Structural and Dynamical Nature of Hydration Shells of the Carbonate Ion in Water: An Ab Initio Molecular Dynamics Study.

The structural and dynamical nature of hydration shells of the carbonate ion in water is investigated through ab initio molecular dynamics simulation. The anisotropic solvation shell structure of the ion is resolved by calculating conically restricted pair distribution and radial/angular correlation functions. The vibrational frequency of OD modes hydrogen-bonded to the ion is found to be smaller than that of bulk water, which means the carbonate ion-water hydrogen bonds are stronger than those between water molecules. Calculations of the orientational and residence dynamics and translational diffusion reveal retarded mobility of the hydration shell water molecules compared to that of the bulk water due to stronger ion-water interactions. It is shown that the rotation of hydration shell water takes place through dual routes of hydrogen bond switching, where an OD bond initially hydrogen-bonded to a carbonate oxygen switches its hydrogen bond to another carbonate oxygen or to a water oxygen. The carbonate ion is found to have a nonzero dipole moment of 1.0 D in water, which can be attributed to its interactions with the fluctuating environment of the surrounding water. The carbonate ion is also found to have a long-range effect on neighboring water molecules that goes beyond the first solvation shell.