Chemistry Reference
In-Depth Information
This binding can be reversible, as demonstrated by the fact that certain cluster
ions are metastable, undergoing CO loss (Eq. (
16
)) [
279
]:
Au
x
C
ðÞ
=þ
!
Au
x
C
ðÞ
=þ
yz
þ
z
CO
ð
16
Þ
y
Reactions with O
2
O
2
is more selective in its reactions with gold cluster ions. Small gold cluster
cations are essentially unreactive, while anions exhibit a pronounced odd-even
effect for
x
in adding O
2
(Eq. (
17
)) [
273
,
275
]. Cluster anions with an unpaired
electron (
x
even) are highly reactive, and it has been suggested that these
undergo electron transfer to form a superoxide anion bound to Au
x
(see Sect.
3.6
and A. Fielicke et al.'s chapter in this topic):
¼
Au
x
O
ðÞ
Au
x
þ
O
2
!
ð
17
Þ
Reactions with N
2
The reactions of Au
x
+
3 and 5) with N
2
have been studied in a variable-
temperature ion trap [
281
]. The addition of multiple N
2
to Au
x
+
is very sensitive to
the temperature. At room temperature no Au
x
(N
2
)
y
+
are observed. For Au
3
+
the ions
Au
3
+
and Au
3
(N
2
)
3
+
are observed at 200 K, while at 100 K only Au
3
(N
2
)
3
+
is found.
In the case of Au
5
+
both Au
5
(N
2
)
3
+
and Au
5
(N
2
)
4
+
are observed at 200 K, and only
the latter ion is observed at 100 K. These nitrogen adducts facilitate absorption of
H
2
and O
2
, as discussed below.
(
x
¼
Reactions with H
2
Three studies of the reactions of gold cluster ions with hydrogen have been reported
using different instruments, which highlight the role of reaction conditions (pres-
sure and temperature). Cox's group used a fast
flow reactor to examine the reac-
tions of gold cluster cations and anion with deuterium. They found that while small
(
n
‐
<
15) gold cations react readily with D
2
via addition of D
2
, no reactions were
observed for the anions [
269
]. Sugawara et al. have noted that in the lower-pressure
regime of an FT-ICR mass spectrometer, no reaction occurs for gold cluster cations
Au
n
+
1-12) with H
2
[
67
]. Under variable-temperature ion-trap conditions,
where the hydrogen adducts can be collisionally cooled with the helium bath gas
[
271
], Au
x
+
(
x
(
n
¼
2-7) exhibit interesting temperature-dependent reactivity patterns
towards molecular hydrogen. At 300 K, only Au
5
+
adsorbs up to 3 molecules of H
2
.
Lowering the temperature to 200 K results in a dramatic change of reactivity.
Although Au
2
+
remains unreactive, the other cluster cations adsorb the following
numbers of hydrogen molecules: Au
3
+
and Au
4
+
up to three; Au
5
+
and Au
6
+
up to
four; and Au
7
+
up to two. Further lowering the temperature to 100 K leads to a
¼
