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

The zero-field splitting parameter D of the localized tripler 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 rho(a) at the radical sites) effects, i.e. D = f(d, rho(a)). The D-calcd values calculated from the theoretical d and rho(a), values determined by semiempirical MO calculations (PM3) for the triplet diradicals 1-11, correlate well (r(2) = 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 tripler diradicals.