JRP CALL information
Supported By

European Commission

Description of the work
This report summarizes the first part of the work performed within the TALISMAN project “Structure and bonding in actinyl nitrate complexes”, performed at INE during one month last summer. The remaining part of the project will be conducted in July – August 2014.

The project is devoted to studies on the structure and bonding in the hexavalent actinyl (An(VI)O2(2+), An = U, Np, Pu) compounds of [AnO2(H2O)2(NO3)2]×nH2O (n = 0, 1, 4), M[AnO2(NO3)3] (M = Cs, Rb, NH4) and [AnO2(H2O)5](ClO4)2. Important in this project is to gain knowledge about the factors influencing the bond strength of the actinyl(VI) ion, AnO2; such as the electronic structure of the actinide ion, the presence of nitrate and water ligands in the equatorial plane, and the hydrogen bonding between these ligands and the neighbouring water molecules.

 

During the first part of the project (summer 2013), [AnO2(H2O)2(NO3)2]×H2O (An = U, Np, Pu) complexes were synthesized, structurally characterized with single-crystal X-ray diffraction and investigated with infrared and Raman spectroscopies [1]. In addition, published vibrational frequencies for [UO2(H2O)2(NO3)2]×4H2O and [UO2(H2O)2(NO3)2] were also used [2-4]. This comprehensive data enabled us to perform a detailed force field analysis including most fundamental vibrational modes of the [AnO2(H2O)2(NO3)2] complexes. Along with these investigations, supporting DFT studies are presently being performed (see below). In the following, we present some key findings from the work performed in 2013 and outline the work planned for the second part of the project, which is to be conducted in July - August this year.

 

The Raman spectra of [AnO2(H2O)2(NO3)2]×H2O recorded at 77 K show numerous of well-resolved bands in the region below 1500 cm-1­­. The corresponding infrared spectra are also rather detailed. The most intense band in the Raman spectra is that associated with the symmetric stretching vibration of the actinyl entity, while in the infrared spectra the strongest band is that of the antisymmetric stretching vibration of the actinyl entity. It is interesting to note that the slight decrease in the An-O bond distance with increasing actinide atomic number on going from uranium to plutonium does not correspond to an increase in the frequency of the mentioned symmetric stretching vibration. Instead, this vibration decreases from about 875 for U to 835 cm-1­­ for Pu, whereas the antisymmetric vibration is nearly constant at 940-950 cm-1­­. This corresponds to a decrease in the An-O stretching force constant from about 730 (U) to 700 (Pu) Nm-1­­. This trend goes against Badger’s rule, which says that the force constant should increase with decreasing bond distance in analogous compounds and complexes [5]. To better understand this phenomenon we are currently performing DFT calculations of molecular clusters containing a central [AnO2(H2O)2(NO3)2] complex.

The N-O stretch force constant of the coordinated nitrate ions was found to be slightly smaller (on the average 6 %) compare to that of a non-coordinated nitrate ion. A similar effect was found for the O-H force constant of the water ligands. However, from the present data it is not possible to conclude which of the three actinide ions influences the N-O force constant most. On the other hand, we may conclude that the force constant of the O-N-O bending is smaller when the nitrate ion is coordinated to Pu compared to when it is coordinated to U.

To continue this project we are planning to perform infrared and Raman measurements on deuterated [AnO2(D2O)2(NO3)2] complexes this year. Upon deuteration the coupling between water librations and other modes may disappear. This, in addition to the fact that the weak bands associated with the An-O vibrational modes of the nitrate and water ligands shift slightly to lower wavenumbers, may allow us to unambiguously localize the An-O(ligand) frequencies.

The second part of the project is also devoted to synthesis and vibrational spectroscopic studies of the fully hydrated actinyl compounds and to actinyl trinitrate compounds, [AnO2(H2O)5](ClO4)2 and M[AnO2(NO3)3] (M = Cs, Rb, NH4), respectively. This will be followed by a force field study using normal coordinate analysis.

References and Notes:

[1] The results obtained in 2013 were partly presented as a poster ("Structure and spectroscopic evidence of hexavalent neptunyl and plutonyl mono- and dinitrate complexes in aqueous nitric acid") at the conference Actinides 2013, Karlsruhe (Germany), July 21-26, 2013; and as the paper "Structure and spectroscopy of hydrated neptunyl(VI) nitrate complexes", Dalton Transactions 2013, 42, 15275.

[2] P. K. Khulbe, et al. J. Phys. Chem. Solids 1992, 53, 639.

[3] P. K. Khulbe, et al. J. Raman Spectroscopy 1989, 20, 283.

[4] J. R. Ferraro, et al. J. Chem. Phys. 1966, 45, 550.

[5] R. M. Badger, J. Chem. Phys. 1934, 2, 128.

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:
Other APF involved in the project: KIT-INE - Laboratories

Description of the work done at the associated pooled facility
Start date of the stay: 7/16/2013
End date of the stay: 8/16/2013
Quantity of access: 36
Access Unit: Days

Other APF visitors of the JRP during the stay
Visitor 2: Mr. Artem Gorbunov (Saint-Petersburg State University)
Visitor 3:
Visitor 4:
Visitor 5: