A Quantum Mechanical Charge Field Molecular Dynamics Study of Fe2+ and Fe3+ Ions in Aqueous Solutions

Ab initio quantum mechanical charge field molecular dynamics (QMCF-MD) simulations have been performed for aqueous solutions of Fe2+ and Fe3+ ions at the Hartree−Fock level of theory to describe and compare their structural and dynamical behavior. The structural features of both hydrated ions are characterized by radial distribution functions that give the maximum probability of the ion−O distance for Fe2+ and Fe3+ ions at 2.15 and 2.03 Å, respectively. The angular distribution functions of both ions prove the octahedral arrangement of six water ligands, whereas the second shells of these ions differ. Both ions show influence on the water molecules beyond the second shells. The structure-forming abilities of both ions are visible from the ligand mean residence times and ion−O stretching frequencies evaluated for both ions. The substantially improved data obtained from these QMCF-MD simulations show better correlation with available experimental results than the conventional quantum mechanics/molecular mechanics molecular dynamics (QM/MM MD) approaches with one hydration shell treated by quantum mechanics.