Hydration Structure and Hydrolysis of U(IV) and Np(IV) Ions: A Comparative Density Functional Study Using a Modified Continuum Solvation Approach

We
studied the hydration and the first hydrolysis reaction of U­(IV)
and Np­(IV) ions in an aqueous environment, applying a relativistic
density functional method together with a recently proposed variant
of a continuum solvation model where the solute cavities are constructed
with effective atomic radii, based on charge-dependent scaling factors.
In this way, one obtains improved solvation energies of charged species.
We demonstrate that solute cavities, constructed with scaled atomic
radii as described, permit one to calculate hydrolysis constants of
acceptable accuracy. As a consequence, one is also able to estimate free hydration energies
of U­(IV) and Np­(IV) in adequate agreement with empirical data. According
to the model calculations, U­(IV) is coordinated by eight to nine water
molecules, while the preferred coordination number of Np­(IV) is 8.
For the highly charged ions under study, the modified solvation model
simultaneously yields improved geometries, hydration energies, and
hydrolysis constants.