Raman spectrometric studies of actinide(V) and -(VI) complexes in aqueous sodium carbonate solution and of solid sodium actinide(V) carbonate compounds

Raman spectra of all the actinide(V) (except protactinium (Pa)) and actinide(VI) complexes in 2 M(sodium carbonate Na2CO3) solutions have been obtained. A resonance Raman effect was observed for neptunium(Np)(VI), plutonium(Pu)(VI), and americium(Am)(VI) and found to be related to the position of the charge-transfer bands observed in the corresponding electronic spectra and to the formal potential of actinide(VI)/actinide(V) couples in carbonate solution. No resonance effect was observed in the Raman spectrum of uranium (U(VI)). The symmetric stretching frequency (nu1) of the MO22+ group was shifted in carbonate solutions as compared to acidic noncomplexing solutions and decreased regularly with an increase in the atomic number of the actinide considered. In contrast, the nu1 frequencies of the MO2+ group for actinide(V) species in carbonate solutions did not vary as the atomic number of the actinide increased. In comparison to the v1 frequencies in acidic solutions, in carbonate solution a small negative shift was observed for nu1 of Np(V) and a positive shift was obtained for nu1 of Am(V). The Raman spectra of Na3MO2(CO3)2 solid compounds were obtained for M = Np, Pu, and Am. The nu1 frequencies of the MO2+ ions decreased linearly with increasing atomic number of the actinide and were higher than the corresponding values for aqueous MO2+ ions. The differences in behavior observed between actinide(V) and -(VI) species in noncompexing media and in carbonate media may be related to possible hydrogen bonding between the oxygen of the actinide(V) oxycation and water molecules. 12 figures, 3 tables