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Title: Computational study of covalency and complexation in actinides using static and dynamic simulation and topological density analysis
Author: Kirker, I. D. J.
Awarding Body: University College London (University of London)
Current Institution: University College London (University of London)
Date of Award: 2013
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The separation of minor actinides such as americium and curium from other actinide and lanthanide-bearing components of used nuclear fuel is a necessary part of post-processing and recycling this fuel into storable components and new fuel material. Separation ratios can be optimised using a comprehensive understanding of the differences between these elements and their aqueous chemistry. This work uses computational simulation to investigate bonding behaviour and covalency differences between the actinides in the (III) and (IV) oxidation states for several ligand systems: first, actinide (III) tris-cyclopentadienyl compounds (\ce{AnCp_3}), comparing with similar work on actinide (IV) tetrakis-cyclopentadienyl compounds, and second, a group of related actinide (IV) betaketoiminate systems (An(ᴬʳNCRCHCRO)₄). Multiple methods are used and compared, including common wavefunction-based methods such as Mulliken and Natural Population Analysis, as well as topological density analysis methods based on the quantum theory of atoms in molecules (QTAIM). Common interpretations give misleading results, and QTAIM-based methods more clearly show the decreasing covalent interaction across the actinides. A study of the aqueous behaviour of plutonium (IV) follows, first obtaining molecular hydrate and hydroxide complex geometries, and then inserting two of these into a dynamic simulated solution. Molecular complex free energies are shown to produce results that are in agreement with experimentally determined coordination number, and matching with trends towards dehydration with increasing hydrolysis degree found in other metal hydroxide complexes. Dynamic simulation results suggest a thermal barrier to equilibration which may be an artifact of the method used. Molecular complexes are also inspected using QTAIM methods, which find an exponential relationship between many of the properties and bond length for the Pu-O bond, and suggest partial multiple bond character.
Supervisor: Kaltsoyannis, N. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available