Investigating Covalency in Uranyl and its Complexes via Exploration of Electronic Excited States

Armstrong, Sapphire and Kerridge, Andrew (2022) Investigating Covalency in Uranyl and its Complexes via Exploration of Electronic Excited States. PhD thesis, Lancaster University.

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Abstract

Herein, computational calculation has been used to study the electronic structure and bonding of uranyl ([UO2] 2+) and its complexes at both the ground and electronically excited state geometries. The optically accessible u!u triplet excitation, allowed via spin-orbit coupling, was investigated throughout. Density Functional Theory (DFT) was used for obtaining optimised geometries and corresponding excitation energies for the transitions of interest. Application of the density based analytical tool, Quantum Theory of Atoms in Molecules (QTAIM) on the generated structures enabled rationalisation of the bonding interactions within the complexes. Here, QTAIM analysis was utilised for the first time to probe an excited state structure of an f-element complex. This became significantly important as although trends between the ground and excited state electronic structures were similar, investigation of the excited state resulted in additional findings which would not have been established by investigation of the ground state alone. Investigation of the covalency in uranyl via symmetry-preserving excited states resulted in trends between the type of covalency dependence and the orbitals involved in the excitation to be established. The covalency in the U-Oyl bond was found to decrease upon equatorial complexation, with charge being transferred onto the uranyl oxygen centres highlighting an increase in the ionic nature of the U-Oyl bond. Investigation into the electronic structure and bonding of bent uranyl complexes enabled the design of theoretical complexes, which although found to not be synthetically viable did have a significant O-U-O bend (O-U-O angle 100). Intramolecular proton transfers within uranyl hydroxide analogues were investigated as an alternative method for obtaining cis uranyl. The energy barrier was lowered in the electronic excited state for all complexes, again highlighting the significance of considering the excited state. Throughout this work, the importance of excited state investigation is established, and these results present a promising starting point for further actinide covalency investigations.