Study of the Elusive Hydration of Pb²⁺ from the Gas Phase to the Liquid Aqueous Solution: Modeling the Hemidirected Solvation with a Polarizable MCDHO Force-Field

The Pb²⁺ presents unique hydration features that make the experimental characterization and its theoretical modeling challenging: classical molecular dynamics (MD) with standard force-fields fails to produce the experimentally determined diffusion coefficient and the EXAFS spectrum. Here we study the hydration of Pb²⁺ in aqueous solution employing a polarizable model compatible with the MCDHO water model. The MCDHO FF for the Pb²⁺–water interaction was fitted to reproduce the configurations and interaction energies of various [Pb­(H₂O)_n]²⁺ clusters obtained with ab initio calculations, with n = 4, 6, and 8. Its use in classical MD simulations yielded qualitative agreement with Born–Oppenheimer molecular dynamics of gas-phase hydrated clusters and MD simulations of the aqueous solution resulted in good agreement with the experimental D_Pb²⁺ and EXAFS spectrum. Analysis of the MD trajectories revealed a labile and very dynamic hemidirected first hydration shell in the aqueous solution with a non-well-defined coordination number CN; nonetheless, it was found that the more probable hydration structures have either 3 or 4 water molecules directly bound to the Pb²⁺ with another 3 or 2 at slightly larger distances. The simulations of the gas-phase [Pb­(H₂O)₂₉]²⁺ cluster were found to capture the main structural features of the diluted aqueous solution.