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

Abstract

The nuclear waste problem is one of the main interests of the rare earth and actinide elements chemistry. Studies of Actinide-containing compounds are at the frontier of the applications of current theoretical methods due to the need to consider relativistic effects and approximations to the Dirac equation in them. Here, we employ four-component relativistic quantum calculations and scalar approximations to understand the contribution of f-type atomic orbitals in the chemical bonding of actinides (Ac) 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. In short, state-of-the-art relativistic calculations were performed at different levels of theory to investigate the relativistic effects and electron correlations on geometrical structures and bonding energies of $Ac$-DOPO$_3$ complexes ($Ac$=Pu, Am, Cf, Bk) : 1) the scalar relativistic zeroth order regular approximation (ZORA) within the hybrid density functional theory (DFT) and 2) the four-component Dirac equation with the Dirac-Hartree-Fock (4c-DHF) and L\'evy-Leblond (LL) Hamiltonians. We show that scalar DFT-ZORA could be used as an efficient theoretical approximation to first approximate the geometry and electronic properties of actinides which are difficult to synthesize or characterize; but knowing that the higher levels of theory, like the 4c-DHF, gives closer results to experiments than the scalar DFT-ZORA. We also performed spin-free calculations of geometric parameters for the Americium and Berkelium compounds.