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

摘要

The D parameter, readily determined by EPR spectroscopy, is a sensitive function of the average distance r of separation between the unpaired electrons in the localized triplet 1,3-diradicals 2 (D proportional to 1/r(3)). For convenience, we have defined the difference between the 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 disubstituted diradicals 2 demonstrates that such triplet diradicals can be described as a composite of two geometrically fixed cumyl radical fragments. The D parameter correlates well with the experimental hyperfine coupling constants (alpha(beta)), with the calculated alpha spin densities (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 as a probe to assess the importance of polar substituent effects in chemical sigma(rad) scales. [References: 16]