MOLECULAR-DYNAMICS SIMULATION STUDY OF LANTHANIDE IONS LN(3-SOLUTION - ANALYSIS OF THE STRUCTURE OF THE FIRST HYDRATION SHELL AND OF THE ORIGIN OF SYMMETRY FLUCTUATIONS() IN AQUEOUS)

We have analyzed molecular dynamics simulations of tripositive lanthan ide ions Ln(3+) in aqueous solution. Combining a variety of new approa ches for the analysis and the visualization of the first hydration she lls, we were able to extract in detail their angular structure and the ir dynamic behavior along the series of Ln(3+) ions. For a heavy lanth anide ion (Yb3+) the eight water molecules of the first hydration shel l form a well-defined square antiprism, whereas for a nine-coordinate light lanthanide ion (Nd3+) the first hydration shell adopts the trica pped trigonal prism geometry. In the middle of the series both geometr ies coexist. Both the square antiprism and the tricapped trigonal pris m rearrange via 90 degrees pseudorotations of the main symmetry axis. The pertaining transition state of lowered symmetry is a dodecahedron for the Yb3+ octaaqua complex and a capped square antiprism for the Nd 3+ enneaaqua complex. The lifetime of a square antiprism between two p seudorotations is 11 ps but amounts to only 2 ps for a tricapped trigo nal prism. The Lifetime of a square antiprism from the simulation of Y b3+ (CN = 8) is in quantitative agreement with the correlation time fo r the fluctuation of the zero-field splitting from experimental EPR sp ectra of the Gd3+ octaaqua complex. This correlation time, of relevanc e for the understanding of the mechanism of the H-1 relaxation of Gd3-based MRI contrast agents, is linked to transient distorsions of the Gd3+ aqua complex from perfect symmetry that were so far assumed to be due to random impacts of solvent molecules. On the basis of our MD-si mulations we can go beyond these general notions and propose a model w ith 90 degrees pseudorotations of the coordination polyhedron as the p rincipal mechanism for distorsions of the first hydration shell.