A Comparison of the Carbon-13 Hyperfine Constants of Carboxylated Cyclohexadienyl and Hydroxycyclohexadienyl Radicals

Article abstract of DOI:10.1021/j100236a030

(13)C ESR spectra of the H atom adducts to pyromellitic and benzenepentacarboxylic acids have been observed.Comparison of the hyperfine constants with those previously obtained for the OH adducts showes essentially identical values at the carboxyl positions and significant differencies for the ring atoms only at the C₃ and C₆ positions.The spin density distribution in the ? system is clearly not affected to any significant extent by OH substitution.In spite of this the (13)C hyperfine coupling constants for C₆, the site of addition are 13 and 23 percent lower, respectively, in the OH adducts.This finding is at variance with correlation of (13)C couplings strictly in terms of the local and adjacent ? spin densities.Since there is no appreciable lost of spin density from the ? system the effects of OH substitution cannot arise directly from transfer of spin density to the C₆ position.Rather these effects must result from changes in the interactions between the ? and ? systems introduced by the substitution.It is suggested that the differencies noted here, and the related drop of ca. 35 percent in the proton hyperfine constants at C₆, result mainly from the loss of symmetry in the OH substituted radicals.