Chemistry Reference
In-Depth Information
anionic gold clusters were widely studied in particular by Lai-Sheng Wang
et al. who have recently reviewed their wide body of work (see [
239
] with
references therein) and included some comparisons to other reports. Briefly, the
structures of the anionic clusters Au
n
(
n
¼
3-15) were reported as 2D flat struc-
tures for
n
12 [
240
]. This was surprising since metal clusters are expected to
form 3D structures in this size range. For
n
¼
12, a mixture of isomers (2D and 3D)
has been reported, and hence this size represents the transition from 2D to 3D
structures for gold cluster anions. In the case of
n
13-15, 3D structures are
expected. Mixtures of isomeric structures were identified for most of the cluster
sizes for
n
¼
¼ 7-15.
Argon tagging and oxygen titration were used to shed more light on the struc-
tures of Au
n
[
241
]. For instance, they were used to resolve the issue of isomeric
Au
10
where four distinct isomers were found to coexist and they were readily
distinguished (Fig.
23
). It should be noted that the formation of a superoxo was
observed and confirmed via PES, upon adsorption of O
2
onto gold cluster anions
(Au
n
) with an even number of gold atoms [
242
,
243
]. In addition, substitution in
Au
n
by isoelectronic Cu or Ag was also used as another method that provide
information on the various isomers [
244
].
Structures of the gold clusters, Au
n
, in the range
n
¼
16-19 [
245
] were also
reported (Fig.
24
). The major isomers found for
n
16-18 were dominated by
hollow cage motifs, with the Au
16
structure being the most interesting. This highly
symmetrical cage structure (T
d
symmetry) shows, amongst others, a break from the
odd-even trend observed for coinage metal cluster ions. Au
19
structure was
resolved and assigned as a single isomer with a pyramidal structure. A transition
from cage structure to pyramidal is observed at Au
18
.
A tetrahedral symmetric structure for Au
20
was reported (Fig.
24
)[
203
]. This
highly symmetric cluster is special, possessing a large HUMO-LUMO gap of
1.77 eV reminiscent of C
60
. Excluding Au
2
and Au
6
, this HUMO-LUMO gap is
the largest amongst all known coinage metal clusters [
238
]. The high symmetry of
this structure led several researchers to attempt the synthesis of ligand-protected
golden pyramids with an Au
20
core. For example, the isolation of the thiolate-
capped Au
20
(SCH
2
CH
2
Ph)
16
cluster has recently been reported [
246
].
The experimental vertical detachment energies and adiabatic detachment ener-
gies measured for Au
n
(
n
¼
¼
2-20) gold cluster anions via PES are listed in
Table
6
.
Few experimental structures of the anions in the range Au
n
n
21-35 have
been proposed [
247
-
249
] to range from pyramidal to tubular to core-shell. Au
21
possesses a pyramidal structure, whereas Au
22,23
were reported to have mixed
isomers: pyramidal and fused planar. Au
24
was found to be tubular and Au
25
with
a core-shell structure. This latter cluster is the smallest anionic gold cluster with an
atom in the core. Larger clusters were reported to have low-symmetry core-shell
structures.
¼
