Actinide Fluorides

The periodic table of the elements, derived from numerous attempts in the 1800s to systematically examine the chemistry found in nature, has seen a stark evolution over the last century. The elements of the actinide series are the frontier of a radical discontinuity in periodicity, driven by a relativistic reorganization of the frontier orbitals. Characterization of the chemistry of these elements with simple model species enables detailed coordination between structural chemistry and theoretical characterization. Such examinations advance understanding of these relativistic shifts. With the ever-growing influence of these effects, periodic trends on the table the elements break down, and synthetic elements beyond uranium offer new opportunities to probe these divergent and nonlinear chemistries. The introductory chapter addresses the history of the synthesis and the initial chemical characterization of the transuranium elements, with a brief basic introduction given to discuss the reaction physics which will follow. These elements share a single common feature in that they all have large Z values, and thus have electronic structures that are significantly altered by both scalar relativistic effects and spin-orbit coupling. These effects scale nonlinearly with increasing Z and create unexpected deviations both across series and down groups of elements. The magnitude of these effects is large enough that orbital energies rearrange and mix in ways that complicate incomplete depictions that are based solely on electron repulsion. The second chapter discusses calculations with nuclear reaction codes to examine a potential production rate improvement that may be derived by super-heavy synthesis with reactions run in inverse kinematics. This result is particularly important for continuing the evolution of the periodic table, as increased production rates enable new spectroscopic analyses of electronic and nuclear structure that can, in future years, potentially rewrite our understanding of relativity at the most extreme scale heretofore produced. The third chapter discusses new structural, spectroscopic, and theoretical features of uranium fluorides that are relevant for fundamental uranium(IV) chemistry and actinide fluoride reactors. The simple system of tetraammonium octafluorouranate is employed to derive fundamental understanding of the uranium-fluorine interaction. The structure is composed of isolated molecules, enabling a clear look at the U4+ (f 2) species in the most ionic bonding environment possible, without the possibility of direct interactions or strong interactions through ligands between uranium centers. Characterization of single-crystals by x-ray diffraction, absorption spectroscopy, and magnetic analysis up to 45 T is interwoven with extensive theoretical treatment by CASSCF. The influence of different active spaces and representations of the structure is examined in the context of the experimental evidence. The Interacting Quantum Atoms method is used to examine the nature of the U-F bond, concluding that there is a non-negligible degree of covalent character (9% of the total bond energy) even with the most ionic simple anion of fluoride. Two new sodium uranium(IV) pentafluorides were synthesized from uranium dioxide, HF, and NaF under mild hydrothermal conditions. Although β-NaUF5·H2O crystallizes in the (lower) monoclinic crystal class, it possesses greater crystal lattice energy than the previously-known orthorhombic α-NaUF5·H2O. Trigonal β-NaUF5 possess significantly different bonding between [UF9]5- moieties than the α-phase, with higher symmetry and greater lattice energy than its orthorhombic structural isomer, which is most directly comparable in structure to Na3,4M2+/3+U6F30. Single-crystal absorption spectra of these compounds are reported and correlated. Simulated powder x-ray diffraction data are also reported and compared to address a (mis)identification of the NaUF5 series that dates back to the Manhattan Project. The final textual chapter extends the methods discussed in previous chapters to the actinide series, with new lithium plutonium fluorides analyzed and placed into the broader context of actinide fluorides, including a new evaluation of an analogous zirconium structure. The structure of Li4ZrF8 was refined from single crystal X-ray diffraction data. Alkali zirconium fluorides are important in nuclear-relevant technologies, and zirconium is commonly employed as an analogue for tetravalent f-block elements. This result is largely consistent with prior reports, but with significant improvements in precision. The similar reaction of 242PuO2 with HF and LiF under hydrothermal conditions results in the formation of Li4PuF8 and LiPuF5. These compounds were structurally characterized using single crystal X–ray diffraction and UV–vis–NIR absorption spectroscopy. The structure of Li4PuF8 consists of [PuF8]4‒ anions that adopt a non-bridged bicapped trigonal prismatic geometry with approximate C2v symmetry. In contrast, LiPuF5 forms a dense three–dimensional network constructed from [PuF9]5‒ units that are bridged by fluoride anions. The Pu4+ cations are found within tricapped trigonal prisms. Extensive theoretical analysis of electronic and bonding interactions is included with comparison between results derived from CASSCF at different levels of theory, QTAIM, IQA, NLMO, and WBO analyses. Covalent interactions in these compounds are examined and intra–molecular trends in covalent and electrostatic interactions are discussed.