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

In this thesis, two rather different types of problems were investigated.\nThe first was a case of anomalous reactivity in the area of carbohydrate synthesis.\nHexopyranosides having the galacto configuration (i.e. C-4?OR axial) display\nvery low SN2 reactivities towards anionic nucleophiles, whereas the\ncorresponding gluco-configured C-6 sulfonates (C-4?OR equatorial) react at rates\ntypical of primary centers. The accepted explanation for this difference involves\nthe repulsive interaction of local dipoles in the transition structure of the galacto\ncompound. This interaction is thought to destabilize the transition structure,\nmaking this reaction difficult. However, there are numerous inconsistencies in\nthe application of this simple model (cases where the model fails to predict the\nobserved behavior). Thus, a computational project was undertaken to examine six\nmodel systems of this type. The energetics and equilibria of the reactants were\ndetermined, including solvation. Reaction pathways and kinetics for various\ndisplacements were computed. Analyses of the calculated charge densities\nallowed for evaluation of any electrostatic interactions. This study revealed a\nnumber of important factors affecting the rates of reaction, while clearly showing\nthat dipole-dipole interactions are very limited in these systems.\nThe second project was in the related areas of molecular polarizability and\nvibrational spectroscopy. Descriptive models have been sought relating the\nstructure and connectivity of molecules to their electronic properties. Researchers\nin the areas of non-linear optics and of conducting polymers require a better\nunderstanding of the effects of structural variations on electronic properties. The\n4\nsimplest models of molecular polarizability and its derivatives with respect to\nmolecular vibrations are grossly inadequate. The highest-level calculations are\ngenerally reliable but are not applicable to even moderately sized systems. Thus\ntrends in these properties were investigated for a large series of molecules.\nCalculations were performed at a variety of theoretical levels in order to\ndetermine the ranges of predicted behavior. Since these calculations predicted\nunusual properties in bicyclo-[1.1.1]-pentane, an experimental project was\ncompleted on this molecule. The results of Raman scattering intensity\nexperiments on bicyclo-[1.1.1]-pentane allowed for evaluation of the performance\nof various computational methods. More importantly, it allowed for the\nconfirmation of some qualitative structure/property relationships.

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