Ab initio study of the structure and dynamics of solvated highly charged metal ions

The influence of highly charged cations on the structure and dynamics of the aqueous phase is investigated by performing Ab Initio Molecular Dynamics (AIMD) simulations on the Fe³⁺ and Ca²⁺ cations and the CaCl₂, FeOH²⁺ and AlOH²⁺ species all solvated by up to 64 waters. A detailed comparison between results of Fe³⁺ and a previous study of the Al³⁺ ion reveal significant changes in the hydrogen bond structure of 1st and 2nd hydration shells between the two systems. Differences are also noticed in the dynamics of the 2nd hydration shell. An orbital interaction is observed between Fe³⁺ and water that is not observed for Al³⁺. The results of the Ca²⁺ AIMD simulation show a more tetrahedral H-bonding structure relative to 3+ cations. Solvent exchange between the coordinating waters of Ca²⁺ and bulk proceeds via the associative interchange mechanism. Ambient and high temperature (near critical, 650K) simulations of the CaCl₂ system show that the Ca²⁺+ and Cl- ions exist as solvent separated ion pairs under ambient conditions while at 650K the Ca-Cl contact pairs are formed. This is accompanied by a significant disruption of the hydrogen bond structure of the solvent. The [FeOH]²⁺ and [AlOH]²⁺ aqueous species are found to display a significant labilizing effect due to the presence of the hydroxide species in the 1st hydration shell. A uniform destabilizing of all 1st shell waters is observed for the AlOH²⁺ species while for the FeOH²⁺ species some hydrating waters feel a stronger effect determined by their position relative to the OH⁻ specie

Ab initio study of the structure and dynamics of solvated highly charged metal ions

The influence of highly charged cations on the structure and dynamics of the aqueous phase is investigated by performing Ab Initio Molecular Dynamics (AIMD) simulations on the Fe³⁺ and Ca²⁺ cations and the CaCl₂, FeOH²⁺ and AlOH²⁺ species all solvated by up to 64 waters. A detailed comparison between results of Fe³⁺ and a previous study of the Al³⁺ ion reveal significant changes in the hydrogen bond structure of 1st and 2nd hydration shells between the two systems. Differences are also noticed in the dynamics of the 2nd hydration shell. An orbital interaction is observed between Fe³⁺ and water that is not observed for Al³⁺. The results of the Ca²⁺ AIMD simulation show a more tetrahedral H-bonding structure relative to 3+ cations. Solvent exchange between the coordinating waters of Ca²⁺ and bulk proceeds via the associative interchange mechanism. Ambient and high temperature (near critical, 650K) simulations of the CaCl₂ system show that the Ca²⁺+ and Cl- ions exist as solvent separated ion pairs under ambient conditions while at 650K the Ca-Cl contact pairs are formed. This is accompanied by a significant disruption of the hydrogen bond structure of the solvent. The [FeOH]²⁺ and [AlOH]²⁺ aqueous species are found to display a significant labilizing effect due to the presence of the hydroxide species in the 1st hydration shell. A uniform destabilizing of all 1st shell waters is observed for the AlOH²⁺ species while for the FeOH²⁺ species some hydrating waters feel a stronger effect determined by their position relative to the OH⁻ specie

Ion Association in AlCl₃ Aqueous Solutions from Constrained First-Principles Molecular Dynamics

The Car–Parrinello-based molecular dynamics (CPMD) method was used to investigate the ion-pairing behavior between Cl^– and Al³⁺ ions in an aqueous AlCl₃ solution containing 63 water molecules. A series of constrained simulations was carried out at 300 K for up to 16 ps each, with the internuclear separation (<i>r</i>_Al–Cl) between the Al³⁺ ion and one of the Cl^– ions held constant. The calculated potential of mean force (PMF) of the Al³⁺–Cl^– ion pair shows a global minimum at <i>r</i>_Al–Cl = 2.3 Å corresponding to a contact ion pair (CIP). Two local minima assigned to solvent-separated ion pairs (SSIPs) are identified at <i>r</i>_Al–Cl = 4.4 and 6.0 Å. The positions of the free energy minima coincide with the hydration-shell intervals of the Al³⁺ cation, suggesting that the Cl^– ion is inclined to reside in regions with low concentrations of water molecules, that is, between the first and second hydration shells of Al³⁺ and between the second shell and the bulk. A detailed analysis of the solvent structure around the Al³⁺ and Cl^– ions as a function of <i>r</i>_Al–Cl is presented. The results are compared to structural data from X-ray measurements and unconstrained CPMD simulations of single Al³⁺ and Cl^– ions and AlCl₃ solutions. The dipole moments of the water molecules in the first and second hydration shells of Al³⁺ and in the bulk region and those of Cl^– ions were calculated as a function of <i>r</i>_Al–Cl. Major changes in the electronic structure of the system were found to result from the removal of Cl^– from the first hydration shell of the Al³⁺ cation. Finally, two unconstrained CPMD simulations of aqueous AlCl₃ solutions corresponding to CIP and SSIP configurations were performed (17 ps, 300 K). Only minor structural changes were observed in these systems, confirming their stability

NWChem: Past, Present, and Future

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