Lanthanides, term

The methods listed thus far can be used for the reliable prediction of NMR chemical shifts for small organic compounds in the gas phase, which are often reasonably close to the liquid-phase results. Heavy elements, such as transition metals and lanthanides, present a much more dilficult problem. Mass defect and spin-coupling terms have been found to be significant for the description of the NMR shielding tensors for these elements. Since NMR is a nuclear effect, core potentials should not be used. 

Solid-State Lasers. Sohd-state lasers (37) use glassy or crystalline host materials containing some active species. The term soHd-state as used in connection with lasers does not imply semiconductors rather it appHes to soHd materials containing impurity ions. The impurity ions are typically ions of the transition metals, such as chromium, or ions of the rare-earth series, such as neodymium (see Lanthanides). Most often, the soHd material is in the form of a cylindrical rod with the ends poHshed flat and parallel, but a variety of other forms have been used, including slabs and cylindrical rods with the ends cut at Brewster s angle. 

The purity of the cerium-containing materials depends on the appHcation as indicated in Table 3, and purity can mean not only percentage of cerium content but also absence of unwanted components. For some uses, eg, gasoline production catalysts, the lanthanides are often used in the natural-ratio without separation and source Hterature for these appHcations often does not explicitly mention cerium. Conversely, particulady in ferrous metallurgy, cerium is often assumed to be synonymous with rare-earth or lanthanide and these terms are used somewhat interchangeably. 

The three series of elements arising from the filling of the 3d, 4d and 5d shells, and situated in the periodic table following the alkaline earth metals, are commonly described as transition elements , though this term is sometimes also extended to include the lanthanide and actinide (or inner transition) elements. They exhibit a number of characteristic properties which together distinguish them from other groups of elements ... 

To avoid this confusion, and because many of the elements are actually far from rare, the terms lanthanide , lanthanon and lanthanoid have been introduced. Even now, however, there is no general agreement about the position of La, i.e, whether the group is made up of the elements La to Lu or Ce to Lu. Throughout this chapter the term lanthanide and the general symbol, Ln, will be used to refer to the fourteen elements cerium to lutetium inclusive, the Group 3 elements, scandium, yttrium and lanthanum having already been dealt with in Chapter 20. 

Electronic absorption spectra are produced when electromagnetic radiation promotes the ions from their ground state to excited states. For the lanthanides the most common of such transitions involve excited states which are either components of the ground term or else belong to excited terms which arise from the same 4f" configuration as the ground term. In either case the transitions therefore involve only a redistribution of electrons within the 4f orbitals (i.e. f—>f transitions) and so are orbitally forbidden just like d—>d transitions. In the case of the latter the rule is partially relaxed by a mechanism which depends on the effect of the crystal field in distorting the symmetry of the metal ion. However, it has already been pointed out that crystal field effects are very much smaller in the case of ions and they... 

Again, however, this is strictly applicable only for free ions. Even though spin-orbit coupling is much less important for the first row of the d block, this formula provides a far less good approximation for d -block complexes than Eq. (5.6) does for lanthanide complexes. The reason is that the ground, and other, terms in these d complexes differ grossly from those of the corresponding free ion. These differences are one result of the crystal field. 

These have been calculated from Caro s spectroscopic analyses [35]. The ligands come from opposite ends of the nephelauxetic series, so for a lanthanide reaction, A rep(irteg) should be relatively large. Even so, although it proves to be the largest contributor to the overall change, AEqs and AEso are significant Quantitative analyses of claimed examples of the tetrad effect must take such terms into account... 

Instead of Bronsted acids, lanthanide triflates can be used to catalyze the reaction of indole with benzaldehyde (Eq. 7.7). The use of an ethanol/water system was found to be the best in terms of both yield and product isolation. The use of organic solvent such as chloroform resulted in oxidized byproducts.17... 

The only lanthanide of which there is no stable isotope — they all decompose with half-lives between 2.6 and 17.7 years. Strong beta-emitters that are used industrially as thickness gauges. Also suitable as an additive for fluorescent materials. Produced artificially in kg amounts and serves as an energy provider for satellites in radionucleide batteries. Tiny batteries are long-term energy sources for pacemakers. 

In their subsequent analysis Baker and Bleaney (ibidem) decided to ignore the last term on the assumption that gdl 3b hv. Although this is a reasonable approximation for lanthanide and actinide integer-spin ions doped in single crystals, it is not usually an acceptable assumption for the broad-line spectra from metalloproteins. Furthermore, the assumption of a A-distribution around zero (i.e., D 0 but all other zero-field interaction parameters are zero) is equally untenable for biomolecules. Therefore, we go for a later extension of the theory, based on a full Equation 12.9 and on (A) 0, for application to metalloproteins (Hagen 1982b). 

Rare earth elements are the general term for 15 kinds of lanthanide elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Py, Ho, Er, Tm, Yb, Lu) together with Sc and Y elements. They prefer trivalent states in the complex formation, though three elements (Eu, Sm, Yb) can assume tri- and divalent stateos and Ce a tri- or tetravalent state. Their ionic radii are fairly large (1.0-1.17 A) and their electronegativities are low (1.1-1.2). In fact, the former are much larger than those of... 

Table 1.2 Non-vanishing crystal field terms (Stevens formalism) for common lanthanide point symmetries.

Inspection of Equation 1.23 and consideration of the properties of 3-y and 6-j symbols confirm that only even A--values contribute to crystal field splitting. Further, it indicates that mixing between levels belonging to different / multiplets can only occur if terms with k site symmetry of the lanthanide, in much the same way as discussed above for the Stevens formalism. 

This method was proposed to study LF parameters of anionic bis(phthalocyan-inato)lanthanides, [TBA][LnPc2] (TBA, tetrabutylammonium). They present an idealized molecular symmetry Did, where the two equivalent Pc rings are placed parallel to each other with a skew angle of 45° (Figure 2.2). For this ideal symmetry, only LF terms with q = 0 do not vanish, and the LF Hamiltonian is... 

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