Aqueous Solutions of Metal Ions - Metallic Systems: A Quantum Chemist's Perspective - page 119

the.lifetime.of.the.Zundel-like.complex.is.extremely.short.(<.0.3.fs).and.the.proton.
migrates. back. to. the. OH − . coordinated. to. the. metal. ion,. forming. a. neutral. water.
molecule.and.the.initial.octahedral.Al(H 2 O) 3+ .geometry.in.aqueous.solution..We.do.
not.observe.a.clear.water.dissociation.mechanism.around.the.Al 3+ .ion,.which.is.in.
agreement.with.our.previous.studies..These.indings.are.in.agreement.with.the.work.
of.Lubin.et.al.,.who.studied.the.octahedral.Al(H 2 O) 3+ .and.the.tetrahedral.Al(OH) - .
ionic.species.in.aqueous.solution.via.CPMD.simulations.and.did.not.observe.a.clear.
water.dissociation.mechanism.in.the.presence.of.Al.ions.[73].
Table.4.3.lists.several.structural.parameters.for.the.Al.ions.in.octahedral.Al(H 2 O) 3+ .
and.trigonal.bipyramidal.Al(OH)(H 2 O) 2+ .structures.in.aqueous.solution.obtained.from.
our.CPMD.simulations..The.structural.parameters.are.in.very.good.agreement.with.
experiments.as.well.as.with.previous.CPMD.simulations.for.these.species.in.aque-
ous.solution..Based.on.our.CPMD.simulation.results,.the.average.distance.between.
the. oxygen. atoms. of. the. irst. shell. water. molecules. (O I ). and. the. Al. ion. in. water. is.
1.95.and.1.98.Å.for.the.octahedral.Al(H 2 O) 3+ .and.the.trigonal.bipyramidal.Al(OH)
(H 2 O) 2+ .structures,.respectively..The.bond.distances.for.the.Al(H 2 O) 3+ .ion.in.water.are.
slightly.larger.than.the.reported.x-ray.data.(1.88-1.90.Å).for.the.octahedral.geometry..
However,.our.values.are.in.excellent.agreement.with.previously.reported.CPMD.simu-
lation.results.for.the.octahedral.Al(H 2 O) 3+ .structure.in.water..As.shown.in.Table.4.3,.
the.Al-O I .bond.distance.does.not.change.signiicantly.for.the.optimized.octahedral.
Al(H 2 O) 3+ .and.the.trigonal.bipyramidal.Al(OH)(H 2 O) 2+ .structures.using.DFT.calcula-
tions.with.a.COSMO..This.inding.indicates.that.the.use.of.explicit.or.implicit.water.
models.does.not.signiicantly.change.the.predicted.hydrated.Al.ion.structures.
The.average.HO-H.bond.lengths.for.the.irst.shell.water.molecules.are.similar,.0.99.
and. 1.01. Å. for. the. octahedral. Al(H 2 O) 3+ . and. trigonal. bipyramidal. Al(OH)(H 2 O) 2+ .
species.in.aqueous.solution,.respectively,.and.are.only.slightly.different.than.results.
obtained. from. static. DFT. calculations. utilizing. a. COSMO. (Table. 4.3).. The. second.
shell.water.molecule.oxygen.atoms.(O II ).to.Al 3+ .distance.for.the.octahedral.Al(H 2 O) 3+ .
complex. in. aqueous. solution. is. 4.12.±.0.02. Å.. This. value. becomes. slightly. smaller.
(4.05.±.0.04. Å). for. the. trigonal. bipyramidal. Al(OH)(H 2 O) 2+ . species. in. water.. These.
indings. are. in. agreement. with. x-ray. data.. In. addition,. the. hydrogen. bond. lengths.
between. the. irst. and. second. shell. water. molecules. around. the. octahedral. Al 3+ . ion.
are.2.69.±.0.02.Å.in.our.CPMD.simulations..This.distance.is.1.4%.smaller.than.the.
distance.in.the.aqueous.Al(OH)(H 2 O) 2+ .solution..The.second.shell.CN.of.water.mol-
ecules.varies.between.8.and.12.and.7.and.14.around.Al.in.octahedral.Al(H 2 O) 3+ .and.
Al.in.trigonal.bipyramidal.Al(OH)(H 2 O) 2+ .arrangements.in.aqueous.solution,.respec-
tively..The.computed.CNs.are.in.agreement.with.experimental.and.theoretical.studies.
In.general,.the.importance.of.polymerization.reactions.of.Al.is.widely.accepted.
as.described.above..However,.a.detailed.molecular-level.understanding.of.the.early.
stages.of.the.polymerization.reactions.is.lacking..Such.information.can.be.useful.in.
the.design.of.more.eficient.techniques.for.wastewater.treatment.or.for.the.removal.
of.metal.ions.before.they.form.the.more.stable.metal.ion.polymers.in.solution,.which.
can.be.toxic.as.explained.in.Section.4.1..In.fact,.few.theoretical.studies.have.success-
fully. studied. possible. reactions. between. two. hydrated. Al. ions.. Qian. et. al.. studied.
the. reaction. mechanism. of. hydrated. Al 3+ . ions. with. static. DFT. calculations. in. the.
gas.phase.and.using.a.COSMO.[91]..They.predicted.that.the.octahedral.Al(H 2 O) 3+ .