Chemistry Reference
In-Depth Information
Sugawara et al. have probed gold hydride clusters formation upon laser ablation
of a gold rod in a H
2
/He mixture [
67
]. The hydride cluster cations Au
n
H
m
+
were
produced for
n
1-7, while bare Au
n
+
clusters were the main products for
n
¼
8.
The authors suggested that the border between
n
7 and 8 hints to the formation of
planar gold hydrides as the structure of bare Au
n
+
changes from planar for
n
¼
¼
7to
three-dimensional for
n
¼
8. Finally, the stoichiometries of the gold hydride cluster
cations Au
n
H
m
+
favour combinations of n and m that yield even electron clusters.
Thus the main gold hydride clusters formed were Au
2
H
5
+
,Au
3
H
6
+
,Au
5
H
6
+
,
Au
6
H
5
+
and Au
7
H
4
+
.
4.2.2 Gold Hydroxide Cluster Ions
Wallace et al. [
303
] have studied the reactions of gold cluster anions with oxygen
under humid conditions in a near-atmospheric-pressure flow reactor. In the absence
of oxygen and at near room temperature, abundant gold hydroxide cluster anions,
Au
x
OH
(
x
2-6), are observed. When oxygen is added, adsorption occurs to
yield, Au
x
OH(O
2
)
. An even/odd size effect is observed for Au
x
OH
which is
different to the bare gold cluster anion Au
x
reactions with O
2
. Thus the binding of
an OH group enhances the reactivity towards molecular oxygen on odd-sized
anionic gold clusters but lowers the reactivity on even-sized ones.
¼
4.2.3 Gold Oxide Cluster Ions
Castleman's group has widely studied the reactions of gold oxide cluster ions in the
gas phase, and much of that work has been reviewed [
304
]. This work has been
inspired by Haruta's discovery of CO oxidation by gold clusters (Eq. (
3
)). Indeed
the focus of all of the bimolecular work has been on developing an understanding of
how charge and cluster stoichiometry influence the reactions of gold oxide cluster
ions, Au
x
O
y
/+
, with CO. For the anions, there are three types of reactions that can
occur [
305
-
309
]: association (Eq. (
42
)), replacement (Eq. (
43
)) and oxidation of
CO (Eq. (
44
)). Which of these reactions occur depends on both
x
and
y
:
ðÞ
Au
x
O
y
þ
CO
!
Au
x
O
y
CO
ð
42
Þ
ðÞ
!
Au
x
O
y
2
CO
þ
O
2
ð
43
Þ
!
Au
x
O
y
1
þ
CO
2
ð
44
Þ
Detailed DFT calculations and molecular dynamics calculations were carried out
on several systems to help rationalise the experimental observations. The results of
DFT calculations on Au
2
O
y
and Au
3
O
y
(
y
1-5), which are shown in Fig.
30
,
reveal that there are three types of oxygen centres in these cluster anions: peripheral
O atoms (e.g. Au
2
O
), bridging O atoms (e.g. Au
2
O
2
) and molecular O
2
(e.g. Au
3
O
). Experimentally it was found that association (Eq. (
42
)) occurs for
¼
