Looking at chemistry through the unifying lens of molecular orbital theory: From extended hypervalent bonding to hydrocarbon chemisorption and the reactivity of organic molecules

This thesis consists of seemingly diverse projects on the nature of bonding in solids, surfaces, and molecules. What unites them is that they have been analyzed with the help of the same language of molecular orbital and perturbation theories. This, perhaps, is the most important message of this work---the unity of chemistry is reached through a theoretical perspective.;In Chapter One we expand the Zintl-Klemm electron counting rules to the electron-rich multi-center networks of heavy late main group elements. We illustrate the similarity between the principles of bonding in these networks and the hypervalent bonding concept applied previously to molecules such as XeF2 or I-3 . We propose optimal electron counts for a one-dimensional linear chain, a two-dimensional square sheet, a simple cubic lattice, a one-dimensional ribbon of vertex sharing squares and for various other topologies. Along the way we reflect on the nature of bonding in a number of complicated intermetallic compounds.;Many ideas developed in Chapter One are applied in Chapter Two to understand the electronic structure of a non-trivial ternary alloy, La12Mn 2Sb30. We hypothetically disassemble its complicated three-dimensional network into simpler pieces, study those, and assemble them back into the whole structure; we call this process "retrotheoretical analysis" in analogy with the well-known concept in synthetic organic chemistry.;A comprehensive molecular orbital theory of H and CH3 chemisorption on the Pt (111) surface is presented in Chapter Three. We demonstrate and explain the binding site preferences for hydrogen, methyl, and ethyl groups on Pt (111). For the two latter adsorbates at high-symmetry positions, we propose a simple molecular orbital model of agostic interactions between corresponding a -C-H bonds and the surface Pt atoms.;The orbital mechanism by which the internal strain of [2.2]paracyclophane is transformed into enhanced reactivity toward the Cr(CO)3 group is studied in detail in Chapter Four. Finally, up to second-order expressions in energies and wavefunctions are derived for non-degenerate and degenerate perturbational molecular orbital theories in Chapter Five. Qualitative and quantitative applications of perturbation theory are illustrated on simple examples.