Date of Award

Spring 1-1-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Josef Michl

Second Advisor

Joel D. Eaves

Third Advisor

David Jonas

Abstract

Bonds of σ type are the most common chemical bonds, dominating bonding in all molecules and defining the framework of a molecule. It has been long known that electrons in σ bonds can delocalize throughout a molecule. However, the nature of σ-electron delocalization is not fully understood due to its complexity. One of the important aspects of σ-electron delocalization is its conformational dependence, first noted in oligosilanes where properties of a molecule change drastically when it changes conformation. This dissertation describes efforts to understand the conformational dependence of σ delocalization in oligosilanes and their heavier analogs using simple intuitive models and density functional theory (DFT) calculations. The first part of the dissertation focuses on studying the conformational dependence of electron delocalization in oligosilanes. It is shown that oligosilanes can exist as two distinct chromophores, σ-delocalized conformers with large dihedral angles and σ-localized conformers with small dihedral angles. Properties of a conformer are dictated by the shape of frontier molecular orbitals, which is different for conformers with small and large dihedral angles. The origin of the conformational dependence is found in the interplay between two delocalization mechanisms, which can lead to enhancement of delocalization or to its complete annihilation. The second part of the dissertation describes characteristics of σ-electron delocalization in linear infinite polysilane, polygermane, polystannane, and polyplumbane. The conformational dependence of electron delocalization is confirmed in all these molecules. Effective hole mass is used to indicate an extent of the delocalization, while a position of the Fermi level in reciprocal space is used to determine the dominant mechanism of delocalization. The simple model used to explain the origin of the conformational dependence of electron delocalization in saturated compounds is verified by comparison with density functional theory calculations. Relatively simple analysis presented here provides an important insight into properties of a complex ubiquitous phenomenon of delocalization of electrons through σ bonds.

Share

COinS