Aqueous Solutions of Metal Ions - Metallic Systems: A Quantum Chemist's Perspective - page 128

formation. of. hydroxyl-ligand-bridged. Al. polymers. via. reaction. between. Al(OH) 2+ .
ions.in.water,.which.may.be.found.in.solutions.with.pH.values.between.3.and.7..In.
addition,.NPA.reveals.that.the.coordination.of.hydroxyl.groups.donates.more.elec-
tron.density.to.Al.than.that.to.the.water.molecules,.which.favors.the.hydroxyl-ligand-
bridged.Al 2 .dimer.over.the.water-molecule-bridged.metal.dimer.in.aqueous.solution..
Another.key.inding.from.the.simulations.is.that.the.hydroxyl-bridged.Al 2 .dimer.is.
more.stable.than.the.water-separated.single.Al.ions.in.water..Furthermore,.a.com-
parison.of.our.CPMD.simulation.results.with.DFT.calculations.utilizing.a.COSMO.
shows. that. the. structures. of. hydrated. octahedral. Al(H 2 O) 3+ ,. trigonal. bipyramidal.
Al(OH)(H 2 O) 2+ ,. and. Al 2 (OH) 2 (H 2 O) 4+ . do. not. change. signiicantly,. indicating. that.
the. use. of. implicit. or. explicit. models. for. water. does. not. signiicantly. change. the.
hydration.structures.for.hydrated.metal.ions..Overall,.our.predicted.Al 2 (OH) 2 (H 2 O) 4+ .
structure. agrees. well. with. x-ray. measurements.. We. ind. that. one. water. molecule.
coordinated. to. Al. in. Al 2 (OH) 2 (H 2 O) 4+ . possesses. a. slightly. longer. distance. to. the.
metal.center.in.comparison.to.the.rest.of.the.water.molecules.coordinated.to.both.Al.
centers..Structural.and.thermodynamic.studies.for.Al 2 (OH) 2 (H 2 O) 4+ .support.previ-
ous.experiments.and.theoretical.investigations.
We. have. demonstrated. that. theoretical. studies. performed. with. the. appropriate.
methods.and.at.an.appropriate.level.of.theory.can.provide.complementary.insights.
into. structural. and. thermodynamic. studies. including. dynamics. of. hydrated. metal.
ions.and.their.polymers.in.aqueous.solution..Such.theoretical.studies.coupled.with.
kinetics.and.spectroscopic.measurements.can.help.to.identify.the.mechanisms.of.the.
reactions.of.metal.ions.in.solution,.which.may.lead.to.eficient.processes.for.remov-
ing. toxic. metal. ions. from. drinking. water. or. for. treating. wastewater.. In. addition,.
metal.ion.solutions.are.important.in.organic.and.medicinal.chemistry.since.they.can.
act.as.catalysts.or.ligands.to.organic.molecules.and.biomolecules..An.understanding.
of.the.solution.properties.including.the.acid-base.chemistry.(pH).of.these.solutions.
along.with.the.geometry.of.the.hydrated.metal.ion.can.help.in.the.design.of.more.efi-
cient.syntheses.and.lead.to.a.more.detailed.understanding.of.coordination.chemistry.
In.summary,.studies.of.the.mechanism.of.water.dissociation.remain.challenging.
with.experimental.techniques.due.to.the.small.size.and.short.timescale.associated.with.
this.process..Ab.initio.molecular.dynamics.simulations.coupled.with.special.sampling.
techniques.enable.computational.study.of.these.mechanisms.as.shown.in.this.chapter.

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