The EPR-spectral D parameter of photochemically generated cyclopentane-1,3-diyl triplet diradicals as a quantitative measure of spin delocalization in vinyl-, phenyl-, and carbonyl-substituted radicals

The zero-field splitting parameter D of the localized cyclopentane-1,3-diyl triplet diradicals T, generated in a 2-methyltetrahydrofuran (MTHF) glass matrix at 77 K through photochemical deazetation of the corresponding azoalkanes 1-13, has been determined by EPR spectroscopy. It is demonstrated that the D parameter reliably reflects the changes caused by the alkyl (simple and oxyfunctionalized), vinyl, and carbonyl substituents of the spin densities at the radical site. Thus, spin delocalization of the substituents and stabilization of the radical center follow the order vinyl > phenyl > carbonyl much greater than alkyl. This trend is confirmed by the excellent linear correlations with the reported cr-hyperfine coupling constants (alpha-hfc) of the corresponding monoradicals and computed semiempirical (PM3 method) spin densities of the radical fragments RI.