Spectroscopic investigations of the interaction between uranyl(VI) and the cellulose degradation products $\alpha$-isosaccharic acid and acetate

In this thesis the interaction between UO${}_2^{+2}$and the cellulose degradation products acetate (AcO${}^{-}$) and α-isosaccharic acid ISA was investigated. Although acetate is formed only in very small amounts during the abiotic degradation of cellulose, it is of particular importance as a model for complex organic ligands with carboxyl groups in the context of studies on metal-ligand interaction. Accordingly, gaps should be closed regarding the spectroscopic characterization of the UO${}_2^{+2}$-AcO${}^{-}$ species. In the literature, it has only been described that the [UO${}_2$(AcO)]${}^{+}$ complex does not luminesce at room temperature and the occurrence of static and dynamic quenching has been demonstrated. The underlying quenching mechanism could not be identified in a follow-up study based on theoretical calculations. The experimental conditions were chosen in this work to capture the three UO${}_2^{+2}$-AcO${}^{-}$ complexes described in the literature. This was investigated using UV/VIS spectroscopy in parallel to the luminescence measurements. Static and time-resolved luminescence spectra were recorded at 20${}^{\circ }$C and at-125${}^{\circ }$C. At low temperatures an intensive luminescence of the [UO${}_2$(AcO)]${}^{+}$-complex could be detected. In order to find an explanation for quenching at room temperature, theoretical calculations were performed, whereby bond strengths as well as the charges of the individual atoms in the ground and lowest excited state were calculated. Thus it could be shown that the excitation of the UO${}_2^{+2}$ unit leads to a weakening of the UO${}_2^{+2}$-AcO${}^{-}$ bond. As a result, the complex can dissociate more easily in the excited state and thus the excitation energy of the UO${}_2^{+2}$ unit can be emitted without radiation. However, the energy barrier of this dynamic process cannot be overcome at low temperatures, which explains the luminescence at -125${}^{\circ }$C. Thus, in the context of this thesis the quenching in the UO${}_2^{+2}$-AcO${}^{-}$-system could be described for the first time and thus the general understanding of photophysical processes in UO${}_2^{+2}$-systems could be extended. Within the scope of the luminescence measurements no third UO${}_2^{+2}$-AcO${}^{-}$ species could be found, therefore UV/VIS measurements were performed at the same UO${}_2^{+2}$ concentration (50 μM). The evaluation was performed under the assumption that either two or three UO${}_2^{+2}$-AcO${}^{-}$ complexes exist. The formation of only two complexes was sufficient to describe the measured UV/VIS spectra. However, this is not consistent with the general assumption in the literature that there are three complexes. Therefore, the potentiometry as the starting point of this assumption was analyzed in detail and discussed under consideration of the chemical properties of the UO${}_2^{+2}$ unit. Results from other studies point to the interaction of UO${}_2^{+2}$ with the protonated form of the ligand, acetic acid. However, this interaction is not taken into account in the evaluation of potentiometric data and thus in determining the average number of coordinating AcO${}^{-}$ molecules per UO${}_2^{+2}$ unit ($\overline{n}$). A consideration would lead to lower values for $\overline{n}$, which may explain the contradiction between the results of this work and those of otherauthors. In the second part of the work the interaction of UO${}_2^{+2}$ with the polyhydroxycarboxylic acid α-isosaccharic acid, the main product of cellulose degradation, was comprehensively characterized. Since the investigations were performed under acidic conditions, the conversion from the acid to the lactone form was an important aspect in this system. By means of NMR measurements it could be shown that the conversion of the open-chain to the lactone form of the ligand proceeds significantly faster in the presence of UO${}_2^{+2}$. It was also shown that this effect is due to the interaction between UO${}_2^{+2}$ and the protonated form of α isosaccharic acid. The theoretically calculated shift in electron density that occurs during the interaction of the two components provided the explanation for the catalytic activity of the UO${}_2^{+2}$ unit during lactone formation. The electron attracting effect reduces the electron density at the C atom of the carboxy group. As a result, the nucleophilic attack by the secondary alcohol on the activated C-atom can occur more easily, which is the speed determining step in this intramolecular reaction. The identification and characterization as well as the description of formation mechanisms of UO${}_2^{+2}$-ISA-complexes on a molecular level was performed by a multi-method approach. UV/VIS, ATR-FTIR, NMR, EXAFS and ESI-MS measurements in combination with theoretical calculations allowed a description of the system from the metal and ligand side.