Aqueous Solutions of Metal Ions - Metallic Systems: A Quantum Chemist's Perspective - page 117

the.ionization.of.the.water.molecule..In.the.following.80.fs,.the.proton.moves.away.
from. the. metal. ion. leaving. the. ion. coordinated. to. hydroxyl. groups. (Figure. 4.1B)..
Both.Zundel.and.Eigen.complexes.are.formed.once.the.proton.moves.to.the.second.
solvation.shell..Within.30.fs,.Zundel.complex.formation.is.observed.in.both.metal.ion.
solutions.via.hydrogen.bond.luctuations.(Figure.4.1C)..The.Eigen.complex.is.formed.
after.50.fs.in.both.solutions.via.proton.transfer.between.second.and.third.hydration.
shells.(Figure.4.1D)..Unlike.the.Al 3+ .solution,.no.immediate.return.path.to.the.undis-
sociated.state.of.water.is.observed.once.the.Zundel.and.Eigen.complexes.are.formed.
around.Cr.and.Fe.ions.in.water.during.our.simulation..We.observe.fast.interconver-
sion.between.the.Zundel.and.the.Eigen.complexes.in.agreement.with.the.theoretical.
and.computational.studies.of.a.proton.in.pure.water.
The. Zundel. complex. is. formed. in. the. second. hydration. shell. and. the. Eigen.
complex. is. located. between. the. second. and. third. shells.. The. Zundel. complex. is.
1.5.Å.closer.to.these.transition.metal.ions.than.the.Eigen.complex..Unlike.earlier.
calculations,.which.proposed.a.symmetric.Zundel.complex.geometry,.we.note.that.
the.Zundel.complex.in.these.transition.metal.ion.aqueous.solutions.is.asymmetric..
The. H + . is. coordinated. to. two. water. molecules. with. average. H + -O. distances. of.
1.16.and.1.19.Å..The.slight.difference.is.inluenced.by.the.close.proximity.of.the.
Fe. and. Cr. ions. in. the. aqueous. solution.. The. oxygen. atoms. that. are. separated. by.
the.hydrogen-bonded.proton.are.separated.by.2.2.Å..This.value.is.not.signiicantly.
different. from. other. computed. values. reported. for. the. Zundel. complex.. In. addi-
tion,. the. asymmetric. binding. of. the. proton. to. the. two. water. molecules. has. been.
reported. in. recent. theoretical. studies. using. state-of-the-art. irst. principles. meth-
ods..The.average.H + -O.distance.is.larger.for.the.Eigen.complex.than.for.the.Zundel.
complex,.with.a.value.of.1.59.Å.in.the.vicinity.of.Fe.and.Cr.ions..This.trend.is.in.
excellent.agreement.with.studies.performed.for.pure.water..Overall,.the.structural.
parameters.that.we.report.here.are.in.excellent.agreement.with.experimental.and.
recent.theoretical.studies.for.pure.water,.with.a.difference.in.structural.parameters.
of.1%-3%.
Calculations.of.the.change.in.the.Gibbs.free.energy.associated.with.proton.trans-
fer.in.aqueous.solutions.of.Fe.and.Cr.using.perturbation.theory.(see.Equation.4.2).
show. that. the. Zundel. complex. is. thermodynamically. more. stable. than. the. Eigen.
complex.by.2.6.and.3.2.kJ.mol −1 ,.respectively..Recalling.that.these.simulations.and.
calculations. were. performed. at. room. temperature,. we. note. that. these. Gibbs. free.
energy.difference.values.are.small,.similar.to. kT .(2.5.kJ.mol −1 ),.which.indicate.that.
the.aqua.ion.complexes.might.coexist.
The. structures. for. Cr 3+ . and. Fe 3+ . in. water. are. all. in. octahedral. arrangements..
Speciically,.we.ind.that.two.and.one.hydroxyl.groups.(OH).are.coordinated.to.the.
Fe 3+ .and.Cr 3+ .ions.on.the.average,.respectively..Our.results.indicate.a.lower.pH.value.
for.the.Fe 3+ .solution.as.compared.to.the.pH.of.the.Cr 3+ .solution..In.fact,.p K a .values.
and. hydrolysis. constant. values. support. these. indings,. which. indicate. that. hydrox-
ylation. should. be. easier. for. the. Fe 3+ . ion. than. for. the. Cr 3+ . ion. in. aqueous. solution..
Structural.parameters.for.the.irst.and.second.shell.water.molecules.around.these.two.
transition.metal.ions.are.presented.in.Table.4.2..The.predicted.structural.properties.
for.the.hydrated.Fe 3+ .and.Cr 3+ .ions.in.water.agree.with.spectroscopic.measurements.
within.0.2%.