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

Abstract

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 ρ_α at the radical sites) effects, i.e. D= f(d, ρ_α). The D_calcd values calculated from the theoretical d and ρ_α values determined by semiempirical MO calculations (PM3) for the triplet diradicals 1–11, correlate well (r²= 0.961) with the D_exp 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.