A Uranyl Peroxide Dimer in the Gas Phase

The gas-phase uranyl peroxide dimer, [(UO₂)₂(O₂)(L)₂]²⁺ where L = 2,2'-trifluoroethylazanediyl)bis(N,N'-dimethylacetamide), was synthesized by electrospray ionization of a solution of UO₂²⁺ and L. Collision-induced dissociation of this dimer resulted in endothermic O atom elimination to give [(UO₂)₂(O)(L)₂]²⁺, which was found to spontaneously react with water via exothermic hydrolytic chemisorption to yield [(UO₂)₂(OH)₂(L)₂]²⁺. Density functional theory computations of the energies for the gas-phase reactions are in accord with observations. The structures of the observed uranyl dimer were computed, with that of the peroxide of particular interest, as a basis to evaluate the formation of condensed phase uranyl peroxides with bent structures. The computed dihedral angle in [(UO₂)₂(O₂)(L)₂]²⁺ is 145°, indicating a substantial deviation from the planar structure with a dihedral angle of 180°. Energies needed to induce bending in the most elementary gas-phase uranyl peroxide complex, [(UO₂)₂(O₂)]²⁺, were computed. It was found that bending from the lowest-energy planar structure to dihedral angles up to 140° required energies of <10 kJ/mol. The gas-phase results demonstrate the inherent stability of the uranyl peroxide moiety and support the notion that the uranyl-peroxide-uranyl structural unit is intrinsically planar, with only minor energy perturbations needed to form the bent structures found in studtite and uranyl peroxide nanostructures.