A theoretical study of uranyl solvation: Explicit modelling of the second hydration sphere by quantum mechanical methods

The inclusion of an explicit second sphere in the hydration of the uranyl ion is investigated by DFT. We study model complexes that contain two water molecules in the second sphere hydrogen-bonded to each water molecule in the first. Compared with single-sphere models, significant changes are observed for the uranium-water first-sphere distance, the uranium-“yl” oxygen distance and the uranyl stretching vibrational frequencies. For each of these observables, agreement with experiment is improved with our new model. Charge transfer to uranyl is substantially enhanced when the second hydration sphere is present. Effects of third and subsequent hydration spheres appear to be small, but the influence of water molecules linked to the apical oxygens by a hydrogen-bonding network is probably not negligible. Models based on a polarizable continuum are less satisfactory, particularly for the vibrational frequencies. The uranyl stretching frequencies are highly correlated with charge transfer from water molecules to the “yl” oxygen atoms and with the uranyl bond length.