Observation and investigation of the uranyl tetrafluoride dianion (UO2F42−) and its solvation complexes with water and acetonitrile

Abstract

Bare uranyl tetrafluoride (UO₂F₄²⁻) and its solvation complexes by one and two water or acetonitrile molecules have been observed in the gas phase using electrospray ionization and investigated by photoelectron spectroscopy and ab initio calculations. The isolated UO₂F₄²⁻ dianion is found to be electronically stable with an adiabatic electron binding energy of 1.10 ± 0.05 eV and a repulsive Coulomb barrier of ∼2 eV. Photoelectron spectra of UO₂F₄²⁻ display congested features due to detachment from U–O bonding orbitals and F 2p lone pairs. Solvated complexes by H₂O and CH₃CN, UO₂F₄(H₂O)_n²⁻ and UO₂F₄(CH₃CN)_n²⁻ (n = 1, 2), are also observed and their photoelectron spectra are similar to those of the bare UO₂F₄²⁻ dianion, suggesting that the solvent molecules are coordinated to the outer sphere of UO₂F₄²⁻ with relatively weak interactions between the solvent molecules and the dianion core. Both DFT and CCSD(T) calculations are performed on UO₂F₄²⁻ and its solvated species to understand the electronic structure of the dianion core and solute–solvent interactions. The strong U–F interactions with partial (d–p)π bonding are shown to weaken the UO bonds in the [OUO]²⁺ unit. Each H atom in the water molecules forms a H-bond to a F atom in the equatorial plane of UO₂F₄²⁻, while each CH₃CN molecule forms three H-bonds to two F ligands and one axial oxygen.