Relativistic Quantum Calculations to Understand the Nature of f-Orbitals and Chemical Bonding of Actinides with Organic Ligands

The nuclear waste problem is one of the main interests of the rare earth and actinide elements chemistry. Actinides are at the frontier of current theoretical methods due to the need to consider relativistic effects and approximations to the Dirac equation. Here, we employ four-component relativistic quantum calculations and scalar approximations to understand the nature of f-orbitals in the chemical bonding of actinides to organic ligands. We studied the relativistic quantum structure of an isostructural family made of Plutonium (Pu), Americium (Am), Californium (Cf), and Berkelium (Bk) atoms with the redox-active model ligand; DOPO (2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate). Crystallographic structures were available to validate our calculations for all mentioned elements except for Cf. State-of-the-art relativistic calculations were performed at different levels of theory to investigate their electronic structure: 1) the zeroth order regular approximation (ZORA) in the hybrid density functional theory (DFT) and 2) the four-component Dirac equation with the Dirac-Hartree-Fock (DHF) and Levy-Leblond (LL) Hamiltonians. We show that DFT-ZORA could be an efficient first approximation to predict and investigate the geometry and electronic properties of actinides that are difficult to synthesize or characterize, and that the higher level of theory, the 4-component DHF, gives closer results to experiments than DFT-ZORA. To the best of our knowledge, this is the first time that such kind of large actinide compounds are studied with highly accurate four-component methods. Finally, we show for the first time, how the participation of 5f atomic orbitals (AO) from actinides on the frontier molecular orbitals (MO) is strongly affected by relativistic effects.