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

Abstract

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.

Molecular dynamics simulation of the Pb(II) coordination in biological media via cationic dummy atom models

“…All obtained structures of Pb-water clusters were characterized by a minimum Pb-O distance in the 2.27-2.43 Å range (Table 1), although, by increasing the number of water molecules (n > 4), we also detected slightly longer Pb-O distance, typically in the 2.47-2.87 Å range (Table 1), consistently with available computational data [55,56]. The calculated values of O-Pb-O angles for the n < 7 species were mostly detected in the range of either 64-75 (proximal) or 132-135 (distal) degrees, in agreement with octahedral-like spatial orientations of the ligands bound at the metal center as also reported by others [55][56][57][58][59][60][61][62]. Interestingly, all water molecules were detected in the first coordination sphere of both n = 7 and n = 9 species, whereas the Pb(II) center of the n = 8 complex was found to be coordinated by only six water molecules (Table 1).…”

“…Preliminary DFT calculations were carried out to investigate in detail the composition and the structure of both Pb(II)-water and Pb(II)-thiol/thiolate binary systems, the latter being small model of the lead coordination occurring in proteins. The investigation of the Pb(II) hydration at high level of theory reported by others [55,56,[58][59][60][61] predicted the holo-directed coordination of up to nine water molecules. In our DFT study, the incremental coordination of water molecules to the Pb(II) showed that, although optimized structures with up to nine H 2 O in the first coordination sphere could be obtained, the most stable [Pb(H 2 O) n ] 2+ complexes were characterized by n = 4 or 6 and, again, holo-directed coordination geometry (Table 1).…”

Nature of High- and Low-Affinity Metal Surface Sites on Birnessite Nanosheets

“…On the basis of EXAFS , and DFT Pb–O distances, we assume that [Pb­(H 2 O) 6 ] 2+ is more abundant than [Pb­(H 2 O) 4 ] 2+ in aqueous solution, although the relative energies suggest otherwise. Given that previous theoretical studies have obtained varying results depending on the level of theory, only DFT-based molecular dynamics simulations in explicit solvent using the same level of theory could clarify the actual situation, but such an investigation is beyond the scope of the present study.…”

“…Given that previous theoretical studies have obtained varying results depending on the level of theory,103 only DFT-based molecular dynamics simulations in explicit solvent using the same level of theory could clarify the actual situation, but such an investigation is beyond the scope of the present study.We found that the lowest energy surface complexes are completely or partially dehydrated at the MnO 2 surface. Therefore, the generic surface reactions for Pb 2+ can be written as…”

Mg(II) and Ca(II) Microsolvation by Ammonia: Born–Oppenheimer Molecular Dynamics Studies

“…Here we present a detailed study of the structural and dynamic properties of [Mg­(NH 3 ) n ] 2+ and [Ca­(NH 3 ) n ] 2+ clusters with n = 2–6, 8, 10, 20, and 27 through Born–Oppenheimer molecular dynamics (BOMD) simulations. We have employed this powerful methodology with great success in the study of the aqueous solvation of Mg 2+ , Ca 2+ , and Pb 2+ cations ,, as well as in the study of the solvation of several types of metallic molecules. − We stress that in these cases nanodroplet cluster models (with around 30 solvent molecules) have quite successfully reproduced the solvation structure observed in the liquid phase.…”