ESR spectra of the carboxylated cyclohexadienyl radicals: 13C hyperfine constants and the effect of carboxylation on the g factors and proton hyperfine constants

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Abstract
The g factors and proton hyperfine constants of 15 multiply carboxylated cyclohexadienyl radicals and all 19 of the carboxylated hydroxycyclohexadienyl radicals have been determined by the in situ radiolysis method. The g factors were measured with sufficient accuracy (±0.00002) to establish that they increase monotonically with total spin density on the positions substituted. The proton hyperfine data are interpreted to indicate that there is a 13% loss in spin density on the ring in the pentacarboxylate radical. Pronounced effects of asymmetric substitution are noted even when this substitution is at a position of low spin density. For the more highly charged hydroxycyclohexadienyl radicals the ESR spectra are sufficiently intense that the 13C containing radicals can be observed at the natural abundance level. Complete sets of 13C hyperfine data are reported for the 1,3,5‐tricarboxylate, the three tetracarboxylate and the pentacarboxylate radicals. Definitive assignment of 13C hyperfine constants to the carboxyl groups attached to the conjugated system is possible. Values of 4.8, 1.3, and 6.8 G are attributed to positions 1, 2, and 3, respectively, in the pentacarboxylate radical. These hyperfine constants parallel almost exactly the proton values in the unsubstituted radical so that one can conclude that they can be used as a good indicator of the spin density on the adjacent ring carbon via a relation of the form aC(CO2−) = QC(CO2−) ρα. Correlation of the 13C and proton hyperfine data give a value of 15.8 G for QC(CO2−). Ring 13C hyperfine constants of ∼ 18, 12(2), 11(2), and 8.5 G are observed for each radical with the largest and smallest values being assigned, respectively, to the carbon atoms at positions 3 and 6. Comparisons among the different sets of values indicate variations from predictions based on the Karplus–Fraenkel treatment of the order of 10% so that specific assignment of the remaining four values is difficult. The hyperfine constants of the methylene protons in the substituted cyclohexadienyl radicals reflect very closely the loss in spin density to the carboxyl groups. Substitution in the hydroxycyclohexadienyl radicals, however, produces a far more pronounced and complicated effect that indicates a strong interaction between the OH group and the carboxyl groups substituted on the adjacent positions.