A coupled EXAFS-molecular dynamics study on PuO

Abstract

The physicochemical properties of the monovalent actinyl cations, PuO2+and NpO2+, in water have been studied by means of classical molecular dynamicsimulations. A specific set of cation-water intermolecular potentials based on ab initiopotential energy surfaces has been built on the basis of the hydrated ion concept. TheTIP4P water model was adopted. Given the paramagnetic character of these actinyls, thecation−water interaction energies were computed from highly correlated wave functionsusing the NEVPT2 method. It is shown that the multideterminantal character of the wavefunction has a relevant effect on the main distances of the hydrated molecular cations.Several structural, dynamical, and energetic properties of the aqueous solutions have beenobtained and analyzed. Structural RDF analysis gives An−Oyldistances of 1.82 and 1.84 Åand An−O(water) distances of 2.51 and 2.53 Å for PuO2+and NpO2+in water,respectively. Experimental EXAFS spectra from dilute aqueous solutions of PuO2+and NpO2+are revisited and analyzed, assumingtetra- and pentahydration of the actinyl cations. Simulated EXAFS spectra have been computed from the snapshots of the MDsimulations. Good agreement with the experimental information available is found. The global analysis leads us to conclude that bothPuO2+and NpO2+cations in water are stable pentahydrated aqua ions.