb (DC) and exact two-component formalisms like X2C-MMF.

Lau Kai-Chi. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references. / Abstracts in English and Chinese. / Abstract --- p.i / Acknowledgements --- p.iii / Table of Contents --- p.iv / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- The Gaussian´ؤ3 Method --- p.1 / Chapter 1.2 --- The G3 Method with Reduced MΦ ller-Plesset Order and Basis Set --- p.2 / Chapter 1.3 --- The Gaussian-3X Method --- p.2 / Chapter 1.4 --- The Modified G2 Method --- p.3 / Chapter 1.5 --- Calculation of Thermodynamical Data --- p.3 / Chapter 1.6 --- Remark on the Location of Transition Structures --- p.4 / Chapter 1.7 --- Scope of the Thesis --- p.4 / Chapter 1.8 --- References --- p.4 / Chapter Chapter 2 --- "A Gaussian-2 and Gaussian-3 Study of Alkoxide Anion Decompositions. I. H2 and CH4 Eliminations of the Methoxide, Ethoxide, i-Propoxide, and t-Butoxide Anions" --- p.6 / Chapter 2.1 --- Introduction --- p.6 / Chapter 2.2 --- Methods of Calcuations --- p.7 / Chapter 2.3 --- Results and Discussion --- p.8 / Chapter 2.3.1 --- Nature of ion-neutral complex --- p.8 / Chapter 2.3.2 --- Initial bond cleavage of alkoxide anions --- p.9 / Chapter 2.3.3 --- Dissociation of alkoxide anions --- p.10 / Chapter 2.4 --- Conclusions --- p.23 / Chapter 2.5 --- Publication Note --- p.25 / Chapter 2.6 --- References --- p.25 / Chapter Chapter 3 --- A Gaussian-2 and Gaussian-3 Study of Alkoxide Anion Decompositions. II. Alkane Eliminations of (CH3)2(C2H5)CO- and (i-Pr)(C2H5)2CO- --- p.28 / Chapter 3.1 --- Introduction --- p.28 / Chapter 3.2 --- Methods of Calculations --- p.29 / Chapter 3.3 --- Results and Discussion --- p.29 / Chapter 3.3.1 --- Initial bond cleavage of alkoxide anions --- p.30 / Chapter 3.3.2 --- Dissociation of alkoxde anions --- p.31 / Chapter 3.3.3 --- General dissociation mechanism of alkoxide anions --- p.35 / Chapter 3.4 --- Conclusions --- p.37 / Chapter 3.5 --- References --- p.37 / Chapter Chapter 4 --- A Gaussian-3 Study of the Photoionization and Dissociative Photoionization Channels of Dimethyl Disulfide --- p.40 / Chapter 4.1 --- Introduction --- p.40 / Chapter 4.2 --- Methods of Calculations --- p.41 / Chapter 4.3 --- Results and Discussion --- p.41 / Chapter 4.3.1 --- Bond cleavage reactions --- p.44 / Chapter 4.3.2 --- Dissociation channels involving transition structures --- p.45 / Chapter 4.4 --- Conclusions --- p.48 / Chapter 4.5 --- References --- p.48 / Chapter Chapter 5 --- A Gaussian´ؤ3 Study of the Photodissociation Channels of Propylene Sulfide --- p.50 / Chapter 5.1 --- Introduction --- p.50 / Chapter 5.2 --- Methods of Calculations --- p.51 / Chapter 5.3 --- Results and Discussion --- p.51 / Chapter 5.3.1 --- The dissociation channels involving transition structures --- p.53 / Chapter 5.3.2 --- The dissociations of sulfur atom --- p.56 / Chapter 5.4 --- Conclusions --- p.58 / Chapter 5.5 --- References --- p.59 / Chapter Chapter 6 --- Thermochemistry of Phosphorus Fluorides: A Gaussian´ؤ3 and Gaussian´ؤ3X Study --- p.60 / Chapter 6.1 --- Introduction --- p.60 / Chapter 6.2 --- Methods of Calculations --- p.62 / Chapter 6.3 --- Results and Discussion --- p.62 / Chapter 6.3.1 --- Comparison of the G3 and G3X methods --- p.62 / Chapter 6.3.2 --- Assessments of the experimental results --- p.65 / Chapter 6.4 --- Conclusions --- p.71 / Chapter 6.5 --- References --- p.71 / Chapter Chapter 7 --- Conclusions --- p.74 / Appendix A --- p.75 / Appendix B --- p.78

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 (Σα2), basicidade (Σβ2), e dipolaridade/polarizabilidade (π2) 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+ , Ιq-Ι e µ 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 (Σα2), basicity (Σβ2) , and dipolarity/polarizability (π2); excess molar refraction and characteristic volume (Vx). Theoretical quantities tested includes: HOMO and LUMO energies; class IV, CM1 model atomic partial charges (q+ and Ιq-Ι) and dipole moment (µ); Famini\'s polarizability index (πF); 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+, Ιq-Ι and µ 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.

Neste trabalho foram estudadas as moléculas conhecidas corno cetonas-gaiola de Cookson e seus derivados pelo uso de técnicas computacionais de modelagem molecular. Estas moléculas participam de cicloadições [2+2], urna classe de reações cujo mecanismo ainda é estudado. As técnicas usadas foram as do Conjunto Completo de Funções de Base (CBS-4), Hartree-Fock, métodos de Teoria do Funcional de Densidade pBP e B3LYP e semi-empírico PM3. As técnicas foram usadas com diferentes conjuntos de base para analisar as conformações das moléculas e avaliar a exatidão do método em reproduzir resultados experimentais de termodinâmica e ressonância magnética nuclear. Com base na concordância dos resultados e com as caraterísticas do método, procura-se inferir quais as características estruturais e interações atômicas que explicam os resultados experimentais e os calculados. Foi estudada também a solvatação das espécies. Os programas empregados foram o Gaussian 94, Spartan 5.0, MOPAC e Amsol 6.6. Os resultados mostraram as interações interatômicas mais relevantes para a determinação das estabilidades relativas das moléculas. Foi considerado o efeito da estrutura no resultado de cálculos de energias e avaliado o método de reações isodésmicas para moléculas de estrutura complexa. / In this work the molecules known as Cookson\'s cage ketones and similar adducts were studied with the use of computational molecular modelling techniques. These molecules participate in cycloadditions [2+2], a class of reactions whose mechanism is still being studied. Toe techniques used were the Complete Basis Set (CBS-4), Hartree-Fock, Density Functional Theory methods pBP and B3LYP and semi-emprical PM3. The techniques were used with some basis sets to analise the molecular conformations and evaluate the agreement of the calculated with the experimental results of thermodynamics and NMR spectroscopy. Based on the agreement between the results and on the characteristics of the methods, it is sought to determine the structural characteristics and atomic interactions that explain the results. The solvation of the species was also studied. The programs used were the Gaussian 94, Spartan 5.0, MOPAC and Amsol 6.6. The results show the more important interactions that determine the relative stabilities of the molecules. The effect of the molecular structure on the energy calculation was studied and the method of isodesmic reactions was evaluated for complex molecules.