The interaction of uranyl ion with acetic, glycolic, malic, tartaric, tricarballylic, and citric acids in aqueous solution has been investigated by means of IR and 13C NMR spectroscopy. The complex formation is reflected in the IR frequency shifts for the COO-stretching vibrations (vas(COO) and vs(COO)) and the asymmetric O=U=O stretching mode (v3). The possible coordination structures of COO groups to the uranyl ion in the individual systems are discussed in terms of the IR frequencies of the vas(COO) and vs(COO) modes. The model calculation of the vas(COO) and vs(COO) frequencies based on a normal-coordinate treatment has indicated that the carboxylate coordination in the uranyl acetate and tricarballylate complexes is assigned to a bidentate structure in which both the two oxygen atoms in a given COO group take part in the coordination and that in the uranyl glycolate, lactate, malate, tartrate, and citrate complexes only one of the two oxygen atoms in a given carboxylate of the individual ligands coordinates to the uranyl ion. The carboxylate 13C NMR signals for all the systems examined have experienced significant shifts going from the individual free ligands to the complex species, which suggests the coordination of the carboxylate groups to the uranyl ion. Consideration of the alcoholic 13C NMR signals of the uranyl complexes involving glycolate, lactate, malate, tartrate, and citrate has indicated that the uranyl ion interacts with the alcoholic oxygen as well as the carboxylate oxygen of the individual ligands. The presence of some uranyl complex species, in which the alcoholic oxygen in a given -COH group coordinates to uranyl ion with dissociation of protons, was revealed by the observation of an unusually large lower magnetic field shift of the alcoholic carbon signal. The possible structures of the main uranyl complex species present in aqueous solution, compatible with the IR and 13C NMR evidence, are proposed and discussed.
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