Beekmeyer, Reece; (2018) Applicability of the Restricted Active Space Self Consistent Field Methodology to problems in transition metal and f-element. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Computational investigations of transition metal and f-element complexes, with their inherent multiconfigurational nature, can present significant challenges to the computational chemist. In this thesis the applicability of the Restricted Active Space Self Consistent Field (RASSCF) methodology to tackle these problems is investigated. This is initially used to investigate poprhyrin ring complexes, upon which there has been a great deal of experimental and computational literature. Calculations on free base porphyrin, regular transition metal porphryin and irregular transition metal porphyrins have been completed using the most commonly used and popular computational method, density functional theory (DFT) and its time-dependent extension, TDDFT. Ground state and vertical excitation energies have been compared and contrasted between DFT/TDDFT and RASSCF. Discrepancies in results between the methods have been identified for the irregular porphryin complexes, particularly in the manganese porphyrin complex where different ground states are predicted with different DFT xc−functionals. The significant covalency exhibited by transition metal complexes makes the selection of appropriate active spaces highly challenging using current methodologies and so this work expanded to investigate how the RASSCF methodology performs in inorganic complexes where there is less interaction between the metal and ligand, such as in f-element complexes. This began with a study of the unusual covalency observed experimentally in cerium and uranium hexachlorides where the bonding was further investigated through the use of the Quantum Theory of Atoms in Molecules (QTAIM) methodology. This was followed by an investigation into the newly synthesised family of divalent actinide and lanthanide complexes with three cyclopentadiene ligands (Cp3) with the RASSCF methodology, the first of its kind using this methodology, again augmented with the use of the QTAIM methodology. Finally, having shown the effectiveness of the RASSCF methodology on f-element complexes this thesis returns to its initial theme with an investigation of lutetium texaphyrin - a f-element expanded porphyrin complex. Ultimately this thesis has demonstrated that the RASSCF methodology can be effectively applied to these systems to gain a deeper understanding of their electronic structure, although quantitatively accurate descriptions sometimes remain out of reach.

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