Electronic spectra and excited states of neptunyl and its [NpO2Cl4]2- complex.

Electronic states and spectra of NpO(2)(2+) and NpO(2)Cl(4)(2-) with a Np 5f(1) ground-state configuration, related to low-lying 5f-5f and ligand-to-metal charge-transfer (CT) transitions, are investigated, using restricted-active-space perturbation theory (RASPT2) with spin-orbit coupling. Restrictions on the antibonding orbital occupations have little influence on the 5f-5f transition energies, but an important impact on the CT states with an open bonding orbital shell. The present calculations provide significant improvement over previous literature results. The assignment of the experimental electronic spectra of Cs(2)NpO(2)Cl(4) is refined, based on our calculations of NpO(2)Cl(4)(2-). Assignments on the basis of bare NpO(2)(2+) are less reliable, since the equatorial Cl ligands perturb the excited-state energies considerably. Calculated changes of the Np-O bond lengths are in agreement with the observed short symmetric-stretching progressions in the f-f spectra and longer progressions in the CT spectra of neptunyl. A possible luminescence spectrum of the lowest quartet CT state is predicted.