Comparison of the carbon-13 hyperfine constants of carboxylated cyclohexadienyl and hydroxycyclohexadienyl radicals

13C ESR spectra of the H atom adducts to pyromellitic and benzenepentacarboxylic acids have been observed. Comparison of the 13C hyperfine constants with those previously obtained for the OH adducts shows essentially identical values at the carboxyl positions and significant differences for the ring atoms only at the C3 and C6 positions. The spin density distribution in the π system is clearly not affected to any significant extent by OH substitution. In spite of this the 13C hyperfine coupling constants for C6, the site of addition, are 13 and 23% lower, respectively, in the OH adducts. This finding is at variance with correlation of 13C couplings strictly in terms of the local and adjacent π spin densities. Since there is no appreciable loss of spin density from the π 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 sigma and π systems introduced by the substitution. It is suggested that the differences noted here, and the related drop of approx. 35% in the proton hyperfine constants at C6, result mainly from the loss of symmetry in the OH substituted radicals. 12 references, 1 figure, 1 table