Lanthanide shift rule

As a general rule the c.t. bands shift to lower energies with increasing oxidation state, whereas Rydberg transitions (such as 4f- -5d transitions) shift to higher energies. It may, therefore, be expected that the lowest absorption bands of the tetravalent lanthanide ions will be due c. t. transitions and those of the divalent lanthanide ions to Af- -5d transitions. 

Fluorine substitution in nucleosides and nucleic bases resulted in stronger interaction between lanthanides and ligands as compared with unsubstituted nucleosides and nucleic bases. The analysis of the fine structure of the bands, based upon the selection rules for the symmetry allowed transitions enabled the understanding of ligand field symmetry in Pr(III) and Nd(III) complexes of fluorinated nucleosides and nucleic bases. The stepwise complexation of Pr(III) Nd(III) was studied by the red shift of the absorption band which indicated a decrease in Ln-O distance in the complex due to the substitution of water by the fluorinated nucleoside ligand. 

Following the initial experiments by Kaiser, Axe (1964) extended the theories of Judd (1962) and Ofelt (1962) to calculate selection rules and intensities of two-photon transitions in solids doped with lanthanides. The theoretical predictions with later extensions by Bader and Gold (1968) laid largely untested as an hiatus occurred in the experimental studies in this area, the emphasis having shifted in this period to the observation of (4f) states buried in allowed (5d) or conduction bands. Interest has since returned following the work of Degenais (1981), Downer et al. (1982) and Down and Bivas (1983) on Gd " in LaFj. As noted in fig. 20, these workers observed a number of violations of the selection rules developed by Axe in both the intensity and polarization dependence and were able to trace the descrepancies to various approximations necessary in the... 

The trivalent state of americium is the stable aqueous oxidation state. Although americium is the homolog of europium, the Am " radius (0.975 A) is closer to that of Nd " " (radius 0.983 A) [76]. It is a convenient rule of thumb that the radii of the light lanthanide ions are nearly identical to the radii of the corresponding actinides shifted three elements to the right in the periodic table, e.g. r(La " ) a r(U ). In some early purification schemes, fission-product promethium accompanied americium. Am(iii) is precipitated by hydroxide, fluoride, phosphate, and oxalate ions from aqueous solution. 

Reactions.—A new method for the determination of absolute configuration in chiral secondary alcohols is based on the changes in chemical shifts observed in their n.m.r. spectra on glycoside formation. Characteristic shift changes for the glycosidic carbon of the sugar, and for C-a and C-/3 of the alcohol, have been summarized as rules. Measurement of lanthanide-induced shifts for the CF3 resonance in the n.m.r. spectra of a-methoxy-a-trifluoromethylphenyl-acetate (MTPA) esters of secondary alcohols has been reported as an... 

The structural picture is better clarified for the dichlorides (see Drake and Rosenblatt 1979 and references therein). Scandium dichloride is known to be linear (Dooh) as indicated by matrix IR isotope shifts and selection rules, while YCI2 is predicted to be bent (C2v configuration). The known lanthanide dichlorides SmCb, EuCl2 and YbCl2 are bent (C2v) and the Cl-R-Cl angles have been determined as 130 , 135 and 140 , respectively. Table 7 lists the vibrational frequencies of the dichlorides of Sm, Eu and Yb assigned on the basis of bent C2v geometry. 

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