Molecular Orbital Theory and Hyperfine Splitting in Electron Spin Resonance Spectra of Semiquinones

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Abstract
Molecular orbital calculations of the unpaired electron density in a number of semiquinone ions have been made as a function of a range of values of the Coulomb integral αO for the oxygen atom and the resonance integral βCO between the oxygen atom and the carbon atom. The unpaired electron densities ρ i were compared with the experimental hyperfine splitting constants ai arising from the proton bonded to carbon atom i by the use of McConnell's relation ai =Qρ i . Excellent agreement is obtained for the proper choice of αO and βCO. In contrast to most experimental tests of molecular orbital theory, the agreement depends rather critically on the choice of these two parameters. The best values found for the parasemiquinone ions are in the neighborhood of αO≅αC+1.2 βCC and βCO≅1.56 βCC. The values of | Q | are close to 22.5 gauss. A different set of parameters is needed for the ortho‐semiquinones. The molecular orbital calculation also fits the experimental splitting constants of the ringprotons in the positions meta and para to the methyl group of the tolusemiquinone ion, but there is a large discrepancy at the position ortho to the methyl group. The results of detailed examination of a number of spectra of semiquinone ions are also reported.

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