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 PARASUBSTITUTED BENZYL-TYPE RADICALS

The zero-field splitting parameters D of the symmetrically disubstitut ed 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-C N, p-NO2), and 5 were determined in 2-methyltetrahydrofuran glass at 7 7 K, The linear plot (m = 0.558, r(2) = 0.993) of the experimental D v alues for the symmetrically disubstituted derivatives versus the corre sponding monosubstituted ones reveals that the electronic substituent effects are additive and implies (except for the magnetic dipolar inte raction) 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 sp in density rho at the benzylic positions and the inter-radical distanc e d in localized diradicals, A good correlation exists between the D p arameter of these triplet diradicals (constant inter-radical distance d) and the EPR hyperfine coupling constants of the corresponding benzy l-type monoradicals, which establishes that the observed electronic su bstituent effects reflect changes in the spin densities at the radical sites. The novel Delta D scale allows us to quantify spectroscopicall y the para substituent effect on the spin delocalization at the benzyl ic position.