Structures and Relative Stabilities of An(IV)-DOTA Complexes from First-Principles and Ab Initio Calculations

The structures and relative stabilities of complexes of the DOTA ligand with tetravalent actinide ions An⁴⁺ (An = Th, Pa, U, Np, Pu) in the gas phase and aqueous solution have been studied by relativistic An pseudopotentials combined with gradient-corrected density functional theory BP86 as well as Møller-Plesset second-order perturbation theory MP2 and the approximate coupled cluster singles and doubles model CC2. The complexes are found to become more stable along the actinide series, and the squared antiprismatic conformation is found to be lower in energy than the twisted squared antiprismatic one. Covalent actinide-ligand bonding contributions are analyzed and found to arise mainly for the actinide-oxygen bonds, and to a lesser extent for the actinide-nitrogen bonds. The performance of 5f-in-core and 5f-in-valence An pseudopotentials is compared and deviations due to the averaging over electronic states arising from the An 5f subconfiguration are analyzed by performing configuration-averaged Hartree-Fock CAHF and subsequent complete active space configuration interaction CASSCI calculations, with and without inclusion of spin-orbit interactions.