Structural and Dynamical Properties and Vibrational Spectra of Bisulfate Ion in Water: A Study by Ab Initio Quantum Mechanical Charge Field Molecular Dynamics

Abstract

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.