Aqueous uranyl complexes 1. Raman spectroscopic study of the hydrolysis of uranyl(VI) in solutions of trifluoromethanesulfonic acid and/or tetramethylammonium hydroxide at 25°C and 0.1 MPa

Raman spectra have been used to identify and characterize aqueous hydroxouranyl(VI) complexes from 0.0038 to 0.647M at pH from 0.24 to 14.96 adjusted witheither HCF3SO3 and/or (CH3)4NOH under ambient conditions. In acidic media(0.24 ≤ pH ≤ 5.63), the existence of four species UO2+2,(UO2)2(OH)3+,(UO2)2(OH)2+2, and (UO2)3(OH)+5 was confirmed. At high uranium concentrations(ΣU ≥ 0.1M) and in strongly acidic solutions (pH ≤ 1.94), one additional weakband was observed at 883±1 cm−1. This band was assumed torepresent thespecies UO2+2 with a reduced hydration number.In neutral and basic solutions(5.63 ≤ pH ≤ 14.96), five complexes were postulated: (UO2)3(OH)−7,(UO2)3(OH)2−8,(UO2)3(OH)4−10,(UO2)3(OH)5−11, andUO2(OH)2−4, based on theassigned symmetrical stretching frequencies of the UO2 group in each complex.(UO2)3(OH)−7 is the dominant species over mostof the pH range (4.53–12.78).The stability ranges of the other trinuclear species are:(UO2)3(OH)2−8 (10.97 ≤pH ≤ 13.83), (UO2)3(OH)4−10 (10.97 ≤ pH ≤ 13.85) and (UO2)3(OH)5−11(12.53 ≤pH ≤ 14.10), which were identified for the first time. Finally, the monomericuranate anion OU2(OH)2−4 dominates in highly basic solution (12.48 ≤ pH ≤14.96). The linear correlation between the symmetrical vibrational frequency v1of the linear O = U = O entity and the average number \(\overline n\) of hydroxide ligandscoordinated to each uranium atom in a given species has been reaffirmed andexpanded:\(v_1 ({\text{cm}}^{{\text{ - 1}}} ) = - 22X\overline n + 870\)The v1 correlation was also used to predict the vibration frequencies of theundetected monomers UO2(OH)+, UO2(OH)o2,UO2(OH)−3 at 848±2, 826±2, and804±2 cm±1, respectively. Characteristic band areas for eachuranyl hydrolyzedspecies were determined by Raman spectra decomposition and their hydrolysisquotients log Q, were calculated. Structures of the four triuranylspecies are proposed.