Building a polarizable pair interaction potential for lanthanoids(III) in liquid water: a molecular dynamics study of structure and dynamics of the whole series.

In this work we have extended our previously presented polarizable pair interaction potential for La(3+)-water [Duvail et al., J. Chem. Phys. 127, 034503 (2007)] to the whole lanthanoid(III) series (Ln(3+)) interacting with water. This was performed taking into account known modification of ionic radius and atomic polarizability across the series and thus changing potential parameters according to that. Our procedure avoids the hard task of doing expensive high level ab initio calculations for all the atoms in the series and provides results in good agreement with experimental data and with ab initio calculations performed on the last atom in the series (Lu(3+), the atom for which the extrapolation should be in principle much crude). Thus we have studied the hydration properties of the whole Ln(3+) series by performing classical molecular dynamics in liquid phase. This systematic study allows us to rationalize from a microscopic point of view the different experimental results on Ln(3+)-water distances, first shell coordination numbers and first shell water self-exchange reactivity. In particular, we found that across the series the coordination number decreases from 9 for light lanthanoids to 8 for heavy lanthanoids in a continuous shape. This is due to the continuous changing in relative stability of the two forms that can be both populated at finite temperature with different probabilities as a function of the Ln(3+) atomic number. The changeover of the Ln(3+) ionic radius across the series resulted to be the main driving physical properties governing not always the Ln(3+)-water distance changing across the series but also the observed coordination number and consequently ligand dynamics.