Actinide excited states

It is remarkable that the PW91 DFT method can reproduce the results of the much more expensive SOCI method so well for the actinide excited states. Given the huge difference in the amount of computer time demanded by these two methods, the application of the DFT method to excited states of other actinide compounds with /" ( > 1) configurations promises to be a challenging venture (because of the problem of state multiplets) but potentially a very fruitful one. 

Accuracy of Electron Correlation Methods for Actinide Excited States WFT and DFT Methods... 

Magnetic measurements of PuFi, between 4.2 and 300 K are consistent at high temperatures with older measurements (10-12). The large temperature dependent diamagnetism observed earlier was not found. Up to 100 K the susceptibility is nearly temperature independent with a value of X ip 2940 x 10-6 emu. The Curie-Weiss behavior near room temperature indicates population of a higher first excited state. The structure of PuFi, is isomorphic with that of UFi, (13), where two different sets of actinide atoms are 8-fold coordinated by a distorted antiprism. 

That magnetic measurements often raise more problems than they solve, is demonstrated for the indicated compound. We prepared a series of [ (C2H5N] i,An(NSC) e compounds (An = Th, U, Np, Pu) with cubic coordination of the actinide ion. We derived a consistent interpretation of the magnetic and optical properties of the uranium and the neptunium compounds (6 ). In the case of Pu we expect an isolated T1 ground state and a first excited state at about 728 cm-1. To our surprise we found a magnetic ground state much more pronounced than in the case of the hexachloro-complex, Fig. 4. 

Studies of actinide photochemistry are always dominated by the reactions that photochemically reduce the uranyl, U(VI), species. Almost any UV-visible light will excite the uranyl species such that the long-lived, 10-lt seconds, excited-state species will react with most reductants, and the quantum yield for this reduction of UQ22+ to U02+ is very near unity (8). Because of the continued high level of interest in uranyl photochemistry and the similarities in the actinyl species, one wonders why aqueous plutonium photochemistry was not investigated earlier. 

Table 1. Ground state and first excited state configurations, and their energy difference, for lanthanides and actinides...

The preceding discussion of the relationships between excited state electronic structure and photochemical reactivity focused primarily upon coordination compounds containing cP or low-spin cP transition metals. These relationships are generally applicable, however, to complexes of other d transition elements, the lanthanides and the actinides. A brief survey of the photochemical reactions of these latter systems is presented below. 

The Absorption Spectra and Excited State Relaxation Properties of Lanthanide and Actinide Halide Vapor Complexes. I. ErCl3(AlCl3L, W.T. Camall, J.P. Hessler, H.R. Hoekstra, and C.W. Williams, J. Chem. Phys. 68, 4304-4309 (1978). 

For the description of the linear and nonlinear optical properties of metallotetrapyrroles, TDDFT methods have proven [133-148] to be an excellent alternative to conventional highly correlated ab initio methods, such as SAC-CI, STEOM-CC, and CASPT2, for which these systems still represent a severe computational challenge, especially when transition metals, lanthanides or actinides are involved. The few highly correlated ab initio calculations dealing with the excited state properties of metallotetrapyrroles that have appeared to date only concern magnesium and zinc porphyrins and porphyrazines [149-151]. Application of TDDFT methods to the electronic spectroscopy of a variety of metallotetrapyrroles, including homoleptic and heteroleptic sandwiches, will be illustrated in this section. 

The CASSCF/CASPT2 method has been designed to deal with quantum chemical situations, where the electronic structure is complex and not well described, even qualitatively, by single configurational methods. The method relies on the possibility to choose an active space of orbitals that can be used to construct a full Cl wave function that describes the system qualitatively correct. When this is possible, the method is capable of describing complex electronic structures quite accurately. Examples of such situations are found in excited states, in particular photochemical reactions that is the subject of this book, but also in transition metal, and actinide chemistry. 

Spectra involving only one / electron are simple, consisting of only a single transition 2F5/2 — 1Fm. For the f configuration (Cm3+ cf. Gd3+) the lowest excited state lies about 4 eV above the ground level, so that these ions show only charge-transfer absorption in the ultraviolet. Most actinide species have complicated spectra. 

Weiss behavior near room temperature indicates population of a higher first excited state. The structure of PuFi is isomorphic with that of UFif (13), where two different sets of actinide atoms are 8-fold coordinated by a distorted antiprism. 

Then, the conduction band structure of an actinide metal appears to be more complicated than that of a transition or rare earth metal because some 5/states are hybridized with the 6 d band. According to A. J. Freeman, for the hghter actinides up to Pu, the degree of the overlap between the 5/wave functions on neighbouring atoms is large thus the bandwidth, because of overlap and the hybridization with the 6d—l s bands, is noticeable. This seems to be in disagreement with the presence of R lines in the My and Mjy emission spectra. Indeed, the R lines involve 5/ states normally empty in the unperturbed metal, and information on the localization of only 5/excited states is obtained by their observation and not on that of 5/states situated below the Fermi level. 

The prospects for actinide lasers, based on available spectroscopic data, is definitely more limited. Although there are a few prospects for visible lasers, the presence of low-lying 6d and electron transfer states can cause intense excited-state absorption, thus limiting oscillation principally to the infrared. Strong ion-host interactions increase the probabilities for radiative and nonradiative transitions and must be carefully considered with respect to the overall operation and efficiency of any practical system. 

Ionization potentials of 6.1941(5) eV for uranium i and 6.2657(6) eV for neptunium ) have been derived from observed Rydberg series using laser techniques and the method described above. These are the most accurate ionization potentials available for actinide elements. Series converging to the first excited state and to the ground state of the ion were observed for both elements. In the case of neptunium, the presence of two series converging to limits 24 cm - - apart (see Fig. 6) helps to confirm the unpublished value. ) for the interval between the two lowest levels of neptunium. 

The use of autoionizing Rydberg levels converging to excited states of the ion to determine ionization potentials has been discussed above. If autoionization resonances as narrow as those found in gadolinium exist in the actinides, it should be possible to determine the isotope shifts and hfs of such features. (isotope shifts for actinides range up to 0.4 cm l per mass unit and odd atomic number actinides exhibit hfs with total widths of 4 to 6 cm l and hfs component spacing of 0.2 cm l or more for some transitions). 

Topics, which have formed the subjects of reviews this year, include the luminescence kinetics of metal complexes in solution, photochemical rearrangements of co-ordination compounds, photochromic complexes of heavy metals with diphenylthiocarbazone derivatives, the photochemistry of actinides, actinide separation processes, and light-induced electron-transfer reactions in solution and organized assemblies. A discussion has also appeared on assigning excited states in inorganic photochemistry. ... 

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