Electronic spectra actinides

More recently, the CASSCF/CASPT2 method with spin-orbit coupling has been applied to a number of problems in actinide chemistry. Some recent examples are the electronic spectrum of U02 [38], the electronic structure of PhUUPh [62], the diactinides Ac2, Th2, Pa2, and U2 [36, 37], etc. Some of this work has recently been reviewed [63]. 

The present method to study heavy element compounds in new and our experience is so far limited to atoms and some small molecules. It has the virtue that we can now use the machinery of CASSCF/CASPT2 for the entire periodic system. The method has been tested for all alkaline, alkaline earth, main group, transition metal atoms and in addition for some of the lanthanides and actinides. The results are promising. One example It has recently been possible to assign the electronic spectrum of the UO2 molecule (more than 150 electronic levels were computed) [33]. A drawback is that for the heaviest elements one has to include a large number of electronic states in order to fully account for the effects of spin-orbit coupling. 

A. S. P. Gomes, C. R. Jacob, F. Real, L. Visscher, and V. Vallet, Towards systematically improvahle models for actinides in condensed phase the electronic spectrum of uranyl in CS2UO2CI4 as a test case, Phys. Chem. Chem. Phys. 15,15153 (2013). 

The first attempts to record the Bk(IV) solution absorption spectrum were hindered by the presence of cerium impurities (92). The positions of the Bk(IV) absorption bands, superimposed on the strong Ce(IV) bands, suggested the assignment of Sf7 for the electronic configuration of Bk(IV), in agreement with the actinide hypothesis. 

Electronic absorption bands in the spectrum of PrCls (aq) reproduced with permission from S.A. Cotton, Lanthanides and Actinides, Macmillan (1991) p. 30. 

Absorption and Fluorescence Spectra. The absorption spectra of actinide and lanthanide ions in aqueous solution and in crystalline form contain narrow bands in the visible, near-ultraviolet, and near-infrared regions of the spectrum (13,14,17,24). Much evidence indicates that these bands arise from electronic transitions within the 4f and 5/shells in which the 4f and bf configurations are preserved in the upper and lower states for a particular ion. 

The first stable actinide complex of carbon monoxide was prepared by reaction of (Me3SiC5H4)3U (Cp sU) with CO in solution or in the solid state to produce Cp 3U(CO), which reversibly dissociates CO. Cp 3U(CO) exhibits a carbonyl stretch in the IR spectrum at 1976 cm, or approximately 170 cm lower than the uco for free carbon monoxide. This is taken as an indication of uranium-to-carbonyl jr-back bonding see Back Bonding). Electronic stracture calculations on the model complex CpsUCO indicated a significant U 5f-CO 2n... 

For thorium there are only estimates of the corresponding potential. An early estimate, of -2.4 V, was based on a relation between this quantity and the frequency of the first electron transfer absorption band in the UV spectrum of an aqueous thorium perchlorate solution (9). However, the spectral measurements did not quite reach the absorption maximum, and the necessary extrapolation introduced some uncertainty. Another value, -3.6 V, was based on the RESPET treatment of J0rgensen (10,11). The adjustable parameters in the RESPET equation were fixed using experimental values for other actinide elements (12). This method yields a value of -0.69 V for U(IV)/(III). Another rather simple method correlates this potential with the number of 5/"electrons for the element and gives -3.41 V for thorium and -0.54 V for uranium (13). A more sophisticated estimate (14), using a method proposed by Nugent et al. (12) (described later), gave -3.8 V for thorium. 

The aim of the present work is to perform a detailed theoretical study of the electronic structures of actinyl nitrates. Relativistic effects are remarkable in the electronic structure and chemical bonding of heavy atoms such as actinide elements[6j. In our previous study, we applied the relativistic discrete variational Dirac-Fock-Slater(DV-DFS) method to study of the electronic structure of uranyl nitrate dihydrate[7]. The accuracy of the DV-DFS method was demonstrate by its ability to reproduce the uranyl nitrate dihydrate experimental X-ray photoelectron spectrum. 

The first measurement of the temperature dependence of an optical line width in an actinide system, Np + in LaC, was recently completed (47). The fluorescence transitions at 671.4 and 677.2 nm were studied from 10 to 200 K. The low temperature limit for the line width of the 677.2 nm transition is 16.5 GHz and is a measure of the width of the first excited crystal-field level of the ground manifold. The 671.4 nm transition has a line width of 0.55 GHz at 10 K. Its temperature dependence is described in terms of an effective three-level scheme for the excited manifold. The parameters are comparable to those found for Pr + in LaF. Further comparison depends upon the details of the phonon spectrum and the electronic states. At low temperatures, the residual width of the 671.4 nm transition was limited by the laser line width. This is consistent with the very narrow line widths observed in Pr +. Additional detailed studies of this type and proper contrast and comparison between lanthanides and actinides may provide the additional information needed to describe the electron-phonon and electron-ligand interactions of the actinides. 

In view of the position of Cm in the actinide series, numerous experiments have been made to ascertain if Cm has only the +3 state in solution no evidence for a lower state has been found. Concerning the +4 state, the potential of the Cm4+/Cm3+ couple must be greater than that of Am44/ Am3+, which is 2.6 to 2.9 V, so that solutions of Cm4+ must be unstable. When CmF4, prepared by dry fluorination of CmF3, is treated with 15M CsF at 0°, a pale yellow solution is obtained which appears to contain Cm4+ as a fluoro complex. The solution exists for only an hour or so at 10° owing to reduction by the effects of a-radiation its spectrum resembles that of the iso-electronic Am34 ion. 

As a second case, we study the 5/ and 5f 6d manifolds of l/ -doped Cs2ZrCl6, with two open-shell electrons [58]. Now, the manifolds contain a large number of electronic states and the general problems that will be faced all over the actinide series appear here. In particular, MS-CASPT2 methods will be used for dynamic correlation instead of demanding variational Cl based methods such as ACPF [82,83]. The calculations suggest several reassignments of the 5f-> 5f experimental spectrum [84,85], predict the 5f 6d levels, and support the... 

Actinide spectra reflect the characteristic features of the 5/ orbitals which can be considered as both containing the optically active electrons and belonging to the core of filled shells. The electronic transition spectra of actinide ions in solution are dominated by the structure of the / levels and transitions within the / shell. Free-atom spectra provide more information about the interactions between the 5/ and the valence electrons. The emission spectra of the free actinide atoms have an enormous number of lines. In the uranium spectrum, about 100,000 lines have been measured, from which about 2500 lines have been assigned. 

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