The absorption spectra of NpO_2~+ species in aqueous solution are investigated theoretically and experimentally, and the spectrum of NpO_2~(2+) species is investigated theoretically. The spectrum of NpO_2~+ in perchloric acid solution was taken from 350 to 1350 nm. Peak positions and optical densities are reported with overall uncertainties of 0.3 nm and 3%, respectively. A more precise value for the extinction coefficient of the most intense line is reported (398 ± 4 M~(-1) cm~(-1) for the 980.2 nm line). The intensities and positions of the electronic transitions of these actinyl complexes are computed from relativistic quantum chemical theory involving relativistic effective core potentials, corresponding spin-orbit operators, and spin-orbit, graphical unitary group configuration interaction. Because all of the low-lying electronic states for the isolated actinyl ions have the same parity, the equatorial ligands must break the inversion symmetry. Thus, model calculations on NpO_2~+ with one, three, and five chloride ligands were carried out; the five-ligand spectrum was quite similar to experimental solution spectra, whereas the one-ligand and three-ligand spectra were not. Calculations on NpO_2(H_2O)_5~+ were then made in order to provide a close comparison with experimental results. Similar calculations on NpO_2(H_2O)_5~(2+) were also carried out but were hampered by the difficulty in doing sufficiently extensive calculations to determine the ground electronic state with the ligands present. Comparisons were made nevertheless, using both of the candidates for ground state. A simplified crystal-field theory is developed to show how the necessary symmetry-breaking orbital mixing, 5fφ with 6dδ, occurs selectively with 5-fold coordination.
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