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5.2.3.2 Decomposition in Aqueous Solution.
The p
K
a
for deprotonation of
peroxynitrous acid (ONOOH) was determined to be 6.5-6.8 [144, 149, 157,
158]. Thus, both ONOOH and its anion, ONOO
−
, exist at physiological pH. The
value for standard gibbs energy for formation, Δ
f
G
°, of ONOOH from the
reaction of HNO
2
with H
2
O
2
was determined as 29.7 kJ/mol. Using the
p
K
a
(ONOOH) = 6.6, Δ
f
G
°(ONOO
−
) was estimated as 67.4 kJ/mol. The bond
strength of O-O bond in HOONO is 90 kJ/mol, weaker than that of hydrogen
peroxide(170 kJ/mol)[88].ONOOHdecomposesrelativelyfast(
k
0
= 1.25 ± 0.05/
second at 25°C) [149, 158, 159]. Comparatively, ONOO
−
is a stable species. The
lifetime of peroxynitrite is ∼1 second at physiological pH [88]. The homolysis
of ONOOH produces
•
NO
2
and
•
OH radicals with a free radical yield of
28 ± 4% [132, 143, 160, 161]. The activation parameters,
E
a
and frequency factor
(
A
), at pH 4.0 were determined as 86.6-92.0 kcal/mol and (0.18-1.0) × 10
16
/
second, respectively [132, 143]. The value of
A
is of the same order of magnitude
as has been determined for hemolytic reactions of peroxides in gas phase and
in nonpolar organic solvents. The activation parameters suggest the homolysis
of ONOOH. The value of
E
a
was similar at pH 14 (90.8-100.8 kJ/mol), while a
lesser value of
A
was determined ((0.08−4.9) × 10
12
/second) [159, 162]. The
activation volume, Δ
V
*, at pH 4.0 was evaluated as 6.0-14.0 cm
3
/mol [163-166],
which also indicates a process of bond breakage.
Figure 5.13 presents the reactions involving the decomposition of ONOOH
and ONOO
−
[132]. The rate constants of the involved reactions are given in
Table 5.4 [108, 120, 132, 143, 167-173]. The reaction of
•
OH with ONOOH is
much slower than the reaction with
NO
−
, forming
NO
−
as a final product (Fig.
5.13). However, the rate constants of
•
OH with ONOO
−
and
NO
−
are similar
(see Table 5.4). The ratio of
[
2
thus determines the contribution
of the reaction of
•
OH with ONOO
−
. Overall, the change in the reaction
pattern of peroxynitrite was observed when the pH approaches the exceeded
p
K
a
of ONOOH. The results were described using the homolysis of ONOOH
with the formation of
•
NO
2
and
•
OH. The homolytic equilibrium between
ONOO
−
and
•
NO + O
2
and other subsequent reactions of these radicals [170]
are depicted in Fig. 5.13. The formation of the powerful electrophile, N
2
O
3
,
also occurred (see Fig. 5.13). The mechanism for the homolysis of the species
into free radicals in aqueous solution was also simulated by applying meta-
dynamics (MT) [143]. The formation of
•
NO
2
and
•
OH through a bond break-
age of peroxide was obtained in the simulation. MT further suggests the
formation of a short-lived cage radical pair with an average lifetime of less
than 1 ns, which could diffuse into the bulk of the solution to result in free
radicals [143]. The molecular dynamics of the decomposition of HONOOH
in water demonstrated nitrate formed 86% of the time from the water-caged
radical pair [174]. Remaining 14% of the time was the solvent-separated
radical pair from the water-caged radical pair [174]. However,
NO
−
did not
form with an increase in pH. Formation of O
2
and
NO
−
at ratios of 1 : 2
occurred at higher pH. The yield of
NO
−
reached to ∼80% at pH 9-10 [132,
162, 175-177].
ONOO / NO
−
] [
−
]
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