Chemistry Reference
In-Depth Information
studies.have.shown.that.Eigen.and.Zundel.complexes.interconvert.in.pure.water.by.
hydrogen-bond. luctuations.. Later,. it. was. shown. that. Warshell's. empirical. valence.
bond.approach.could.also.be.used.to.obtain.a.mechanistic.description.of.Grotthuss.
diffusion.in.liquid.water.[41].
Proton. transfer. via. the. Grothuss. mechanism. including. the. formation. of. Zundel.
and.Eigen.complexes.is.the.currently.accepted.view..The.pH.of.pure.water,.which.
is.related.to.the.autoionization.of.pure.liquid.water.and.thus.to.the.proton.transfer.
mechanism,. is. of. considerable. interest.. Geissler. et. al.. studied. water. autoinoization.
using.CPMD.simulations.coupled.with.transition.path.sampling.(TPS).calculations.
[24]..The.pH.of.ambient.water.implies.that.a.water.molecule.will.spontaneously.dis-
sociate. once. every. 10.h. on. the. average.. Thus,. water. dissociation. is. indeed. a. rare.
event,.and.one.that.would.not.be.feasible.to.study.without.TPS..Geissler.et.al..showed.
that.the.spontaneous.dissociation.of.a.water.molecule.leads.to.the.propagation.of.a.
proton.through.the.solution.along.a.network.of.hydrogen.bonds..Once.the.H
3
O
+
.and.
OH
−
. are. separated. from. each. other,. they. may. form. Zundel. or. Eigen. complexes. in.
the.aqueous.solution.or.may.recombine..These.events.occur.on.a.timescale.of.about.
100.fs..Only.when.a.luctuation.is.coincident.with.another.hydrogen.bond.breaking.
in.the.hydrogen-bonded.network.along.which.the.two.charged.aqua.ions.separate.is.
fast.recombination.not.possible.[24]..Recently,.we.applied.CPMD.simulations.with.
TPS.calculations.on.metal.ion.solutions.(aqueous.solutions.of.Fe
3+
,.Cr
3+
,.As
3+
,.and.
As
5+
).[65,66]..We.implemented.classical.simulation.techniques,.such.as.perturbation.
theory,.for.calculating.the.Gibbs.free.energy.associated.with.proton.transfer.in.these.
simulations..We.were.able.to.observe.the.mechanism.of.water.dissociation.around.
these.transition.metal.ions.and.found.both.Eigen.and.Zundel.complexes.in.the.solu-
tions.of.Fe
3+
,.Cr
3+
,.As
3+
,.and.As
5+
.ions..We.predicted.the.hydration.structures.of.these.
metal.ions.including.the.impact.of.the.dissociated.water.on.their.structures..For.the.
Al
3+
.solution,.fast.recombination.of.the.H
3
O
+
.and.OH
−
.ions.prevented.us.from.observ-
ing.the.Grotthuss.mechanism.or.the.Eigen.and.Zundel.ions.in.the.solution.[65,66].
In.general,.Al.is.the.most.abundant.metal.in.the.Earth's.crust..An.understanding.
of.hydrated.Al.structures.is.of.prime.interest.in.wastewater.treatment,.neurodegen-
erative.diseases,.catalysis,.and.environmental.sciences..For.example,.it.is.known.that.
Al.is.the.principal.toxic.substance.that.kills.the.ish.in.the.northern.United.States..
Al.affects.the.function.of.ish.gills.by.causing.ion.regulatory.and.respiratory.dys-
functions..Furthermore,.it.has.been.hypothesized.that.the.Al.ion.changes.the.confor-
mation.of.amyloid.β.(Aβ).peptides.and.is.able.to.trigger.the.mis-folding.mechanism.
of.Aβ.peptides.in.the.brain,.which.has.been.implicated.in.Alzheimer's.disease..Many.
studies.of.aqueous.Al.ions.and.their.reactions.with.organic.compounds.and.biomol-
ecules.have.been.performed.[1,67-70]..Nevertheless,.a.thorough.explanation.of.the.
reaction.mechanism.of.aqueous.Al.ions.or.Al.polymers.with.biomolecules,.such.as.
Aβ.peptide,.using.ab.initio.molecular.dynamics.is.still.lacking.
The.structure.of.a.single,.solvated.Al
3+
.ion.that.may.exist.at.high.or.low.pH.has.been.
studied.extensively.[71,72]..Experimental.and.computational.chemistry.studies.agree.
that.the.Al
3+
.ion.coordinates.with.surrounding.irst.shell.water.molecules.in.an.octahe-
dral.and.tetrahedral.arrangement.for.pH.values.lower.than.3.and.higher.than.7,.respec-
tively.[72-75]..However,.the.coordination.chemistry.between.a.single.Al
3+
.ion.and.irst.
shell.water.molecules.for.pH.values.ranging.between.3.and.7.has.been.the.subject.of.
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