Solving the Hydration Structure of the Heaviest Actinide Aqua Ion Known: The Californium(III) Case

In summary, the first MC simulation of the trivalent cation of californium, based on an exchangeable hydrated ion-water intermolecular potential, has been shown to extend and improve the hydrated ion model. Likewise, the CfLIII-edge EXAFS spectrum of an acidic 1 mm Cf(ClO4)3 aqueous solution recorded under optimized experimental conditions has greatly improved the signal/noise ratio of the only previously recorded spectrum. The comparison of the experimental EXAFS spectrum with the two computed ones, obtained from two different intermolecular potentials that predict eight (BP86) or nine (MP2) water molecules in the first coordination shell, leads to the conclusion that the lowest hydration number is preferred. Then, as CfIII is the heaviest actinide aqua ion for which there is experimental information, the actinide contraction is supported by the present study. (For UIII, RU-O=2.56 Angstroms, and CN=9±1; for PuIII, RPu-O=2.51 Angstroms and CN=9±1; for CmIII, RCm-O=2.47 Angstroms and CN=9±1). The role of the second hydration shell is important in defining the structure and dynamics of the CfIII aqua ion, but the contribution of second-shell water molecules to the EXAFS signal as back-scatters is marginal. Finally, this work gives an illustrative example of the benefits which can be achieved from the combination of experimental X-ray absorption spectroscopy and computer simulations. The usefulness of the simultaneous analysis of the results as well as the importance of the structural statistical average has been clearly demonstrated herein. Each technique independently was not adequate. We believe that this study traces out a still poorly explored combined methodology which may be extremely useful for many other complexes and chemical problems. A systematic theoretical and experimental examination of the other known actinide cations on the same basis should be undertaken to confirm the findings presented herein. (authors)