Exploring the Borderland between Physics and Chemistry: Theoretical Methods in the Study of Atomic Clusters - Metallic Systems: A Quantum Chemist's Perspective - page 372

the.big.bang.method.are.described.and.illustrated..We.discuss.the.theoretical.tools.
used. in. cluster. studies. to. describe. the. chemical. bonding. and. to. predict. chemical.
reactivity..In.particular,.we.emphasize.the.use.of.the.electron.localization.function.
(ELF).and.the.Fukui.function.for.these.purposes..We.also.include.a.brief.discussion.
of.recent.reports.of.unusual.physical.properties.of.metallic.clusters.and.the.applica-
tion.of.density.functional.theory.(DFT).methods.to.shed.light.on.the.physics.of.these.
phenomena..Of.special.interest.is.magnetism.observed.in.clusters.and.ilms.of.gold,.
a.typically.diamagnetic.metal.
11.1 INTRODUCTION
In.recent.times,.there.have.been.many.investigations.that.lie.at.the.border.between.
physics.and.chemistry..It.is.not.new.to.these.sciences;.it.has.happened.in.the.past.with.
many.other.disciplines,.such.as.the.kinetic.theory.of.gases,.low.temperature.studies,.
theory.of.solutions,.the.study.of.physical.properties.of.compounds,.molecular.struc-
ture.studies.[2,.and.references.therein]..There.is.invariably.a.great.deal.of.work.that.
must.be.done.before.disciplines.at.the.frontier.of.two.sciences.become.an.integral.
part.of.one.another..The.experimental.and.theoretical.study.of.atomic.clusters.is.a.
modern.discipline.that.lies.between.physics.and.chemistry..It.is.a.discipline.in.the.
making;.simply.inding.an.unambiguous.deinition.of.atomic.clusters.is.a.dificult.
task.
Most.researchers.“deine”.a.cluster.similar.to.the.deinition.given.by.Connerade.
et.al..[3],.“a.group.of.atoms.(structural.subunits).bound.together.by.interatomic.forces.
is.called.a.cluster..There.is.no.qualitative.distinction.between.small.clusters.and.mol-
ecules,.except.perhaps.that.the.binding.forces.must.be.such.as.to.permit.the.molecule.
(system).to.grow.much.larger.by.stacking.more.atoms.or.molecules.(subunits).of.the.
same.type.if.the.system.is.to.be.called.a.cluster.”.It.is.worth.mentioning.that.from.the.
chemist's.perspective,.any.arrangement.of.atoms.without.a.covalent.bond.should.not.
be.considered.a.molecule..A.typical.covalent.bond.implies.binding.energies.greater.
than.15.kcal/mol.and.bond.distances.smaller.than.2.0.Å..In.fact,.the.nature.of.chemi-
cal.bonding.in.clusters.can.vary.strongly..For.instance,.magnesium.dimer.is.a.van.
der.Waals.system,.as.are.Mg 3 .and.Mg 4 ..Magnesium.clusters.with.more.than.10.atoms.
are.more.covalent.in.nature.and.the.largest.clusters.at.some.point.should.experience.
a.transition.to.metallic.bonding.characteristic.of.a.solid..In.reality,.the.binding.forces.
in. atomic. clusters. may. be. metallic,. covalent,. ionic,. hydrogen-bonded,. or. van. der.
Waals.in.character.
The.structural.and.compositional.diversity.of.clusters.is.huge..All.the.units.(com-
ponents).of.a.cluster.are.not.necessarily.atoms.or.monatomic.ions..For.example,.a.
cluster.may.be.an.arrangement.of.a.metal.core.with.one.or.several.atoms.surrounded.
by. molecules,. so-called. ligands,. or. may. be. exclusively. formed. by. molecules.. It.
should.be.noted.that.ligands.play.a.stabilizing.role.in.chemically.synthesized.metal.
clusters,.and.consequently,.ligands.substantially.modify.the.physical.and.chemical.
properties. of. the. clusters.. Throughout. this. chapter. we. shall. refer. to. bare. clusters.
(atomic.clusters).with.a.single.component,.unless.otherwise.stated..The.properties.
of. the. atomic. clusters. can. vary. from. atomic. to. bulk. properties,. but. are. also. often.
unique.depending.on.their.size.and.elemental.composition..Many.clusters.with.sizes.