Quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations of multiple-charged transition metal ions in aqueous solution

Classical and combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations have been performed for evaluating structural, dynamical properties of Cd2+, Hg2+, Co3+, Cr3+, and Ti3+ in aqueous solution. Octahedral solvate complexes with a coordination number of 6 for the first hydration shell result from QM/MM molecular dynamics simulations for Cd2+, Co3+, Cr3+ and Ti3+ ions, whereas a value of 6.2 associated with a distorted octahedral structure was found for Hg2+. The first shell distances of 2.33A, 2.42A, 1.97A, and 1.92A, obtained from QM/MM molecular dynamics simulations of Cd2+, Hg2+, Co3+, and Cr3+, respectively, are in good agreement with experimental values. A value of 2.03 A with side peaks at 1.99A, 2.12A, and 2.14A is obtained for Ti3+ from the QM/MM simulation, after including the second hydration shell in the QM region. A fast dynamical Jahn-Teller effect was observed for this ion in the QM/MM simulations. The hydration energies obtained from QM/MM simulation for Cd2+, Hg2+, Co3+, Cr3+, and Ti3+ are -458 kcal/mol, -553 kcal/mol, -1144 kcal/mol, -1108 kcal/mol, and -1068 kcal/mol, respectively, all in good agreement with experimental hydration enthalpies. Water exchange processes in the first hydration shell were only observed for Hg2+, with a mean ligand residence time of 87 ps. For all ions, numerous water exchange processes between second hydration shell and bulk were observed. The mean residence times of water in the second shell obtained from QM/MM-MD are 10 ps, 13 ps, 22 ps, 55 ps and 85 ps for Cd2+, Hg2+, Cr3+, Co3+, and Ti3+, respectively. (author)