Actinyl Ions in Cs2UO2Cl4

Abstract

The electronic energy levels of the uranyl ion (UO22+) and the neptunyl ion (NpO22+) in the crystalline environment of Cs2UO2Cl4 are studied theoretically and compared with the spectroscopic work of Denning and co-workers. A layered-cluster computational method is used. The valence electrons of the actinyl ion and the nearest-neighbor chloride ions are treated explicitly, the closest cesium ions are replaced by all-electron core potentials, and all ions further away are replaced by point charges. The cluster is approximately spherical overall and contains 1873 ions. For the electrons treated explicitly, we use relativistic quantum chemical theory, including relativistic effective core potentials, corresponding spin−orbit operators, and spin−orbit, graphical unitary group configuration interaction. The effects of the crystalline environment on bond distances, vibrational frequencies, excitation energies, energy splittings, and wave function character are examined. Shifts are generally more accurate than absolute values, and the electron correlation treatment is generally the limiting factor in the accuracy.