JT-Spin density

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

It is well known that delocalized stable radicals may have potential for the construction of solid state conducting materials. The phenylalenyl radical 40 has been considered a good candidate with its spin density delocalized over 13 carbons in its jt-conjugated system. Unfortunately, 40 exists in equilibrium with its dimer and it decomposes at modest temperatures. To overcome the dimerization problem, Goto et al. and Koutentis et al. synthesized substituted radicals 41 and 42. 

The different shift mechanisms may be understood in more detail by considering the effect of the magnetic field on the populations and energies of the different crystal orbitals (Figure 7a). Transfer of electron density via the 90° interaction arises due to a direct delocalization of spin density due to overlap between the half-filled tzg. oxygen jt, and empty Li 2s atomic orbitals (the delocalization mechanism. Figure 7b).This overlap is responsible for the formation of the tzg (antibonding) molecular orbital in a molecule or the tzg crystal orbital (or band) in a solid. No shift occurs for the 180° interaction from this mechanism as the eg orbitals are empty. 

The radical anions of various phenyldiphosphaalkenes (Scheme 11) were studied by EPR. Their reduction is easier than that of monophosphaalkenes and is dependent on the nature of the isomer. Both EPR spectra and DFT calculations showed that in the radical anion the unpaired electron belongs to a Jt orbital and that its delocalization is dependent on the relative position of the two phos-phaalkene moieties and on the nature of the bridging group. The spin density on the phosphaalkene carbon is higher for the meta compound than for the ortho and para compounds. 

These structures may be viewed as distorted from the Bj-type geometries via a second-order JT-type mechanism or, alternatively, as Aj-type with the substituents at the wrong carbon atom. The calculations suggest that the radical cation state preference can be fine-tuned by appropriate substituents and predict substantial differences in spin-density distributions. These predictions should be verifiable by an appropriate spectroscopic technique (ESR or CIDNP) and might be probed via the chemical reactivity of the radical cations (vide infra). 

Fig. 2.18. Spin polarization (through a bonding) and direct delocalization (through jt bonding) mechanisms for unpaired spin density transfer from an sp2 carbon to a fluorine nucleus.

The distribution of the total spin density in radicals was calculated using the UB3LYP16-31g method. In systems 1, 3, 4, and 8-11, the spin density is localized on the -S-N-S- fragment with minor spin distribution onto the Jt-framework, presumably through a spin polarization mechanism. For instance, in 1,3,2-dithiazolyl 1, virtually all positive spin density is localized on the -S-N-S- array due to the two polar resonance structures shown in Equation (2). A very similar spin distribution is observed in ring-fused derivatives 3, 4 and 9-11 <2001PCA7615>. 

The postulation of the cyclic structure of the complex is mainly intuitive. The active center exhibits a certain localized stress which is expressed mainly in the lability of active bonds or in the existence of non-compensated char or spin density. In the interaction with the unsaturated jt-system of the monomer, the possibility of delocalization of the corresponding functional stress appears on the intermolecular level. This delocalization is probably most effective for the cyclic structure of the complex. 

Fig. 30.6 Spin density distribution of the 1V54B and 1V540 models of the Cua site in the a and Jt oxidized states in the gas phase. All isovalue surfaces are set at 0.001 e/A. Blue and green surfaces represent positive and negative spin densities, respectively. Molecular structures are shown in thin lines...

Ayy is the principal value for the C—proton in a Jt-electron radical with spin density p = 1, and vo is the free proton frequency. When the external field is applied parallel fo the stretching direction (y) the maximum ENDOR frequency v+ is estimated to be c.a. 21 MHz at which point the signal changes its slope. By substituting the experimental values of Ayy and (v -vo) in Eq. (7.33), p(0) is evaluated. Due to the uncertainty of the exact value for Ayy the value of p(0) ranges between 0.11 and... 

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