A comprehensive molecular analysis of a simple aqueous complexing system. U(VI) acetate. selected to be independently investigated by various spectroscopic (vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance spectroscopy) and quantum chemical methods was achieved by an international round-robin test (RRT). Twenty laboratories from six different countries with a focus on actinide or geochemical research participated and contributed to this scientific endeavor. The outcomes of this RRT were considered on two levels of complexity: first, within each technical discipline, conformities as well as discrepancies of the results and their sources were evaluated. The raw data from the different experimental approaches were found to be generally consistent. In particular, for complex setups such as accelerator-based X-ray absorption spectroscopy, the agreement between the raw data was high. By contrast, luminescence spectroscopic data turned out to be strongly related to the chosen acquisition parameters. Second, the potentials and limitations of coupling various spectroscopic and theoretical approaches for the comprehensive study of actinide molecular complexes were assessed. Previous spectroscopic data from the literature were revised and the benchmark data on the U(VI) acetate system provided an unambiguous molecular interpretation based on the correlation of spectroscopic and theoretical results. The multimethodologic approach and the conclusions drawn address not only important aspects of actinide spectroscopy but particularly general aspects of modern molecular analytical chemistry.

The alkali metal ions Li+, Na+ and K+ have a profound influence on the stoichiometry of the complexes formed in uranyl(VI)-peroxide-hydroxide systems, presumably as a result of a templating effect, resulting in the formation of two complexes, M[(UO2)(O-2)(OH)](2)(-) where the uranyl units are linked by one peroxide bridge, mu-eta(2)-eta(2), with the second peroxide coordinated "end-on", eta(2), to one of the uranyl groups, and M[(UO2)(O-2)(OH)](4)(3-), with a four-membered ring of uranyl ions linked by mu-eta(2)-eta(2) peroxide bridges. The stoichiometry and equilibrium constants for the reactions: M+ + 2UO(2)(2+) + 2HO(2)(-) + 2H(2)O -> M[(UO2)(O-2)(OH)] 2 - + 4H(+) (1) and M+ + 4UO(2)(2+) + 4HO(2)(-) + 4H(2)O -> M[(UO2)(O-2)(OH)](4)(3-) + 8H(+) (2) have been measured at 25 degrees C in 0.10 M (tetramethyl ammonium/M+)NO3 ionic media using reaction calorimetry. Both reactions are strongly enthalpy driven with large negative entropies of reaction; the observation that Delta H(2) approximate to 2 Delta H(1) suggests that the enthalpy of reaction is approximately the same when peroxide is added in bridging and "end-on" positions. The thermodynamic driving force in the reactions is the formation of strong peroxide bridges and the role of M+ cations is to provide a pathway with a low activation barrier between the reactants and in this way "guide" them to form peroxide bridged complexes; they play a similar role as in the synthesis of crown-ethers. Quantum chemical (QC) methods were used to determine the structure of the complexes, and to demonstrate how the size of the M+-ions affects their coordination geometry. There are several isomers of Na[(UO2)(O-2)(OH)](2)(-) and QC energy calculations show that the ones with a peroxide bridge are substantially more stable than the ones with hydroxide bridges. There are isomers with different coordination sites for Na+ and the one with coordination to the peroxide bridge and two uranyl oxygen atoms is the most stable one.

The constitution and equilibrium constants of ternary uranyl(VI) peroxide carbonate complexes [(UO2)(p)(O-2)(q)(CO3)(r)](2(p-q-r)) have been determined at 0 degrees C in 0.50 M MNO3, M = Li, K, and TMA (tetramethyl ammonium), ionic media using potentiometric and spectrophotometric data; O-17 NMR data were used to determine the number of complexes present. The formation of cyclic oligomers, "[(UO2)(O-2)(CO3)](n)", n = 4, 5, 6, with different stoichiometries depending on the ionic medium used, suggests that Li+, Na+, K+ and TMA ions act as templates for the formation of uranyl peroxide rings where the uranyl-units are linked by mu-eta(2)-eta(2) bridged peroxide-ions. The templating effect is due to the coordination of the M+-ions to the uranyl oxygen atoms, where the coordination of Li+ results in the formation of Li[(UO2)(O-2)(CO3)](4)(7-), Na+ and K+ in the formation of Na/K[(UO2)(O-2)(CO3)](5)(9-) complexes, while the large tetramethyl ammonium ion promotes the formation of two oligomers, TMA[(UO2)(O-2)(CO3)] 5 9-and TMA[(UO2)(O-2)(CO3)](6)(11-). The NMR spectra demonstrate that the coordination of Na+ in the five-and six-membered oligomers is significantly stronger than that of TMA(+); these observations suggest that the templating effect is similar to the one observed in the synthesis of crown-ethers. The NMR experiments also demonstrate that the exchange between TMA[(UO2)(O-2)(CO3)](5)(9-) and TMA[(UO2)(O-2)(CO3)](6)(11-) is slow on the O-17 chemical shift time-scale, while the exchange between TMA[(UO2)(O-2)(CO3)](6)(11-)and Na[(UO2)(O-2)(CO3)](6)(11-) is fast. There was no indication of the presence of large clusters of the type identified by Burns and Nyman (M. Nyman and P. C. Burns, Chem. Soc. Rev., 2012, 41, 7314-7367) and possible reasons for this and the implications for the synthesis of large clusters are briefly discussed.

We measured with high accuracy the effective charge of a uranium (VI)-AMP complex by electrophoretic NMR (eNMR). Using the same method, the degree of counterion association is also assessed which leads to a quantitative determination of the nominal charge which then provides the degree of ligand deprotonation in the complex. This demonstrates a new application of eNMR for resolving structural details of supramolecular complexes.

The focus of this study is on the identification of precursors in solution that might act as building blocks when solid uranyl(VI) poly-peroxometallate clusters containing peroxide and hydroxide bridges are formed. The precursors could be identified by using carbonate as an auxiliary ligand that prevented the formation of large clusters, such as the ones found in solids of fullerene type. Using data from potentiometric and NMR (O-17 and C-13) experiments we identified the following complexes and determined their equilibrium constants: (UO2)(2)(O-2)(CO3)(4)(6-), UO2(O-2)CO32-, UO2(O-2)(CO3)(2)(4-), (UO2)(2)(O-2)(CO3)(2)(2-), (UO2)(2)(O-2)(2)(CO3)(2-) and [UO2(O-2)(CO3)(5)(10-). The NMR spectra of the pentamer show that all uranyl and carbonate sites are equivalent, which is only consistent with a ring structure built from uranyl units linked by peroxide bridges with the carbonate coordinated "outside" the ring; this proposed structure is very similar to [UO2(O-2)(oxalate)](5)(10-) identified by Burns et al. (J. Am. Chem. Soc., 2009, 131, 16648; Inorg. Chem., 2012, 51, 2403) in K-10[UO2(O-2)(oxalate)](5)center dot(H2O)(13); similar ring structures where oxalate or carbonate has been replaced by hydroxide are important structure elements in solid poly-peroxometallate complexes. The equivalent uranyl sites in (UO2)(2)(O-2)(2)(CO3)(2-) suggest that the uranyl-units are linked by the carbonate ion and not by peroxide.

Possible mechanisms for intermolecular exchange between coordinated and solvent water in the complexes Y(TTA)(3)(OH2)(2) and Y(TTA)(3)(TBP)(OH2) and intermolecular exchange between free and coordinated HTTA in Y(TTA)(3)(OH2)(HTTA) and Y(TTA)(3)(TBP)(HTTA) have been investigated using ab initio quantum chemical methods. The calculations comprise both structures and energies of isomers, intermediates and transition states. Based on these data and experimental NMR data (Part 2) we have suggested intimate reaction mechanisms for water exchange, intramolecular exchange between structure isomers and intermolecular exchange between free HTTA and coordinated TTA. A large number of isomers are possible for the complexes investigated, but only some of them have been investigated, in all of them the most stable geometry is a more or less distorted square anti-prism or bicapped trigonal prism; the energy differences between the various isomers are in general small, less than 10 kJ mol(-1). 9-coordinated intermediates play an important role in all reactions. Y(TTA)(3)(OH2)(3) has three non-equivalent water ligands that can participate in ligand exchange reactions. The fastest of these exchanging sites has a QM activation energy of 18.1 kJ mol(-1), in good agreement with the experimental activation enthalpy of 19.6 kJ mol(-1). The mechanism for the intramolecular exchange between structure isomers in Y(TTA)(3)(OH2)(2) involves the opening of a TTA-ring as the rate determining step as suggested by the good agreement between the QM activation energy and the experimental activation enthalpy 47.8 and 58.3 J mol(-1), respectively. The mechanism for the intermolecular exchange between free and coordinated HTTA in Y(TTA)(3)(HTTA) and Y(TTA)(3)(TBP)(HTTA) involves the opening of the intramolecular hydrogen bond in coordinated HTTA followed by proton transfer to coordinated TTA. This mechanism is supported by the good agreement between experimental activation enthalpies (within parenthesis) and calculated activation energies 68.7 (71.8) and 35.3 (38.8) kJ mol(-1). The main reason for the difference between the two systems is the much lower energy required to open the intramolecular hydrogen bond in the latter. The accuracy of the QM methods and chemical models used is discussed.

The mechanism, rate constant, and activation parameters for the exchange between uranyl(VI)) oxygen and water oxygen in tetramethyl ammonium hydroxide solution, TMA-OH, have been determined using O-17 NMR magnetization transfer technique. In the concentration range investigated, the predominant complex is UO2(OH)(4)(2-). The experimental rate equation, rate = k(ex)[TMA-OH](free)[U(VI)](2)(total) indicates that the exchange takes place via a binuclear complex or transition state with the stoichiometry [(UO2(OH)(4)(2-))(UO2(OH)(5)(3-)]. The rate-determining step most likely takes place between the axial "yl" oxygens and the equatorial hydroxides. The experimental Gibbs energy of activation, Delta G(double dagger) = 60.8 +/- 2.4 kJ/mol is in good agreement with the value, Delta A(double dagger) approximate to Delta G(double dagger) = 52.3 +/- 5.4 kJ/mol, found by Buhl and Schreckenbach in a recent Car-Parrinello molecular dynamics study, indicating that their proposed "shuttle" mechanism may be applicable also on the proposed binuclear transition state.