6 research outputs found
Recommended from our members
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 AlCl<sub>3</sub> 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<sup>–</sup> and Al<sup>3+</sup> ions in an aqueous AlCl<sub>3</sub> 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><sub>Al–Cl</sub>) between the
Al<sup>3+</sup> ion and one of the Cl<sup>–</sup> ions held
constant. The calculated potential of mean force (PMF) of the Al<sup>3+</sup>–Cl<sup>–</sup> ion pair shows a global minimum
at <i>r</i><sub>Al–Cl</sub> = 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><sub>Al–Cl</sub> = 4.4 and 6.0 Å. The positions of the free energy minima coincide
with the hydration-shell intervals of the Al<sup>3+</sup> cation,
suggesting that the Cl<sup>–</sup> ion is inclined to reside
in regions with low concentrations of water molecules, that is, between
the first and second hydration shells of Al<sup>3+</sup> and between
the second shell and the bulk. A detailed analysis of the solvent
structure around the Al<sup>3+</sup> and Cl<sup>–</sup> ions
as a function of <i>r</i><sub>Al–Cl</sub> is presented.
The results are compared to structural data from X-ray measurements
and unconstrained CPMD simulations of single Al<sup>3+</sup> and Cl<sup>–</sup> ions and AlCl<sub>3</sub> solutions. The dipole moments
of the water molecules in the first and second hydration shells of
Al<sup>3+</sup> and in the bulk region and those of Cl<sup>–</sup> ions were calculated as a function of <i>r</i><sub>Al–Cl</sub>. Major changes in the electronic structure of the system were found
to result from the removal of Cl<sup>–</sup> from the first
hydration shell of the Al<sup>3+</sup> cation. Finally, two unconstrained
CPMD simulations of aqueous AlCl<sub>3</sub> 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