Quantum chemical computations of heterogeneous selective oxidation, STM images, and multiple bond reactions

Chapter one of this thesis describes first principles electronic structure computations performed to understand the mechanism of molecular oxygen activation by vanadyl pyrophosphate. The process is believed to play a key role in the catalytic oxidation of n-butane to maleic anhydride. The results obtained demonstrate that the mechanism involves at least two layers of vanadyl pyrophosphate crystal. Based on the computed energetics for small clusters, we propose an activation mechanism which involves the transfer of one oxygen atom from the first to the second layer of the crystal concerted with dioxygen activation by the first layer.; Chapter two describes a novel ab-initio computational technique, called GVB-RCI, which correctly describes the stretching and dissociation of multiple bonds and provides smooth potential energy surfaces for most chemical reactions. The technique is a special case of Multi Configuration SCF that does not have the Perfect Pairing restriction and still scales well with the size of the system. The capabilities and limitations of GVB-RCI are illustrated in the case of a few simple chemical reactions.; Chapter three contains a theoretical model describing the Scanning Tunneling Microscopy (STM) imaging of molecules adsorbed on graphite. The model is applicable to a variety of different molecules with reasonable computational effort, and provides images that are in qualitative agreement with experimental results. The model predicts that topographic effects will dominate the STM images of alkanes on graphite surfaces. The computations correlate well with the STM data of functionalized alkanes, and allow assessment of the structure and orientation of most of the functionalized alkanes that have been studied experimentally. In addition, the computations suggest that the highly diffuse virtual orbitals, despite being much farther in energy from the Fermi level of the graphite than the occupied orbitals of the adsorbed molecules, may play an important role in determining the STM image contrast of such systems.