Her, L.J.L.; (2008) Theoretical studies of actinyl bonding. Doctoral thesis , University of London.

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Abstract

This thesis involves the computational study of uranium, neptunium, plutonium, and americium complexes in aqueous and non-aqueous solution. It seeks answers to specific experimental questions, to provide additional information to experiments, and to make predictions that experimentalists can use to design or abstain from new experiments. The work mainly uses density functional theory, as this method shows good scaling with system size. This is important because actinides have a large number of electrons, and the ligands in this work are often very large. The family of compounds with the formula U02(H2O) (OH)m 2 ,7 ( +w=5) are studied, to investigate how the changing equatorial ligand field affects the uranyl ion as hydroxide ligands replace water ligands. The investigation involves uranyl stretching vibrations, orbital analysis, charge analysis, and bond orders. I evaluate how solvent models affect the geometry and uranyl stretching vibrations. The cis and trans isomers of the U02Cl2(Cy3PNH)2 and U02Cl2(Cy3PO)2 (Cy = cyclohexyl) exist in equilibrium, even though one expects the bulky phosphinimine and phosphine oxide ligands to show large repulsion in a cis configuration. It is unknown experimentally whether the trans or the cis isomer is the major species. N-donor ligands displace O-donor ligands, when added to solution of U02Cl2(Cy3PO)2, i.e. the N-donor ligands form stronger bonds to uranium than the O-donor ligands. I investigate which isomer is the major species, and explain why the cis isomer exists at all. The origin of the stronger N-donor ligand bonds is studied by orbital analysis, energy decomposition, and electron densities. This study is extended to include all halide ligands, and uranium is substituted by neptunium, plutonium, and americium. I use the concepts of electron localisation and electron density differences on the systems above, to further study the actinyl axial and equatorial bonding in greater detail. I compare this analysis with the results from more traditional methods, e.g. charge analysis. I also investigate if it is possible to form stable c/s-uranyl compounds and neptunyl complexes with cation-cation interactions.

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