Extension of computational chemistry to the study of lanthanide(III) ions in aqueous solution: Implementation and validation of a continuum solvent approach

A set of atomic radii used for the construction of solute cavities in the f ramework of the polarizable continuum model (PCM) is extended and validated with the aim of supporting the investigation of lanthanide(III) complexes in aqueous solution. The parameterization of the atomic radii for the whole Ln(III) series is performed by minimizing the differences between the expe rimental and the calculated standard hydration free energies of the ions ca lculated at the HF level. The optimized radii show a remarkable linear rela tionship with effective ionic radii and well reproduce the experimental hyd ration free energies also when electron correlation effects are included in the calculations. We have next validated a mixed discrete continuum model in which a supermolecule formed by the ion and by water molecules in the fi rst hydration shell is immersed in a polarizable continuum. The molecular s tructures, the relative stability of the octa- with respect to the nonahydr ated species, and the ion hydration free energies have been calculated for the neodymium(III) and ytterbium(III) aqueous ions. Results are in agreemen t with experimental evidence, both from structural and energetic standpoint s. The molecular structures optimized including surrounding effects are in better agreement with the experimental structures than the in vacuo geometr ies. Moreover, the results show that the energetic properties of these syst ems in aqueous solution can be effectively calculated by using the structur es optimized in vacuo, and including correlation effects in the gas-phase r eaction of complex formation.