Theoretical and experimental investigations in chemistry: Part 1- SN2 Reactivity at C-6 in hexopyranosides, Part 2- Polarizability and raman intensities in hydrocarbons

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2005-05-19T13:59:04Z
Authors
Dawes, Richard
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Abstract
In this thesis, two rather different types of problems were investigated. The first was a case of anomalous reactivity in the area of carbohydrate synthesis. Hexopyranosides having the galacto configuration (i.e. C-4?OR axial) display very low SN2 reactivities towards anionic nucleophiles, whereas the corresponding gluco-configured C-6 sulfonates (C-4?OR equatorial) react at rates typical of primary centers. The accepted explanation for this difference involves the repulsive interaction of local dipoles in the transition structure of the galacto compound. This interaction is thought to destabilize the transition structure, making this reaction difficult. However, there are numerous inconsistencies in the application of this simple model (cases where the model fails to predict the observed behavior). Thus, a computational project was undertaken to examine six model systems of this type. The energetics and equilibria of the reactants were determined, including solvation. Reaction pathways and kinetics for various displacements were computed. Analyses of the calculated charge densities allowed for evaluation of any electrostatic interactions. This study revealed a number of important factors affecting the rates of reaction, while clearly showing that dipole-dipole interactions are very limited in these systems. The second project was in the related areas of molecular polarizability and vibrational spectroscopy. Descriptive models have been sought relating the structure and connectivity of molecules to their electronic properties. Researchers in the areas of non-linear optics and of conducting polymers require a better understanding of the effects of structural variations on electronic properties. The 4 simplest models of molecular polarizability and its derivatives with respect to molecular vibrations are grossly inadequate. The highest-level calculations are generally reliable but are not applicable to even moderately sized systems. Thus trends in these properties were investigated for a large series of molecules. Calculations were performed at a variety of theoretical levels in order to determine the ranges of predicted behavior. Since these calculations predicted unusual properties in bicyclo-[1.1.1]-pentane, an experimental project was completed on this molecule. The results of Raman scattering intensity experiments on bicyclo-[1.1.1]-pentane allowed for evaluation of the performance of various computational methods. More importantly, it allowed for the confirmation of some qualitative structure/property relationships.
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Spectroscopy, Raman intensities, Sn2 Reactivity
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