Energy levels, transitions actinides

It will be noted that the transitions are often broader than those found in the spectra of lanthanide complexes - and indeed the later actinides, see Section 12.2.4. The 5f energy levels are more sensitive to coordination number than are the corresponding levels in the lanthanides since there are bigger crystal-field effects, one sees pronounced differences between the spectra of 6-coordinate [UCle] and of U" +(aq) (Figure 12.5), leading to the conclusion that the uranium(iv) aqua ion was not six coordinate (most recent EX-AFS results suggest a value of 9 or 10, see Table 11.1). Figure 12.6 displays another example of the difference in spectra between similar complexes of different coordination number. 

We have calculated sets of theoretical energy levels for the trivalent actinides and lanthanides and correlated these levels with transitions observed in the solution absorption spectra of these elements. Using the eigenvectors resulting from this energy level calculation, we have computed the theoretical matrix elements required to account for the observed band intensities in the two series of elements. The extent to which the theoretical calculations can be correlated with experimental results has been discussed, and some applications for the intensity relationships are pointed out. 

The two rows beneath the main body of the periodic table are the lanthanides (atomic numbers 58 to 71) and the actinides (atomic numbers 90 to 103). These two series are called inner transition elements because their last electron occupies inner-level 4/orbitals in the sixth period and the 5/orbitals in the seventh period. As with the d-level transition elements, the energies of sublevels in the inner transition elements are so close that electrons can move back and forth between them. This results in variable oxidation numbers, but the most common oxidation number for all of these elements is 3+. 

The virtues of the current scheme are relatively reliable predictions of the energy level positions, effective parameters that vary systematically across a series, and wave functions that may be utilized for additional calculations. The prediction of energy levels has aided the experimental study of new systems such as Gd3+ in CaF2 (15). The systematic variation of parameters across a series has been used to estimate parameters for the initial analysis of an ion. The properly admixed wave functions will improve the transition probability analysis of the actinides. 

In order to understand the photochemical reactions of metal complexes at the molecular level, it is necessary to know both the number and the energy levels of the spectroscopic states of the complex. The first step in developing a state model is to know the coordination number and structure of the complex about the metal center. For complexes of the lanthanide and actinide ions the coordination number is commonly 8 or 9, but for transition metal complexes a coordination number of 6 is that most frequently observed. 

This chapter deals with the electronic properties of isolated actinide atoms and ions, observed in the vapor phase at low density. The free atoms have all or most of the valence electrons present, and the spectra are due essentially to changes in the quantum numbers of the valence electrons. This is in contrast to the spectra of actinides in crystals or in solution, where the spectra are largely due to transitions within the 5f shell. In both cases, the energy level structure is dominated by the structure of the Sf shell, but in different ways. In crystals, the actinide ions are exposed to the electric field of the surrounding ions, which produces a Stark effect on the levels. The magnitude of the effect is relatively small because the field has high symmetry and, moreover, the Sf electrons are shielded from it by the 6s and... 

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