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.

Abstract

The D parameter (EPR zero-field splitting) of the 3,3',5,5'-tetrasubstituted triplet diradicals 6 (X = X' = H, NO(2), CH(3), OAc, OCH(3), NH(2), and OH) were determined in a MTHF matrix at 77 K and serves as a spectroscopic 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 diradicals 6 versus the 3,3'-disubstituted triplet diradicals 5 demonstrates the additivity of the electronic meta-substituent effect in the corresponding 3,5-disubstituted cumyl monoradicals 4 and is corroborated by theoretical (PM3-AUHF) spin density calculations for the latter. Thus, the combined use of the experimental D parameter and semiempirically calculated alpha spin density has provided for the first time the unambiguous demonstration of the additivity of electronic effects exerted by meta-substituents in the cumyl monoradicals 4.