| Abstract
| - The EPR-spectral zero-field splitting parameter D of the localized heterovinyl-substituted 1,3-cyclopentanediyl triplet diradicals T, generated in a 2-methyltetrahydrofuran (MTHF) glass matrixat 77 K through the photochemical deazetation of the corresponding azoalkanes 1−13, is aquantitative measure of the spin-density (ρ) variation by the substituents at the radical site in the1-heteroallylic radicals. From these data, the radical-stabilizing ability of a variety of nitrogen-containing groups has been assessed, which includes imino and hydrazonyl functionalities. Theradical stabilization in the heteroallylic radical fragment follows the order X = O < NMe < CH2<CHMe ≪ NOH ≈ NOMe ≪ NNHCHO ≈ NNHC(O)NH2< NPh ≈ NNMe2< NNH2< CHPh <NNHPh. The lowest D values have been found for the hydrazonyl-substituted derivatives, whichimplies the lowest spin density at the carbon center and, thus, the most efficacious radicalstabilization through spin delocalization. This superdelocalization may be rationalized in terms ofnitrogen-centered radical-cationic structures. Localization of the spin at the terminal atom is resistedthrough the electronegativity effect (O < N < C).
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