Cumyl monoradicals

The aforementioned analysis makes it evident that no special electronic effects (captodative stabilization, spin polarization, etc.) play a significant role in the localized triplet diradicals 9-11. Nevertheless, the question arises whether the electronic substituent effects in such triplet diradicals are additive, that is, whether the substituents on the two aryl groups act independently of one another. This feature was tested by comparison of the AD values of the unsymmetrical monosubstituted triplet diradicals 9 with those of the corresponding symmetrical disubstituted derivatives 10. Indeed, a linear correlation (Fig. 8) with a slope of nearly one-half (m = 0.558) unequivocally establishes that the electronic effects on the D parameter of localized triplet diradicals are additive [7], Thus, the localized triplet diradicals 9-11 may be considered to be electronically a composite of two independent (except for dipolar interactions) cumyl-type monoradicals, embedded in the molecular framework of the planar cyclopentane- 1,3-diyl ring system. This fact allows us to assess electronic substituent effects on the cumyl monoradicals 14 from the experimentally determined D parameter of the cyclopentane-1,3-diyl triplet diradicals 9-11 through the changes of the spin densities. 

Figure 12. Theoretical model to assess the RSE in the cumyl monoradicals 14.

Additionally, the excellent linear correlation (r2 = 0.991, not shown) of the calculated a spin densities of the 3,5-disubstituted cumyl radicals versus the 3-substituted radicals 14 with a slope of m = 2.00 again substantiates the additivity of electronic effects in the cumyl monoradicals exerted by meta substituents. 

Spin densities (p) are theoretical quantities, defined as the sum of the squared atomic orbital coefficients in the nonbonding semi-occupied molecular orbital (SOMO) of the radical species (Hiickel theory). For monoradical species, the spin density is connected to the experimental EPR hyperfine coupling constant a through the McConnell equation [38]. This relation provides the opportunity to test the spin density dependence of the D parameter [Eq. (8)] for the cyclopentane-1,3-diyl triplet diradicals 10 by comparing them with the known experimental hyperfine coupling constants (ap) of the corresponding substituted cumyl radicals 14 [39]. The good semiquadratic correlation (Fig. 9) between these two EPR spectral quantities demonstrates unequivocally that the localized triplet 1,3-diradicals 9-11 constitute an excellent model system to assess electronic substituent effects on the spin density in cumyl-type monoradicals. 

Figure 19. The D values of the triplet diradicals 12 versus the theoretical (PM3-AUHF/CI) a spin densities (p ) of the cumyl-type monoradicals 16.

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