The trend in magnetic moments of the +3 lanthanides, in order of increasing atomic number, exhibits a double-humped curve, with a minimum at Sm 3+ and a maximum at Dy 3+ and Ho 3+, but trends in the EPR spectra are more complicated. Most lanthanide ions in biological systems or in molecules that are used to probe biological systems are in the +3 oxidation state, so the discussion in this chapter focuses on the +3 ions. In this chapter, we provide a brief overview of EPR of lanthanides with an emphasis on relaxation times to guide applications. There has been a resurgence of interest in recent years in EPR of lanthanides, especially the relaxation behavior, because of new applications, some of which are discussed in other chapters of this volume. EPR of the lanthanides is also discussed in ( Carlin 1986). 298 detailed treatment of Ce 3+, p.308 example of the non-Kramers Pr 3+ p.216, 312, 324). A fundamental source for theory of lanthanide EPR and experimental examples is Abragam and Bleaney 1970 (Kramers degeneracy, p.289 spin-orbit coupling p.283 hyperfine, p. 1994a, b) provide comprehensive reviews of the literature up to their publication dates. The books by ( Abragam and Bleaney 1970), ( Orton 1968), and ( Standley and Vaughan 1969), and the 1994 chapters by ( Bertini et al. Early electron paramagnetic resonance (EPR) studies of lanthanides focused on g values and relaxation mechanisms of Ln 3+ ions in high-symmetry crystalline lattices at low temperatures and on the effects of Ln 3+ ions on 1H relaxation in room-temperature aqueous solutions.
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