Chemistry Reference
In-Depth Information
3.1.1 Collision-Induced Dissociation and Photodissociation of Gold
Cluster Ions in the Gas Phase
Bare Gold Cluster Cations and Anions
Schweikhard's group has widely studied the fragmentation reactions of bare gold
cluster anions and cations in a Penning ion trap using a range of activation
techniques including CID [
170
-
178
], PD [
176
,
179
-
195
] and electron-induced
dissociation (EID) [
196
-
201
]. The latter studies are discussed in Sect.
3.1.2
below.
The collision of positively charged gold clusters, Au
n
+
n
23 atoms),
with rare gases, results in dissociation via loss of a single gold atom, (Eq. (
7
),
z
(2
+1), or two gold atoms. The loss of two atoms occurs most probably through
the emission of a dimer, (Eq. (
8
),
z
¼
+1), rather than by a sequential evaporation.
The minimum kinetic energies of clusters required to induce dissociation exhibit a
pronounced odd-even effect. Clusters with an even number of delocalised electrons
are more stable than those with an odd number. This was observed for Au
n
+
,
n
¼
23, monomer loss is the main fragmentation pathway
suggesting that the binding energies of all cluster cations in this range lie above the
dimer binding energy:
15. When 15
n
z
z
½
Au
n
!
½
Au
n
1
þ
Au
ð
7
Þ
z
!
½
Au
n
2
þ
Au
2
ð
8
Þ
z
ð
z
þ
1
Þ
e
½
Au
n
!
½
Au
n
þ
ð
9
Þ
Ervin's group have examined the fragmentation patterns and extracted the bond
dissociation energies of small gold anionic clusters, Au
n
, using the energy-
resolved threshold collision-induced dissociation (
n
¼
2-7) as well as the time-
resolved photodissociation lifetime (
n
6, 7) techniques [
176
]. In both cases the
main fragmentation channels are found to be loss of Au atom (Eq. (
7
),
z
¼
¼
1),
which is favoured for odd electron anions (i.e. when
n
¼
even) or Au
2
(Eq. (
8
),
z
odd). The disso-
ciation energies extracted from the two techniques were in good agreement and
show an even-odd alternation for the loss of an atom from gold cluster anions,
D
0
(Au
n
1
-Au). Finally, photoelectron loss (Eq. (
9
)) appears to be important for
[Au
6
]
and was inferred via depletion of the signal for the cluster anion. Table
3
summarises the fragmentation methods used and the key observations in the study
of bare gold cluster ions.
¼
1), which dominates for even electron ions (i.e. when
n
¼
Ligated Gold Cluster Cations
One of the first studies in which a ligated gold cluster cation was subjected to CID
appears to be that of Wang et al. [
202
], who studied the gas-phase fragmentation of
[Au
20
(PPh
3
)
8
]
2+
under conditions of sustained off-resonance irradiation collision-
induced dissociation (SORI-CID) in an FT-ICR. Losses of
n
PPh
3
ligands (where
