JRP CALL information
Supported By

European Commission

Short description of the work
The structure and bonding in hydrated and anhydrous actinyl (AnO22+; An = U, Np, U) nitrate complexes was studied by a combination of experimental methods (XRD, IR, Raman) on crystal compounds and theoretical methods (DFT) on small and large clusters and periodic crystals. Complete assignment of the experimental vibrational frequencies for [AnO2(NO3)2(H2O)2]×x(H2O) (x = 0, 1, 4) and M[UO2(NO3)3] (M = NH4, K, Rb, Cs) was achieved and force field studies of these compounds was undertaken. In all isotypic series the An-O2NO bond distance increases but bond strength down the actinide series. The most important contribution to this is the decrease of the actinide ionic radii. It was also found that the local structure plays an important role for the actinyl An=O bond strength. In the hydrated compounds the An=O bond strength was found to be weakened by hydrogen bonds to water in the second shell, while in the anhydrous compounds it was rather the interactions with the monovalent cations of higher polarization ability that weakened the An=O bond.
The effect of hydrogen bonding to water molecules in the second shell was further studied by DFT on the [AnO2(NO3)2(H2O)2]×(H2O)n (n = 0, 2, 6, 10) clusters. The results showed that the An-OH2 bond strength depends on the composition of the second shell and on intra- and intermolecular hydrogen bonding. The participation of coordinated water molecules in hydrogen bonding leads to numerous chelate cycles, with a significant increase of An–OH2 bond strengths and a minor weakening of the An-O2NO bonds. It should be noted that some of the Pu(VI) complexes deviate slightly from the trend set by U(VI) and Np(VI). This is likely due to the poorly described multiconfigurational character of the wave function of Pu(VI).
A systematic DFT study on [AnO2(NO3)(H2O)3]+, [AnO2(NO3)2(H2O)2], and [AnO2(NO3)3]- complexes was also performed. The An–O2NO stretch force constant was found to decrease with increasing number of nitrate ions in the mono-, di-, and trintrate complexes. This is most likely due to a decrease of the bond strength arising from the interplay between the difference in the coordination strengths of the water and nitrate ligands and steric effects. In all three series the An-O2NO stretch force constant increased with decreasing actinide ionic radius from U(VI) to Pu(VI), which was most noticeable for the mononitrate complexes where the steric repulsion between the ligands in the equatorial plane is the smallest.
A most interesting result for all isostructural series studied is that the violation of Badger’s rule for the An=O bonds. According to this rule the force constant of the An=O bonds should increase on going from U to Pu as the interatomic distances decrease, but the reverse trend was observed, the force constants decreased, both from the experimental data and from the DFT calculations. This appears to be a general rule in actinyl(VI) complexes. This has been recognized in the literature but the mechanism behind the effect is not understood as yet. To gain better understanding about this mechanism is one of the aims in our next Talisman proposal.

Main visitor contact data
Name: Dr Mikhail Skripkin
Organisation: Saint-Petersburg State University

JRP Identification
JRP nr: TALI-C01-07
JRP title: Structure and bonding in actinyl nitrate complexes
JRP scope: Scope 1: Actinide separation chemistry

Visited Associated Pooled Facility
Visited APF during the stay: KIT-INE - Laboratories
Name of the APF Contact Person: Dr. Thorsten Schafer

Other APF and organisation involved in the JRP
Other organisations involved: Institute of Transuranium elements (Karlsruhe; Dr. Cristo Apostolidis, Dr. Olaf Walter)
Other APF involved in the project:

Description of the work done at the associated pooled facility
Start date of the stay: 7/16/2014
End date of the stay: 8/22/2014
Quantity of access: 41
Access Unit: Days

Other APF visitors of the JRP during the stay
Visitor 2: Mr. Artem Gorbunov (Saint-Petersburg State University)