Aqueous Solutions of Metal Ions - Metallic Systems: A Quantum Chemist's Perspective - page 109

theoretical.predictions.challenging..Another.reason.lies.in.the.nature.of.the.solution.
environment.itself..Even.though.water.is.the.most.abundant.and.widely.used.solvent.
on. our.planet,. and. despite. the. great.amount.of. experimental.and. theoretical.study.
on.water,.there.is.currently.considerable.debate.over.the.structure.of.pure.water.at.
room. temperature. due. to. differing. interpretation. of. experimental. studies. such. as.
x-ray. absorption. and. neutron. diffraction. [20,21].. Quantum. chemistry. calculations.
are.also.employed.on.both.sides.of.this.debate..Direct.measurement.of.the.local.elec-
tronic.structure.of.solvents.is.immensely.dificult..Some.information.can.be.obtained.
from.x-ray.absorption.studies.of.liquids,.which.may.be.used.to.calculate.the.radial.
distribution. function,. a. quantity. that. is. directly. comparable. to. the. data. that. may.
be. extracted. from. simulations.. However,. radial. distribution. functions. are. usually.
spherically. averaged. and. information. associated. with. anisotropic. and. asymmetric.
characters.is.lost.[22,23].
Another.area.that.has.been.examined.by.state-of-the-art.theoretical.and.experi-
mental.techniques.is.the.autoionization.of.water.and.the.dissociation.of.water.in.dense.
systems.[24-35]..From.a.theoretical.standpoint,.special.sampling.techniques.coupled.
with. ab. initio. molecular. dynamics. simulations. can. provide. unprecedented. insight.
into.dynamical.events.in.water.[24]..One.might.be.surprised.to.learn.that.the.quest.
to.understand.proton.transfer.in.water.has.been.going.on.for.more.than.200.years..
Grotthuss.introduced.the.concept.of.“structural.diffusion”.more.than.200.years.ago.
and.described.the.mechanism.of.proton.transfer.along.a.network.of.hydrogen.bonds..
Some.of.the.most.important.aspects.of.understanding.the.behavior.of.the.proton.in.
solution,.and.probably.the.link.to.a.deeper.understanding.of.acid.and.base.chemistry.
in. aqueous. solution,. include. the. structure. and. energetics. of. hydrated. protons. that.
form. during. the. course. of. the. dynamics. in. dense. aqueous. solutions. [36-43].. Our.
modern. understanding. of. hydrated. protons. is. dominated. by. the. well-known. Eigen.
and. Zundel. complexes.. In. the. middle. of. the. nineteenth. century,. Eigen. proposed. a.
H 9 O + .proton.complex.in.which.the.hydronium.ion.(H 3 O + ).is.coordinated.to.surround-
ing.water.molecule.via.hydrogen.bonding.interactions.(Figure.4.1),.and.Zundel.pro-
posed. a. H 5 O + . proton. complex. in. which. a. proton. (H + ). is. coordinated. to. two. water.
molecules. via. hydrogen. bonding. interactions. in. dense. water. (Figure. 4.1). [44-46]..
The. structural. properties. and. associated. energetics. of. these. proton. complexes. in.
dense.aqueous.solutions.containing.various.inorganic.and.organic.systems.are.still.
being.investigated.and.debated.just.as.in.the.case.of.pure.water.solutions.[47-51].
Proton. transfer. studies—mostly. in. pure. water. and. in. organic. solutions—have.
beneited. from. the. application. of. ab. initio. molecular. dynamics. simulations. dur-
ing. the. last. decade.. In. particular,. Car-Parrinello. molecular. dynamics. (CPMD).
simulations. have. been. extremely. useful. in. studies. of. hydrogen-bonded. com-
plexes.[52-58]..The.electronic.structure.of.the.Zundel.complex.in.pure.water.has.
been. studied. using. irst. principles. calculations. that. ind. the. favored. position. of.
the.proton.in.the.middle.of.the.two.water.molecules..This.arrangement.is.termed.
“symmetrically. coordinated.”. It. is. clear. that. a. quantitative. description. including.
microscopic.properties.with.hydrogen.bond.interactions.in.the.presence.of.solvent.
luctuations. is. required. for. the. proper. description. of. liquid. water.. The. introduc-
tion. of. AIMD. simulations,. such. as. Born-Oppenheimer. and. CPMD. simulations,.
enable.dynamics.studies.of.proton.transfer.with.solvent.luctuations.including.the.