Analysis Of Chemical Bonding Using Ab Initio Valence Bond Theory

Abstract

In this thesis, theoretical chemical research is presented in which the Valence Bond (VB) Theory plays a central role. For the last three chapters, the VB method is used, in combination with Magnetically Induced Ring Currents, to analyze the aromaticity of several conjugated molecules. The electronic wave function for the (delocalized) π system can be expressed in Kekulé structures. With VB, properties like resonance energy and contribution of each Kekulé-like structure can be computed and, with that, aromatic behavior can be investigated and understood. The structures and orbitals used to construct VB wave functions are not necessarily orthogonal. Therefore, VB calculations are quite often costlier in terms of computing time than the more common orthogonal Molecular Orbital (MO) based methods. In the second chapter a method is presented to reduce the computing time. During the construction of the Brillouin matrix, fewer matrix elements are computed and some are replaced by Fock matrix elements. These elements are computed in much less time than regular Brillouin matrix elements. In its current form, the Fock matrix extension can be very well used in the analysis of planar conjugated molecules like the ones described above. However, for the application to other, non-planar molecules more investigation will be required.