The EPR D-parameter of 1,3-diarylcyclopentane-1,3-diyl triplet diradicals as a probe for steric substituent effects in benzyl-type radicals

The zero-field splitting D parameter of a set of ortho-substituted 1,3-diarylcyclopentane-1,3-diyl triplet diradicals 3 has been determined by EPR spectroscopy in a 2-methyltetrahydrofuran glass matrix at 77 K. While for meta- and para-substituted triplet diradicals 3 the D parameter is only a function of the electronic nature (spin donor versus spin acceptor) of the substituent, for the ortho-substituted derivatives the steric influence must also be considered. We show herein that the steric effect of the ortho substituent diminishes delocalization of the cumyl spin density into the aromatic ring through twisting of the pi system out of conjugation with the cumyl radical center; indeed, the steric effect may outweigh the electronic influence of the substituent. The computed (PM3) spin density of the minimum-energy conformer reproduces well the experimentally observed steric effect on the D parameter of the triplet diradicals 3. These results demonstrate that the EPR-spectroscopic D parameter serves as a sensitive probe for steric as well as electronic substituent effects.