Nucleophile Assistance of Electron-Transfer Reactions between Nitrogen Dioxide and Chlorine Dioxide Concurrent with the Nitrogen Dioxide Disproportionation

Article abstract of DOI:10.1021/ic0347891

The reaction of chlorine dioxide with excess NO₂^- to form ClO₂^- and NO₃^- in the presence of a large concentration of ClO₂^- is followed via stopped-flow spectroscopy. Concentrations are set to establish a preequilibrium among ClO₂, NO₂^-, ClO₂^-, and an intermediate, NO₂. Studies are conducted at pH 12.0 to avoid complications due to the ClO₂^-/NO₂^- reaction. These conditions enable the kinetic study of the ClO₂ reaction with nitrogen dioxide as well as the NO₂ disproportionation reaction. The rate of the NO₂/ClO₂ electron-transfer reaction is accelerated by different nucleophiles (NO₂^- > Br^- > OH^- > CO₃^2- > PO₄^3- > ClO₂^- > H ₂O). The third-order rate constants for the nucleophile-assisted reactions between NO₂ and ClO₂ (k^Nu, M ^-2 s^-1) at 25.0 °C vary from 4.4 × 10 ⁶ for NO₂^- to 2.0 × 10³ when H₂O is the nucleophile. The nucleophile is found to associate with NO₂ and not with ClO₂ in the rate-determining step to give NuNO₂⁺ + ClO₂^-. The concurrent NO₂ disproportionation reaction exhibits no nucleophilic effect and has a rate constant of 4.8 × 10⁷ M^-1 s ^-1. The ClO₂/NO₂/nucleophile reaction is another example of a system that exhibits general nucleophilic acceleration of electron transfer. This system also represents an alternative way to study the rate of NO₂ disproportionation.

Full text of DOI:10.1021/ic0347891

Inorg. Chem. 2003, 42, 7938−7944
Nucleophile Assistance of Electron-Transfer Reactions between
Nitrogen Dioxide and Chlorine Dioxide Concurrent with the Nitrogen
Dioxide Disproportionation
Robert H. Becker, Jeffrey S. Nicoson, and Dale W. Margerum*
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-2084
Received July 8, 2003
The reaction of chlorine dioxide with excess NO -
-
-
2
2 3
to form ClO and NO in the presence of a large concentration
of ClO -
is followed via stopped-flow spectroscopy. Concentrations are set to establish a preequilibrium among
2
-
-, and an intermediate, NO
reaction. These conditions enable the kinetic study of the ClO
disproportionation reaction. The rate of the NO /ClO electron-transfer reaction is accelerated by different
ClO
2
-
, NO
2
, ClO
2
2
. Studies are conducted at pH 12.0 to avoid complications due to the
reaction with nitrogen dioxide as well
-
ClO
as the NO
nucleophiles (NO
assisted reactions between NO
when H O is the nucleophile. The nucleophile is found to associate with NO
determining step to give NuNO
effect and has a rate constant of 4.8 × 10 M s . The ClO
system that exhibits general nucleophilic acceleration of electron transfer. This system also represents an alternative
way to study the rate of NO disproportionation.
2
/NO
2
2
2
2
-
2
-
-
-
2-
3-
2
>Br >OH >CO
3
>PO
4
2 2
> ClO >HO). The third-order rate constants for the nucleophile-
Nu
-2 -1
6
-
to 2.0 × 103
2
and ClO
2
(k , M s ) at 25.0 °C vary from 4.4 × 10 for NO
and not with ClO
2
2
2
2
in the rate-
+
-
2
+ ClO
2
. The concurrent NO
2
disproportionation reaction exhibits no nucleophilic
/NO /nucleophile reaction is another example of a
7
-1 -1
2
2
2
Introduction
Recently, it was reported that many different nucleophiles
Nu’s) catalyze the electron transfer from ClO to BrO where
third-order rate constants increased 4 orders of magnitude
from the weakest nucleophile studied (H O) to the strongest
Br ). It was postulated that the nucleophile effect need not
be specific to the system studied but may be a more common
phenomenon. Since then, the decomposition of ClO in base
disproportionation, performed by pulse radiolysis methods,
showed scattered data and gave different interpretations of
the overall reaction mechanism. Yet, many of the results in
(
2
2
5
these two papers are in agreement. The present work
represents an alternative way to explore the rate of the NO
2
2
-
1
disproportionation reaction in solution. The NO
does not exhibit nucleophile assistance.
2
/NO
2
path
(
-
Stanbury et al. showed that the reaction of NO
2
and ClO
2
2
-
proceeds via eq 1, and showed that ClO
2
suppresses the
2
reaction order
was found to exhibit a similar nucleophile effect, albeit on
6
reaction. They observed an increase in ClO
a more limited scale of only strong bases acting as nucleo-
2
philes. The present study of the reaction between ClO
2
and
k1
-
-
NO
2
represents a third instance in which nucleophiles
ClO + NO { } ClO + NO2
(1)
2
2
2
k-1
accelerate electron transfer between nonmetal dioxide spe-
cies. This system exhibits effects from a range of nucleo-
philes comparable to that seen by Wang et al.1
-
when different concentrations of ClO
2
were added. They
-1
determined the forward reaction rate constant (k
s ), and used reduction potentials to establish a value of
1
) 153 M
2
The disproportionation of NO to nitrite and nitrate is a
-
1
7
concurrent reaction that occurs in solution along with the
2 2 2
NO /ClO
/Nu reaction. Two previous studies3 of NO
,4
(
(
3) Treinin, A., Hayon, E. J. Am. Chem. Soc. 1970, 92, 5821-5828.
4) Gr a¨ tzel, M.; Henglein, A.; Lilie, J.; Beck, G. Ber. Bunsen-Ges. Phys.
Chem. 1969, 73, 646-653.
*
Author to whom correspondence should be addressed. E-mail:
margerum@purdue.edu.
(5) Trace Atmospheric Constituents; Schwartz, S. E. Ed.; Advances in
Environmental Science and Technology 12; John Wiley: New York,
1983; pp 41-47.
(
1) Wang, L.; Nicoson, J. S.; Huff Hartz, K. E.; Francisco, J. S.; Margerum,
D. W. Inorg. Chem. 2002, 41, 108-113.
(2) Odeh, I. N., Francisco, J. S.; Margerum, D. W. Inorg. Chem. 2002,
(6) Stanbury, D. M.; Martinez, R.; Tseng, E.; Miller, C. E. Inorg. Chem.
1988, 27, 4277-4280.
41, 6500-6506.
7938 Inorganic Chemistry, Vol. 42, No. 24, 2003
10.1021/ic0347891 CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/01/2003

Nucleophile Assistance of Electron Transfer
0
.016 for the equilibrium constant, K
exploited this equilibrium to study the reactions of NO
ClO as well as NO disproportionation kinetics.
1
) k
1
/k-1. We have
2
with
2
2
Experimental Section
Chlorine dioxide stock solutions were made as described
previously.8 Stock ClO
to slow ClO volatilization and light decomposition. Concentrations
of ClO stock solutions were determined via UV/vis spectroscopy
359 ) 1230 M- cm ). NaClO
to remove Na CO via precipitation of BaCO
To increase yields of recovered NaClO
removed via freeze-drying with a Virtis Freezemobile 12 lyophilizer.
,9
solutions were stored in the dark at 4 °C
2
2
2
1
-1 8
(
ꢀ
2
(80% pure, Fluka) was purified
upon addition of
, water was
2
3
3
8,10,11
BaCl
2
.
2
2
The purity of the solid NaClO (99.3%) was determined via UV/
vis spectroscopy (ꢀ260 ) 154 M- cm ). NaNO
1
-1 8
purity (99.5%)
2
-
was checked via titration of excess OCl with previously standard-
12
ized Na S O
2 2 3
NaClO
.
Recrystallized and gravimetrically standardized
was used for ionic strength (µ) control where µ for all
4
T
Figure 1. (a) kobsd dependence on chlorite concentration under second-
order ClO2-loss conditions. (b) 1/kobsd dependence on chlorite concentration
solutions was equal to 1.0 M. Carbonate-free NaOH was prepared
from Ar-sparged boiled water and saturated NaOH solution. The
under second-order ClO2-loss conditions. Conditions for both graphs are
stock NaOH was stored under Ar to keep atmospheric CO
entering the solution.
2
from
-
0
.02 M NO2 , pH 9.80, 5.0 mM [CO3]T, µ ) 1.0 M (NaClO4), and T )
25.0 °C. The solid line in (b) shows the curve fit to a/[ClO -] + b/[ClO -]2,
where a ) 100(7) s and b ) 4.0(3) M s .
2
2
-
1
-1
For pH measurements a ROSS combination electrode was used.
This electrode was calibrated via titration with previously standard-
ized HClO
4
and NaOH to allow for determination of pH (-log
+
1
[H ]). The pK
w
value is 13.60 for the conditions used. UV/vis
spectra were taken with a Lambda-9 UV/vis-near-IR spectropho-
tometer. Kinetic studies were performed on an Applied Photophys-
ics stopped-flow spectrometer (0.962 cm optical path length). All
data points shown are an average of at least five replicates.
Chromatographic data were collected for the species produced in
this reaction by using a Dionex DX-500 HPLC instrument. Samples
were injected through a 25 µL injection loop to quaternary amine
2 3
anion exchange guard and separation columns. Na CO (9.0 mM)
was used as the eluent at a flow rate of 1 mL/min. Species were
quantified via conductivity detection after background suppression
with an ASRS-Ultra suppressor which has a self-regenerating
Figure 2. kobsd dependence on nitrite concentration under second-order
-
-
-
ClO2-loss conditions, at 0.06 M ClO2 , 0.0512 M Br , 0.025 M OH , µ )
1.0 M (NaClO4), and T ) 25.0 °C. The solid line shows the curve fit to
-
- 2
4
-2 -1
kobsd ) k3′[NO2 ] + k4′[NO2 ] , where k3′ ) 5.0(5) × 10 M
s
and k4′
current of 100 mA. Conditions for the product study were set to
6
-3 -1
)
4.2(2) × 10 M
s .
-
be similar to those used for kinetic determinations ([ClO
2
] ) 0.06
-
-
M, [NO
2
] ) 0.020 M, [OH ] ) 0.020 M). After reaction
completion, aliquots were added to a 0.1 M [OH ] solution to
-
-
to second order as the ClO
2
concentration increases.
-
-
Stanbury et al. observed this increase in reaction order, but
minimize the ClO
2
/NO
2
reaction.
6
-
did not mention its significance. When [ClO
2
] is between
becomes
Conditions for most of the kinetic studies were set so the loss of
ClO
2
was second order. Chlorite concentration was kept at 0.060
0.040 and 0.10 M at pH 10, the rate of loss of ClO
2
-
M, and [OH ] ) 0.025 M (pH 12.0). High pH was used to eliminate
second order. Figure 1a shows the suppression of the rate
-
the need for added buffers and to eliminate interference from the
with increasing ClO2 concentration under these conditions.
-
-
reaction, which is catalyzed by Cl- and H .
+ 13
NO
2
/ClO
2
The data in Figure 1b indicate that the chlorite suppression
-
is greater than first order. Conditions with [ClO
2
] less than
Results and Discussion
0
.040 M were insufficient to maintain preequilibrium condi-
tions (i.e., both a first-order and a second-order dependence
in ClO loss is observed, where the first-order dependence
comes from the ClO
tion greater than 0.10 M at pH 10 the loss of ClO
longer second order because of a competing reaction between
-
A. Kinetics. The reaction between ClO
2
and NO
2
is first
-
6
order in both reactants at low [NO
2
]. In the presence of
2
-
-
chlorite, the reaction order in ClO changes from first order
2
2 2 2
/NO reaction). With ClO
concentra-
is no
2
(
7) Stanbury, D. M. AdVances in Inorganic Chemistry; Academic Press:
New York, 1989; Vol. 33, pp 69-138.
-
-
ClO
2
and NO
2
. When the pH is increased to 12.0, a
(
(
8) Furman, C. S.; Margerum, D. W. Inorg. Chem. 1998, 37, 4321-4327.
9) Masschelein, W. J. J. Am. Water Works Assoc. 1984, 76, 70-76.
-
second-order loss of ClO
concentrations. Thus, at pH 12, the ClO
2
is observed even at higher ClO
2
(
(
10) F a´ bi a´ n, I.; Gordon, G. Inorg. Chem. 1992, 30, 3785-3787.
-
-
2
/NO
2
reaction is
11) Peintler, G.; Nagyp a´ l, I.; Epstein, I. R. J. Phys. Chem. 1990, 94, 2954-
very slow and is not a factor in the kinetics, so most of our
2958.
(
12) Johnson, D. W., Margerum, D. W. Inorg. Chem. 1991, 30, 4845-
851.
13) Lengyel, I.; G a´ sp a´ r, V.; Beck, M. T. J. Phys. Chem. 1988, 92, 137-
40.
studies were conducted at this pH. Figure 2 shows the
4
-
observed rate constant dependence on [NO
2
] for second-
(
1
order ClO
2
loss. Under these conditions the dependence on
Inorganic Chemistry, Vol. 42, No. 24, 2003 7939

Becker et al.
pathway from dominating the loss of ClO . This
low level of NO also enables us to set [NO (where [NO
) [NO ] + 2[N ]) equal to [NO ] because the overall
concentration of N under our conditions is very low. The
NO dependence is first order as described in eq 3 and second
order as described in eq 4 (after consideration of eq 2). This
explains the nitrite dependence shown in Figure 2. The
preequilibrium in eq 1 necessitates that the loss of nitrite
[
NO
2
-] is greater than first order, with both first- and second-
NO
2
/NO
2
2
order contributions.
2
]
2 T
2 T
]
B. Effect of Nucleophiles. The nucleophiles studied in
2
2
O
O
2 4
4
2
-
-
-
2-
3-
-
the present work (Br , Cl , OH , CO
ClO
effect on the rate of the ClO
NO
increase linearly with nucleophile concentration at pH 12.0.
Hydroxide is used to control the pH of the solutions, but it
also acts as a nucleophile. Studies with high chloride
concentrations and much lower hydroxide concentration (pH
1
3
, PO
4
, NO
2
,
-
2
, and H
2
O) were chosen because of their demonstrated
2
1
2
/BrO
2
reaction. For the ClO
2
/
-
-
2
system, all nucleophiles except Cl caused the rate to
parallels the loss of NO
follows an a[NO
-] + b[NO
dependence as shown in Figure 1a (data are fit to an
2
. The fit of the data in Figure 2
- 2
15
-
2
2 2
] equation. The ClO
-
-
- 2
0) also failed to show a Cl effect.
C. Mechanism. The mechanism in eqs 1-5 is proposed
a/[ClO
mechanism. With consideration of the preequilibrium (eq 1),
the NO /ClO /Nu reaction in eq 3 yields an inverse first-
order dependence in chlorite while the NO hydrolysis
2
] + b/[ClO
2
] equation) also is explained by the
to explain the experimental observations. The rate-determin-
ing steps in this mechanism are eqs 3 and 4.
2
2
2
reaction in eq 4 gives an inverse second-order dependence
as shown in eq 7. Equation 5 is a rapid reaction where the
nucleophile is regenerated to complete the catalytic electron
K1
-
-
ClO + NO2
{\ } ClO2 + NO2
(1)
(2)
(3)
2
-
K2
\
transfer by forming NO
ClO
to emphasize the species present in the transition state. To
the best of our knowledge, the reaction between ClO and
NO in aqueous solution has not been studied, although the
3
. The reaction between NO
2
and
NO + NO {
} N O
2
2
2
4
2
(eq 3) may occur in more than one step, but is written
k3Nu
+
-
NO + ClO + Nu
8 NuNO2 + ClO2
2
2
2
2
k4
reaction has been observed in nonaqueous solvents and in
N O + H O
9
8 NO3- + NO2- + 2H+
(4)
(5)
2
4
2
16-19
the gas phase.
However, the reported rate constants in
these media are much smaller than we observe in aqueous
fast
NuNO2+ + H O
8 NO3- + Nu + 2H+
Nu
2
solution. The ∑k
3
[Nu] term in eqs 6 and 7 represents the
pathway when a nucleophile is present
rate constant for the k
3
Equation 4 is the hydrolysis of N
2 4
O , which when coupled
at a specific concentration ([Nu]).
to the equilibrium in eq 2 has been studied previously as
-
The association of the nucleophile with NO
2
yields ClO
2
-5
-
the NO
2
disproportionation reaction.3 With added ClO
2
,
-
and NO
3
as products. If the nucleophile were to associate
eq 1 is under rapid preequilibrium conditions with low levels
2
. The equilibrium in eq 2 is also established rapidly.
The resulting rate expression, based on the above mechanism,
1,2
-
-
with ClO
2
instead, as was seen previously, NO
2
and ClO
3
5
of NO
would be the products. Electrode potentials for the two
possible reactions shown in eqs 8 and 9
is shown in eq 6. The loss of ClO
2
is second order in the
-
NO + ClO + 2OH f
Nu
-
2
2
2
d[ClO2]
K (
1
∑k
[Nu])[NO ][ClO ]
3 2 2
-
)
+
NO2- + ClO3 + H O (E ° ) 1.51 V) (8)
-
2
b
-
dt
[
ClO ]
2
2
- 2
2
2
-
K K k [NO ] [ClO ]
NO
2
+ ClO
2
+ 2OH f
1
2
4
2
(6)
ClO2- + NO3- + H O (E ° ) 1.89 V) (9)
-
2
2
[ClO ]
2
b
(
where the acidic reduction potentials are converted to basic
two pathways represented by eqs 3 and 4 when the
preequilibria in eqs 1 and 2 are considered. The observed
rate constant for this system, eq 7, is composed of two
potentials and the solubility equilibrium of NO is em-
ployed
cally favorable. We indicate in the mechanism that NO3 is
formed preferentially over ClO
study for our system, which shows that nitrate is the major
product (∆[ClO
because ClO is lost in eq 1 as well as in eq 3. Small amounts
2
7,20,21
) indicate that both pathways are thermodynami-
-
Nu
14
reaction pathways (k
3
2 4
and K k ). Computer modeling with
-
3
. This is based on a product
Nu
-
2
2
1
- 2
K (
∑k3
[Nu])[NO ] K K k [NO ]
1
2
4
2
kobsd
)
+
(7)
]/∆[NO ] ) 1.96(4)). The 2:1 ratio occurs
-
-
2
- 2
2
3
[ClO ]
[ClO ]
2
2
known parameters for K
1
, K
shows that the reactions, eq 3 and the product of eqs 2
and 4, proceed at comparable rates. While the NO dispro-
portionation rate constant (K ) is approximately 100 times
larger than the NO /ClO reaction rate constant (k
low levels of NO
in this system (∼1 × 10 M) keep the
2 4
k , and approximate values of
(
15) SigmaPlot V. 8.0 for Windows; SPSS Inc.: Chicago, IL, 2002.
Nu
k
3
(16) Wallington, T. J.; Cox, R. A. J. Chem. Soc., Faraday Trans. 2 1986,
82, 275-289.
2
(
17) Martin, H.; Gareis, R. Z. Elektrochem. 1956, 60, 959-964.
2 4
k
(18) Martin, H.; Kohnlein, E. Z. Phys. Chem. (Muenchen) 1958, 17, 375-
Nu
398.
2
2
3
), the
(19) Martin, H.; Fuchs, G.; Wartini, L. Ber. Bunsen-Ges. Phys. Chem. 1963,
-
6
2
67 (9), 984-987.
(20) Bard, A. J., Parsons, S. R., Jordan, J., Eds. Standard Potentials in
Aqueous Solution; Marcell Dekker: New York, 1985; p 129.
(
14) Specfit Global Analysis V. 2.12 reV. B; Spectrum Software Associ-
ates: Marlborough, MA, 1999.
(21) Lee, Y. N.; Schwartz, S. E. J. Phys. Chem. 1981, 85, 840-848.
7940 Inorganic Chemistry, Vol. 42, No. 24, 2003

Nucleophile Assistance of Electron Transfer
of chlorate are detected as products, but the concentrations
produced are too low to evaluate and are within the error of
zero for the method used. We, therefore, consider the reac-
tion in eq 8 to be a very minor pathway and will focus
solely on the reaction in which the nucleophile associates
with NO
2
.
D. Resolution of Reaction Pathways. The NO
2
/NO
2
pathway in the proposed mechanism (eqs 2 and 4) has no
nucleophile dependence. Initially, this was not known and
necessitated the inclusion of an additional factor in the second
Nu
Figure 3. k3′ dependence on bromide concentration, where the slope is
term in eq 7 (∑k
4
[Nu]) to account for the possibility that
Br-
-
Nu
-
K1k3 /[ClO2 ] and the intercept is K1(∑k3 [Nu])/[ClO2 ]. Data points
represent the coefficients from the first-order nitrite term of several different
nitrite dependencies conducted at different bromide concentrations.
the NO /NO pathway also depends on nucleophiles. In this
2
2
case a simple kobsd vs [Nu] study would not lead to separable
rate constants. Because of the possibility that eq 4 is assisted
by nucleophiles, two different strategies were employed to
1
. Lack of Nucleophile Dependence for the NO
2
/NO
2
Nu
2
- 2
Nu
Pathway. Plots of k ′ (∑(k [Nu])K K /[ClO ] ) vs [Nu]
study the system and to resolve the two rate constants (k
3
4
4
1
2
2
Nu
-
show no trend as the nucleophile concentrations increase for
all nucleophiles studied (see the Supporting Information
and K
2
k
4
). A ClO
2
dependence at varying nucleophile
Nu
Nu
concentrations will separate the k
because the k
3
path from the k
4
path
-
Nu
-
Nu
figures) except for Br , which inhibits the reaction (as
3
path will vary as 1/[ClO
2
] while the k
4
-
2
discussed later). Because the k ′ term shows no dependence
path will vary as 1/[ClO
2
] . Due to the limited range of
-loss kinetics
] must be large enough to maintain preequilib-
4
-
on nucleophile concentration, the summation term is dropped
[ClO
2
] that will yield pure second-order ClO
2
2
- 2
-
4 4 1 2 2
and k ′ becomes k K K /[ClO ] . This leads to the mech-
(i.e., [ClO
2
2 2
anism given in eqs 1-5, and to eq 7. Because the NO /NO
rium conditions), and because of the rapid drop in rate with
-
-
pathway is now known to be independent of nucleophile
concentration, evaluation of the nucleophile dependencies
directly from eq 7 is possible. As detailed from eq 7, plots
increasing [ClO
2
], a NO
2
dependence approach was chosen
-
instead. Thus, a NO
phile concentration is performed. The k
order nitrite concentration dependence, while the k
would have a [Nu][NO
limitations with respect to [NO
loss kinetics as long as NO
To separate the k
2
dependence at each specific nucleo-
Nu
3
path has a first-
Nu
Nu
of kobsd vs [Nu] have slopes related to k3 , where [Nu]
4
path
2
] dependence. This method has no
-
2
represents the concentration of the nucleophile with variable
Nu
-
2 4
concentration. Once values of k3 are known, the K k rate
2
] for second-order ClO
remains in excess.
/Nu reaction) rate constant
2
-
-
constant is determined from the intercept after subtraction
2
Nu
Nu
of the contributions from all other nucleophiles (∑k3 [Nu′]).
3
(ClO
2
/NO
2
Nu
from the k
4
(NO
2
disproportionation) rate constant, a set
This lack of a nucleophile dependence in the K
2 4
k path
-
of five separate kobsd vs [NO
2
] data points are obtained at a
raises the question as to why nucleophiles have no effect on
specific nucleophile concentration. This procedure is carried
out for several different nucleophile concentrations. Each of
the plots, representing a nitrite dependence at a set nucleo-
phile concentration, is fit to a combined first-order and
second-order rate expression. Figure 2 shows a fit to the data
the rate-determining step of the NO
2
/NO
2
pathway. We
2
reaction assist
hypothesize that nucleophiles in the ClO
2
/NO
+
+
in the formation of NO
from the NO /ClO
2
. The NuNO
2
species, which forms
+
2
2
/Nu reaction, stabilizes the NO
2
species
by association of the nucleophile. On the other hand, in eq
2, two NO molecules can form a relatively stable N
molecule. Nucleophiles are not needed to stabilize this
species. N is a known species that has a millisecond
lifetime in aqueous solution. From the intercept of a plot
of k ′ vs [Nu], a value of the NO disproportionation rate
-
-
- 2
at a [Br ] of 0.051 M (kobsd ) k
3
′[NO
2
-
] + k
], and if a k
4
′[NO
2
Nu
] ). In
path
2
2 4
O
Nu
this case, k
3
′ ) K
1
(∑k
3
Nu
[Nu])/[ClO
2
4
2
- 2
exists, k ′ ) K
4
1
K (∑k
2
4
[Nu])/[ClO
2
] . As shown in eq 10,
2 4
O
4
-
K1k3 [Br-] K ( k3 [Nu′])
Br
Nu
1
∑
4
2
k3′ )
+
(10)
-
2
-
constant is obtained. For all nucleophiles studied the average
[
ClO ]
[ClO ]
2
7
-1 -1
value of K
)
2
k
4
equals 4.8(4) × 10 M
s
(T ) 25.0 °C, µ
1.0 M), which is in general agreement with previously
obtained values. Our study represents a new method, via
the summation in k
3
′ can be divided into two terms, one
5
representing the desired nucleophile under study (in this case
-
Br-
-
-
use of stopped-flow spectroscopy, for determining the NO2
Br ), K
all other nucleophiles, [Nu′], present, K
Figure 3 is a plot of k ′ vs the concentration of bromide.
With known values of K
1
k
3
2
[Br ]/[ClO ], and a second term that represents
Nu
-
disproportionation rate constant. It also reaffirms the pulse
1 3 2
(∑k [Nu′])/[ClO ].
3
-5
radiolysis results, where the scatter and uncertainty may
lead to speculation as to the validity of the rate constant.
Our experiment represents many individual measurements
3
-
Nu
1
Nu
and [ClO
2
], the values of k
represents the k rate constant
for the specific nucleophile of interest. All plots of k ′ vs
3
are determined, where k
3
3
2
of the NO disproportionation rate constant (during different
nucleophile studies), so we are confident in its value.
3
specific nucleophile concentration, except chloride, show a
linear increase as expected from the rate expression in eq
2. Bromide Inhibition. Unlike all other nucleophiles
-
10 (see the Supporting Information figures). The intercepts
studied, bromide exhibits a trend in the k
4
′ vs [Br ] plot. As
-
of these plots incorporate the contributions from all other
[Br ] increases, the value of k
4
′ decreases linearly (Figure
nucleophiles in the system ([Nu′]).
4). We hypothesize that bromide can form a strong enough
Inorganic Chemistry, Vol. 42, No. 24, 2003 7941

Becker et al.
Figure 5. k4′ dependence on chlorite concentration for the nitrite-as-a-
2
- 2
NO
-
2
-
nucleophile study. Data points are fit to k4′ ) K1 K2k4/[ClO2 ] + K1k3
/
Figure 4. Decrease in rate of the NO2/NO2 pathway with increasing [Br ].
-
2
4
-1 -1
NO
-
2
Nu
- 2
2
Br-
-
- 2
[ClO2 ], where K1 K2k4 ) 1.33(9) × 10 M
s
and K1k3
2
) 7(1) ×
k4′ ) K1 K2(∑k4 [Nu′])/[ClO2 ] + K1 K2k4 [Br ]/[ClO2 ] . For all data
4
-2 -1
-
-
-
-
-
10 M
s
. For all data points, 0.008 M < [NO2 ] < 0.024 M, [OH ] )
.025 M, µ ) 1.0 M (NaClO4), and T ) 25.0 °C.
points, 0.008 M < [NO2 ] < 0.024 M, [ClO2 ] ) 0.06 M, [OH ] ) 0.025
M, µ ) 1.0 M (NaClO4), and T ) 25.0 °C. Data are fit to a/(1 + 2bx +
0
2
2
b x ), an equation generated by using eq 11 and [NO2]T, were a equals the
Br-
H O
2
intercept and b equals K
.
hydroxide plot enable the determination of k
0 M s ). The values of the third-order rate constants
increase by a factor of two thousand from the weakest nu-
3
(1.0(3) ×
5
-1 -1
1
Table 1. Summary of Rate Constants for the Reaction between ClO2
a
and NO2
-
cleophile (H
in Table 1.
2
O) to the strongest nucleophile (NO
2
) as listed
nucleophile
H2O
nb
k3Nu, M-2 s-1 nucleophile
nb
k3Nu, M-2 s-1
3
e
2-
6
0
3.04
d
2c .0(5) × 10
CO3
d
1.0(2) × 10
F. Nitrite as a Nucleophile. Nitrite is also studied as a
possible nucleophile in this system. Because all previous
nucleophile studies were conducted as nitrite dependencies,
a large nucleophile dependence in nitrite would alter the rate
constants obtained. The nucleophilicity (n) of nitrite was
previously estimated as 4.2 (i.e., 4 orders of magnitude more
O). Nitrite is a strong nucleophile compared
to other nucleophiles studied, and so a large NO
possible. Equation 12 restates eq 7 if nitrite acts as a
-
-
6
Cl
ClO2
PO4
OH
4.2
1.1(4) × 10
-
6
-
6
0.10(3) × 10
Br
3.89 1.5(4) × 10
3
-
6
-
22
6
d
0.8(4) × 10
NO2
4.2
4.4(6) × 10
a
b
Conditions: 25.0 °C, µ ) 1.0 M. n is the nucleophilicity defined as
Nu H O
n ) log(k3 /k
2
) for CH3X substitution, where values were obtained
c
from ref 24. This value is not measurable due to the effects of other
d
e
H O
2
nucleophiles present at high pH. Values are not available.
k
/55.5 M
22
-
2
-1
reactive than H
to give M
s
.
2
-
2
effect is
complex with NO
2
to sequester NO
2
in a relatively unreactive
-
form (i.e., NO
2
Br ) (eq 11) and hinder NO
2
/NO
2
bond
-
Nu
-
NO2
- 2
KBr-
K (
∑k
[Nu′])[NO ]
K1k
[NO ]
1
3
2
3
2
-
-
Br + NO { } NO Br
(11)
k
obsd
)
+
+
2
2
-
-
[ClO ]
[ClO ]
2
2
2
- 2
formation at the nitrogens. If the sequestered form of NO
2
K K k [NO ]
1
2
4
2
-
is NO
2
Br , the total concentration of NO
2
, before NO
2
loss
(12)
-
2
[
ClO ]
2
in reaction eqs 3 and 4, equals [NO
2
]
T
2 T
, where [NO ] )
-
Br-
[
NO
NO
By solving for NO
this term into the k
dependence in the coefficient for the [NO
A fit of the bromide dependence data to such a term yields
2
] + [NO
2
Br ]. The equilibrium constant (K ) between
nucleophile. The second term, which represents the nitrite-
as-a-nucleophile term, comes from the summation term in
-
-
2
and this unreactive form equals [NO
with these two equations, and by adding
portion of eq 6, an inverse bromide
2 2
Br ]/[NO ][Br ].
2
Nu
eq 7. The ∑k
3
[Nu′] term in eq 12 represents all nucleophiles
4
-
-
present with the exception of NO
nucleophile, an additional second-order term in nitrite arises,
which will not manifest itself in the k ′ constant because k
represents only first-order nitrite dependencies. This is also
2
. When NO
2
is a
-
2
2
] term results.
3
3
′
Br-
-1
-
K
) 1.3(2) M . This NO
for the nucleophile association with NO
ClO as exhibited by the product study.
E. Determination of k
of k ′ (eq 10) increase linearly with the concentration of the
2
Br species is further evidence
2
rather than with
Nu
why nitrite is not listed as a contribution to the ∑k
intercept terms for Br and OH in the previous section.
dependencies are
concentrations. The high pH of
the study prevents interference from the ClO
concentrations are used. The first term
, while the second and third
terms are second order in nitrite. This enables fits of the
3
[Nu′]
2
-
-
Nu
3
Values. The experimental values
-
To study the nitrite effect, NO
conducted at several ClO
2
3
-
-
2
selected nucleophile as shown in Figure 3 for [Br ]. The
relatively large error bars arise because these data points
represent the first-order coefficient of the nitrite dependence
-
-
2
/NO
2
reac-
-
tion, so higher ClO
in eq 12 is first order in NO
2
-
2
fit (eq 7). Similar results are obtained for a series of
Nu
nucleophiles with slopes that permit the evaluation of k
3
-
- 2
k
obsd data to a k
chlorite concentration. The k
3
′[NO
2
] + k
4 2
′[NO ] equation for each
′ coefficients from the above
(Table 1). The intercepts in these plots represent the sum of
4
-
the rates from all the other nucleophiles present ([Nu′]) in
equation are plotted vs [ClO
in eq 13. A fit of the k ′ values in Figure 5 resolves k
2 4
and K . The K value obtained is in agreement with K k
2 4
] (Figure 5), where k ′ is listed
the system. In the case of bromide, the intercept equals
NO -
ClO -
-
OH-
-
4
3
2
H O
k
3
2
+ k
3
2
[ClO
2
] + k
3
[OH ]. When hydroxide is
k
2 4
2
k
4
H O
the nucleophile of interest, the intercept equals k
3
2
+
ClO -
-
ClO -
2
k
3
2
[ClO
2
]. The k
3
rate constant (obtained from a
(
22) Margerum, D. W.; Schurter, L. M.; Hobson, J.; Moore, E. E. EnViron.
-
nucleophilic study of ClO
2
) and the intercept from the
Sci. Technol. 1994, 28, 331-337.
7942 Inorganic Chemistry, Vol. 42, No. 24, 2003

Nucleophile Assistance of Electron Transfer
-
NO2
2
K1k3
K K k
1 2 4
k4′ )
+
(13)
-
2
- 2
2
[
ClO ] [ClO ]
values obtained from other nucleophile studies. The value
NO -
6
-2 -1
of k
3
2
is 4.4(6) × 10 M s . This value is the largest
obtained of any nucleophile studied in our system. Therefore,
-
NO
2
, acting as a nucleophile, does indeed contribute to the
rates when other nucleophiles are studied. The lack of a third-
-
order nitrite term, when the kobsd vs [NO
2
] data are fitted,
Figure 6. kobsd dependence on chlorite concentration for the chlorite-as-
is further evidence of no nucleophile assistance in the NO
2
/
-
2
-
a-nucleophile study. Data points are fit to kobsd ) a/[ClO2 ] + b/[ClO2 ]
NO pathway.
2
-1
-1
-1 -1
+
c, where a ) 9.2(3) M s , b ) 43(5) s , and c ) 40(10) M
s . For
-
-
Because the term representing nitrite as a nucleophile is
all data points, [NO2 ] ) 0.024 M, [OH ] ) 0.025 M, µ ) 1.0 M (NaClO4),
and T ) 25.0 °C.
-
second order with respect to NO
2
concentration (second
term in eq 12), its effect will only manifest itself in the k
constant and not the k ′ constant of other nucleophiles
studied. Therefore, the value of k
4
′
nucleophile-associated metastable intermediates range from
3
+
a +1 species (H
a doubly negative species (PO
nucleophile. In the case of OH , the intermediate is HONO
2
ONO
2
) when water is the nucleophile to
4
′ needs correction to
2
-
3-
NO -
-
-
NO
) when PO
is the
4
2
4
account for the K
1
k
3
2
/[ClO
2
] term. Because [ClO
2
] is
-
2
,
always kept constant at 0.06 M, this correction is constant
which at this pH rapidly deprotonates to form products. Other
nucleophiles behave in a similar manner, where the nucleo-
phile-associated species hydrolyze in a rapid step that does
not appear in the rate expression.
and can be subtracted from the overall k
4
′ term for each
7
-1 -1
nucleophile. The value of 4.8(4) × 10 M
s
2 4
for K k
given earlier has been corrected to reflect the finding that
-
NO
2
also acts as a nucleophile.
Wang et al. discussed two association mechanisms for
nucleophile-assisted electron-transfer reactions.1 Similar
G. Chlorite as a Nucleophile. Chlorite, present in all
previous nucleophile studies at 0.06 M, may also act as a
-
nucleophile. Because the ClO
2
concentration is constant,
alternative mechanisms may occur for the ClO
In the first possible mechanism ClO associates with NO
and in a second step the nucleophile binds to the ClO NO
2
2 2
/NO /Nu path.
its effect on the rate will not vary, which is different from
2
2
,
-
the nitrite case. When eq 7 is rewritten with ClO
2
as a
2
nucleophile, eq 14 results. The first term represents the
species. This association would then facilitate the electron
transfer, after which occurs a heteronuclear bond cleavage
Nu
-
-
+
K (
∑k3
[Nu′])[NO ]
to form ClO
involves NO
with ClO
both mechanisms occur. They observed a ClO
reaction with another ClO while concurrently a strongly
basic nucleophile associates to a Cl species, leading to
2 2
and NuNO . The second possible mechanism
-
1
2
ClO2
-
kobsd ) K1k
3
[NO ] +
2
+
2
/Nu association initially, followed by reaction
-
2
[ClO ]
-
2
. Odeh et al. showed that in the ClO
2
/OH system
2
1
- 2
2
K K k [NO ]
2
Nu species
2
4
2
(14)
-
2
2
[
ClO ]
2
2 4
O
-
products. In our system, the inhibition of the NO
2
/NO
2
path-
dependence of ClO
2
as a nucleophile. It derives from the
-
Nu[Nu] term in eq 7, which leaves the ∑k
parameter in eq 14 to represent the sum of all nucleophiles
present in the system except ClO
determined directly from kobsd vs [ClO
. These data are fit to an a/[ClO
equation, and that fit is shown in Figure 6, where a ) K
Nu
way with the formation of NO
association with NO
of ClO NO first, followed by nucleophile addition.
I. Lack of Nucleophile Assistance for the NO
Pathway. The NO /NO pathway (K ) exhibits no evidence
of a nucleophile effect. This indicates that nucleophiles are
not involved in the rate-determining step as in the ClO
NO reaction. Because the establishment of an equilibrium
between NO
for the NO /NO
2
Br suggests prior nucleophile
∑
k
3
3
[Nu′]
2
, but this does not rule out the formation
-
2
2
2
. Rate constants can be
-
/NO
2
2
] experiments, Figure
2
-
2
-
2 4
k
6
2
] + b/[ClO ] + c
2
2
2
2
1
K
2
-
2
Nu
-
ClO -
2
k [NO
4
2
] , b ) K
1
(∑k
3
[Nu′])[NO
2
], and c ) K
k
1 3
from
2
/
eq 14. From these parameters, K
2
k
4
6
agrees with the previous
2
ClO -
-2 -1
5
results and k
3
2
is 0.10(3) × 10 M s . Chlorite is not
2
and N
2 4
O is rapid, the rate-determining step
a very strong nucleophile in this system, but its effect may
2
2
pathway is a combination of eqs 2 and 4.
ClO -
still be notable due to its high concentration. A value of k
multiplied by the chlorite concentration (0.06 M) is sub-
tracted from the intercept value for the k
3
2
Because eq 4 is the slow step for this pathway, the lack of
any nucleophile dependence indicates that the only “nucleo-
phile” that affects the reaction is water, with a rate constant
-
3
′[OH ] study to
H O
7
-1 -1
yield k
H. Nucleophile Assistance for the ClO
The rate expression in eq 6, for the mechanism in eqs 1-5,
shows a nucleophile dependence for the k step but not for
the K steps. The transition state for the k step is (ClO
NO
3
2
.
(K
2
k
4
) of 4.8(4) × 10 M s . A hydroxide effect would
-
2
/NO
2
Reaction.
be expected since the nucleophilicity of OH is 4 orders of
magnitude larger than that of water, but none is seen.
Similarly, no other strong bases show an effect in this
3
2
k
4
3
2
-
2 4
reaction. Treinin and Hayon reported that the N O hydrolysis
q
2
Nu) , where the presence of the nucleophile in the
reaction is independent of pH, which confirms our observa-
+
3
transition state facilitates electron transfer to give NuNO
2
tion. A similar conclusion was reached when the absorption
-
and ClO
2
. For the nucleophiles studied, the charges of the
of gaseous NO
x
(where NO
x
) N O
2 4
2 3
+ N O ) into an alkali
Inorganic Chemistry, Vol. 42, No. 24, 2003 7943

Becker et al.
solution was studied.23 The lack of a nucleophile effect for
the NO /NO pathway suggests that any nucleophile as-
assists the transfer of an electron to the oxidizing species.
The third-order rate constants for the ClO /NO /Nu reaction
2 2
2
2
sistance, if present, must occur in a subsequent rapid step.
increase by 3 orders of magnitude from the water-assisted
reaction to the nitrite-assisted reaction. Only chloride shows
no increase in rate with increases in concentration. The effect
Conclusions
of chloride on the rate may not be large enough to prevent
Several nucleophiles accelerate the electron transfer from
-
OH from overwhelming it. On the other hand, the NO
2
/
2 2
NO to ClO as shown in eq 15. This is the third such
NO
portionation of NO
no increase in rate is observed with added nucleophiles. This
indicates that the N hydrolysis reaction is not assisted
by nucleophiles or bases. The current system represents a
new way to study the NO disproportionation reaction.
2
pathway is not assisted by nucleophiles. The dispro-
example where expansion of the coordination of a nonmetal
2
occurs as a rate-determining step, but
q
+
2
-
NO + ClO + Nu f (NuNO ClO ) f NuNO + ClO2
2
2
2
2
2
O
4
(15)
dioxide facilitates an electron transfer. In the previous two
examples, the nucleophile associates with ClO . In the present
case, the nucleophile associates with NO , which indicates
the nucleophilic effect is a general phenomenon that is not
limited to only Nu/ClO interactions. In all three cases the
nucleophile associates with the reducing species, and this
2
2
Acknowledgment. This work was supported by National
Science Foundation Grant CHE-0139876 and Brooks Fortune
Endowment Fellowships to Purdue University (R.H.B. and
J.S.N.).
2
2
Supporting Information Available: Figures of kinetic data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(
23) Katima, J. H. Y.; Azapagic, A.; Handley, D. Trans. Inst. Chem. Eng.,
Part B 1992, 70, 39-43.
24) Hine, J. Physical Organic Chemistry; McGraw-Hill: New York, 1962;
(
p 161.
IC0347891
7944 Inorganic Chemistry, Vol. 42, No. 24, 2003