LOCALIZED TRIPLET DIRADICALS AS A PROBE FOR ELECTRONIC SUBSTITUENT EFFECTS IN BENZYL-TYPE RADICALS - THE DELTA-D SCALE

The D parameter, readily determined by EPR spectroscopy, is a sensitiv e function of the average distance r of separation between the unpaire d electrons in the localized triplet 1,3-diradicals 2 (D proportional to 1/r(3)). For convenience, we have defined the difference between th e D values of the benzylic substituent X and the parent system (X = H) , i.e. Delta D = D-H - D-X, as a measure of spin delocalization by the aryl group at the radical site. The additivity of the Delta D values of the monosubstituted diradicals 1 versus the symmetrically disubstit uted diradicals 2 demonstrates that such triplet diradicals can be des cribed as a composite of two geometrically fixed cumyl radical fragmen ts. The D parameter correlates well with the experimental hyperfine co upling constants (alpha(beta)), with the calculated alpha spin densiti es (rho(alpha)), rind the calculated resonance stabilization energies (RSE) for substituted cumyl radicals. These results manifest that the novel Delta D scale constitutes a reliable spectral tool to determine electronic substituent effects in benzyl-type radicals and may serve a s a probe to assess the importance of polar substituent effects in che mical sigma(rad) scales.