Electron spin resonance of semiquinones: Spin-density distribution and carbonyl sigma-pi parameters

Abstract

The spin-density distributions in the 1,4-benzosemiquinone and 2,5-dioxy-1,4-benzosemiquinone ions have been estimated by measuring the proton and ¹³C splittings and using reasonable choices for the values of the sigma-pi parameters which relate the splittings to the spin densities for C x C and C x H bonds. The determination of the signs as well as the magnitudes of the ¹³C splittings was required to obtain the spin densities. With Q_CH^H=-27.0 G in the relation a_i^H=Q_CH^Hρ_i ^∏ between proton splitting a_i^H and carbon-atom-pi-electron spin density ρ_i^∏, the sigma-pi parameters for the ¹³C splitting at a carbonyl position were found to be Q_CO^C=17.7 G and Q_OC^C=-27.1 G. The values of Q_CO^C and Q_OC^C depend on the value assumed for Q_CH^H. Contrary to earlier predictions made on the basis of molecular orbital calculations, the sign of the ¹³C splitting from the 2 position of the ρ-benzosemiquinone ion was found to be negative. This discrepancy results because the MO calculations were performed so as to reproduce proton splittings rather than the entire spin-density distribution, a procedure that is insensitive to the spin densities on the carbonyl carbon and oxygen atoms. New MO calculations were performed to fit the spin densities at all the positions in the radical. The results show that calculations fitted to proton splittings alone are inadequate for predicting spin densities at "blind" positions such as those at the carbon and oxygen atoms of a carbonyl group, or the nitrogen and oxygen atoms of a nitro group. Carbon-13 splittings were also observed in the anthrasemiquinone ion in both ethanol and 1,2-dimethoxyethane (DME) solutions. The sign of the 9-position ¹³C splitting changes from +0.70 G in ethanol to -0.47 G in DME solution. By using the sigma-pi parameters for the carbonyl group carbon atom, the spin densities were determined for this radical and MO calculations performed to reproduce the spin-density distributions. The variation of the splittings in the p-benzosemiquinone ion with solvent composition could be accounted for quantitatively by using the newly determined sigma-pi parameters.