Ligand hyperfine interactions

FIGURE 5.2 A schematic model of multiple X Y interactions. Black dots are unpaired electrons the central, big black dot is the point of EPR observation. Straight lines are interactions a single straight line symbolizes the electronic Zeeman interaction S B double lines represent central and ligand hyperfine interactions S I triple lines are zero-field interactions S S between electrons (i) around a single metal (ii) at different centers within a molecule and (iii) at centers in different molecules. 

The example of nitrogen lines in the spectrum of cobalamin points to the necessity of also writing out resonance conditions for the presence of ligand hyperfine interaction. In general we have ... 

The spin Hamiltonian used to model the deuterium ligand hyperfine interaction consisted of nuclear Zeeman, electron-nuclear hyperfine and nuclear quadrupole terms. 

Moment reductions have been measured for NiO (56) and for KNiFa (82) by powder diffraction i) and ligand hyperfine interactions in KNiFs (83) and KMgFs (84) by NMR and ESR, and recently in i O-doped MgO by ENDOR (77). The results are given in Table 3. As already mentioned, until the determination of fa =8.5% in MgO, it seemed clear from both the neutron and the resonance data that for this divalent ion, oxides showed similar covalency to fiuorides, with a hgand-to-metal transfer of approximately 0.2e [Eq. (2.23)] (Mn2+, but not Fe3+ data could be similarly interpreted). 

The chemical behavior of the trivalent rare earths, the low magnetic ordering temperatures of most rare earth compounds with unfilled 4/ shells ), and the ligand hyperfine interactions observed in spin resonance measurements ) all indicate predominantly ionic behavior. This is presumably the result of the shielding of the 4/ electrons from the chemical environment by the 5s 5p shell. This shielding is also reflected in the narrow-line optical spectra of the trivalent... 

A number of other small contributions to the ligand hyperfine interaction have been discussed by Marshall (458, 487). These are, however, usually very small, and any estimated contributions from such terms are generally no larger than the errors in the hyperfine coupling itself. 

The theory of LHFI with regard to covalency has been well discussed by Owen and Thornley< 25> js only briefly mentioned here. Ligand hyperfine interactions are in principle observable when ligand nuclei have a nonzero nuclear spin (I), this being a sensitive probe of the distribution of spin density. The majority of experiments have, in fact, been concerned with F, and it is unfortunate that the most abundant oxygen isotope, 0 has 1 = 0, although results are now being obtained for transition metal ions in crystals enriched with 170 (I = 5/2)( 4.65). 

We have shown earlier [11] that the direct calculation of (ft9>)-pairs is also applicable to systems with a rhombic g- and coaxial A-tensor, the latter arising from a metal or strong ligand hyperfine interaction already resolved in EPR. Presently,... 

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