Electron transfer processes—X: Radical ions with hydrogen bridges derived from 1,1′,3,3′-tetraketo-Δ2,2′-biindan, 5,6,11,12-naphthacenetetrone, 1,4,9,10-anthracenetetrone, and 1,4,5,8-naphthalenetetrone

Radical anions and cations have been prepared by the reduction of the title compounds or their diprotonated derivatives. The same radical ions have been observed by oxidation of the dihydro derivatives. Electron spin resonance hyperfine splitting constants have been assigned on the basis of Hückel and McLachlan calculations and selective substitutions. The hyperfine splitting by the bridging hydrogen atoms in the radical anions was much weaker (0·5 to 0·7 gauss) than for the corresponding radical cations (2 to 3 gauss) for the naphthacene, anthracene, and naphthalene dihydrotetrones. This is interpreted to be a result of orbital symmetry since, in the highest occupied MO for the anion but not for the cation, a nodal plane passes through the bridging hydrogen atom