Dynamics and Structure of Ln(III)−Aqua Ions: A Comparative Molecular Dynamics Study Using ab Initio Based Flexible and Polarizable Model Potentials

Abstract

Aqueous solutions of a light (Nd3+), a middle (Gd3+), and a heavy (Yb3+) lanthanide ion were studied using ab initio based flexible and polarizable analytical potentials in classical molecular dynamics simulations to describe their thermodynamic, structural, and dynamic features. To avoid the spurious demise of O-H bonds, it was necessary to reparametrize an existing water model, which resulted in an improved description of pure water. The good agreement of the results from the simulations with the experimental hydration enthalpies, the Ln(III)-water radial distribution functions, and the water-exchange rates validated the potentials, though the r(Ln-Ow) distances were 6% longer than the experimentally determined values. A nona-coordinated state was found for Nd3+ in 95% of the simulation, with a tricapped trigonal prism (TCTP) geometry; the corresponding water-exchange mechanism was found to be of dissociative interchange (Id) character through a short-lived octa-coordinated transition state in a square antiprism (SQA) geometry. An octa-coordinated state in SQA geometry was found for Yb3+ in 99% of the simulation, and the observed exchange events exhibited characteristics of an interchange (I) mechanism. For Gd3+ an equilibrium was observed between 8-fold SQA and 9-fold TCTP coordinated states that was maintained by the frequent exchange of a water molecule from the first hydration shell with the bulk, thus producing significant deviations from the ideal geometries, and a fast exchange rate. Though strong water-water interactions prevented a full alignment of the dipoles to the ion's electric field, the screening was found large enough as to limit its range to 5 A; water molecules further apart from the ion were found to have the same dipole as the molecules in the bulk, and a random orientation. The interplay among the water-ion and the water-water interactions determined the different coordination numbers and the different dynamics of the water exchange in the first hydration shell for each ion.

Hydration of gas-phase ytterbium ion complexes studied by experiment and theory

“…However, structures with five water molecules in the first shell and a sixth in the second shell were located and determined to be high energy in all cases (see Figs. 9,10,11,12,13).…”

“…A number of these studies applied molecular dynamics simulations of trivalent lanthanides in solution to understand the hydration of the bare metal cation [11][12][13][14][15]. Other studies focused on modeling trivalent lanthanide hydrates in the gas phase.…”

Toward accurate solvation dynamics of lanthanides and actinides in water using polarizable force fields: from gas-phase energetics to hydration free energies

“…In addition, many recent studies highlighted the failure of traditional fixed charge force fields to capture the main physical effects that govern interaction for highly charged ions in polar solvents [8][9][10][11][12][13][14][15][16][17][18][19].…”

Ions and the Protein Surface Revisited: Extensive Molecular Dynamics Simulations and Analysis of Protein Structures in Alkali-Chloride Solutions

“…[17][18][19][20] Moreover, classical force-fields with fixed partial charges are constantly under ongoing development, with recent contributions dealing with calcium, 21 alkali metal ions, [22][23][24][25] halides, 22,24 heavy metal ions 26 and lanthanide ions. 27 Over the recent years, several studies have aimed at an understanding of the specificity of salts to the protein surface. In a comprehensive study, Jungwirth and co-workers have used MD simulations, quantum chemistry and conductivity measurements to quantify the preference of the protein surface to Na + over K + .…”