Symmetry Breaking and Hydration Structure of Carbonate and Nitrate in Aqueous Solutions: A Study by Ab Initio Quantum Mechanical Charge Field Molecular Dynamics

The ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism was applied to simulate carbonate and nitrate anions in aqueous solution. The out-of-plane (nu(2)) spectra obtained from the velocity autocorrelation functions (VACFs) and the torsion angle-time functions indicate that the symmetry of carbonate is reduced from D-3h to a lower degree by breaking up the molecular plane, whereas the planarity of nitrate anion is retained. The calculated frequencies are in good agreement with the Raman and IR data. Carbonate shows a stronger molecular hydration shell than the nitrate anion with the average molecular coordination numbers of 8.9 and 7.9, respectively. A comparison with the average number of ion-solvent hydrogen bonds (H-bonds) indicates the extra water molecules within the hydration shell of carbonate (similar to 2) and nitrate (similar to 3), readily migrating from one coordinating site to another. The mean residence times for water ligands in general classify carbonate and nitrate as moderate and weak structure-making anions, while the specific values for individual sites of nitrate reveal local weak structure-breaking properties.