Chemistry Reference
In-Depth Information
CO
•−
+
CO
2
−
→
CO
•−
+
CO
2
−
(5.6)
3
4
4
4
CO
•−
+
CO
•−
→
C O
2
−
k
=
1 0 10
.
×
9
/M/s
.
(5.7)
3
4
2
7
7
•− −
is high and the
determined value at pH 12.5 was 1.58V [34, 40]. The calculated value obtained
using
ab initio
was in modest agreement with this experimental value [35].
The monomeric
CO
•−
form of the radical was also suggested [35]. A value of
1.78V was determined for the
CO ,H /HCO
2
5.1.1.2 Reactivity.
The reduction potential of
CO /CO
3
•− −
pair at pH 7.0 [41]. This indi-
cates rapid reactions with biomolecules, causing possible damage to protein
and nucleic acids [1]. Literature shows determination of second-order rate
constants for the reactions of
CO
•−
with a wide range of compounds [25, 31,
39, 42-50]. Table 5.1 gives the rate constants for some of the inorganic species.
The measured rate constants ranged from ≤1.0 × 10
6
/M/s to 8.0 × 10
9
/M/s
[25, 40, 42, 44, 47, 48, 51]. The rate constants for reactions of
CO
•−
with
metal complexes increased with a decrease in the reduction potential of the
metal complex. This is expected for electron transfer reactions. The activation
energies of the studied compounds ranged from −11.4 to 18.8 kJ/mol [40].
The effect of temperature on the rate constants was also determined to know
the mechanism of the reactions. The pre-exponential factors ranged from
6.4 to 9.7. The decrease in activation energies of the inorganic species with
an increase in the driving force for the reactions suggests an outer-sphere
electron transfer.
+
3
3
TABLE 5.1. Rate Constants and Arrhenius Parameters for the Reactions of
CO
•−
with Various Inorganic Species [25, 40, 42, 44, 47, 48, 51]
Species
pH
k
298
(/M/s)
E
a
log
A
≤1.0 × 10
6
O
2
11.0
-
-
H
2
O
2
-
8.0 × 10
5
-
-
-
6.0 × 10
7
-
-
HO
−
-
0.4-1.5 × 10
9
-
-
O
−
-
6.0 × 10
7
-
-
O
−
I
−
11.4
2.5 × 10
8
−11.4 ± 0.6
6.4
11.0
8.0 × 10
9
-
-
NH
•
6.6 × 10
5
NO
−
11.4
7.1 ± 1.1
7.1
11.4
3.4 × 10
7
6.8 ± 0.3
8.7
ClO
−
SO
2−
11.4
2.9 × 10
7
3.9 ± 1.3
8.2
SCN
−
11.4
1.6 × 10
5
18.8 ± 4.6
8.5
11.4
3.6 × 10
8
6.5 ± 1.4
9.7
Fe CN
(
)
4−
3.5 × 10
7
Mo CN
(
)
4−
11.2
8.4 ± 1.1
8.4
11.2
2.4 × 10
8
3.3 ± 0.8
8.9
W CN
(
)
4−
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