Electron Spin Resonance Spectra of Cyclic Ketyls and Ortho-Semiquinones

Electron spin resonance spectra are reported for the radical anions of fluorenone, 2,7-difluorofluorenone, 4,5-phenanthrylene ketone, 0-phenanthrenequinone, acenaphthenequinone, and benzil. The radicals were generated electrolytically in dimethylformamide solution. Electrolytic reduction of the hydrocarbons fluorene, 4,5-methylenephenanthrene, and 9,10-dihydrophenanthrene resulted in the oxidation of the methylene groups to carbonyl groups. Presumably this oxidation was caused by trace amounts of oxygen in the system. Approximate self-consistent configuration-interaction molecular orbital calculations of the McLachlan type were performed to obtain the spin densities in the radical anions. Good agreement between calculated spin densities and experiment for the planar radicals was obtained by increasing the carbonyl-ring resonance integral above the usual values, and in some cases further improvement resulted from a small decrease in the inter-ring resonance integral.