Dynamics of a highly charged ion in aqueous solutions: MD simulations of dilute CrCl aqueous solutions using interaction potentials based on the hydrated ion concept

Structural and dynamical properties of dilute aqueous solutions containing a trivalent cation have been determined by means of Molecular Dynamics simulations. The concept of hydrated ion has been used when considering aqueous solutions of Cr, [Cr(HO)] being the cationic entity interacting in solution. An ab initio Cr hydrate-water interaction potential previously developed [J. Phys. Chem. 1996, 100, 11748] and a new one describing the Cr hydrate-Cl interactions have been used with a TIP4P water model to carry out simulations of the system Cr(HO)Cl + 512HO. To examine the role of anions, simulations without chloride ions were performed as well ([Cr(HO)] + 512HO). To investigate the influence of shape and size of the hydrated cation, two additional models of trivalent cation have been studied using the simplest concept of spherical ion. Ad hoc charged sphere-water interaction potentials for the latter situations were built. RDFs, hydration numbers, vibrational spectra of the intermolecular modes, translational self-diffusion coefficients for ions and water molecules in the different hydration shells, interdiffusion coefficients, mean residence times, and rotational diffusion coefficients and correlation times for the hexahydrate and water molecules are obtained and discussed. Comparison of dynamical properties of Cr aqueous solutions with those obtained from simulations of Cr hexahydrate strongly supports the validity of the hydrated ion model for this cation. The examination of rotational mobility leads to the conclusion that the hydrate ion rotates following Debye's rotational model. Advantages and drawbacks of the hydrated ion approach to deal with solvation of highly charged cations of transition metals are examined. The structural consequences of adopting a spherical shape for cation when developing potentials are quite different when either the bare or hydrated radius is considered; thus, whereas the small sphere overestimates the first shell coordination number, the big sphere overestimates the second hydration shell, promoting a clathrate structure. Specially designed EXAFS measurements of a set of Cr(NO) aqueous solutions 0.1 M in hydrochloric and hydrobromic acids were carried out and analyzed to investigate the possibility of detecting the halide anion in the first or second hydration shell. Simulations agree with experimental results in the sense that the counterion of Cr hexahydrate is placed in dilute acidic solutions beyond the second hydration shell.