Date of Award

9-15-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Hoffmann, Mark R.

Abstract

Ground and low-lying excited electronic states of difluorodiazirine (F 2CN2) were investigated using the recently revised variant of second-order Generalized Van Vleck Perturbation theory. It was shown that the CN2 ring is retained in all the low-lying excited states and that the compound is highly multireference in nature. The results are expected to be useful in describing the mechanism of the ultraviolet photolysis of this compound. Difluorophosphaazirine (F2CNP) is isovalent with F2CN 2 and presents useful similarities and differences. In mapping the potential energy surface of F2CNP, the geometrical parameters and harmonic vibration frequencies of six new local minima, besides the previously studied cyanodifluorophosphine, were investigated at the MP2/cc-pVDZ level. Relative energies were obtained at the CCSD(T)/cc-pVTZ level. It was shown that difluorophosphaazirine, isocyanodifluorophosphine, and one additional isomer are stable, relative to the dissociation products; two of the six new isomers were shown to be metastable and the remaining isomers were found to be nearly isoenergetic with the dissociation products. The peptide cyclo(L-Pro)3 was studied at the SCF and DFT levels to compare the effects of theory level on predicted geometric structure. The peptide was also investigated at the SCF level by enclosing it in a solvent cavity. Results were compared with the aforementioned SCF results (in vacuum) to understand the effects of solvent on the geometry and the energy differences between the two conformers of the peptide. Within the limitations of the currently available ab initio program packages, it was shown that there are no major differences between the results in vaccum and in the presence of the solvent. Complementing the studies of nitrogen in ring systems was an additional study in which diphenylamine, which contains a bridging N between two rings, was investigated. The dependence of the predicted molecular geometries and harmonic vibrational frequencies of diphenylamine and related compounds on the level of electronic structure theory, especially, MP2 in comparison to various popular density functional theory (DFT) methods, was investigated.

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