title: Cation-cation interactions in [(UO2)2(OH)n](4-n) complexes.
abstract: The structures and bonding of gas-phase [(UO2)2(OH)n](4-n) (n = 2-6) complexes have been studied using density functional theory (DFT), MP2, and CCSD(T) methods with particular emphasis on ground state structures featuring cation-cation interactions (CCIs) between the uranyl groups. An interesting trend is observed in the stabilities of members of this series of complexes. The structures of [(UO2)2(OH)2](2+), [(UO2)2(OH)4], and [(UO2)2(OH)6](2-) featuring CCIs are found at higher energies (by 3-27 kcal/mol) in comparison to their conventional μ2-dihydroxo structures. In contrast, the CCI structures of [(UO2)2(OH)3](+) and [(UO2)2(OH)5](-) are respectively degenerate with and lower in energy than the structures with the μ2-dihydroxo format. The origin of this trend lies in the symmetry-based need to balance the coordination numbers and effective atomic charges of each uranium center. The calculated IR vibrational frequencies provide signature probes that can be used in differentiating the low-energy structures and in experimentally confirming the existence of the structures featuring CCIs.
authors.affiliation: Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory , Richland, Washington 99352, United States.
authors.forename: Samuel O
authors.initials: SO
authors.lastname: Odoh
authors.forename: Niranjan
authors.initials: N
authors.lastname: Govind
authors.forename: Georg
authors.initials: G
authors.lastname: Schreckenbach
authors.forename: Wibe A
authors.initials: WA
authors.lastname: de Jong