Chemistry Reference
In-Depth Information
electrolysis. The pulse radiolysis technique has been used widely to study
enzymatic reactions in which the radiolysis of water is performed under high-
energy electrons (Eq. 4.1):
H O
•
OH e H H H O
aq
,
−
,
•
,
,
.
(4.1)
2
2
2
2
If water contains molecular oxygen and sodium formate, the primary radi-
cals are converted to
O /HO
2
•−
•
(reactions 4.2-4.5) [16]:
2
e
−
+ →
O
O
•−
k
= ×
2 10
10
/M/s
(4.2)
aq
2
2
2
H O
•
+ →
HO
•
k
= ×
5 10
9
/M/s
(4.3)
2
2
3
•
OH HCOO
+
−
→
COO
•−
+
H O
k
= ×
5 10
9
/M/s
(4.4)
2
4
COO
•−
+ →
O
CO O
+
•−
k
= ×
5 10
9
/M/s
.
(4.5)
2
2
2
5
Vacuum ultraviolet (VUV) photolysis (165-185 nm) generates the primary
radicals,
•
OH and H
•
, which undergo reactions (4.3)-(4.5) to produce
O
•−
[16].
UV photolysis of H
2
O
2
forms
O /HO
•−
•
by the following reactions (Eqs.
2
2
4.6-4.8):
+ →
•
H O
h
ν
2
OH
(4.6)
2
2
•
•
H O
+
OH HO H O
→
+
(4.7)
2
2
2
2
−
•
•−
HO
+
OH O
→
+
H O
.
(4.8)
2
2
2
Reactions of semiquinone radicals with O
2
resulted in superoxide ions [17].
A study on the electrochemical production of
O
•−
from O
2
in concentrated
NaOH solutions has also been performed [18]. The electroreduction of O
2
in
nonaqueous aprotic solvents, such as dimethyl formamide, dimethyl sulfoxide,
acetonitrile, propylene, and acetone, also generates
O
•−
[19, 20]. An aprotic
solution of
O
•−
can also be prepared by adding KO
2
into a “crown” ether.
Recently, room-temperature ionic liquid as a solvent has been applied to
produce
O
•−
[21, 22].
4.1.2 Properties
Superoxide is produced in several oxidation reactions occurring in chemical
and biological systems. For example, superoxide is the first species produced
in the respiratory chain by an electron transfer reduction of oxygen [23]. The
thermodynamic parameters of formation of superoxide have been determined,
which may help in evaluating processes that lead to its generation and disap-
pearance.
∆
H
f
o
were determined as −36.0 and −24.7 kJ/mol at 298 K for
HO
•
and
O
•−
, respectively [24].
S
o
values were calculated as 138 and 79.5 J/mol [24].
Enthalpy, entropy, and free energy of hydration for the
O
•−
species have also
been calculated as −398 kJ/mol, −124 J/K/mol, and −356 kJ/mol, respectively
[25]. The diffusion coefficient for the
O
•−
species in 0.1 M NaOH and 10% (v)
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