Structure and dynamics of the chromate ion in aqueous solution. An ab initio QMCF-MD simulation.

An ab initio quantum-mechanical charge-field molecular-dynamics (QMCF-MD) simulation of the chromate ion in aqueous solution at ambient temperature was performed to study the structure and dynamics of this ion and its hydration shell. In contrast to conventional quantum-mechanical molecular-mechanics molecular-dynamics (QM/MM-MD) simulations, the QMCF-MD approach offers the possibility of investigating composite systems with the accuracy of a QM/MM method but without the time-consuming construction of solute-solvent potential functions. The data of the simulation give a clear picture of the first hydration shell of the chromate anion, which consists of 14 water molecules. The mean distance between the oxygen atoms of the chromate and the hydrogen atoms of water is 1.82 A. Each chromate oxygen atom is in average coordinated to 2.6 water molecules. The first-shell mean ligand residence time was evalulated as 2.2 ps; the vibrational frequency of the nu(OH) mode was found to be 185 cm(-1). Several structural parameters such as the radial distribution functions, angular distribution functions, and coordination number distributions enable a full characterization of the embedding of the chromate ion in the solvent water. The dynamics of the hydration structure are described by mean residence times of the water molecules in the first hydration shell, distance plots, and velocity autocorrelation functions.