Chemistry Reference
In-Depth Information
Fig. 8 IR-MPD spectra and
predictions of harmonic
IR spectra for isomers of
Au
4
Y-Au
6
Y(
light sphere
,
Y;
dark spheres
, Au;
black
sphere
, Xe). Relative
energies are given in
eV. From [
76
]
and experiment for the Au
4
Y and Au
5
Y clusters is excellent and the well-resolved
and narrow (bandwidth limited) lines are accurately reproduced by the harmonic
calculations. This is not so for Au
6
Y where the experimental spectrum consists of
broad features which are not adequately reproduced in the harmonic prediction.
This, it transpires, arises from an isomerisation occurring on the timescale of the
experiment between limiting forms of structure I passing through an intermediate of
D
6
h
symmetry. Interestingly,
this intermediate structure is predicted to be a
-aromatic system in the anionic cluster as it possesses 10 e
(cf
.
H¨ckel's 4
n
+ 2 electron rule). Further calculations reveal that such a cluster
should possess an appreciable ring current as determined by calculating nucleus-
independent chemical shifts, a hallmark of aromaticity [
76
].
All of the low-energy isomers calculated for the Au
n
Y clusters are given in Fig.
9
with the structures favoured on the basis of experiment and theory highlighted. In
general, the clusters are seen to adopt structures in which the number of Au-Y
bonds is maximised with the centred 7-member ring structure, which forms at
Au
7
Y, being a favoured motif for the larger clusters. This motif is not observed
for pure gold clusters, but is stabilised by the incorporated Y due to it being a 10 e
system (as a neutral subunit).
delocalised
˃
