Spin polarization mechanism

In the early 1990s, a new spin polarization mechanism was posPilated by Paul and co-workers to explain how polarization can be developed m transient radicals in the presence of excited triplet state molecules (Blattler et al [43], Blattler and Paul [44], Goudsmit et al [45]). While the earliest examples of the radical-triplet pair mechanism (RTPM) mvolved emissive polarizations similar in appearance to triplet mechanism polarizations, cases have since been discovered m which absorptive and multiplet polarizations are also generated by RTPM. 

This spin polarization mechanism was first advanced by McConnell and Chestnut [8], who proposed the equation... 

Fig. 2.17. Spin polarization mechanisms arising from unpaired spin density on a 2p orbital, illustrated for a l3C nucleus. (A) Polarization through a Is orbital (B) polarization through a 2s orbital (C) polarization on a nearby carbon through a o bond.

C data are available on several ligands. In Tables 2.3 and 2.4 the relative 13C hyperfine shifts of pyridine and pyridine-/V-oxide are reported [22,37]. In both cases it appears that there is alternation in the shifts, which would indicate n spin delocalization. However, the ratio between the shifts of protons and carbons at each position are different one from the other, and differ from what would be expected on the basis of a simple spin polarization mechanism. It is concluded that more than one mechanism is operative. 

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>. 

A subset of electron-hole radical pairs exhibits features of Spin Correlated Radical Pair (CRRP) electron spin polarization mechanism [101] which can be observed at somewhat longer times via light/field modulated (LFM) EPR measurements. This technique is only sensitive to the light dependent part of the EPR spectrum on the time scale of the light modulation frequency (millisecond regime, insert Fig. 1.15). Using LFM EPR it was observed that both the transitions of the holes localized on the surface modifier and electrons localized on the Ti02... 

MO) method used to evaluate ps and with the type of species to which the expressions are applied, but it is usually found that QN - 10 QCn - (Their signs are probably opposed, QN being positive while QCN is negative on account of the qualitative arguments to be advanced later (see Section III,A,1, for example), Eq. (3) may be approximated to Eq. (5) for pyridine and comparable systems ... 

The utility of CIDEP in photochemistry was greatly enhanced when it was realized (131) that the radical-pair mechanism is not the exclusive spin polarization mechanism. Initial triplet spin polarization produced by the different intersystem-crossing rates to the excited triplet sublevels can be "transferred" to radicals formed by the photochemical reaction of the polarized triplet. 

Regarding the relative intensities of the observed spin polarization mechanisms, it is also important to note that the [3-hyperfine interactions in these radicals are conformationally modulated, and this process can also quench RPM polarization. In a qualitative way, we can consider the modulation process to be a relaxation mechanism that exchanges magnetization between different nuclear spin orientations. Since these different orientations can have opposite phases of RPM polarization, the exchange of emissive and absorptive lines can cancel the intensity of the transitions. 

An excellent introduction to this topic is provided by La Mar. (40) This review clearly distinguishes between the three possible spin-transfer mechanisms, namely ligand (L) -> metal (M) a-spin transfer, L -> M 7t-spin transfer, and M L 7r-spin transfer. In addition to these direct spin delocalization mechanisms, where a fraction of unpaired electron density originally associated with the metal is actually placed into an MO that includes the ligand to some extent, there are indirect spin delocalization or spin polarization mechanisms. These involve essentially a redistribution of electron spin in an MO otherwise... 

More recently, Marcellus et al. (47) have questioned the usefulness of dividing spin density distributions into direct spin delocalization and spin polarization mechanisms. Both are non-observables and not uniquely defined in most calculations. The authors show that, if both unrestricted and restricted HF INDO calculations are performed, the spin densities due to spin delocalization Pain) and spin polarization Pp(n) may be precisely defined respectively as ... 

The low values exhibited by imidoyls (1.20-1.85 G) could be accounted for through a spin polarization mechanism that induces negative spin density at the nitrogen, somewhat balancing the positive spin density resulting from resonance effect. In subsequent papers [5d,f], Roberts used isonitriles as the source of various a-heteroatom-substituted imidoyl radicals and calculated some rate constants for both the radical addition steps and the -fragmentation processes (Scheme 4, routes... 

While each of the previous examples illustrated just one of the electron spin polarization mechanisms, the spectra of many systems involve polarizations from multiple mechanisms or a change in mechanism with delay time. 

Fig. 1.11 Electron spin polarization mechanism for hyperfine interaction...

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