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Fe(C
2
H
2
)
Fe(CCH
2
)
Fe
2
(C
2
H
2
)
Fe
2
(CCH
2
)
I-1
I-2
II-1
II-2
II-3
II-4
II-5
II-6
Fe
3
(C
2
H
2
)
Fe
3
(CCH
2
)
Fe
4
(C
2
H
2
)
Fe
4
(CCH
2
)
III-1
III-4
III-7
III-10
IV-1
IV-4
IV-7
IV-10
IV-13
III-2
III-5
III-8
III-11
IV-2
IV-5
IV-8
IV-11
IV-14
III-3
III-6
III-9
III-12
IV-3
IV-6
IV-9
IV-12
IV-15
FIGURE 9.30
Initial.structures.used.to.study.the.adsorption.of.an.acetylene.or.a.vinylidene.
molecule. on. iron. clusters.
Fe /F
n n
+
. (
n
.=.1-4).. (Reprinted. from. Chretien,. S.. et. al.,.
J. Chem.
Phys
.,.119(23),.December.5,.12279,.2003..With.permission.)
.single-point.calculations.using.deMonKS3P4)..For.the.gas-phase.reaction,.activation.
energies.of.182.7.and.199.3.kJ/mol.are.obtained.with.the.PP86.and.PLAP3.function-
als,.respectively..These.two.DFT.results.bracket.the.CCSD(T).value.of.187.4.kJ/mol..
The.values.of.the.isomerization.energy.and.the.activation.energy.are.shown.in.Table.
VI.of.Ref..[217],.shown.here.as.Figure.9.32..The.missing.values.are.due.to.our.inabil-
ity.to.converge.some.of.the.transition.states.with.Gaussian94..The.activation.ener-
gies.are.all.very.high.and.greater.than.the.energy.that.would.be.available.from.the.
adsorption.of.a.single.acetylene.molecule.on.the.clusters..Consequently,.we.believe.
that.vinylidene.will.not.be.involved.in.the.reaction.mechanism.
In.the.third.paper,.Chrétien.and.Salahub.go.as.far.as.the.then.current.technology.
would.take.them.toward.mapping.out.the.mechanism.for.the.formation.of.benzene.
from.acetylene.on.the.small.clusters..A.schematic.representation.of.the.mechanisms.
considered.is.shown.in.Figure.9.33.
All.of.these.steps.were.considered.in.a.truly.extensive.(but.of.course.not.exhaustive).
series.of.calculations.that.involved.some.381.starting.structures.and.more.than.2000.
unconstrained. geometry. optimizations. for. the. various. spin. states.. Some. transition.
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