Chemistry Reference
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spectrum of [Au
25
(SCH
2
CH
2
Ph)
18
], [Au
38
(SCH
2
CH
2
Ph)
24
] and
[Au
68
(SCH
2
CH
2
Ph)
34
], a fragment corresponding to a loss of [Au
4
(SCH
2
CH
2
Ph)
4
]
appeared in positive mode, although the crystal structure of [Au
25
(SCH
2
CH
2
Ph)
18
]
did not show [Au
4
(SCH
2
CH
2
Ph)
4
] as a prominent feature [
148
-
150
].
A complementary technique which has been used for the analysis of ligand-
protected AuNPs in the gas phase is the ion mobility spectrometry (IMS). Separa-
tion selectivity in IMS reflects the ion size and more specifically ion surface area. In
IMS ions are injected into a gas-filled drift tube where they experience numerous
low-energy collisions with a background gas which separates ions based on the
ion-neutral collision cross sections (CCS). Smaller ions elute faster than larger ions
which experience more collisions. IMS has been used to measure the diameter of
ligand-protected AuNPs. Following the report of Jarrold [
151
], Kappes and col-
leagues [
152
] published two reports in 2002 on positively charged gold clusters
(
25 atoms). The ion CCS of each cluster provides convincing circumstantial
evidence for assigning geometries. The ability to distinguish between various
three-dimensional geometries illustrates the structural capabilities of IMS for
small gold clusters. Building on this foundation, Harkness et al. [
140
] have reported
the first application of combined ion mobility spectrometry-mass spectrometry
(IMS-MS) to the analysis of ligand-protected AuNPs. By integrating mass and
ion CCS separation, gold-thiolate ions can be isolated from nearly isobaric but
larger organic ions (i.e. chemical noise). IM-MS is well suited for studying frag-
ments generated from ligand-protected AuNPs, because of the signal-to-noise
enhancement and structural characterisation capability. Their study of AuNPs
protected by tiopronin or phenylethanethiolate by MALDI-IM-MS revealed signif-
icant features. In the negative ion mode, many of these fragments correlate to
capping structural motifs proposed previously. In the positive ion mode, the frag-
ment ions are nearly identical to the positive ions generated from the gold-thiolate
AuNP precursor complexes. This suggests that energetic processes during laser
desorption/ionisation induce a structural rearrangement in the capping gold-thiolate
structure of the AuNP. This results in the generation of positively charged gold-
thiolate complexes similar to the precursors of AuNP formation by reduction and
negatively charged complexes which are more representative of the AuNP surface.
The structures of small gold clusters in the gas phase have also been established
by a combination of mass spectrometry, vibrational spectroscopy and theoretical
calculations. Bare gold metal clusters with one or two krypton ligands are formed
by means of laser vaporisation from a gold rod in a continuous flow of helium and
krypton (1.5% Kr in He) at 100 K. The molecular beam is overlapped with a pulsed
FIR beam delivered by the Free Electron Laser for Infrared eXperiments (FELIX).
The neutral complexes are analysed in a time-of-flight mass spectrometer. The
subsequent heating of the complex results in the evaporation of a loosely bound
krypton ligand and a depletion of the corresponding mass spectrometric signal.
Recording the mass spectrometric signal while scanning the wavelength of FELIX
leads to depletion spectra, from which absorption spectra are reconstructed. The
geometries of the Au
x
clusters were established by comparing the experimental
spectrum to the calculated vibrational spectra for multiple isomers predicted by
density functional theory (DFT) calculations. For Au
7
Fielicke et al. proposed a
<
