Localized triplet diradicals as a probe for electronic substituent effects in benzyl-type radicals: The ΔD scale

Abstract

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 ∝ 1/r3). For convenience, we have defined the difference between the D values of the benzylic substituent X and the parent system (X = H), i.e. ΔD = DH - DX, as a measure of spin delocalization by the aryl group at the radical site. The additivity of the ΔD 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 (aβ), with the calculated α spin densities (ρα), and the calculated resonance stabilization energies (RSE) for substituted cumyl radicals. These results manifest that the novel ΔD 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 σrad scales.