Carbohydrate and Trivalent Iron Ion Interactions in the Gas Phase and in Aqueous Solution - Metallic Systems: A Quantum Chemist's Perspective - page 96

3.4.2 c oordination c hemiStry S tudieS of m and
f e 3+ in a queouS S olution
Chemical.reactivity.calculations.using.the.condensed.FFs.(Table.3.1).on.a.particular.
geometry.of.M.in.water.extracted.from.CPMD.simulations.indicate.that.the.O2.atom.
(Figure.3.1).with.a. f HOMO. value.of.0.18.possesses.a.strong.nucleophilic.character..The.
NPA.shows.that.the.O2.atom.has.a.larger.partial.negative.charge.than.the.ring.oxy-
gen.(O5).and.methoxy.oxygen.(O6).in.aqueous.solution.(see.Table.3.2)..This.analysis.
indicates. that. the. hydroxyl. oxygen. atoms. of. aqueous. M. have. a. better. Lewis. base.
reactivity.than.the.ring.and.methoxy.oxygen.atoms..This.inding.is.in.agreement.with.
our.results.obtained.for.M.in.the.gas.phase.(see.above),.which.show.O2,.O3,.and.O4.
atoms.as.being.reactive.toward.Fe.coordination.
The. CPMD. simulations. of. M. in. aqueous. solution. show. that. a. water. molecule.
is.coordinated.to.the.O2.atom.after.14.ps.and.stays.coordinated.for.the.rest.of.the.
simulation.(56.ps)..This.inding.supports.the.FF.analysis,.which.shows.that.the.O2.
atom.is.the.most.reactive.site.of.M.toward.Fe.coordination..The.coordination.of.a.
water.molecule.to.the.O2.atom.and.electronic.and.steric.effects.seem.to.decrease.
the.reactivity.of.the.O2.atom.toward.Fe.coordination.in.water..CPMD.simulations.
show.that.a.single.Fe 3+ .ion,.which.is.initially.coordinated.to.six.water.molecules.in.
an.octahedral.arrangement,.coordinates.to.the.O3.and.O4.atoms.within.27.ps.and.
stays.coordinated.for.the.rest.of.the.simulation.(43.ps).as.shown.in.Figure.3.5..CPMD.
simulations.further.show.that.two.water.molecules.bind.to.the.carbohydrate-coor-
dinated.single.Fe 3+ .ion.in.aqueous.solution.within.31.ps,.with.two.water.molecules.
and.two.hydroxyl.atoms.(O3.and.O4).of.M.placed.in.a.square-planar.arrangement.
around.the.metal.ion.(see.Figure.3.5)..In.other.words,.these.simulations.show.that.
the.Fe 3+ .ion.interacts.with.two.types.of.ligands;.the.carbohydrate.hydroxyl.groups.
and.water.molecules.
The.partial.charges.indicate.that.the.O3.atom.possesses.a.slightly.smaller.partial.
negative.charge.in.the.vicinity.of.the.Fe 3+ .ion.(Table.3.2)..The.distances.between.the.
Fe 3+ .ion.and.O3.and.O4.atoms.are.3.1.±.0.1.and.4.0.±.0.1.Å,.respectively..NPA.results.
support.this.inding.and.indicate.a.smaller.negative.charge.for.O3.than.for.O4..This.
effect.is.a.result.of.Fe.coordination..A.comparison.with.our.results.for.the.gas.phase,.
which.are.presented.above,.shows.that.the.Fe.and.O3.and.O4.distances.are.slightly.
larger.(about.10%).in.water.than.those.in.the.gas.phase..This.result.may.be.due.to.
the.coordination.of.additional.water.molecules.to.Fe..The.calculated.binding.energy.
for.the.Fe 3+ .ion.coordination.to.the.aqueous.carbohydrate.is.-98.7.kJ.mol −1 ,.indicat-
ing.that.the.single.transition.metal.ion.coordination.is.thermodynamically.favorable.
In. summary,. in. comparison. to. the. studies. of. M-Fe 3+ . in. the. gas. phase,. we. ind.
that.the.presence.of.water.impacts.the.coordination.of.the.Fe 3+ .ion.to.the.carbohy-
drate..Our.gas-phase.studies.show.a.coordination.of.the.metal.ion.to.O2,.O3,.and.O4.
atoms,.but.the.solution-phase.CPMD.simulations.show.that.the.transition.metal.ion.
is.coordinated.to.O3.and.O4,.and.not.to.O2..It.is.found.that.a.water.molecule.is.coor-
dinated.to.O2.instead,.with.an.intermolecular.hydrogen.bond.distance.of.2.8.±.0.2.Å.
(Figure. 3.5. and. Table. 3.4).. Results. regarding. inter-. and. intramolecular. hydrogen.
bonding.interactions.are.discussed.in.the.following.paragraphs..Our.CMD.simula-
tions.(Figure.3.5).do.not.seem.to.detect.a.coordination.between.M.and.Fe.in.water.