ELECTRONIC EFFECTS OF PARA-SUBSTITUENTS AND META-SUBSTITUENTS ON THE EPR-D PARAMETER IN 1,3-ARYLCYCLOPENTANE-1,3-DIYL TRIPLET DIRADICALS - A NEW SPECTROSCOPIC MEASURE OF ALPHA-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-subs tituted 1,3-arylcyclopentane-1,3-diyl triplet diradicals 6. The D valu es are a sensitive function of electronic substituent effects; for con venience, the Delta D scale was defined as the difference D-H - D-X. S pin accepters decrease while spin donors increase the D value relative to the unsubstituted reference system (D-H) Theoretical (PM3-AUHF) al pha spin densities (rho alpha) for the corresponding cumyl monoradical s 7 display a good linear dependence (r(2) = 0.963) when plotted again st the D parameters of the triplet diradicals 6. The radical stabiliza tion energies (RSE) of the cumyl radicals 7 were semiempirically calcu lated as the energy difference between in-plane (full conjugation) and perpendicular (no conjugation) conformations of the aryl groups and s hown to correlate linearly (r(2) = 0.947) against the experimental D p arameter for the corresponding triplet diradicals 6. These linear corr elations, i.e., D versus rho alpha and versus RSE, demonstrate that th e D parameter of the localized triplet diradicals 6 reflects reliably electronic substituent effects in benzyl-type monoradicals. The spectr oscopic Delta D scale correlates poorly with the reported chemical sig ma(rad) scales, unless polar corrections (Hammett sigma(pol) values) a re made by means of a two-parameter Hammett treatment. Then a good lin ear correlation (r(2) = 0.921) of the Delta D values versus the Creary sigma(rad) scale applies; as expected, the radical effects dominate(r ho(rad) = 1.00 versus rho(pol) = 0.41). The advantages of the new EPR- spectroscopic Delta D scale are that polar effects are nominal and the D parameter can be measured experimentally with sufficient accuracy t o probe even small and subtle electronic effects through changes in th e a spin densities.