Aqueous Solutions of Metal Ions - Metallic Systems: A Quantum Chemist's Perspective - page 123

(A) (B)

(C) (D)

FIGURE 4.4 Illustration.of.the.water.dissociation.mechanism.around.both.As.ions.in.aque-
ous.solution.at.room.temperature.obtained.from.CPMD.simulations.coupled.with.TPS.cal-
culations..The.water.molecules.were.removed.to.provide.a.clear.presentation..Various.steps.
in.the.mechanism.of.water.dissociation.about.As 3+ .are.depicted..The.mechanism.begins.with.
the.hydroxylation.of.the.As.ion.(A).and.leads.to.the.formation.of.a.Zundel.complex.in.the.
irst.shell.(B)..A.proton.transfer.leads.to.the.disappearance.of.the.Zundel.complex.in.the.irst.
shell.and.to.the.formation.of.another.Zundel.complex.in.the.second.shell.(C)..Finally,.proton.
migration.leads.to.the.formation.of.an.Eigen.complex.(D)..(Coskuner,.O..et.al.,. Dynamic and
Structural Properties of Aqueous Arsenic Solutions ,.1187,.2009..Wiley-VCH.Verlag.GmbH.&.
Co..KGaA..Reproduced.with.permission.)

oxygen.atoms.to.which.the.proton.is.coordinated).of.2.59.Å..These.structural.param-
eters.do.not.deviate.signiicantly.from.previous.calculations.for.pure.water.and.tran-
sition. metal. ion.. It. is. interesting. to. note. that. we. do. not. observe. a. distinct. water.
dissociation.event.around.the.Al 3+ .ion.because.the.charged.water.ions.rapidly.form.
the. neutral. state. of. water. between. the. second. and. third. shells.. This. inding. does.
not.indicate.that.water.dissociation.does.not.occur.in.Al.solutions..However,.it.indi-
cates.that.special.sampling.methods.with.extremely.large.transition.pathway.sam-
pling.may.be.required.to.predict.the.possible.pathways.of.water.dissociation.around.
hydrated.poly-Al.ions.
The.structural.parameters.of.the.Al 2 (OH) 2 (H 2 O) 4+ .complex.are.depicted.in.Table.
4.3..Based.on.our.CPMD.simulation.results,.the.average.distance.between.the.two.Al.
ions.in.Al 2 (OH) 2 (H 2 O) 4+ .is.3.05.Å.