Chemistry Reference
In-Depth Information
2 Characterization
Identification and characterization of PGCs have been achieved in terms of
nuclearity (size), molecular formulae, and geometrical structures by various ana-
lytical techniques. Among them, single-crystal X-ray crystallography is still the
most definitive technique to elucidate the geometrical structures, but it is limited to
the clusters that give high-quality crystals suitable for the structure determination.
Recently mass spectrometry coupled with soft-ionization techniques (e.g., electro-
spray ionization (ESI), matrix-assisted laser desorption ionization (MALDI)) are
found to be powerful tools to obtain direct information about the molecular mass
and formulae of cluster species. ESI-mass spectroscopy is especially useful since it
can analyze solution samples, allowing the direct analyses of the reaction mixtures.
However, one must note that the observed peak, especially before the purification
procedure, may be a result of fragmentation/recombination in the gas phase, and the
results sometimes strongly depend on the measurement conditions (e.g., voltage).
Further, mass spectra give only molecular formula information so the geometrical
structures must be deduced with the aid of other characterization methods.
1
H and
31
P NMR spectra are useful to investigate the dynamics of the coordinated ligands
on the cluster surface, but are not suitable to obtain definitive molecular structures.
TEM has been used to estimate the size and nuclearity of large clusters but the
application for small clusters is limited because of the resolution limitation.
Electronic absorption (UV-visible) spectroscopy is classical but has been most
conveniently utilized as a characterization tool. The spectral features of cluster
species in the subnanometer-regime are unique to individual clusters and are
sensitive to the structures/nuclearity of the cluster compounds, so it is possible to
identify unknown species based on the spectral pattern of structurally identified
authentic samples. Further, since the spectral profiles are closely correlated with the
electronic structures, they are useful to understand the geometry- or nuclearity
(size)-dependent profiles of PGCs. This aspect will be discussed in Sect.
5
.
3 Synthesis
Generally, PGCs are synthesized in homogeneous solution either by the reduction
of ionic gold sources (e.g., gold(I) phosphine complex, tetrachloroaurate) or by the
post-synthetic methods involving the growth and/or etching of preformed PGCs.
Other methods have also been known, but most of the synthetic procedures reported
to date fall into these two categories. The representative examples were summa-
rized in Schemes
1
,
2
,
3
,
4
,
5
,
6
,
7
,
8
, and
9
in terms of the reaction types.
