Electronic Effects of para- and meta-Substituents on the EPR D Parameter in 1,3-Arylcyclopentane-1,3-diyl Triplet Diradicals. A New Spectroscopic Measure of α Spin Densities and Radical Stabilization Energies in Benzyl-Type Monoradicals

The zero-field splitting D parameter was determined in a 2-MTHF glass matrix at 77 K for a large set (35 derivatives) of para- and meta-substituted 1,3-arylcyclopentane-1,3-diyl triplet diradicals 6. The D values are a sensitive function of electronic substituent effects; for convenience, the ΔD scale was defined as the difference D_H − D_X. Spin acceptors decrease while spin donors increase the D value relative to the unsubstituted reference system (D_H). Theoretical (PM3-AUHF) α spin densities (ρ_α) for the corresponding cumyl monoradicals 7 display a good linear dependence (r² = 0.963) when plotted against the D parameters of the triplet diradicals 6. The radical stabilization energies (RSE) of the cumyl radicals 7 were semiempirically calculated as the energy difference between in-plane (full conjugation) and perpendicular (no conjugation) conformations of the aryl groups and shown to correlate linearly (r² = 0.947) against the experimental D parameter for the corresponding triplet diradicals 6. These linear correlations, i.e., D versus ρ_α and versus RSE, demonstrate that the D parameter of the localized triplet diradicals 6 reflects reliably electronic substituent effects in benzyl-type monoradicals. The spectroscopic ΔD scale correlates poorly with the reported chemical σ_rad scales, unless polar corrections (Hammett σ_pol values) are made by means of a two-parameter Hammett treatment. Then a good linear correlation (r² = 0.921) of the ΔD values versus the Creary σ_rad scale applies; as expected, the radical effects dominate (ρ_rad = 1.00 versus ρ_pol = 0.41). The advantages of the new EPR-spectroscopic ΔD scale are that polar effects are nominal and the D parameter can be measured experimentally with sufficient accuracy to probe even small and subtle electronic effects through changes in the α spin densities.