Effects of the first hydration sphere and the bulk solvent on the spectra of the f2 isoelectronic actinide compounds: U4+, NpO2+, and PuO22+

Abstract

The electronic spectra of the 5f² isoelectronic actinide compounds U⁴⁺, NpO₂⁺, and PuO₂²⁺ have been investigated theoretically both in gas phase and in solution. In the latter case the solvent was modelled by a saturated first hydration sphere, five water molecules for NpO₂⁺, and PuO₂²⁺ and eight for U⁴⁺, and a continuum model describing the remaining solvent. The transition energies and oscillator strengths were obtained at the spin–orbit level using the relativistic wave function based multi-configuration methods CASPT2 (complete active space with second-order perturbation theory) and MRCI + DC (Davidson corrected multi-reference configuration interaction), followed by a spin–orbit CI based on a dressed effective spin–orbit Hamiltonian. This study is an attempt to contribute to an enhanced understanding of the electronic structure of tetravalent actinide ions and actinyl(V) and (VI) ions. The spin–orbit MRCI and spin–orbit CASPT2 transitions energies have been compared for the bare ions, leading us to the conclusion that the spin–orbit CASPT2 approach is reasonably accurate and can be used with confidence for the calculation of the hydrated species. The first hydration sphere and the bulk solvent lift degeneracies, but the effect on the transition energies is fairly small for the two actinyl ions, while it is larger, up to several thousands of wave numbers for U⁴⁺. The calculations allowed us to make assignments of the experimentally observed absorption spectra for all species. The computed transition energies and intensities compared favourably with experiment.