Multiply aromatic clusters via ab initio genetic algorithm

The concept of aromaticity has been known since its introduction by August Kekule in 1865. While pi-aromaticity and pi-antiaromaticity are well studied stabilizing/destabilizing effects in both organic and inorganic systems, aromaticity and antiaromaticity of the sigma-type have not been investigated as well or used as widely. The phenomena of multiple aromaticity and antiaromaticity, when delocalization of both pi- and sigma-types is responsible for the chemical bonding, are even more unusual in chemistry. However, the electron-deficient nature of many elements in the Periodic Table requires delocalized bonding in their clusters, molecules, and crystals, and often multiple aromaticity and antiaromaticity solely rule the structure and electronic properties of such systems. The primary aim of this Dissertation was to study the chemical bonding in the cluster compounds of main group elements and to extend the concepts of aromaticity and antiaromaticity further into inorganic chemistry. Multiple aromaticity and antiaromaticity were found to be the keys to the stability, shape, and properties of the studied clusters.; The other aspect of the dissertation is methodological. Being a natural precursor to the elucidation and interpretation of the chemical bonding in various species, the search for global minima on the potential energy surfaces of these systems is one of the crucial problems in modern quantum chemistry. The ab initio Gradient Embedded Genetic Algorithm (GEGA) program for the elucidation of the structure of global minima of clusters was developed, tested, and applied to the considered cluster systems.