Other lanthanides

The NMRD profiles show small and field independent relaxivity values for proton frequencies lower than 50 MHz (Fig. 5.57), thus indicating that xc is indeed provided by rs. The fitting made with Eqs. (3.32)-(3.37) of different sets of nuclear relaxation measurements provides the values for rv reported in Fig. 5.58. These very short values confirm an efficient Orbach relaxation 

Lanthanides have been used as substitutes for calcium in calcium binding proteins since the early days in NMR [155,156]. Remarkable information was obtained on systems such as Yb(III)-substituted parvalbumin (which contains a typical calcium binding site called EF-hand [157,158]) from ID spectroscopy alone [159-164], later complemented by 2D spectroscopy [165]. More recently, pseudocontact shifts and longitudinal proton relaxation times have been used to 

Analogously, the pseudocontact shifts were used to refine the solution structure of the Ce(III), Dy(III) and Yb(III) derivatives of monolanthanide-substituted Calbindin D9k. Since the three lanthanides span a wide range of magnetic anisotropies, the refinement was effective in shells from the metal of 5—15 A for Ce(III), 9-25 A for Yb(III), 13-40 A for Dy(III), as useful pseudocontact shifts were observed in these shells (Fig. 5.59) [167]. Therefore, by using different lanthanides it was possible to enlighten shells at variable distances from the metal itself. 

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Little is known of the toxicity of terbium. It should be handled with care as with other lanthanide elements. 

Extensive efforts have been made to develop catalyst systems to control the stereochemistry, addition site, and other properties of the final polymers. Among the most prominant ones are transition metal-based catalysts including Ziegler or Ziegler-Natta type catalysts. The metals most frequentiy studied are Ti (203,204), Mo (205), Co (206-208), Cr (206-208), Ni (209,210), V (205), Nd (211-215), and other lanthanides (216). Of these, Ti, Co, and Ni complexes have been used commercially. It has long been recognized that by varying the catalyst compositions, the trans/cis ratio for 1,4-additions can be controlled quite selectively (204). Catalysts have also been developed to control the ratio of 1,4- to 1,2-additions within the polymers (203). 

Bastnasite has been identified in various locations on several continents. The largest recognized deposit occurs mixed with monazite and iron ores in a complex mineralization at Baiyunebo in Inner MongoHa, China. The mineral is obtained as a by-product of the iron ore mining. The other commercially viable bastnasite source is the Mountain Pass, California deposit where the average Ln oxide content of the ore is ca 9%. This U.S. deposit is the only resource in the world that is minded solely for its content of cerium and other lanthanides. 

Production of Cerium Derivatives. Moderately pure (90—95%) cerium compounds can be made from rare-earth chloride through oxidation with, for example, hypochlorite to produce an iasoluble cerium hydrate. The other lanthanides remain ia solutioa. The hydrate, oa calciaatioa, coaverts to Ce02. 

The role of cerium in these lighting phosphors is not as the emitting atom but rather as the sensitizer. The initial step in the lighting process is the efficient absorption of the 254 nm emission Ce ", with broad absorption bands in the uv, is very suitable. This absorbed energy is then transferred to the sublattice within the crystalline phosphor eventually the activator ion is fed and emission results. Cerium, as a sensitizer ion, is compatible in crystal lattices with other lanthanide ions, such as Eu and Tb, the usual activator atoms. 

While there have been many non-isothermal studies of the decompositions of lanthanide oxalates, fewer detailed kinetic investigations have been reported. The anhydrous salts are difficult to prepare. La, Pr and Nd oxalates decompose [1097] to the oxide with intervention of a stable oxycarbonate, but no intermediate was detected during decomposition of the other lanthanide oxalates. The product CO disproportionates exten-... 

The a—time curves for the vacuum decomposition at 593—693 K of lanthanum oxalate [1098] are sigmoid. Following a short induction period (E = 164 kJ mole-1), the inflexion point occurred at a 0.15 and the Prout—Tompkins equation [eqn. (9)] was applied (E = 133 kJ mole-1). Young [29] has suggested, however, that a more appropriate analysis is that exponential behaviour [eqn. (8)] is followed by obedience to the contracting volume equation [eqn. (7), n = 3]. Similar kinetic characteristics were found [1098] for several other lanthanide oxalates and the sequence of relative stabilities established was Gd > Sm > Nd > La > Pr > Ce. The behaviour of europium(III) oxalate [1100] is exceptional in that Eu3+ is readily reduced... 

Lanthanum fluoride (and fluorides of some other lanthanides) has an unusual type of defect (see Section 6.3.2), namely Schottky defects of the molecular hole type (whole LaF3 molecules are missing at certain sites). Charge carriers (F ) are formed as the result of interaction of LaF3 with this hole, leading to dissociation with formation of LaF2+ and F . 

The yttrium, lanthanum and other lanthanide salts exploded after dehydration during heating to above 300°C. 

Many other lanthanide-based initiators have been shown to polymerize MMA, including lanthanocene amides,464 168 alkoxides,469 substituted indenyl and fluorenyl bivalent ytterbocenes,470,471 hexamethylphosphoric triamide thiolates,472 and allyl, azaallyl, and diazapentadienyl complexes.473... 

As described in Section 9.1.2.2.3, several lanthanocene alkyls are known to be ethylene polymerization catalysts.221,226-229 Both (188) and (190) have been reported to catalyze the block copolymerization of ethylene with MMA (as well as with other polar monomers including MA, EA and lactones).229 The reaction is only successful if the olefin is polymerized first reversing the order of monomer addition, i.e., polymerizing MMA first, then adding ethylene only affords PMMA homopolymer. In order to keep the PE block soluble the Mn of the prepolymer is restricted to <12,000. Several other lanthanide complexes have also been reported to catalyze the preparation of PE-b-PMMA,474 76 as well as the copolymer of MMA with higher olefins such as 1-hexene.477... 

Bis(ethylacetoacetonate)-lanthanide(III) alkoxides, represented by structure (314), also initiate the well-controlled ROP of CL.895 Mn increases linearly with conversion (with Mw/Mn<1.10 throughout), and increasing [M]0/[I]o- Kinetic analysis implies a first order dependence on the lanthanide initiator, consistent with a non-aggregated active site. Block copolymers with moderately narrow polydispersities (1.25-1.45) have also been prepared using these initiators. NMR spectroscopy confirms well-controlled block sequences suggesting that these initiators are less susceptible to transesteriflcation than other lanthanide alkoxides. Initiation occurs exclusively at the alkoxide bond, and the tris(ethylacetoacetonate) analogs are inactive under the same conditions. 

Several other lanthanide complexes have been tested for ROP activity with varying degrees of success. Some of these are summarized in Table 2. 

A deeper analysis [80] on this Cu - Dy chain showed that NN and NNN interactions are efficient above Tc and that inter spin-ladder ferromagnetic interaction is observed in this family of compounds [79-82]. Attempts were made to use the 3d tectons with other lanthanide precursors, such as hexanuclear lanthanide clusters. Regular chains were obtained with Dy-Cu ferromagnetic interaction but their close packing prevents observation of SCM behaviour [83]. 

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