Physical and theoretical studies of localized organic biradicals

• Main Author: Goldberg, Aaron Henry
• Format: Others
• Language: en
• Published: 1983
• Online Access: https://thesis.library.caltech.edu/5755/1/Goldberg_a_1983.pdf; Goldberg, Aaron Henry (1983) Physical and theoretical studies of localized organic biradicals. Master's thesis, California Institute of Technology. doi:10.7907/w5h0-qg02. https://resolver.caltech.edu/CaltechTHESIS:04292010-114150617 <https://resolver.caltech.edu/CaltechTHESIS:04292010-114150617>

Section I. Introduction. Section II. Singlet-triplet energy gaps in localized 1,3-biradicals have been investigated using ab initio electronic structure theory. Analysis of both the molecular orbital and generalized valence bond wavefunctions allows one to follow the complex interplay between through-bond and through-space interactions. When the two effects are of similar magnitude, a triplet ground state is possible. However, when one significantly dominates the other, a singlet ground state is expected. Extension of the analysis to other systems and the implications of the results for experimental studies of localized biradicals are discussed. Section III. Several unsuccessful attempts to observe the triplet cyclobutane- and cyclohexane-1,3-diyls through low temperature photolysis of the corresponding azo and azoxy compounds are reported. The reasons for these failures are considered together with their implications for the observation of other localized biradicals. Section IV. Photolysis of either 7-(2,3-diazabicyclo[2.2.1]hept-2-ene) spirocyclopropane or 7-(2,3-diazabicyclo[2.2.1]hept-2-ene) spiro-5'-bicyclo[2.1.0]pentane in a glassy matrix at 8°K leads to observation of a triplet ESR spectrum.. Both signals exhibit zero-field splitting parameters which are typical of delocalized systems (|D/hc| = 0.0255 and |E/hci| = 0.003 cm^(-1)). In addition, they are stable at temperatures well in excess of 85°K. Several possible sources of the two spectra are discussed. Section V. A program for the ab initio calculation of zero-field splitting parameters in localized biradicals is developed. The results for several 1,3-biradicals are presented.