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

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 similar to 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)(n)(2-) and UO2F4(CH3CN)(n)(2-) (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)pi bonding are shown to weaken the U=O bonds in the [O=U=O](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.