Photochemically generated cyclopentane-1,3-diyl triplet diradicals as model systems for the assessment of spin delocalization in heteroaryl-substituted benzyl-type monoradicals through the EPR spectral D parameter

Abstract

The zero-field D parameters of a comprehensive set of cyclopentane-1,3-diyl triplet diradicals 2 were determined at 77 K in a 2-MTHF glass matrix. The D values were found to be dependent on the heteroaryl substituents in the decreasing order 3-furyl 3-thienyl > 4-pyridyl ≈ 3-N-oxypyridyl > 2-pyridyl ≈ 3-pyridylium > 3-pyridyl ≈ phenyl 2-pyridylium 4-pyridylium 2-furyl 2-thienyl 4-N-oxypyridyl. Good linear correlations were obtained with the reported a_α coupling constants (r² = 0.953) for the benzyl monoradicals 4 and with the semiempirically calculated (PM3) α spin densities (r² = 0.928) for the cumyl monoradicals 3. To rationalize the observed electronic effects, for convenience the ΔD_Ar scale was defined as a measure of the spin-delocalizing ability of the different heteroaryl substituents relative to the phenyl group as reference. The hitherto unknown electronic effects of N-oxidation and protonation for the different pyridyl regioisomers as well as the regioisomeric effects of the furyl and thienyl substituents, are experimentally reflected accurately by the changes in the D parameter of the triplet diradicals 2 and explained theoretically with the help of MO calculations for the corresponding monoradicals 3.