ELECTRONIC ABSORPTION-SPECTRA OF SOLID NEPTUNIUM(V) HYDROXIDE COMPOUNDS

Electronic spectra of Np(V) hydroxide compounds of general formula MNpO2(OH)2.xH2O (I) (M = Li-Cs) and M2NpO2(OH)3.xH2O (II) (M = Na-Cs) are recorded in the range 980-1100 nm on a SF-20 spectrophotometer as NaCl pellets pressed at 6.10(3) atm. In this spectral region, I has no absorption bands. Conversely, strong narrow absorption bands with epsilon about 200 liter/(mole-cm) and splitting of the principal f-f-transition for certain ones are observed for II. The peak maxima shift to longer wavelength relative to the band of hydrated NpO2+. The shift increases as the radius of the alkali metal decreases. The increase in the intensity is connected with a change of NpO2+ coordination number from 6 (or 4) to 5. The shift and splitting of the bands is explained by the nature of the ligand, the strength of bonds formed by them, and the generation of nonequivalent NpO2+ cations in the structures owing to their different orientation and degree of interaction through the "yl" oxygens with water molecules.