Charge transfer in uranyl(VI) halides [UO2X4]2- (X = F, Cl, Br, and I). A quantum chemical study of the absorption spectra.

The electronic spectra of uranyl(VI) coordinated with four equatorial halide ligands, [UO2X4]2- (X = F, Cl, Br, and I), have been calculated at the all-electron level using the multiconfigurational CASPT2 method, with spin-orbit coupling included through the variational-perturbational method. The halide-to-uranyl charge-transfer states were taken into account in the calculation by including ligand orbitals in the active space. In order to do that, it is assumed that the charge transfer takes place from only one of the four ligands. Two models, which in principle can describe this, were investigated: the first one makes use of a localizing technique and the second one replaces three ligands by ab initio model potentials (AIMPs). The basis set dependence was investigated by using two different basis sets for the halides, of triple-zeta and quadruple-zeta quality. The localization procedure turned out to be strongly basis set dependent, and the most stable results were obtained with ab initio model potentials. The ground state is a closed shell singlet state, and the first excitation is from the bonding sigma(u) orbital on uranyl to the nonbonding delta(u) orbitals, except for the [UO2I4]2- complex, where the first excited state has a mixed character of charge transfer from the I- and the sigma(u)(1)phi(u)(1) configuration. In [UO2F4]2- there is no charge transfer excitation below 50,000 cm(-1) , while in [UO2Cl4]2- it appears around 33,000 cm(-1) and in [UO2Br4]2- around 23,000 cm(-1) . A blueshift of the spectra, from F- to I-, is observed. The calculations compare reasonably well with available experimental results.