Chemistry Reference
In-Depth Information
comparable.to.
kT
,.which.indicates.that.both.conigurations.are.likely.to.exist.in.aque-
ous.solution..The.hydroxyl.oxygen.atoms,.which.are.connected.to.Al.ions,.are.hydro-
gen.bonded.to.water.molecules.that.are.between.the.Al.centers,.which.in.turn.interact.
with.the.neutral.water.molecules.coordinated.to.the.neighboring.Al.ion.via.hydrogen.
bonding.(Figures.4.3B.through.D)..We.observe.an.elongation.of.the.Al-OH
2
.bond.
of.0.2.Å.after.1.3.ps.(Figure.4.3D),.which.is.in.agreement.with.our.previous.studies..
The. proton. attached. to. the. water. molecule. forms. a. Zundel-like. complex. with. the.
neighboring.water.molecule.in.which.the.proton.is.coordinated.both.to.the.oxygen.
atom. of. the. neutral. water. molecule. and. to. the. oxygen. atom. hydrogen. bonded. to.
the. Al. ion. (Figure. 4.3D).. Next,. the. Al-OH
2
. bond. distance. for. the. Al-coordinated.
water. molecule,. which. participates. in. a. Zundel-like. structure,. increases. by. 0.3. Å.
and. the. Zundel-like. complex. moves. to. the. second. shell. via. proton. transfer. where.
it.forms.Eigen-like.complexes.between.the.second.and.third.shells.within.the.next.
0.2.ps. (Figure. 4.3E).. The. system. does. not. interconvert. between. Zundel. and. Eigen.
complexes.and.an.immediate.return.to.the.neutral.state.of.water.is.observed.in.the.
next.few.femtoseconds.unlike.the.proton.transfer.mechanism.found.in.aqueous.solu-
tions.of.transition.metal.ions.(see.above)..We.observe.an.elongation.of.another.H-OH.
bond. coordinated. to. the. second. Al. center,. and. a. second. Zundel-like. complex. for-
mation..The.distance.between.Al.ions.decreases.to.3.6.Å.in.this.structure.and.the.
irst.hydroxyl.oxygen.atom.coordinates.to.the.next.Al.center.(Figure.4.3F)..It.takes.
a.slightly.shorter.time.(<0.4.ps).for.the.second.hydroxyl.oxygen.atom.to.coordinate.
to.the.neighboring.Al.center.(Figure.4.3G)..Once.the.aqua.ions.move.to.the.second.
and. third. shells,. another. water. molecule. from. the. second. shell. coordinates. to. one.
Al.center.within.the.next.1.4.ps.at.a.distance.of.2.12.Å.from.the.Al.center.(Figure.
4.3D).. The. coordination. of. the. second. shell. water. molecule. to. the. second. Al. ion.
takes.0.6.ps.longer.compared.to.the.coordination.of.a.neutral.water.molecule.to.the.
irst.Al.center.(Figure.4.3G)..In.addition,.the.distance.from.this.coordinating.water.
molecule. oxygen. to. the. Al. center. is. slightly. larger. with. a. value. of. 2.17. Å.. After.
5.8.ps,.the.distance.between.the.Al.ions.becomes.smaller.(3.05.Å).and.the.aqueous.
dimer.Al
2
(OH)
2
(H
2
O)
4+
.structure.is.formed.with.an.average.Al-(OH)-Al.bond.angle.
of. 103.2°.. Calculations. of. free. energy. indicate. that. the. Al
2
(OH)
2
(H
2
O)
4+
. structure.
is.thermodynamically.preferred.(by.10.4.kJ.mol
−1
).over.the.existence.of.the.water-
separated.single.Al(OH)(H
2
O)
2+
.species.with.Al.ions.separated.by.5.2.Å.in.aqueous.
solution. (Figure. 4.4).. Overall,. these. results. support. previous. experimental. obser-
vations. and. quantum. mechanical. calculations,. which. proposed. hydroxyl-bridged.
poly-Al.structures.in.water..Our.simulations.indicate.that.both.water.exchange.and.
substitution.reactions.take.place.in.the.formation.of.Al.ion.dimers.in.aqueous.solu-
tion. at. room. temperature. and. a. pressure. of. 0.1.MPa,. supporting. previous. reaction.
mechanisms.proposed.on.the.basis.of.static.quantum.calculations.
For. the. Zundel-like. complex,. which. we. observe. in. the. second. shell. for. a. very.
short. time,. we. note. that. the. proton. is. asymmetrically. coordinated. to. the. neutral.
water.molecule.oxygen.atom.and.to.the.oxygen.atom.attached.to.Al.with.H
+
-O.dis-
tances.of.1.18.and.1.15.Å,.respectively..The.O-O.distance.(water.molecule.oxygen.
atom.to.an.oxygen.atom.attached.to.Al).is.2.22.Å..The.Eigen.complex.that.is.formed.
for.a.very.short.time.between.the.second.and.third.shells.has.H
+
-O.distances.of.1.59.
and.1.62.Å.with.an.average.O-O.distance.(the.distance.between.the.water.molecule.
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