The D parameter (EPR zero-field splitting) of localized 1,3-cyclopentanediyl triplet diradicals as a measure of electronic substituent effects on the spin densities in para-substituted benzyl-type radicals

The zero-field splitting parameters D of the symmetrically disubstituted and unsymmetrically monosubstituted 1,3-dialyl-1,3-cyclopentanediyl tripler diradicals 1, 2 (X = p-MeO, p-Me, p-Cl, p-NH2, p-CO(2)Me, p-CN, p-NO2), and 5 were determined in 2-methyltetrahydrofuran glass at 77 K, The linear plot (m = 0.558, r(2) = 0.993) of the experimental D values for the symmetrically disubstituted derivatives versus the corresponding monosubstituted ones reveals that the electronic substituent effects are additive and implies (except for the magnetic dipolar interaction) that each benzyl-type radical site acts independently in the localized diradicals, This additivity permits us to view these triplet diradicals as a composite of the two separate monoradical components and allows us to assess valuable electronic properties of benzyl-type monoradicals from the D parameter of the triplet diradical species. A theoretical analysis shows that the D parameter is a measure of the spin density rho at the benzylic positions and the inter-radical distance d in localized diradicals, A good correlation exists between the D parameter of these triplet diradicals (constant inter-radical distance d) and the EPR hyperfine coupling constants of the corresponding benzyl-type monoradicals, which establishes that the observed electronic substituent effects reflect changes in the spin densities at the radical sites. The novel Delta D scale allows us to quantify spectroscopically the para substituent effect on the spin delocalization at the benzylic position.