Aqueous Solutions of Metal Ions - Metallic Systems: A Quantum Chemist's Perspective - page 125

4.3.3 m echaniSm of W ater d iSSociation in a queouS
S olutionS of a S 3+ and a S 5+ i onS
To.study.the.mechanism.of.water.dissociation.in.As 3+ .and.As 5+ .solutions,.we.employed.
CPMD.simulations.coupled.with.TPS.calculations.as.in.our.previous.investigations.
of.the.mechanism.of.water.dissociation.induced.by.Fe 3+ .and.Cr 3+ .ions.and.the.impact.
of.dissociated.waters.on.the.hydration.structure.and.coordination.chemistry.of.these.
metal.ions..Unlike.our.Al.ion.solution.studies,.but.similar.to.our.Fe.and.Cr.solution.
studies,.we.observe.proton.transfer.in.As.solutions..In.agreement.with.our.studies.
of.Fe.and.Cr.solutions,.in.As.solutions.the.dissociation.mechanism.is.initiated.via.
an.elongated.H-OH.bond.that.subsequently.breaks.and.releases.a.proton..This.leads.
to. hydroxylation. of. the. As. ion. in. water. (see. Figure. 4.4A).. After. the. release. of. the.
proton,.proton.transfer.via.hydrogen.bonding.interactions.initiates.Zundel.and.Eigen.
complex.formation.in.water.for.the.aqueous.solutions.of.both.As 3+ .and.As 5+ .ions..We.
note.that.the.Zundel.complex.is.formed.irst.via.hydrogen.bonding.of.the.proton.to.
two.surrounding.water.molecules.(see.Figure.4.4B)..It.takes.the.proton.60.fs.to.form.
the.Zundel.complex.in.water.around.the.As 3+ .ion,.whereas.this.time.is.11.fs.longer.
around.the.As 5+ .ion..A.second.Zundel.complex.is.formed.between.the.irst.and.sec-
ond.shells.after.38.fs.in.the.vicinity.of.the.As 3+ .ion..The.same.mechanism.is.observed.
around.the.As 5+ .ion.within.the.following.33.fs.(see.Figure.4.4C)..It.takes.the.system.
slightly.longer.(51.fs).in.comparison.to.the.dynamics.observed.in.the.irst.and.second.
shells.to.move.the.proton.between.the.second.and.third.shells.and.to.form.the.Eigen.
complex.(see.Figure.4.4D)..Based.on.our.CPMD/TPS.calculations,.we.observe.that.
the.Eigen.complex.is.also.formed.around.the.As 5+ .ion,.between.the.second.and.third.
shells..The.proton.transfer.that.leads.to.the.formation.of.the.Eigen.complex.takes.a.
slightly.shorter.time.(42.fs).in.the.vicinity.of.the.As 5+ .ion.as.compared.to.the.As 3+ .ion..
These.variations.in.time.might.be.attributed.to.differences.in.ionic.charge.
In.agreement.with.our.studies.on.the.mechanism.of.water.dissociation.induced.by.
Fe 3+ .and.Cr 3+ .ions,.we.ind.that.the.Zundel.complex.is.the.irst.water.ion.complex.to.
be.formed.and.that.the.Eigen.complex.is.formed.afterward.in.the.aqueous.solutions.
of.both.As.ions..We.note.that.the.Zundel.complex.is.formed.in.the.irst.and.second.
shells,.whereas.the.Eigen.complex.is.formed.in.the.second.and.third.hydration.shells.
in.the.vicinity.of.both.As 3+ .and.As 5+ .ions.in.aqueous.solution..Furthermore,.the.same.
trends.were.also.obtained.from.our.CPMD/TPS.calculations.for.Fe 3+ .and.Cr 3+ .ions.in.
aqueous.solution..However,.we.did.not.distinguish.a.clear.water.dissociation.mecha-
nism.in.the.aqueous.solutions.of.Al.ions,.which.might.be.attributed.to.the.lack.of.
partially.or.half-illed.d.orbitals,.the.ionic.radius.of.the.metal.ion,.or.the.differences.
in.charge.transfer.as.compared.to.the.transition.metal.ion.solutions.
To.understand.the.thermodynamic.preference.between.the.Zundel.and.Eigen.com-
plexes.formed.in.the.vicinity.of.the.As 3+ .and.As 5+ .ions.in.aqueous.solution,.we.per-
formed.calculations.of.free.energy.(Equation.4.2)..These.calculations.show.that.both.
Zundel.and.Eigen.complexes.can.exist.in.the.solution.with.a.free.energy.difference.of.
less.than.0.7.kJ.mol −1 .around.the.As 3+ .ion.and.that.the.Zundel.complex.is.only.slightly.
more.stable.than.the.Eigen.complex..For.the.aqueous.solution.of.the.As 5+ .ion,.we.ind.
that.the.free.energy.difference.becomes.larger;.the.Zundel.complex.is.preferred.over.
the.Eigen.complex.by.2.8.kJ.mol −1 ,.which.shows.the.same.thermodynamic.trend.that.