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An ab initio and semiempirical study of the effects of structure and heteroatom substitution on the ground state multiplicities of pi-conjugated radicals and nitrenes
Abstract
The first systematic semiempirical computational study of the effects of structure and heteroatom substitution on the ground state multiplicities of pi-conjugated radicals and nitrenes has been carried out. Geometry optimization was performed with UHF wavefunctions for the highest spin multiplicity of each diradical or polyradical using the MNDO or AM1 hamiltonians. Individual state energies were computed with INDO-CISD or AM1-CI methods. The ground state multiplicity was found to depend primarily on the pi topology. A parity model based on the alternation of electron spins, provides a readily interpretable physical mechanism for both antiferromagnetic and ferromagnetic coupling of unpaired electrons in piconjugated systems. Heteroatoms are found to cause minor perturbations of the parity rules conducing to both high- and low-spin ground states, depending on whether the heteroatom is a radical bearing site or acting as a through-bond coupler. A singlet ground state is predicted for ethano- and methano-bridged oxyallyl systems computed at the 6-31G* pi-SDTQ-CI level of theory. Although the bridged oxyallyls are isoelectronic with trimethylenemethane derivatives, the strong carbonyl bond perturbs the TMM system. Electronically, we found the bridged oxyallyls to be diradicaloid in nature; intermediate between diradical and zwitterionic. This interpretation is supported experimentally by a number of chemical trapping studies. The products of ring-closure, bicyclopentanone and bicyclobutanone, were found to be energetically competitive with the ring opened forms. The geometries and ground state multiplicities of cyclic pentamethylenepropane (PMP) derivatives were predicted at the 6-31G* SCF-CI level of theory. From both spin density and qualitative molecular orbital analyses, PMP belongs to the disjoint class of diradicals. The ground state is found to be a triplet by approximately 1.5 kcal/mol. The ring-closed forms are found to be lower in energy than the diradical open-shell states. The first ab initio computational studies of pi-conjugated dinitrenes was carried out. Dinitrenes are found to require multiconfiguration SCF and CI methods for their correct computational description. meta-Phenylenedinitrene is predicted to be a ground state quintet by 3-5 kcal/mol relative to the triplet state. para-Phenylenedinitrene is found to have nearly degenerate triplet and singlet states.
Subject Area
Physical chemistry|Chemistry
Recommended Citation
Ichimura, Andrew Shohaku, "An ab initio and semiempirical study of the effects of structure and heteroatom substitution on the ground state multiplicities of pi-conjugated radicals and nitrenes" (1992). Doctoral Dissertations Available from Proquest. AAI9233073.
https://scholarworks.umass.edu/dissertations/AAI9233073