Actinyl cations are particularly important in actinide chemistry, e.g., the actinyl(V) cations NpO2+, PuO2+, and AmO2+ in aqueous media, as well as of course the U(VI) uranyl cation, UO22+. Gas phase actinide dioxide cations, AnO2+, have also been previously observed experimentally for An = Pa – Cm. The present work reports coupled cluster studies of two distinctly different intermolecular interactions involving actinide dioxide molecules and cations - (a) cation-cation complexes involving two actinyls with An=U – Cm and (b) the interaction of UO2q+ and NpO2q+ with either CO or N2. In each case new all-electron correlation consistent basis sets are used with the DKH3 scalar relativistic Hamiltonian at the CCSD(T) level of theory. Spin-orbit coupling is included at the 4-component Dirac-Hartree-Fock level. In actinyl CCI complexes the oxo-oxygen on one actinyl is weakly bonded to the central An of the other. The overall stability is the result of a competition between charge transfer stabilization and Coulombic repulsion, which is more subtle than the previously accepted Lewis acid-base explanation. All CCIs in the gas phase are characterized by strong coulombic repulsions upon approach of the two monomer cations, but in many cases a kinetically stable CCI results, with calculated local minima having well depths as large as ~15 kcal/mol, which is accompanied by an overall ~0.6e (NPA) charge transfer. The present gas phase results agree qualitatively with the CCI stabilities observed experimentally in condensed phases. Small molecule bonding and activation with actinides have been previously studied by experiment in the gas phase. In particular, actinide-containing molecules/cations have been shown to form thermodynamically stable bonds with small molecules and sometimes even activate them. In the present work UO20/1+/2+ and NpO20/1+/2+/3+ were chosen to interact with CO and N2. Accurate structures and bonding enthalpies will be reported with stabilities ranging from ~15 to 40 kcal/mol. C-O and N-N bond lengths in the complexes are lengthened relative to the separated molecules by up to ~0.06 Å, which can be regarded as a weak activation.

Coupled cluster studies of intermolecular interactions involving actinide dioxide cations: Cation-cation interactions (CCIs) and complexes with small molecules (CO and N2)

255th National Meeting (2018)

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