Ab initio STO-3G* MO calculations for the diphosphoric acid (H₄P₂O₇) and disulfuric acid (H₂S₂O₇) molecules reproduce the bridging bond length and angle averages observed in solid phosphates and sulfates. Potential energy curves calculated for both molecules relate the observed bridging bond length and angle variations in their solids. Orbital population analyses for both molecules suggest that all five 3d-orbitals play a role in bonding instead of just the two proposed by Cruickshank (1961). The accuracy with which the molecular geometries account for the averages and trends observed in related solids supports the assertion that the local bonding forces in solid phosphates and sulfates behave as though they are short range. / M.S.

The modified electron gas (MEG) model has been used to generate ionic model CaCO₃ crystals in the calcite, aragonite, diopside, and perovskite structure types. For calcite and aragonite, the model predicts shorter CO bonds and larger bulk moduli than observed. Modeling of the thermochemistry of CaCO₃ does not reproduce the observed thermochemistry even qualitatively. The model predicts that the hypothetical diopside structure type is the most stable form CaCO₃ among the four structure types. These discrepancies may illustrate the significance of CO bond covalency in determining the physico-chemical properties of CaCO₃. The MEG model has also been used to generate model alkali halide crystals in the sphalerite, rocksalt, and CsCl types in an exploration of the reliability of the radius ratio rule. The MEG model predicts the correct cation coordination numbers for 13 of 16 alkali halides, whereas the radius ratio rule predicts the correct coordination numbers in at most 9 of the same 16 alkali halides. Analyses of the model crystal structures suggests that energy minimization is more important than packing efficiency in determining the most stable structures for ionic crystals. The molecular orbital (MO) model has been used to determine minimum-energy geometries and electron density distributions in sulfate hydroxyacid molecules. These molecules have been used to model the bonding in sulfate crystals. SO bond lengths calculated for H₂SO₄ and H₂S₂O₇ correlate linearly with fractional s-characters of the bonds, as in sulfate crystals. With increasing S coordination number, the bonded radii of S and O, as determined from electron density maps, increase at the same rate, contrary to the common assumption of constant anion H₂S₂O₇ shows a relatively large change in energy as its SOS angle is deformed from its minimum-energy value (125.6°) to l80°, in conformity with the small variation among observed SOS angles. In contrast, SiOSi and POP angles show relatively wide variations in crystals and molecules. This suggests that polysulfates may be less amenable than polysilicates or polyphosphates to polymorphism or glass formation. Other properties of H₂SO₄ are also calculated and compared with experimental observations and previous calculations. / Ph. D.

Thesis (PhD)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: Density functional theory (DFT) has been used to investigate the effect of the surrounding molecules on the structure of selected boron-nitrogen compounds. It was found that a very limited number of molecules, orientated according to the experimental crystal structure, are needed to successfully reproduce the large changes in structure witnessed when HCN–BF3 and CH3CN–BF3 crystallises. Specifically, the addition of seven molecules shortens the B–N distance by 0.735 °A in (HCN–BF3)8 and 0.654A° in (CH3CN– BF3)8. Accompanying the large changes in B–N bond length are equally large changes in the N–B–F angle. Investigation of the structure of these complexes in terms of localised electron pairs shows that the availability of lone pairs, in close proximity to the B–N bond axis, plays an important role in the bond change. Through delocalisation of the fluorine lone pairs the antibonding σ ∗(B–N) orbital becomes increasingly occupied as the N–B–F angle lessens and vice versa. Further, an investigation of the specific effects of dipole-dipole interactions was performed by applying uniform electric fields of varying strength along the donor-acceptor bond axis of a series of compounds of the form X–Y; X=H3N, HCN, CH3CN; Y = BF3, BH3, SO3. All complexes investigated show sensitivity to the external electric field, however, only the compounds having nitrile donors and acceptors with fluorine atoms produce large changes, which in turn are dominated by a very sudden large change in B–N bond length occurring in a very narrow range of changing field strength. Analysis of the changes in bond character reveals that HCN–BF3 and CH3CN–BF3 have long bonds in the gas phase, formed primarily through electrostatic interaction between the donor and acceptor. In the short bond in the condensed phase the bond character changes considerably through the introduction of strong electron sharing interactions, i.e. covalent or orbital interactions. Fundamental changes in the nature of the bond, catalysed by surrounding molecules, thus lie at the heart of the large phase-dependent changes in these species. / AFRIKAANSE OPSOMMING: ’n Kohn-Sham elektrondigtheidsteorie (DFT) studie is gedoen op die effek van die omliggende molekules in die kristalstruktuur van sekere molekules wat boor-stikstof bindings bevat. Daar is gevind dat ’n klein aantal molekules, georienteer soos in die eksperimentele kristalstruktuur, benodig word om die groot veranderinge in stuktuur te veroorsaak wat eksperimenteel waargeneem word wanneer HCN–BF3 en CH3CN–BF3 kristaliseer. Spesifiek, die byvoeging van sewe molekules verminder die B–N bindingslengte met 0.735 °A in (HCN–BF3)8 en 0.654 A° in (CH3CN–BF3)8. Die groot veranderinge in B–N bindingslengte gaan saam met ewe groot veranderinge in die N–B–F hoek. ’n Ondersoek van die struktuur van die molekules in terme van gelokaliseerde elektronpare wys dat die beskikbaarheid van alleenpare, wat naby die B–N bindingsas lˆe, ’n belangrike rol speel in the verandering in bindingslengte. Deur delokalisasie van die fluoor alleenpare word die antibindende σ ∗(B–N) orbitaal toenemend beset soos die N–B–F hoek afneem en omgekeerd. Verder is die spesifieke effek van dipool-dipool interaksies ondersoek deur uniforme elektriese velde aan te lˆe langs the donor-akseptor bindingsas van ’n reeks komplekse van die vorm X–Y; X = H3N, HCN, CH3CN; Y = BF3, BH3, SO3. Al die komplekse toon sensitiwiteit teenoor die eksterne elektriese veld, maar net die verbindings wat nitriel akseptore en fluoor atome aan the donor fragmente het, toon groot veranderinge, wat op hulle beurt weer oorskadu word deur ’n skielike verandering in the B–N bindingslengte in ’n nou band van veranderende veldsterkte. Analise van die veranderinge in bindingskarakter toon dat HCN–BF3 en CH3CN–BF3 lang bindings in die gasfase het, wat hoofsaaklik gevorm word deur elektrostatiese interaksies tussen die donor en akseptor fragmente. In die kort binding in die kristalfase is daar ’n aansienlike verandering in the karakter as gevolg van die intrede van sterk elektrondelingsinteraksies, m.a.w. kovalente of orbitaalinteraksies. Fundamentele veranderinge in the manier wat die binding saamgestel word, wat gekataliseer word deur omliggende molekules, is dus die oorsaak van die groot faseafhanklike veranderinge.

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The thermochemistry of several species, and the kinetics of various H atom radical reactions relevant to atmospheric and combustion chemistry were investigated using ab initio theoretical techniques and the flash photolysis / resonance fluorescence technique. Using ab initio quantum mechanical calculations up to the G3 level of theory, the C-H bond strengths of several alkanes were calculated. The bond strengths were calculated using two working reactions. From the results, it is apparent that the bond strengths decrease as methyl groups are added to the central carbon. The results are in good agreement with recent experimental halogenation kinetic studies. Hydrogen bond strengths with sulfur and oxygen were studied via CCSD(T) theory, together with extrapolation to the complete basis set limit. The results for the bond dissociation energies (ground state at 0 K, units: kJ mol-1) are: S-H = 349.9, S-D = 354.7, HS-H = 376.2, DS-D = 383.4, and HO-H = 492.6. These data compare well with experimental literature. The rate constants for the isotopic reactions of H + H2S, D + H2S, H + D2S, and D + D2S are studied at the QCISD(T)/6-311+G(3df,2p) level of theory. The contributions of the exchange reaction versus abstraction are examined through transition state theory. The energy of NS was computed via CCSD(T) theory, together with extrapolation to the complete basis set limit. The results were employed with three working reactions to find ΔfH0(NS) = 277.3 ± 2 kJ mol-1 and ΔfH298(NS) = 278.0 ± 2 kJ mol-1. This thermochemistry is consistent with, but much more precise than, earlier literature values. A kinetic study of the reaction of H + CH2CCl2 was conducted over the temperature range of 298 - 680 K. The reaction was found to be pressure dependent and results of the rate constants and their interpretation via unimolecular rate theory are presented.

Ligações de hidrogênio é um tema que tem despertado o interesse da comunidade científica desde o final do século XIX. Sua importância é enorme nos processos ligados à vida, como por exemplo na estabilização das estruturas de DNA e na manutenção da água em seu estado líquido. Várias metodologias teóricas foram desenvolvidas para o estudo de sistemas moleculares e das ligaçõas de hidrogênio, entre elas está o emprego de cálculos de teoria de perturbação de muitos corpos (MBPT). Uma alternativa aos cálculos moleculares com MBPT, que tem crescido em termos de aplicação e confiança, é o emprego da teoria do funcional da densidade (DFT). Nesta tese, calculamos propriedades de sistemas hidrogênio-ligados em clusters e líquidos, usando ambas as metodologias DFT e MBPT. Entre as propriedades consideradas estão constantes rotacionais, momentos de dipolo, energias de ligação, deslocamentos espectroscópicos quando da formação do complexo e espalhamento de luz. Parte desta tese é dedicada a salientar as diferenças entre as propriedades de um cluster otimizado e a estrutura de um líquido gerada por simulação de Monte Carlo. Comparamos os resultados obtidos para o complexo uréia-água nestas duas situações e reforçamos o fato de que líquido e cluster são situações físicas distintas, cujas propriedades também são diferentes. Os sistemas estudados foram HCN, CH IND. 3CN, HC IND. 3N, HC IND. 2NC, HCN...H IND. 2O, CH IND. 3CN...H IND.2O, (CH IND. 3) IND.3CCN...H IND. 2O e (NH IND. 2) IND. 2CO...H IND. 2O. Dos resultados obtidos nesta fase, verificamos que DFT é de fato uma alternativa completamente viável para a obtenção de propriedades de moléculas e biomoléculas hidrogênio-ligadas. / Hydrogen bonding is a topic of interest in the scientific community since the end of the XIX century. Its importance is enormous in processes related to life as, for example, the stabilization of DNA structures and the maintenance of water in its liquid state. Several theoretical methodologies were developed to study molecular systems and hydrogen bonds, among them is the use of many-body perturbation theory (MBPT). An alternative to MBPT, that has gained confidence, is the employment of the density functional theory (DFT). In the present thesis we calculate properties of hydrogen-bonded systems in clusters and liquids using both methodologies, DFT and MBPT. Among the properties considered are rotational constants, dipole moments, biding energies, spectroscopic shifts upon complex formation and light scattering. Part of this thesis is dedicated also to point out the difference between the properties of an optimized cluster and a liquid structure generated by Monte Carlo simulation. We compare the results obtained for the urea-water complex in these two situations and reinforce the fact that liquid and cluster are different physical situations, whose properties are also different. The systems studied were HCN, CH IND.3CN, HC IND.3N, HC IND.2NC, HCN H IND.2O and (NH IND.2)IND.2CO H IND.2O. from the results obtained in this thesis, we verify that DFT is indeed feasible to obtain properties of hydrogen bonded molecules and biomolecules.