Chemistry Reference
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need to be explicitly considered in order to explain the observed far-IR-MPD
spectra. For all the sizes shown, theory predicts the presence of isomers within
about 0.02 eV; however in all cases, only a single isomer (top row in Fig.
7
)is
needed to satisfactorily explain the observed spectra [
60
].
3.3 Doped Species
In addition to studies on pure Au clusters, there has been a wealth of investigations,
both experimental and theoretical, on gold clusters containing impurity atoms. By
introducing such dopants, new physical properties may emerge, e.g. related to
modifications of the geometric and/or electronic structure. Depending on the dopant
element, it may act as an electron donor or acceptor in essence changing the number
of valence electrons in the cluster [
63
-
71
]. Clearly such a topic has the potential to
be massive in scope and here we focus on the remarkable case of Au
16
as being
exemplary [
72
-
75
].
Au
16
is found to form a stable cage as an anion. For the transition metal elements
(including the isoelectronic Cu and Ag), these dopant atoms can be incorporated
inside this hollow Au
16
cage with the s electrons of the metal being donated to the
valence orbitals of the gold cluster. For 1 e
donor species (Cu, Ni and Ag), this
results in a stable
anionic
18 e
valence electron system with the slight further
complication in the case of Ni arising due to the additional d-electrons [
73
]. For the
other d-metals, they instead donate 2 electrons into the valence electron system,
i.e. the 18 valence electron system is realised in the
neutral
species, with the anionic
clusters showing the opening of a new electronic shell. Notably, and of great
interest for the development of future nanomaterials, the resultant transition metal
centres do not experience a quenching of the latent spin moments arising from the
d-electrons in such an interaction. This provides the tantalising possibility that the
atomic-like magnetism can be preserved and protected by encapsulation into a
golden cage [
73
].
Lastly, Au
16
has been doped with the main group elements Si, Ge and Sn. With
these elements, an altogether different binding motif is observed. Instead of
endohedral doping, they displace a Au atom from the cage structure; in the latter
two cases, this atom is relocated to a capping site elsewhere on the gold cage,
whilst for Si this displaced atom becomes a “dangling” atom bound to the Si itself.
This is in line with the recently observed strong Au-Si bonds and observation of
gold-based silane analogues [
65
-
67
,
75
].
Comparable experimental information for the neutral species is scarce. To the
best of our knowledge, the only such data relates to the small yttrium-doped gold
clusters [
76
,
77
] and a sample of the recorded IR-MPD spectra (measured by
monitoring depletion of the parent xenon complexes) as well as comparisons to
predicted harmonic spectra are presented in Fig.
8
. The agreement between theory
