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

We have analyzed molecular dynamics simulations of tripositive lanthanide ions Ln(3+) in aqueous solution. Combining a variety of new approaches for the analysis and the visualization of the first hydration shells, we were able to extract in detail their angular structure and their dynamic behavior along the series of Ln(3+) ions. For a heavy lanthanide ion (Yb3+) the eight water molecules of the first hydration shell form a well-defined square antiprism, whereas for a nine-coordinate light lanthanide ion (Nd3+) the first hydration shell adopts the tricapped trigonal prism geometry. In the middle of the series both geometries coexist. Both the square antiprism and the tricapped trigonal prism 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 Nd3+ enneaaqua complex. The lifetime of a square antiprism between two pseudorotations is 11 ps but amounts to only 2 ps for a tricapped trigonal prism. The Lifetime of a square antiprism from the simulation of Yb3+ (CN = 8) is in quantitative agreement with the correlation time for the fluctuation of the zero-field splitting from experimental EPR spectra of the Gd3+ octaaqua complex. This correlation time, of relevance 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-simulations we can go beyond these general notions and propose a model with 90 degrees pseudorotations of the coordination polyhedron as the principal mechanism for distorsions of the first hydration shell.