Relativistic Quantum Chemistry Applied to Actinides

Abstract

Of the many available computational approaches, density functional theory is the most widely used in studying actinide complexes. This is generally because it incorporates electron correlation effects and is computationally inexpensive for modestly sized compounds. The first chapter of this thesis is an introductory chapter in which some basic concepts of electronic structure theory are discussed. The rest of this thesis is a compilation of several studies of the structural and electronic properties of a range of actinide compounds using predominantly density functional theory. The performances of the basis set/relativistic components as well as the density functional component of theoretical calculations were examined in Chapters 2 and 3 respectively. In Chapters 4, 5, 6 and 7, the electronic structures and properties of actinide species in the environment were explored. The speciation of actinyl aquo-hydroxo species at increasing pH values were studied in Chapter 4. In Chapter 5, the structural and electronic properties of uranyl peroxo complexes with other environmentally important ligands were studied. The adsorption of uranyl complexes to geochemical surfaces was studied in Chapter 6. In addition, the mechanistic pathways to the reduction of these complexes on surfaces and alcohols were examined. In chapter 7, the complexes formed by the uranyl moiety with the aquo and fluoride ligands were studied in gas and aqueous phases. The interactions of uranyl pentafluoride with a protein were examined using a hybrid QM/MM approach. Overall these studies (Chapters 4, 5, 6 and 7) provided valuable insights into the speciation and reduction of actinide species in the environment. In Chapter 8, the properties of novel pentavalent uranium complexes were studied using density functional theory. These complexes have promising roles in the retardation of uranium, via U(VI)-U(IV) reduction, in the environments of nuclear storage repositories. In Chapter 9, the existence of cation-cation interactions in an hexavalent bis-uranyl hydroxo complex was examined using density functional theory and wavefunction methods. In Chapter 10, a summary of the works compiled in this thesis is presented. Future directions for work on the chemistry of actinide complexes were also included in this chapter.