Oxygen−17 Hyperfine Splitting Constants in the ESR Spectra of p-Semiquinones—17O

Oxygen−17 hyperfine splitting constants have been determined for a number of semiquinones—¹⁷O. The results were correlated with the spin density distribution in the radicals. It was found that the splitting constants a_O can be very well fitted to an expression of the form, $a_O{\mathrm{=Q}}_{\mathrm{OC}}{\hspace{0.17em}}^O{\hspace{0.17em}}_{\mathrm{\rho O}}{\mathrm{+Q}}_{\mathrm{CO}}{\hspace{0.17em}}^O{\hspace{0.17em}}_{\mathrm{\rho C}},$ where _ρO and _ρC are pπ spin densities on the oxygen and carbon atoms of the carbonyl group, and the Q's are sigma—pi interaction constants. Q_OC ^O is about −40 G, but the analysis does not allow the value of Q_CO^O to be determined, although its magnitude is certainly much less than that of Q_OC ^O. These interaction constants are discussed in terms of the general theory of Karplus and Fraenkel. Solvent effects on a_O were observed, especially in going from apropic to hydrogen‐bonding solvents and are discussed in terms of radical—solvent interaction at the carbonyl oxygen. In the Appendixes molecular orbital parameters for oxygen and chlorine are derived from the analysis of proton hyperfine splitting constants in simple and chloro‐substituted semiquinones.