Understanding the structure and bonding of chemical systems is a fundamental yet central practice of chemistry. While many ab initio calculation methods have been developed extensively and intensively, the development of bonding analysis to unravel the chemical interactions from calculation results is rather left behind, mostly due to the incompatibility between the delocalized molecular orbitals arising from first principle calculations and the localized bonding models that are intuitive and more often adopted by chemists. While classical two-center-two-electron bonding situations are well described by existing bonding analysis tools, there are many examples that do not fall into this scenario. In this dissertation, a new bonding analysis tool, namely the Principal Interacting Orbital...[ Read more ]

Understanding the structure and bonding of chemical systems is a fundamental yet central practice of chemistry. While many ab initio calculation methods have been developed extensively and intensively, the development of bonding analysis to unravel the chemical interactions from calculation results is rather left behind, mostly due to the incompatibility between the delocalized molecular orbitals arising from first principle calculations and the localized bonding models that are intuitive and more often adopted by chemists. While classical two-center-two-electron bonding situations are well described by existing bonding analysis tools, there are many examples that do not fall into this scenario. In this dissertation, a new bonding analysis tool, namely the Principal Interacting Orbital (PIO) analysis, will be proposed and extensively discussed across various systems. The principal interacting orbitals identified by the PIO analysis are semi-localized orbitals which form a sparse yet compact representation of the orbital interactions between fragments while still preserving mathematical rigor and chemical intuitiveness. The PIO analysis also has the advantage of being smooth with respect to reaction coordinate and hence can be applied to chemical reactions to trace the orbital interactions along the reaction. With the aid of the established PIO analysis, a number of group 14 cluster compounds have been investigated, which are known to have delocalized bonding patterns and are difficult to analyze with other methods. A fragment-based bonding model has been proposed on top of the valence orbitals of a common fragment identified by PIO analysis. Still, there are many compounds with distinct bonding patterns. To understand the delocalized skeletal bonding interactions in gold nanoclusters and the spin polarization effect on a number of spin-polarized systems, two more bonding analysis tools have been proposed, to specifically pinpoint subtle bonding features present in these systems.