Aqueous Solutions of Metal Ions - Metallic Systems: A Quantum Chemist's Perspective - page 127

4.4 CONCLUSION
We.report.the.results.of.CPMD.simulations.coupled.with.TPS.calculations.for.study-
ing.the.mechanism.of.proton.transfer.in.aqueous.solutions.of.Fe 3+ ,.Cr 3+ ,.Al 3+ ,.As 3+ ,.
and.As 5+ .ions..The.CPMD/TPS.technique.enables.studies.of.bond.breaking.and.bond.
forming.with.great.accuracy.using.irst.principles.electronic.structure.methods.for.
rare. events. such. as. water. dissociation. induced. by. metal. ions.. We. found. that. both.
Zundel. and. Eigen. complexes. are. formed. in. the. metal. ion. solutions.. However,. we.
were. not. able. to. locate. a. dissociated. state. of. water. in. the. vicinity. of. Al. ions.. Our.
simulations.show.that.a.Zundel.complex.is.formed.in.the.irst.and.second.hydration.
shells. and. the. Eigen. complex. is. formed. in. the. second. and. third. hydration. shells.
around. the. metal. ions.. We. note. that. the. formation. of. aqua. ion. complexes. and. the.
structural.properties.of.these.aqua.ions.depend.on.several.factors:.charge.and.size.
of. the. metal. ion. and. the. number. of. d. orbital. electrons.. In. addition,. we. ind. that.
the. hydration. structure. around. the. metal. ions. is. inluenced. by. the. behavior. of. the.
solvent..According.to.our.studies,.water.dissociation.affects.the.solvated.metal.ion.
geometries..Fe 3+ .and.Cr 3+ .ions.form.octahedral.geometries.with.average.numbers.of.
coordinated.hydroxyl.groups.of.2.1.and.1.7.for.the.Fe 3+ .and.Cr 3+ .ions,.respectively..
These.results.indicate.a.slightly.lower.pH.for.the.Fe 3+ .ion.solution.than.that.for.the.
Cr 3+ .solution..This.trend.is.supported.by.hydrolysis.constant.and.p K a .values..For.the.
As. solutions,. we. found. that. these. metal. ions. form. tetrahedral-like. structures,. and.
that.a.ifth.water.molecule.becomes.coordinated.to.the.As 5+ .ion.and.then.released.
into.the.second.shell.(and.vice.versa).during.our.simulation..These.simulations.show.
that.the.average.number.of.hydroxyl.groups.coordinated.to.the.As 3+ .and.As 5+ .ions.is.
3.2.and.3.5,.respectively..These.results.suggest.a.lower.pH.for.the.As 5+ .solution.than.
for.the.As 3+ .solution..Atomic.force.spectroscopy,.dialysis,.and.kinetics.experiments.
support.these.indings.[96,97]..We.calculated.the.Gibbs.free.energy.associated.with.
proton.transfer.in.metal.ion.solutions.utilizing.the.perturbation.method.with.ab.ini-
tio.molecular.dynamics.simulations..These.simulations.predict.the.Zundel.complex.
to. be. preferred. over. the. Eigen. complex. for. the. Fe 3+ ,. Cr 3+ ,. and. As 5+ . ion. solutions..
However,.this.trend.is.different.in.the.vicinity.of.the.As 3+ .ion,.and.both.Zundel.and.
Eigen.complexes.may.exist.at.the.same.time.in.this.solution.since.the.difference.in.
the.Gibbs.free.energy.difference.is.very.small.(0.7.kJ.mol −1 ).
To. address. the. effect. of. pH. on. metal. ion. solution. properties. and. to. study. the.
dimerization.molecular.mechanism.of.Al 3+ .ions.in.water,.we.employed.CPMD.simu-
lations. and. DFT. calculations. for. pH. values. between. 3. and. 7.. The. insights. gained.
from.this.study.lend.additional.support.for.the.trigonal.bipyramidal.Al(OH)(H 2 O) 2+ .
structure,.which.was.reported.by.recent.experimental.and.CPMD.simulation.studies..
We.do.not.observe.a.clear.water.dissociation.mechanism.around.single.and.poly-Al.
ions..Our.simulations.show.that.(short-lived).charged.states.for.water.convert.to.the.
neutral.state.of.water.around.the.aqueous.Al 2 (OH) 2 (H 2 O) 4+ .structure..The.structural.
parameters. obtained. for. the. short-live. Zundel. and. Eigen. complexes. are. similar. to.
those.reported.for.pure.water.and.for.transition.metal.ion.solutions.
The.hydrated.single.Al.ion.structures.are.in.good.agreement.with.the.available.
experiments. and. support. the. kinetics. experiments. that. report. a. penta-coordinated.
single. Al. ion. for. solutions. with. pH. between. 3. and. 7. [98].. Our. simulations. show.