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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Date
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
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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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Keywords
Spectroscopy, Raman intensities, Sn2 Reactivity