Importance of Counteranions on the Hydration Structure of the Curium Ion

Using density functional theory based ab initio molecular dynamics and metadynamics, we show that counterions can trigger noticeable changes in the hydration shell structure of the curium ion. On the basis of the free energies of curium-water coordination, the eight-fold coordination state is dominant by at least 98% in the absence of counteranions and in the presence of chloride and bromide counteranions. In addition, the solvent hydrogen bond (HB) lifetimes are relatively longer. In contrast, the solvent hydrogen bond (HB) lifetimes are relatively shorter in the presence of perchlorate counteranions, with the nine-fold and eight-fold states existing in an 8/2 ratio, which is in good agreement with the reported ratio measured by X-ray scattering experiments. To our knowledge, this is the first time that molecular simulations have shown that counteranions can directly affect the first hydration shell structure of a cation.