Cyclopentane-1,3-diyl diradicals

Figure 22.12. Cyclopentane-1,3-diyl (16a) has been both calculated " and found to have a triplet ground state, and the 1,3-diphenyl derivative (16b) has also been found to have a triplet ground state. In contrast, calculations on 16c and 16g have predicted both diradicals should have singlet ground states, and experiments on derivatives of 16d, 16e, and 16f found these three diradicals do, in fact, have singlet ground states.

IV. The Influence of Heteroaryl Substituents on the D Parameter of Cyclopentane-1,3-diyl Triplet Diradicals, 238... 

The most prominent representatives of sufficiently persistent diradical intermediates are the cross-conjugated (non-Kekule type) trimethylene-methanediyl (1) [10] diradical (e.g., the Berson diradical 2 as its cyclic derivative [11]), or the diradicals 3 and 4 [12] and the localized triplet cyclobutane-1,3-diyl (5) [13], cyclopentane-1,3-diyl (6) [14 16], or 4,5-diazacyclopentane-l,3-diyl (7) [17] diradicals (Fig. 1). We have focused our attention on the localized type, namely, the structurally related and cyclo-pentane-annellated cyclopentane-1,3-diyl triplets of 6 [Eq. (1)], specifically... 

A typical first-order derivative spectrum of randomly oriented triplet diradicals is given schematically in Figure 4, which shows how readily the D and E values can be read out. The six high-field signals belong to the Ams = +1 transitions, whereas the forbidden Ams = 2 transition appears at half-field. The latter is characteristic for triplet diradicals and serves as a criterion for their unambiguous assignment. Illustrative examples are the triplet cyclobutane-1,3-diyls (5) [13], the cyclopentane-1,3-diyls (6) [14 16], and the 4,5-diazacyclopentane-1,3-diyls (7) [17], whose zfs parameters are summarized in Table 1. 

Of the known triplet states, the cross-conjugated non-Kekule diradicals are classified as delocalized triplet diradicals (e.g., 14). In contrast, the radical centers in localized triplet diradicals (e.g., 5-7) are not joined through a n system. For the localized 1,3-diphenyl-substitutcd cyclopentane-1,3-diyl triplet diradicals 6b and c, a planar five-membered ring with coplanar arrangement of the aryl groups is expected to allow the maximum contribution of the benzylic resonance stabilization, which is worth about 50-54 kJ mol 1 [28], For such aryl-substituted triplet diradicals (e.g., 9-12), the magnetic dipole interaction of the two uncoupled spins is given by Eq. (6). 

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. 

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. 

IV. THE INFLUENCE OF HETEROARYL SUBSTITUENTS ON THE D PARAMETER OF CYCLOPENTANE-1,3-DIYL TRIPLET DIRADICALS... 

Besides spin densities, the D parameter also depends on the interspin distance dAB [Eq. (8)]. Thus, structural effects on the D parameter may be probed through variations of the interspin distance dAB at constant spin densities pA and pB. The influence of such structural features on the D parameter is most prominently displayed by the two localized diradicals 5d [13] and 6b [15] (Table 5) for which the 1,3-diphenyl substitution provides essentially constant spin densities at the radical sites. Thus, any changes in the D parameter of these triplet diradicals should derive from geometrical effects on the interspin distance dAB. Optimized geometries from MO calculations for the cyclobutane-1,3-diyl (5d) gave an interspin distance of 208.5 pm and for the cyclopentane-1,3-diyl (6b) a value of 237.1 pm [64], which accounts nicely for the observed trends in the experimental D values that is, a lower D value (Table 5) for the latter due to the inverse d3 dependence [Eq. (8)]. 

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