Paramagnetism lanthanide ions

Local Structure of the Eu2+ Impurity. From the experimental perspective, the doping of lanthanide ions into solid state materials can be probed by different instrumental technics such as nuclear magnetic resonance (NMR),44 extended X-ray absorption fine structure (EXAFS),45,46 or electron paramagnetic resonance (EPR),47 which instead of giving a direct clue of the local geometry offers only data that can be corroborated to it. From the theoretical point of view,... 

At the beginning of this century slow magnetic relaxation in the paramagnetic phase of a ID system was reported for a Co2+ nitronyl-nitroxide chain [58]. The dynamics was under many respects very similar to that of SMMs and these ID systems were later named SCM to underline the analogies [9]. Since then an intense research activity, though not as spread out as for SMMs, has been devoted to SCMs. The interested reader can find extensive literature on the subject, including exhaustive reviews and a book chapter [59-63]. The aim here is to provide some basic concepts of the phenomenon and to introduce some selected examples of SCMs based on lanthanide ions. 

Fig. 15 Magnetization curves for paramagnetic species with various J = 5-values. These data represent a landmark in that the magnetization of a hydrocarbon molecule [15 m = 4] surpassed for the first time those of transition-metal and lanthanide ions, represented here by NH4Fe(S04)2-12Hj0 and Gd2(S04)3-8H20, respectively.

A compound that is able to influence the relaxation times of water protons has to be paramagnetic. In the Periodic System paramagnetic ions are to be found amongst the transition metals and the rare earth metals (lanthanides). However, it was well known, that the free ions of heavy metals are toxic. Lanthanide ions form soluble complexes with ligands such as phospholipids, amino acids and proteins that are present in plasma. The liver and the skeleton are the major sites of accumulation of free metal ions. Uptake in the liver is mediated by the hepa-tocytes [2]. 

Axial symmetry according to Bleaney s approach is maintained in polymetallic lanthanide complexes when the metals lie on the molecular threefold or fourfold axes. For n magnetically non-coupled lanthanide ions packed along the symmetry axis, contact and pseudo-contact contributions can be considered as additive and the original model-free equation (eq. (47)) is transformed into eq. (61) in which the sum runs over the n paramagnetic centres, each being located at the origin of its own reference frame associated with a specific set of axial coordinates O 1 and r" (the z axis corresponds to the molecular symmetry axis, fig. 54),... 

Thirdly, related plots according to eq. (68) involving H4 clearly exhibit two straight lines, one for the large (Ce-Tb) and one for the small (Er-Yb) lanthanide ions with an abrupt transition between them (fig. 57b). This behaviour contrasts with that observed for Hl-3,6,7 and apparently leads to opposite conclusions in which two different structural arrangements should be invoked. However, both metallic centres significantly contribute to the paramagnetic shift... 

In order to characterize the active site structure of Ca ATPase from sarcoplasmic reticulum, we have employed Gd + as a paramagnetic probe of this system in a series of NMR and EPR investigations. Gadolinium and several other lanthanide ions have been used in recent years to characterize Ca + (and in some cases Mg2+) binding sites on proteins and enzymes using a variety of techniques, including water proton nuclear relaxation rate measurements (35,36,37), fluorescence (38) and electron spin resonance (39). In particular Dwek and Richards (35) as well as Cottam and his coworkers (36,37) have employed a series of nuclear relaxation measurements of both metal-bound water protons and substrate nuclei to characterize the interaction of Gd + with several enzyme systems. 

The nature of the interaction between the lanthanide ions in the triple-deckers was investigated using hetero-dinuclear complexes composed of a diamagnetic Ym and a paramagnetic trivalent lanthanide ion (Fig. 11), PcYPcLnPc and PcLnPcYPc (abbreviated as [Y, Ln] and [Ln, Y], Pc = dianion of 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine) [47-50]. 

Assuming that the relaxation of a given proton is due to the paramagnetism of the lanthanide ions then for the elements other than gadolinium the Solomon-Bloembergen equations for the water relaxation rates, 1/Tj or 1/T2, reduce to... 

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