Chemistry Reference
In-Depth Information
Table 9 Gas-phase products and rates constants for the reactions of gold dimers with neutral
substrates
Au
2
+
rate
a
Au
2
+
Product(s) Au
2
rate
b
Au
2
product(s)
a
Au
2
rate
c
Au
2
product(s)
Substrate
NR
c
NR
b
NR
b
NR
b
8
c
Au
2
(O)
2
O
2
83
0.24
d
NO
+
+Au
2
N
b
NR
b
N
2
O
NS
NS
NR
b
NR
b
N
2
NS
NS
NS
NS
2.9
e
Au
2
CH
4
+
NR
b
NR
b
CH
4
NS
NS
1
c
Au
2
(CO)
CO
NR
NR
2.6
0.9 Au
2
(CO)
5.1
60
b
Au(NH
3
)
+
NH
3
22
4
2
(NH
3
)
NS
NS
C
2
H
4
NS
NS
230
50 Au
2
(C
2
H
4
) R R
Au
2
(H
2
)
3
+f
H
2
NS
1.4
0.3 Au
2
(H)
2
NS
NS
a
Second order, rate constant with units of 10
11
cm
3
s
1
b
Data collected in a fast-flow reactor at room temperature under thermalised conditions
[
270
]. Termolecular rates reported units of 10
30
cm
6
s
1
c
Data collected in a variable-temperature ion trap [
268
,
271
]
d
Data collected in an ion trap [
272
]
e
Data collected in an ion trap at 300 K [
265
]
f
Data collected in an ion trap at 100 K [
271
]
NR
¼
no reaction observed, which sets a limit on the bimolecular rate constants of less than
5
10
15
cm
3
s
1
at 6 Torr He
NR
no reaction,
NS
not studied
4.1 Reactions of Bare Gold Cluster Ions: Overview
The total electron count on bare gold clusters can be manipulated in two ways:
1. Clusters of gold atoms which are neutral, cationic or anionic have different
electron counts. Apart from the role of total electron count, the presence or
absence of a charge influences the collisional rate constant. Thus the collisional
rate constants of singly charged ions are substantially larger than those of
analogous neutrals.
One of the first studies to have systematically examined the gas-phase reac-
tions of a range of neutral substrates (D
2
,CH
4
and O
2
) with gold clusters Au
n
(
n
2-25), in their cationic, neutral and anionic forms, revealed that the cluster
charge and electron count can have a pronounced effect on their reactivities
[
269
]. For the reactions with D
2
, the reactivity follows the order Au
n
+
(reactive
for
n
¼
Au
n
(nonreactive). For
O
2
, the reactivity follows the order Au
n
(clusters where n is even are reactive)
>>
¼
>
Au
n
(only
n
¼
>>
2-15)
3 and 7 reactive)
Au
n
+
(only
n
10 reactive).
To further highlight the role of electron count and charge on reactivity, we
have summarised from the literature, data for the reactions of the gold dimer
cation, neutral and anion with a range of neutral substrates (Table
9
).
2. Pioneering work by the groups of Cox and Ervin on the gas-phase reactivity of
small gold clusters with substrates has shown that the addition or removal of
single Au atom can have a profound effect on reactivity [
273
,
274
]. Figure
26
shows that the relative reactivities of bare anionic gold clusters with both carbon
¼
