Molecular orbital models for electronic and structural properties of organic and inorganic materials.

Abstract

The relationship between electronic structure, electronic conductivity properties and crystal structure geometry are explored within a quantum mechanical framework. Extended Huckel tight binding calculations were conducted on the $\alpha$-phases of bis(ethylene)tetrathiofulvalene (abbreviated BEDT-TTF or ET for short), the phase (Ni(S,G-dihydro-1,4-dithin-2,3-dithiol)$\rm\sb2)\sb3(AuBr\sb2)\sb2$ (abbreviated (Ni(dddt)$\rm\sb2)\sb3(AuBr\sb2)\sb2)$ and the phases (dithiophenetetrathiofulvalene)$\sb2$M(maleonnitriledithiolate)$\sb2$, M = Ni, Pt and Au (abbreviated (DT-TTF)$\sb2$M(mnt)$\sb2$) in order to understand the physical properties of these materials. It was found that the $\alpha$-phases of BEDT-TTF possess electronic conduction bands which are highly sensitive to small changes of the intermolecular interactions. This fact was linked to the large variation in physical properties exhibited by these phases. The salt (Ni(dddt)$\sb2)\sb3$(AuBr)$\sb2$ was found to be a pseudo-one-dimensional conductor with a stable metallic state due to Ni-S contacts between molecules. The metal-insulator transition observed in the salts (DT-TTF)$\sb2$M(mnt)$\sb2$ was found to be related to small slidings of the molecules in the donor DT-TTF layer of the crystal. This molecular motion alters the intermolecular contacts and causes the system to enter into an insulating state. The second portion of this thesis deals with the relationship between crystal and molecular topology and electronic structure. Using the $\mu\sb2$-Huckel method crystal structure/electronic structure relationships for elemental solids and electron deficient borane and carborane cluster were explored. It was found that the elemental structure of the fourth row of the periodic table could be rationalized in terms of arguments based on the local atomic coordination environment. The $\mu\sb2$-Huckel method was found to properly model the shape of the potential energy surface for the molecular ions $\rm B\sb8H\sb8\sp{2-},\ B\sb9H\sb9\sp{2-},\ B\sb{10}H\sb{10}\sp{2-},\ B\sb{11}H\sb{11}\sp{2-},\ C\sb2B\sb8H\sb{10}\sp{2-}\ and\ C\sb3B\sb8H\sb{11}\sp-.$ Simple theoretical models are derived to explain the structure and isomerization chemistry of $\rm B\sb8H\sb8\sp{2-},\ .B\sb{11}H\sb{11}\sp{2-},\ C\sb2B\sb8H\sb{10}\sp{2-}\ and\ C\sb3B\sb8H\sb{11}\sp-.$