Neptunyl Compounds Research Articles

The crystal and coordination chemistry of neptunium in all its oxidation states: An expanded structural hierarchy of neptunium compounds

Structures of 388 neptunium-bearing crystal structures are sorted into an expanded structural hierarchy and examined. The majority of structures contain Np(V) (149), Np(VI) (99), or Np(IV) (91) whereas structures containing Np(II), Np(III), Np(VII), or more than one oxidation state are rare. Coordination geometry of Np(V) and Np(VI) compounds is dominated by the approximately linear neptunyl cation. Actinyl–actinyl interactions (AAIs) are most prevalent in Np(V) compounds and are observed in 48% of these extended structures. Prismatic coordination geometries are commonly observed in Np(IV) compounds. Np(III) organometallic complexes display a variety of coordination modes. Coordination chemistry of Np(VII) compounds is dominated by the tetra-oxido core. Updated average bond length values are provided for Np(IV), Np(V), Np(VI), and Np(VII) compounds and updated bond valence parameters for Np(IV) are reported. A graphical notation system for neptunyl structures coordinated by organic ligands was developed to facilitate discussion and identify similarities between neptunyl compounds coordinated by oxyanions and neptunyl compounds coordinated by organic ligands.

Electronic Transitions and Vibronic Coupling in Neptunyl Compounds

The low-lying electronic transitions of the neptunyl (NpO(2)(2+)) ion are characterized as either charge transfer (CT) or intra- 5f. Comparison of these classes of electronic transitions reveals significantly different photophysical properties, especially in vibronic coupling. An empirical model developed for analyses of uranyl CT vibronic transitions is used here to simulate the absorption (excitation) spectra of neptunyl in two compounds of different chemical compositions and structural symmetries. Analyses reveal that CT vibronic coupling in neptunyl has the same characteristics as that in typical uranyl analogues. The primary profile of the CT spectra is similar for neptunyl respectively with respect to chloride- and oxide-neptunium bonding interactions. On the other hand, vibronic coupling to the CT transitions is significantly different from that of f-f transitions, even within a given neptunyl compound. Electronic energy levels, vibronic coupling strength, and frequencies of various vibration modes were evaluated for transitions to the excited states of different origins in the region from 8000 cm(-1) to 21000 cm(-1) for two neptunyl compounds.

NEPTUNYL COMPOUNDS: POLYHEDRON GEOMETRIES, BOND-VALENCE PARAMETERS, AND STRUCTURAL HIERARCHY

The structures of 59 inorganic Np 5+ and Np 6+ neptunyl compounds are examined and placed within a structural hierarchy, with special attention to the relationships of these structures with U 6+ uranyl compounds. Forty-three Np 5+ neptunyl compounds containing square, pentagonal and hexagonal bipyramids have structural units consisting of isolated polyhedra (2), finite clusters of polyhedra (1), chains of polyhedra (12), sheets of polyhedra (16), and frameworks of polyhedra (12). Cation–cation interactions occur in 18 of these, resulting in a dramatic departure of their topologies from those of U 6+ uranyl compounds. Compounds with cation–cation interactions have structural units consisting of chains (4), sheets (4) and frameworks (10) of polyhedra. Those Np 5+ compounds that lack cation–cation interactions exhibit topologies that are similar or identical to those of U 6+ compounds. Sixteen Np 6+ neptunyl compounds have structural units consisting of isolated polyhedra (1), finite clusters of polyhedra (3), chains of polyhedra (4), and sheets (8) of polyhedra. None of these structures exhibit cation–cation interactions, and their topologies are similar to U 6+ uranyl compounds. Geometries of Np 5+ and Np 6+ polyhedra are examined, and updated bond-valence parameters, R o = 2.035 and b = 0.422, are provided for Np 5+ .

Identification of actinide molecule complexes: A new vibrational spectroscopic approach at the free-electron laser facility FELBE

Photothermal beam deflection (PTBD) spectroscopy using a free-electron laser (FEL) as a coherent pulsed pump source was applied for recording infrared spectra of actinide molecule complexes. We demonstrate that reliable spectra of samples containing uranyl and neptunyl compounds can be obtained in the mid-infrared region which was verified by conventional FT-IR spectroscopy. Since photothermal techniques are generally capable of detecting very low absorptions we tried to evaluate the minimum content of actinide ions which can still be detected by our setup of the PTBD experiment. It was found that the limit is obviously given by the background absorption of the hygroscopic KBr matrix of the samples which is originated by residual water. Furthermore, we present an overview about the technical equipment of the FEL-laboratory which is suitable for spectroscopic investigations in actinide research.

Mössbauer spectroscopic studies of 237Np in aqueous solutions

We applied Mössbauer spectroscopy to 237Np in aqueous solutions to study chemical states. A sample solution was adjusted to 50 mg/ml of Np(V) concentration in 0.6 M nitric acid solution. This solution was put into original designed containers for Mössbauer spectroscopy. Mössbauer spectra were measured in the transmission geometry at 11 K using cryostat, after containers with a sample solution was mounted to a sample holder and soaked into liquid nitrogen to freeze quickly. Mössbauer spectrum was observed 16 absorption lines with quadruple splitting and magnetic splitting. The value of isomer shift of Np(V) in 0.6 M nitric acid was -17.4(1) mm/s at 11 K. Isomer shift value reflects valence and coordination number. It was found that Np(V) had 7 coordination numbers in 0.6 M nitric acid solution from values of isomer shift that had been measured for neptunyl compounds.

Mössbauer spectroscopy of actinides: Some applications to solid state chemistry

Applications of Mössbauer spectroscopy to the study of electronic structure properties of non-conducting compounds of neptunium are reviewed. First a brief introduction into the basics of Mössbauer spectroscopy and hyperfine interactions is given. This is followed by a discussion of isomer shift data. Nevt, paramagnetic hyperfine splittings and the influence of spin dynamics on the Mössbauer spectrum are presented together with a few pertinent evamples. A discussion of some recent results on neptunyl compounds follows. Finally some remarks are given concerning heptavalent neptunium and the charge states of neptunium incorporated in glasses.