Chemistry Reference
In-Depth Information
3.5 Trapped Ion Electron Diffraction
TIED was also used in the structural determination of gold cluster ions. The
analysis of the diffraction measurements is appropriate since it directly relates to
the spatial arrangement of the scattering atoms. This technique however suffers
from limitations due to, for example, the atomic scattering intensities which limit
the lower end of detection and the inability to distinguish between isomers belong-
ing to the same structural family.
Gold cluster anions were studied by Schooss and Kappes et al. [
235
] as well as
by Parks et al. [
236
]. The work of Kappes et al. was recently reviewed [
169
]. Briefly,
the structures of the anions Au
n
(
n
11-20) have been determined by TIED and
compared to theoretically calculated structures. The smallest cluster studied,
Au
11
, possessed a flat C
s
structure as was predicted by theory [
237
]. The structures
of Au
n
(
n
¼
¼
11, 13) were reported to be in agreement with IMS studies (see
before) and PES (Sect.
3.6
). Au
n
(
n
16-20) possess a cagelike structural motif,
in agreement with previous PES studies. Parks et al.'s work on gold cluster anions
Au
n
(
n
¼
¼ 11-24) reported [
236
] structures that were also compared to DFT
calculations. For Au
n
(
n
11-12), structures were found that match those deter-
mined via IMS [
209
]. For Au
13
, however, a mixture of isomers was reported, in
disagreement with the previously mentioned TIED work. In addition the 3D
structures reported for Au
n
(
n
¼
14, 15) were in stark contrast to the previously
mentioned TIED work. Structures for Au
n
(
n
¼
16-20) were in agreement with
other TIED reports highlighting their cagelike structures (Fig.
22
). Au
21
and
Au
24
were reported to possess an elongated cage and a single-wall tube-like
structure, respectively (Fig.
22
).
Structures for gold cluster cations, Au
11-20
+
, were also studied by TIED. The 3D
structures of Au
11-13
+
were found to be in close agreement to those reported by
IMS. Au
14-17
+
were found to have layered structures, whereas a change in the
structural motif to decorated cagelike structures is observed for Au
18-20
+
with
Au
20
+
possessing 2 isomers.
In many cases, the minimum-energy structures obtained by calculations were not
consistent with the experimental scattering function. It was rather calculated struc-
tures with higher energies that fitted the experimental data.
¼
3.6 Photoelectron Spectroscopy
Another technique used in determining the structures of anionic clusters is PES. We
should note that experiments conducted in the last century generated useful data,
however failed to yield detailed structural information. In all cases, an odd-even
oscillation in the values of the electron affinity was observed [
233
,
238
]. Bare
