Heinrich M. Harrer

Localized triplet diradicals as a probe for electronic substituent effects in benzyl-type radicals: The Δ D scale

The D parameter, readily determined by EPR spectroscopy, is a sensitive function of the avera e distance r of separation between the unpaired electrons in the localized triplet 1,3-diradicals 2 (D cc llr ). For convenience, we have defined the difference between the D values of the benzylic substituent X and the parent system (X = H), i.e. AD = D, - D,, as a measure of spin delocalization by the aryl group at the radical site. The additivity of the AD values of the monosubstituted diradicals 1 versus the symmetrically disubstituted diradicals 2 demonstrates that such triplet diradicals can be described as a composite of two geometrically fixed cumyl radical fragments. The D parameter correlates well with the experimental hyperfine coupling constants (ap), with the calculated a spin densities (p,), and the calculated resonance stabilization energies (RSE) for substituted cumyl radicals. These results manifest that the novel AD scale constitutes a reliable spectral tool to determine electronic substituent effects in benzyl-type radicals and may serve as a probe to assess the importance of polar substituent effects in chemical (3,d scales. B

Photochemically generated cyclopentane-1,3-diyl triplet diradicals asmodel systems for the assessment of spin delocalization inheteroaryl-substituted benzyl-type monoradicals through the EPR spectralD parameter

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 aα 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 ΔDAr 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.

The D parameter (zero-field splitting) as a direct measure of structural end electronic effects in localized triplet 1,3-diradicals

The zero-field splitting parameter D of the localized triplet diradicals 3–11, generated in a 2-methyltetrahydrofuran (MTHF) glass matrix at 77 K through photochemical deazetation of the corresponding azoalkanes, have been determined by EPR spectroscopy. It is demonstrated that the D parameter depends on structural (changes in the interspin distance d between the spin-bearing sites) effects, owing to differing ring size annelation, as well as electronic (changes in the spin density ρα at the radical sites) effects, i.e. D= f(d, ρα). The Dcalcd values calculated from the theoretical d and ρα values determined by semiempirical MO calculations (PM3) for the triplet diradicals 1–11, correlate well (r2= 0.961) with the Dexp values determined experimentally. Interestingly, heteroatom substitution at the 4,5-position in the cyclopentane-1,3-diyl moiety causes only a nominal spin delocalization onto the nitrogen atoms for the diradicals 11, which are also classified as localized triplet diradicals.