Molecular orbital calculations of substituted aromatic anion radical spin densities

Abstract

The work in this dissertation has been undertaken to develop a method for determining heteroatom parameters to be used in a modified Huckel molecular orbital calculation. The heteroatom parameters derived from this work have been studied to determine their usefulness in predicting coupling constants of anion radicals containing one or more heteroatoms. In order to calculate heteroatom parameters, a least-squares procedure has been incorporated into the standard HMO method for molecules with heteroatoms. This least-squares procedure varies heteroatom parameters to obtain a minimum difference between calculated coupling constants and experimental coupling constants for as many as four molecules at a time. The experimental coupling constants used in this work, with the exception of heterocyclic N-oxide coupling constants, have been obtained from the literature. To determine the usefulness of heteroatom parameters obtained from this modified HMO technique, coupling constants resulting from the calculation have been compared to coupling constants calculated using an omega prime method and an unstricted Hartree-Fock self-consistent field technique. In the experimental section of this work, a number of heterocyclic N-oxides have been synthesized from their parent heterocycles. All the N-oxides which produced stable anion radicals when electrolyzed have been examined using electron spin resonance techniques. The coupling constants obtained from the electron spin resonance experiments are compared with calculated coupling constants obtained from the least-squares HMO method. The heteroatom parameters generated by the least-squares HMO method produce coupling constants which agree with experimental coupling constants very well, considering the level of approximation involved in any HMO calculation. Comparison of the results of the least-squares HMO calculations with the omega prime and SCF calculations shows that the least-squares HMO technique results in coupling constants which agree with experiment as well as coupling constants resulting from the more sophisticated molecular orbital techniques.