Multi-Configurational Investigation of Thermolytic Pathways of Highly Strained Ring Systems
Date of Award
Ph.D. in Chemistry
The isomerization pathways of model high energy structures are of interest because of their relation to high energy density fuels. Electron resonance has been found to greatly affect the relative activation barriers for several isomerization pathways, and the major goal of this research is to accurately describe its role in determining the relative barriers for strain energy release pathways. This research is centered around the potential energy surfaces (PES's) for ? bond breaking and ? bond rotation in these highly strained structures. Of particular interest was how would resonance and or electronegativity affect the allowed/disallowed nature of the activation barriers in these systems. The two model systems of benzvalene and dihydrobenzvalene have been explored previously. These tricyclic structures were shown to be able to undergo a pericyclic ring opening mechanism. That particular process can either occur in either a conrotatory manner or a disrotatory manner. The disrotatory process was found to be Woodward-Hoffman symmetry allowed in benzvalene, but the conrotatory process was found to be Woodward-Hoffman symmetry allowed in dihydrobenzvalene. The only difference between these two structures in the presence or absence of a ? bond. So much of this work was focused on attempting to deliberately manipulate the allowed and disallowed nature of these type of pericyclic reactions in an attempt to see if the line that is drawn between the allowed and disallowed processes of a particular molecular system could be "blurred". It was found throughout this work that the Woodward-Hoffman symmetry rules held true for these types of isomerization mechanisms. It was also interesting to see that in certain extreme cases such as the thermal isomerization of 3,4-diaza-diium-dihydrobenzvalene to 1,2-diaza-diium-dihydropyridazine the activation barriers (disrotatory and conrotatory) can be brought to be nearly isoenergetic with each other (70.9 kcal/mol vs. 67.2 kcal/mol). In one particular thermal isomerization pathway of 3-aza-benzvalene to pyridine the barriers were found to have actually reversed with TSconB have an activation barrier of 38.2 kcal/mol while that of TSdisB was found to be 39.7 kcal/mol at the MRMP/cc-pVTZ level of theory. The PES for ? bond rotation was explored in more detail for several trans pyrans and pyridines and also for trans cyclohexene and several of its functional isomers. The ground state geometries for each structure were computed using the multi-configurational self consistent field method (MCSCF) with a cc-pVDZ basis set, while energies were computed using multi-reference Möller-Plesset second order perturbation theory (MRMP2) with a cc-pVTZ basis set. A multiconfigurational wavefunction was necessary in order to properly describe the ? bond breaking processes as well as the ?-bond breaking and reforming processes that occur during the isomerization pathways under study. For the minima on the PES's, energies were also calculated using either the quadratic configuration interaction singles double with iterative triples (QCISD(T)) method or the coupled cluster singles doubles with iterative triples (CCSD(T)) method with the cc-pVTZ basis set.
Veals, Jeffrey Dwayne, "Multi-Configurational Investigation of Thermolytic Pathways of Highly Strained Ring Systems" (2012). Electronic Theses and Dissertations. 292.