Aqueous Solutions of Metal Ions - Metallic Systems: A Quantum Chemist's Perspective - page 112

much.debate.and.recent.studies.such.as.kinetics.experiments.and.ab.initio.molecular.
dynamics.simulations.show.that.a.single.Al 3+ .ion.prefers.to.coordinate.to.ive.ligands.
in. this. pH. range. [76].. Furthermore,. previous. theoretical. studies. of. the. dimerization.
reaction.mechanism.of.Al 3+ .ions.in.water.have.been.performed.using.an.octahedral.
geometry,.which.suggests.that.these.polymerization.mechanisms.can.be.proposed.for.
low. pH. values.. However,. most. relevant. chemical. and. environmental. reactions. of. a.
single.or.poly-Al 3+ .species.occur.at.pH.values.between.3.and.7..Two.questions.of.inter-
est.are.the.mechanism.of.dimerization.and.the.structure.of.the.resulting.dimer.in.water,.
that.is,.the.arrangement.of.the.water.molecules.and.ions.around.the.Al.dimer.
In. addition. to. studies. of. the. mechanism. of. water. dissociation. induced. by. vari-
ous.metal.ions,.we.present.the.structures.of.Al(H 2 O) 3+ .and.Al(H 2 O) 4 OH 2+ .in.water.
and. investigate. polymerization. in. water. via. the. modeling. of. two. Al(OH)(H 2 O) 2+ .
species.in.aqueous.solutions.using.CPMD.simulations.and.irst.principles.calcula-
tions.utilizing.a.continuum.model.for.water.(COSMO)..These.studies.yield.impor-
tant. insights. into. the. structure,. stability,. and. coordination. chemistry. of. Al 3+ . ions.
in.aqueous.solution..We.ind.that.the.reaction.between.two.Al(OH)(H 2 O) 2+ .ions.in.
aqueous.solution.yields.the.hydroxyl-ligand-bridged.Al 2 (OH) 2 (H 2 O) 4+ .complex.and.
that. solution. dynamics. is. a. key. factor. in. dimer. formation.. This. study. shows. that.
water.exchange.and.substitution.reactions.are.important.in.the.formation.of.Al.ion.
polymers. in. water.. Here,. we. demonstrate. the. impact. of. the. dynamics. and. solvent.
molecules.on.the.formation.of.the.Al 2 (OH) 2 (H 2 O) 4+ .complex.in.aqueous.solution.(for.
pH.values.varying.between.3.and.7).
4.2 METHODS
Dynamics. calculations. on. a. molecular. system. involve. the. time. propagation. of. a. set.
of.nuclei.according.to.an.equation.of.motion..In.the.usual.case.where.the.equation.of.
motion.is.Newton's.equation,. F .=. ma ,.the.force.( F ).is.required..The.force.is.evaluated.
as.the.negative.at.the.irst.derivative.of.the.potential.energy.with.respect.to.the.atomic.
coordinates. of. the. molecular. system.. In. classical. molecular. dynamics. calculations,.
the.potential.energy.surface.is.usually.some.sort.of.classical.force.ield.[59,77,78]..In.
ab.initio.dynamics.calculations,.the.potential.energy.surface.is.computed.using.some.
irst.principles.method.such.as.Hartree-Fock.or.density.functional.theory.(DFT)..The.
dynamics. calculations. reported. in. this. chapter. were. performed. using. CPMD.. The.
CPMD. method. is. one. of. the. most. popular. methods. for. simulations. of. liquids. and.
solution.systems..Among.the.many.reasons.for.its.popularity.is.its.eficiency,.which.
permits.dynamics.calculations.on.systems.containing.up.to.a.few.hundred.atoms.over.
tens.of.picoseconds..The.most.popular.application.of.CPMD.consists.of.Kohn-Sham.
DFT.with.a.plane.wave.basis.set.for.valence.electrons.and.pseudopotentials.for.core.
electrons.[37,41]..Thus,.CPMD.is.ideally.suited.for.the.present.study.as.it.is.an.eficient.
method.for.performing.dynamics.calculations.while.maintaining.the.beneits.of.the.
DFT.treatment.of.metal.atoms..The.CPMD.methodology.has.been.validated.for.use.in.
liquid.and.solution.studies..Simulations.of.pure.water.solutions.by.Geissler.et.al..[24].
and.previous.simulations.of.metal.ions.in.water.are.of.particular.interest.
All. CPMD. simulations. of. Al 3+ ,. Cr 3+ ,. Fe 3+ ,. As 3+ ,. and. As 5+ . ions. in. water. were.
performed.in.cubic.cells.containing.64.water.molecules.(the.same.number.of.water.