Magnetic hyperfine interactions mechanism

Photolysis of ketones in micelles with simultaneous application of an external magnetic field permits a C isotope enrichment. (Cf. Section 6.1.5.5.) This is the case because C nuclei have a magnetic moment and thus accelerate the spin inversion by the hyperfine interaction mechanism. (Cf. Example 4.9.) Due to the more efficient recombination of radicals containing C, the initial product formed after photolysis in a back reaction is C enriched (TUrro et al., 1980b). 

The effects of electronic relaxation on the magnetic hyperfine interaction have already been briefly alluded to and will now be discussed in more detail. It was seen in Section 3.5 that the hyperfine field is usually generated by the polarising effects of unpaired electron spins. The direction of the field will be related to that of the resultant electronic spin of the atom. This spin direction is not invariant but can alter or flip after a period of time by one of several mechanisms this is the relaxation phenomenon. 

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. 

By this statement we mean that a-Pr does not order magnetically via the normal RKKY mechanism which governs the other lanthanides. a-Pr does order, however, at 40 mK via a hyperfine interaction between the 4f electrons and the nuclei (Bjerrum-Moiler et al. 1982). 

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