Ligand hyperfine structure

Figure 2.1 Ligand hyperfine structure in the ESR spectrum of Na2[(Ir, Pt)Cl6].6H20. (Reproduced with permission from Proc. R. Soc., London, Ser. A, 1953, 219, 526.)...

In the ESR spectra of adsorbed oxovanadium(IV) ions on minerals, Information on the nature of the adsorbed species is obtained from the g-values and V hyperfine coupling constants, but ligand hyperfine structure is seldom, if ever, observed. With ENDOR much smaller hyperfine splittings can be observed than with ESR and it is possible to measure hyperfine coupling from nuclear spins in... 

In a second experiment,ENDOR measurements were performed in the optically populated excited p5/2> Es/2 state of Tm " in Cap2, using the same apparatus. The ENDOR transitions were monitored via the circular polarization of the fluorescence. The authors obtained the ligand hyperfine structure constants A, = 4.83 (3) MHz and Ap = 3.59 (3) MHz of the first shell of fluorine neighbors, thus providing the first ENDOR results of an optically excited state of an impurity center. 

Table II I4I, 149-162) consists of a summary of 9-factors, D values and hyperfine coupling constants observed for ions of the first transition series. A molecular orbital (MO) treatment of the metal ion and ligand orbitals has been discussed by Stevens 163) and Owen 164) to account for covalent bonding and resulting hyperfine structure from hgands of transition element ions. Expressions derived for g-factors and hyperfine coupling constants from a MO treatment allow an estimation of the amount of charge transfer of metal electrons to ligand orbitals. Owen 164) has given a MO treatment of Cr +, Ni++ and Cu++ assuming no t bonding.

The use of nitric add as a solvent for ESR spectra once again highlights the problem of dissodation or partial dissodation of the heterocyclic N ligands. Thus, dissolution of [Ag(bipy)2]S2Og in a minimum quantity of concentrated nitric add produced spectra which, on the basis of the observed nitrogen hyperfine structure, can only be assigned to ds-Ag(bipy)X2-type species.499... 

The hyperfine-structure from nuclear magnetic moments on the electron spin resonance curve was first interpreted by Owen and Stevens in the case of IrClg-. There is no doubt that this gives a perfect qualitative proof for the delocalization of the partly filled shell. However, it is less clear whether there is a simple equivalence between the ligand nuclear influence and b in eq. (19). The point is that the partly filled shell has to be orthogonal, in a very complicated way, on all the previously filled shells such as Is and 2s of the X atoms. 

Since the Ti /H2C>2 reduction is an important system for producing OH radicals and has been used for many studies involving alcohols, it is perhaps interesting to note some of the ESR properties of the Ti+++ ion. The aqueous Ti+++ ion is believed to be coordinated as Ti(H2O)g and has no observable ESR spectrum because of the very short relaxation time. However, if the symmetry is reduced to tetragonal or lower, the orbital momentum should be completely quenched and a narrow ESR line is expected. This phenomenon has been observed in water-alcohol solutions (157). Recently Bolton and coworkers (158) have further observed some proton hyperfine structure from the water ligands of the aqueous Ti+++ complex in a 20% butyl alcohol-water solution. There has been a standing controversy in the interpretation of the spectra detected in the Ti(Il I), Ti(IV)-H2C>2 system (159-162). 

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