Electronic Effects of <i>para</i>- and <i>meta</i>-Substituents on the EPR <i>D</i> 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 <i>D</i> parameter was determined in a 2-MTHF glass matrix at 77 K for a large set (35 derivatives) of <i>para</i>- and <i>meta</i>-substituted 1,3-arylcyclopentane-1,3-diyl <i>triplet diradicals</i> <b>6</b>. The <i>D</i> values are a sensitive function of electronic substituent effects; for convenience, the Δ<i>D</i> scale was defined as the difference <i>D</i><sub>H</sub> − <i>D</i><sub>X</sub>. Spin acceptors decrease while spin donors increase the <i>D</i> value relative to the unsubstituted reference system (<i>D</i><sub>H</sub>). Theoretical (PM3-AUHF) α spin densities (ρ<sub>α</sub>) for the corresponding cumyl monoradicals <b>7</b> display a good linear dependence (<i>r</i><sup>2</sup> = 0.963) when plotted against the <i>D</i> parameters of the <i>triplet diradicals</i> <b>6</b>. The radical stabilization energies (RSE) of the cumyl radicals <b>7</b> 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 (<i>r</i><sup>2</sup> = 0.947) against the experimental <i>D</i> parameter for the corresponding <i>triplet diradicals</i> <b>6</b>. These linear correlations, i.e., <i>D</i> <i>versus</i> ρ<sub>α</sub> and <i>versus</i> RSE, demonstrate that the <i>D</i> parameter of the localized <i>triplet diradicals</i> <b>6</b> reflects reliably electronic substituent effects in benzyl-type monoradicals. The spectroscopic Δ<i>D</i> scale correlates poorly with the reported chemical σ<sub>rad</sub> scales, unless polar corrections (Hammett σ<sub>pol</sub> values) are made by means of a two-parameter Hammett treatment. Then a good linear correlation (<i>r</i><sup>2</sup> = 0.921) of the Δ<i>D</i> values <i>versus</i> the Creary σ<sub>rad</sub> scale applies; as expected, the radical effects dominate (ρ<sub>rad</sub> = 1.00 <i>versus</i> ρ<sub>pol</sub> = 0.41). The advantages of the new EPR-spectroscopic Δ<i>D</i> scale are that polar effects are nominal and the <i>D</i> parameter can be measured experimentally with sufficient accuracy to probe even small and subtle electronic effects through changes in the α spin densities.