Theoretical Actinide Molecular Science

Interest in the chemistry of the early actinide elements (notably uranium through americium) usually results either from the nuclear waste problem or the unique chemistry of these elements that result from 5f contributions to bonding. Computational actinide chemistry provides one useful tool for studying these processes. Theoretical actinide chemistry is challenging because three principal axes of approximation have to be optimized. These are the model chemistry (the choice of approximate electron-electron correlation method and basis sets), the approximate relativistic method, and a method for modeling solvent (condensed phase) effects. In this Account, we arrange these approximations in a three-dimensional diagram, implying that they are relatively independent of each other. A fourth level of approximation concerns the choice of suitable models for situations too complex to treat in their entirety. We discuss test cases for each of these approximations. Gas-phase data for uranium fluorides and oxofluorides such as UF(6) and UO(2)F(2) show that GGA functionals provide accurate geometries and frequencies while hybrid density functional theory (DFT) functionals are superior for energetics. MP2 is seen to be somewhat erratic for this set of compounds, and CCSD(T) gives the most accurate results. Three different relativistic methods, small-core effective core potentials (SC-ECP), ZORA, and all-electron scalar, provide comparable results. The older large-core ECP (LC-ECP) approach is consistently worse and should not be used. We confirmed these conclusions through studies of the actinyl aquo complexes [AnO(2)(OH(2))(5)](n+), (An = U, Np, or Pu and n = 1 or 2) that are also used to test solvation models. As long as the first coordination sphere of the metal is included explicitly, continuum solvation models are reliable, and we found no clear advantage for the (costly) explicit treatment of the second coordination sphere. Spin-orbit effects must be included to reproduce the correct trend in An(VI)/An(V) reduction potentials. We propose a multistep mechanism for the experimentally observed oxygen exchange of UO(2)(2+) cations in highly alkaline solutions present in tank wastes. This process involves an equilibrium between [UO(2)(OH)(4)](2-) and [UO(2)(OH)(5)](3-), followed by formation of the stable [UO(3)(OH)(3)](3-) intermediate that forms from [UO(2)(OH)(5)](3-) through intramolecular water elimination. The [UO(3)(OH)(3)](3-) intermediate facilitates oxygen exchange through proton shuttling. We explain the experimentally observed stabilization of the pentavalent oxidation state of actinyl ions by macrocyclic ligands (such as 18-crown-6) as an effect of solvation: the large macrocycle screens the positive charge of the ion from the polarizable solvent. Alkyl-substituted isoamethyrin complexes are bent despite being aromatic because of steric factors, rather than fit/misfit criteria regarding the actinyl ion. By application of an efficient DFT code, actinide molecules with more than 100 atoms can now be studied routinely. "Real" chemical questions can be answered as long as we take great care to apply methods that are accurate with respect to the three axes of approximation described above. While the exclusive focus of this Account has been on the early actinide elements, these conclusions also apply elsewhere in the periodic table.

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