Chemistry Reference
In-Depth Information
experimental. techniques,. new. theoretical. methods,. and. high-speed. computers. at.
reasonable. cost. (see,. e.g.,. Refs.. [94-98]. and. references. therein).. However,. deter-
mining.the.geometrical.structure.of.TM.clusters.remains.still.a.dificult.task,.and,.
with. only. a. few. exceptions,. the. structures. are. still. unsolved.. Only. with. detailed.
structural.information.can.models.for.reactive.processes.of.these.clusters.be.devel-
oped..Given.the.lack.of.experimentally.available.data.for.these.systems,.theoreti-
cal.methods.have.to.be.used.to.determine.the.structure..A.major.drawback.for.the.
theoretical.investigations.of.TM.clusters.is.the.required.accuracy.in.the.treatment.of.
electron.correlation.that.signiicantly.increases.the.computational.effort.in.Hartree-
Fock-based. methods.. The. development. of. density. functional. methods. and. related.
computer. codes. has. changed. this. situation. considerably,. and. it. is. now. possible. to.
perform. reliable. calculations. of. small. clusters. at. reasonable. computational. cost.
[30,99-107].
The.possibility.of.combining.theoretical.calculations.with.available.experimental.
data. from. pulsed-ield-ionization-zero-electron-kinetic-energy. (PFI-ZEKE). photo-
electron. spectroscopy. to. resolve. the. structure. of. small. TM. clusters. has. attracted,.
in. the. past. few. years,. more. and. more. attention.. A. combination. of. DFT. and. har-
monic.Franck-Condon.factor.calculations.has.been.used.to.determine.the.structures.
of.some.substituted.TM.clusters.[108-111]..The.PFI-ZEKE.technique.was.also.used.
to.determine.accurate.adiabatic.ionization.potentials.of.bare.V
3
.and.V
4
.clusters.[112]..
The.observation.of.a.PFI-ZEKE.spectrum.of.V
3
.with.well-resolved.vibrational.bands.
has. opened. the. possibility. for. a. structure. determination. via. the. above-described.
technique.
Additional.relevant.literature.[113-142].about.vanadium.clusters.may.be.summa-
rized. as. follows.. Divanadium. has. been. experimentally. well. determined. [113,119]..
The. uncertainty. concerning. the. nature. of. its. chemical. bonding. has. been. bril-
liantly.cleared.up.with.detailed.gas-phase.electronic.spectroscopy.experiments.by.
Langridge-Smith.et.al..[114]..Under.high.resolution,.the.rotational.structure.of.the.
observed. bands. could. be. assigned,. and. the. ground. state. of. V
2
. was. determined. to.
be.
3
∑
,. with. a. bond. length. of. 1.77. Å.. Later. on,. Spain. et. al.. extended. this. work.
into. the. infrared. [115].. Moreover,. Spain. and. Morse. have. used. two-color. resonant.
two-photon. ionization. (R2PI). spectroscopy. to. measure. the. dissociation. energy. of.
V
2
,.and.a.value.of.
D
0
.=.2.75.eV.was.obtained.[116]..The.matrix-isolated.resonance.
Raman.spectra.of.divanadium.have.been.extensively.investigated.by.Moskovits.et.
al.. [117],. and. a. frequency. of. 537.cm
−1
. was. assigned. to. the. V
2
. ground. state.. Later.
on,.James.et.al..recorded.photoionization.eficiency.(PIE).spectra.to.determine.the.
adiabatic.ionization.potential.of.V
2
.as.6.36.eV.[118]..They.were.also.able.to.assign.
the.ground.state.of.
V
2
+
.as.
4
g
∑
.[118]..With.the.available.information.of.the.intermedi-
ate.states.and.ionization.potentials,.Hackett.et.al..measured.the.PFI-ZEKE.spectra.
of. the.
V
2
+
. ground. state. [118].. Simard. et. al.. carried. out. a. detailed. analysis. of. the.
rotational.spectra.and.determined.the.bond.length.[98].of.the.
4
g
∑
.ground.state.to.
be.1.73.Å..For.larger.vanadium.clusters,.up.to.22.atoms,.ionization.threshold.ener-
gies.have.been.reported.[120]..Vanadium.clusters.have.also.been.included.in.sev-
eral.reactivity.studies.[121-128]..Stern-Gerlach.measurements.of.vanadium.clusters.
have.been.performed.by.Douglass.et.al.,.and.small.moments.have.been.determined.
g
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