Intensities in the spectra of actinyl ions

The absorption spectra of NpO2+ species in aqueous solution are investigate d theoretically and experimentally, and the spectrum of NpO22+ species is i nvestigated theoretically. The spectrum of NpO2+ in perchloric acid solutio n was taken from 350 to 1350 nm. Peak positions and optical densities are r eported with overall uncertainties of 0.3 nm and 3%, respectively. A more p recise value for the extinction coefficient of the most intense line is rep orted (398 +/- 4 M-1 cm(-1) for the 980.2 nm line). The intensities and pos itions of the electronic transitions of these actinyl complexes are compute d from relativistic quantum chemical theory involving relativistic effectiv e core potentials, corresponding spin-orbit operators, and spin-orbit, grap hical 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 calculati ons on NpO2+ with one, three, and five chloride ligands were carried out; t he five-ligand spectrum was quite similar to experimental solution spectra, whereas the one-ligand and three-ligand spectra were not. Calculations on NpO2(H2O)(5)(+) were then made in order to provide a close comparison with experimental results. Similar calculations on NpO2(H2O)(5)(2+) were also ca rried out but were hampered by the difficulty in doing sufficiently extensi ve calculations to determine the ground electronic state with the ligands p resent. Comparisons were made, nevertheless, using both of the candidates f or ground state. A simplified crystal-field theory is developed to show how the necessary symmetry-breaking orbital mixing, 5f phi with 6d delta, occu rs selectively with 5-fold coordination.