The ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism was applied to simulate the bisulfate ion, HSO4-, in aqueous solution. The averaged geometry of bisulfate ion supports the separation of six normal modes of the O*-SO3 unit with C3v symmetry from three modes of the OH group in the evaluation of vibrational spectra obtained from the velocity autocorrelation functions (VACFs) with subsequent normal coordinate analyses. The calculated frequencies are in good agreement with the observations in Raman and IR experiments. The difference of the averaged coordination number obtained for the whole molecule (8.0) and the summation over coordinating sites (10.9) indicates some water molecules to be located in the overlapping volumes of individual hydration spheres. The averaged number of hydrogen bonds (H-bonds) during the simulation period (5.8) indicates that some water molecules are situated in the molecular hydration shell with an unsuitable orientation to form a hydrogen bond with the ion. The mean residence time in the surroundings of the bisulfate ion classify it generally as a weak structure-making ion, but the analysis of the individual sites reveals a more complex behavior of them, in particular a strong interaction with a water molecule at the hydrogen site.

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 (ν(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 (∼2) and nitrate (∼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.

Articles you may be interested inThe structure of CaCl 2 aqueous solutions over a wide range of concentration. Interpretation of diffraction experiments via molecular simulation Structural and dynamical properties of Co(III) in aqueous solution: Ab initio quantum mechanical/molecular mechanical molecular dynamics simulation A study of aqueous solutions of lanthanide ions by molecular dynamics simulation with ab initio effective pair potentials Pair-potential, three-body corrected and combined ab initio quantum mechanics/molecular mechanics molecular dynamics ͑QM/MM-MD͒ simulations have been performed for Hg 2ϩ in water. The ion's hydration structure was evaluated in terms of radial distribution functions, coordination numbers, and angular distributions, together with changes in ligand structure upon binding of water molecules. An octahedral solvate complex with coordination number of 6/22 for first/second shell results from QM/MM-MD simulations, with average Hg 2ϩ -O distance 2.42 Å for the first shell. The pair-potential simulation gives strongly wrong results, the inclusion of three-body effects corrects many, but not all of the errors. The hydration enthalpy of Ϫ553Ϯ3 kcal/mol obtained from the QM/MM-MD simulation seems a reasonable value for dilute solution, when compared with experimental estimations.