Spin density distributions and g values in semiquinones

From a study of several semiquinone systems, we have shown that their g values can be expressed in terms of the π-electron spin densities on the oxygens by a linear equation. This suggests the possibility of estimating the spin densities on the oxygens directly from the g values in similar systems. The spin density distribution used in establishing such a relation had been determined with the help of a new set of Q parameters for the 13C and 17O hyperfine splittings associated with the C′2CO fragment. Recognition of the existence of hydrogen bonds in solvents such as water or ethanol, by introducing an additional parameter QOOH is a new feature of this work. Q0OH=6.0 G in water or ethanol. QOOH=0 in aprotic solvents. Reliability of our Q parameters and the spin density distributions determined with their help, has been demonstrated by comparing the predicted and the observed hyperfine splittings/spin densities, not only in semiquinones, but also in 2,4,6 tri-t-butyl phenoxy radical and in benzophenone potassium ketyl. (Other authors have recommended substantially different sets of Q parameters.) It has been possible to estimate QCCC′=22.2 G, for sp3 hybridized C″ as in −C″H3, using the spin density distribution in durosemiquinone (DSQ−). Spin densities in DSQ− in ethanol were obtained with the help of our g values. Our value of QCCC″ is considerably less than the theoretical estimate of Strauss and Fraenkel, but is consistent with the predictions of Fessenden.