DYNAMICS OF A HIGHLY-CHARGED ION IN AQUEOUS-SOLUTIONS - MD SIMULATIONS OF DILUTE CRCL3 AQUEOUS-SOLUTIONS USING INTERACTION POTENTIALS BASEDON THE HYDRATED ION CONCEPT

Structural and dynamical properties of dilute aqueous solutions contai ning a trivalent cation have been determined by means of Molecular Dyn amics simulations. The concept of hydrated ion has been used when cons idering aqueous solutions of Cr3+, [Cr(H2O)(6)](3+) being the cationic entity interacting in solution. An ab initio Cr3+ hydrate-water inter action potential previously developed [J. Phys. Chem. 1996, 100, 11748 ] and a new one describing the Cr3+ hydrate-Cl- interactions have been used with a TIP4P water model to carry out simulations of the system Cr(H2O)(6)Cl-3 + 512H(2)O. To examine the role of anions, simulations without chloride ions were performed as well ([Cr(H2O)(6)](3+) + 512H( 2)O). To investigate the influence of shape and size of the hydrated c ation, two additional models of trivalent cation have been studied usi ng the simplest concept of spherical ion. Ad hoc charged sphere-water interaction potentials for the latter situations were built. RDFs, hyd ration numbers, vibrational spectra of the intermolecular modes, trans lational self-diffusion coefficients for ions and water molecules in t he different hydration shells, interdiffusion coefficients, mean resid ence times, and rotational diffusion coefficients and correlation time s for the hexahydrate and water molecules are obtained and discussed. Comparison of dynamical properties of Cr3+ aqueous solutions with thos e obtained from simulations of Cr3+ 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 rota tes following Debye's rotational model. Advantages and drawbacks of th e hydrated ion approach to deal with solvation of highly charged catio ns of transition metals are examined. The structural consequences of a dopting a spherical shape for cation when developing potentials are qu ite different when either the bare or hydrated radius is considered; t hus, whereas the small sphere overestimates the first shell coordinati on number, the big sphere overestimates the second hydration shell, pr omoting a clathrate structure. Specially designed EXAFS measurements o f a set of Cr(NO3)(3) aqueous solutions 0.1 M in hydrochloric and hydr obromic acids were carried out and analyzed to investigate the possibi lity of detecting the halide anion in the first or second hydration sh ell. Simulations agree with experimental results in the sense that the counterion of Cr3+ hexahydrate is placed in dilute acidic solutions b eyond the second hydration shell.