Photoelectron spectroscopy and the electronic structure of the uranyl tetrachloride dianion: UO2C(4(2-)

The uranyl tetrachloride dianion (UO(2)Cl(4)(2-)) is observed in the gas phase using electrospray ionization and investigated by photoelectron spectroscopy and relativistic quantum chemical calculations. Photoelectron spectra of UO(2)Cl(4)(2-) are obtained at various photon energies and congested spectral features are observed. The free UO(2)Cl(4)(2-) dianion is found to be highly stable with an adiabatic electron binding energy of 2.40 eV. Ab initio calculations are carried out and used to interpret the photoelectron spectra and elucidate the electronic structure of UO(2)Cl(4)(2-). The calculations show that the frontier molecular orbitals in UO(2)Cl(4)(2-) are dominated by the ligand Cl 3p orbitals, while the U-O bonding orbitals are much more stable. The electronic structure of UO(2)Cl(4)(2-) is compared with that of the recently reported UO(2)F(4)(2-) [P. D. Dau, J. Su, H. T. Liu, J. B. Liu, D. L. Huang, J. Li, and L. S. Wang, Chem. Sci. 3 1137 (2012)]. The electron binding energy of UO(2)Cl(4)(2-) is found to be 1.3 eV greater than that of UO(2)F(4)(2-). The differences in the electronic stability and electronic structure between UO(2)Cl(4)(2-) and UO(2)F(4)(2-) are discussed.