MOLECULAR-DYNAMICS SIMULATION STUDY OF LANTHANIDE IONS LN(3-SOLUTION INCLUDING WATER POLARIZATION - CHANGE IN COORDINATION-NUMBER FROM 9 TO8 ALONG THE SERIES() IN AQUEOUS)

Classical molecular dynamics simulations have been carried out for lan thanide ions Ln(3+) in aqueous solution. For the Ln(3+)-water interact ion we propose a new three-body potential function that takes into acc ount the mean polarization of water molecules in the first hydration s hell and that has been fitted to ab-initio results. By a usual pair po tential approach we can reproduce the experimental distance of the fir st maximum in the cation-oxygen radial pair distribution function, but the first hydration shell is by far too unstable from both a structur al and a dynamical point of view. Inclusion of a polarization term lea ds to a perfect agreement with coordination numbers from neutron diffr action studies as well as to decreased kinetic lability of the first h ydration shell that is more consistent with experimental evidence. Not ably, a coordination number of 8.5 is obtained for the middle of the l anthanide series and corresponds to an equilibrium between a 9-coordin ated and an 8-coordinated Ln(3+)-aqua ion. Water exchange rate constan ts from computer simulations are reported for the first time for Ln(3)-aqua ions. A maximum of the exchange rate constants in the middle of the series is in agreement with the current interpretation of experim ental data, based on the change of relative stability of the ennea and octa aqua ions along the series.