ADDITIVITY OF THE ELECTRONIC META-SUBSTITUENT EFFECT IN 3,5-DISUBSTITUTED CUMYL RADICALS ASSESSED BY THE EPR-D PARAMETER OF 1,3-ARYLCYCLOPENTANE-1,3-DIYL TRIPLET DIRADICALS

The D parameter (EPR zero-field splitting) of the 3,3',5,5'-tetrasubst ituted triplet diradicals 6 (X = X' = H, NO2, CH3, OAc, OCH3, NH2, and OH) were determined in a MTHF matrix at 77 K and serves as a spectros copic tool for the determination of electronic substituent effects in multiply-substituted benzyl-type monoradicals through its spin density dependence. The linear correlation (m = 2.00 +/- 0.01, r(2) = 0.974) of the experimental D values of these meta-disubstituted triplet dirad icals 6 versus the 3,3'-disubstituted triplet diradicals 5 demonstrate s the additivity of the electronic meta-substituent effect in the corr esponding 3,5-disubstituted cumyl monoradicals 4 and is corroborated b y theoretical (PM3-AUHF) spin density calculations for the latter. Thu s, the combined use of the experimental D parameter and semiempiricall y calculated a spin density has provided for the first time the unambi guous demonstration of the additivity of electronic effects exerted by meta-substituents in the cumyl monoradicals 4.