A chemical oscillator based on the photoreduction of 2-methyl-1,4- benzoquinone

Article abstract of DOI:10.1021/jp108186q

This study investigated bromate-2-methyl-1,4-benzoquinone photoreaction in a batch reactor, in which the photoreduction of 2-methyl-1,4-benzoquinone plays a vital role in initiating and sustaining the reaction process. Transient oscillatory phenomena are observed over broad reaction conditions, but a phase diagram in the bromate and sulfuric acid concentration plane shows that the nonlinear behavior is more sensitive to the ratio of bromate and acid than their absolute concentrations. Light intensity and the photosensitive substance 2-methyl-1,4-benzoquinone exhibited opposite effects on the induction time of these oscillations, implying that illumination does more than just reduce 2-methyl-benzoquinone. The oscillatory behavior has been qualitatively reproduced with a generic model developed for bromate-aromatic compounds systems.

Full text of DOI:10.1021/jp108186q

J. Phys. Chem. A 2010, 114, 13347–13352
13347
A Chemical Oscillator Based on the Photoreduction of 2-Methyl-1,4-benzoquinone
,
†
,†,‡
Takashi Amemiya* and Jichang Wang*
Graduate School of EnVironment and Information Sciences, Yokohama National UniVersity, Yokohama,
2
40-8501, Japan, and Department of Chemistry and Biochemistry, UniVersity of Windsor,
Windsor, ON N9B 3P4, Canada
ReceiVed: August 28, 2010; ReVised Manuscript ReceiVed: NoVember 18, 2010
This study investigated bromate-2-methyl-1,4-benzoquinone photoreaction in a batch reactor, in which the
photoreduction of 2-methyl-1,4-benzoquinone plays a vital role in initiating and sustaining the reaction process.
Transient oscillatory phenomena are observed over broad reaction conditions, but a phase diagram in the
bromate and sulfuric acid concentration plane shows that the nonlinear behavior is more sensitive to the ratio
of bromate and acid than their absolute concentrations. Light intensity and the photosensitive substance
2
-methyl-1,4-benzoquinone exhibited opposite effects on the induction time of these oscillations, implying
that illumination does more than just reduce 2-methyl-benzoquinone. The oscillatory behavior has been
qualitatively reproduced with a generic model developed for bromate-aromatic compounds systems.
1
. Introduction
The photochemistry of 1,4-benzoquinone (Q) and its deriva-
bromate-based oscillators. Preliminary mechanistic study of the
bromate-Q reaction confirmed the production of bromo-
benzoquionones and bromo-hydroquinones, where the bromi-
nation took place at the neighboring site of a ketone group. How
critical these bromide-production steps are in the emergence of
spontaneous oscillations in the bromate-Q photochemical
system remains to be understood. To gain further insights into
such a process, in particular its impact on oscillatory behavior,
in this research we investigated the photochemical reaction
behavior of 2-methyl-1,4-benzoquinone (mQ) in acidic bromate
solution, where the presence of a methyl functional group at
the neighboring site of the ketone group affects the bromination
of benzoquinones and hydroquinones. Alteration in the produc-
tion of bromide ion and the consumption of bromine due to the
presence of a methyl function group shall therefore provide new
insights into the mechanism of the bromate-Q photochemical
oscillator. Studies on the photolysis of mQ in aqueous solution
tives has attracted a great deal of attention in the past three
decades.1 These studies have unveiled, for example, that the
photochemical behaviors of Q are highly dependent on the
nature of the medium, characterized by n f π* triple states,
and rapid and near-unity quantum yield for intersystem crossing.5
While different reaction paths have been proposed to account
for the photoreduction of Q in aqueous solution, all these
-6
,6
2
mechanisms agree on the production of 1,4-hydroquinone (H Q),
a substance that has broad applications in both industries and
household products.7 It is used, for example, as corrosion
inhibitors in boilers, process regulators in polystyrene manu-
facture, and photochemical developers and stabilizers in printing.7
,8
,8
The broad application of H
oxidized to Q. From the perspective of nonlinear chemical
dynamics, the oxidation of H Q by acidic bromate is particularly
interesting, since such a process is autocatalytic and can thus
2
Q is due to fact that it can be readily
2
suggest that 2-methyl-1,4-hydroquinone (mH
2
Q) is a final
Q can be oxidized
by acidic bromate through an autocatalytic approach similar to
6
product. Our investigation shows that mH
2
9
-11
potentially form a photochemical oscillator.
Recent studies have indeed reported oscillatory behavior in
2
the H Q system and thus may form a chemical oscillator with
1
2,13
the bromate-Q photoreaction.
Because light is crucial in
bromate.23 In comparison to 1,4-benzoquine, mQ has a stronger
yellow color and thus shall be more sensitive to the illumination
of visible light. As shown in the following, chemical oscillations
were indeed achieved in the bromate-mQ photoreaction.
initiating and sustaining the reaction chain, nonlinear dynamics
in the bromate-Q system can be readily manipulated all the
way to a nonreactive state, offering the feasibility of studying
perturbed behavior over a broader dynamic range that could
not be achieved with a regular photosensitive chemical system.14
It is interesting to point out that perturbed nonlinear dynamics
has attracted increasing attention in the past two decades, and
various new phenomena that do not exist in perturbation-free
environments have been reported in those perturbed media.1
Because light offers a convenient approach to implement various
spatiotemporal forcing protocols, photosensitive chemical oscil-
lators have become a favorite model system for scientists.
Existing investigations have shown that understanding the
periodic inhibition of the autocatalytic cycle by bromide ions
represents a key step in deciphering the core mechanism of
2
. Experimental Section
All reactions were performed in a quartz cell with an internal
dimension of 2.5 × 2.5 × 3.0 cm. The reaction cell was
thermostatted at a constant temperature of 25.0 ( 0.1 °C by a
circulating water bath (Yamato, cooling circulator CTA 400s).
The progress of reaction was monitored by a Pt electrode
coupled with a Ag/AgCl/KCl reference electrode, which was
4-22
2 4
either connected to the reaction cell through a K SO salt bridge
or was directly immersed in the reaction solution. With the
above two different ways of connecting the reference electrode,
there was no qualitative difference in the reaction behavior (e.g.,
induction time and number of peaks). Data presented in the
following was obtained by directly inserting a Ag/AgCl/KCl
reference electrode into the reaction solution. Potential was
*
Corresponding author. E-mail: amemiyat@ynu.ac.jp (T.A.); jwang@
uwindsor.ca (J.W.).
†
Yokohama National University.
University of Windsor.
‡
1
0.1021/jp108186q  2010 American Chemical Society
Published on Web 12/07/2010

1
3348 J. Phys. Chem. A, Vol. 114, No. 51, 2010
Amemiya and Wang
Figure 1. Time series of the bromate-mQ photoreaction in a stirred
batch reactor. Reaction conditions were [NaBrO ] ) 0.06 M, [H SO
1.6 M, and [mQ] ) 0.011 M. The light intensity was 0.52 W/cm .
Arrow indicates the time when the illumination was switched on.
3
2
4
]
2
)
recorded with a laptop computer connected to the electrodes
through an electrometer (He-104, Hokuto, Denko).
The illumination was provided by a Lightningcure LC8 from
Hamamatsu. The light source is equipped with an internal light
feedback to maintain constant light intensity at a preselected
level, and the intensity (a reference value) is shown on an LCD
screen. An optic fiber connected to the light source was
positioned at 1.5 cm away from the quartz cell and the absolute
light intensity at the cell was measured with an optical
photometer (model 1916-C) and a detector (model 818P-
Figure 2. Influence of mQ concentration on (a) the number of peaks
and (b) the induction time of the bromate-mQ oscillations. Other
reaction conditions were [NaBrO
3
]2 ) 0.06 M, [H
2 4
SO ] ) 1.6 M, and
0
01-12) from Newport. Stock solutions NaBrO
3
(Wako, > 99%,
the light intensity was 0.52 W/cm .
1.5 M) and sulfuric acid (Wako, 97%, 6 M) were prepared with
deionized water from Millipore (18.2 MΩ). mQ (98%, Wako)
solution was prepared daily and was kept in a covered bottle
after the preparation. Throughout this research, the stirring rate
was maintained at about 700 rpm.
Figure 2 shows the influence of mQ concentration on the
bromate-mQ reaction behavior, where concentrations of
3 2 4
NaBrO and H SO were kept constant at 0.06 and 1.6 M,
2
respectively, and the light intensity was equal to 0.52 W/cm .
Figure 2a illustrates that increasing mQ concentration first led
to a rapid increase and then a gradual decrease in the total
number of oscillation peak. No oscillations could be seen for
mQ concentration below 0.004 or above 0.038 M. On the other
hand, as seen in Figure 2b, the induction time increases
monotonically with mQ concentration. We would like to point
out that increasing light intensity caused a monotonic decrease
of the induction time. The opposite effects of light intensity
and mQ concentration on the oscillatory behavior suggest that
illumination may have affected other reaction processes as well.
As pointed out earlier, mQ has strong absorptions in both UV
and visible regime. We have employed a cutoff filter with a
cutoff wavelength of 350 nm to repeat the above experiments
and obtained the same photoreaction behavior, implying that
the absorption of visible (yellow) light by mQ is responsible
for the above photochemical process.
3
. Experimental Results
Redox potential of the nonilluminated mQ, bromate, and
sulfuric acid mixture soon reached a plateau and stayed flat in
time, suggesting that there was no reaction taking place in the
reactor. Such an observation is consistent with literature reports
that benzoquinone, an oxidation product of hydroquinone,
1
0,23
cannot be further oxidized by acidic bromate.
Figure 1 shows
that, upon turning on illumination, there is a sharp drop in the
Pt potential. Since the potential drop has been observed
throughout this research, it is most likely a result of the
photoreduction of mQ. Over the next 15 min, the Pt potential
in Figure 1 increased gradually. Spontaneous oscillations
emerged shortly after the potential started decreasing again.
The time interval from the start of the reaction to the moment
when oscillations emerged varies between 10 and 40 min. Such
a long induction time is similar to what has been reported in
9,12,23
other uncatalyzed bromate-oscillators.
After the oscillatory
Figure 3 summarizes the dependence of the photochemical
reaction dynamics on the concentrations of bromate and acid
in a two-dimensional phase plane, where filled circles represent
the conditions at which spontaneous oscillations have been
achieved in the batch system. This phase diagram shows that
at the low bromate concentration the acid concentration suitable
for spontaneous oscillations is wider than that at high bromate
concentration. When bromate concentration becomes very low
(<0.015 M), no spontaneous oscillation could be achieved. This
band shaped phase diagram implies that the ratio of bromate
behavior disappeared, the Pt potential gradually returned to the
original value. Our experiments show that when the light
intensity was too low (e.g., 0.33 W/cm ), no spontaneous
2
oscillations could be achieved, where the reaction behavior was
qualitatively the same as that presented in Figure 1, except that
the oscillatory evolution was replaced by a smooth decrease.
Increasing light intensity shortened the induction time, for
2
2
example, from 21 min at 0.52 W/cm to 11 min at 1.05 W/cm .
When the light intensity became too high, however, the number
of oscillation decreased.

Chemical Oscillator Based on Photoreduction of mQ
J. Phys. Chem. A, Vol. 114, No. 51, 2010 13349
to point out that the induction time varies between 19 and 23
min across the phase diagram in Figure 3, suggesting that
bromate and acid concentrations do not have any significant
influence on the induction time. This is in contrast to the great
influence of mQ and light intensity on the induction time. The
above result further highlights the importance of the photore-
duction process in this new chemical oscillator.
4
. Modeling
.1. A Generic Model for the Reaction of Bromate and
mH Q. In 1979, Orb a´ n, K o¨ r o¨ s, and Noyes developed a general
model for uncatalyzed bromate-aromatic compounds reac-
4
2
24
tions. The original mechanism, which is referred to as the OKN
model in the following, is composed of 16 reaction steps, i.e.,
Figure 3. Phase diagram of the bromate-mQ reaction in the bromate
and sulfuric acid concentration space. Other conditions are [mQ] )
1
0 steps K1-K10 in Scheme I and six steps K11-K16 in
2
0
.011 M and the light intensity was 0.52 W/cm .
Scheme II (Table 1). The 10 reaction steps K1-K10 in Scheme
I and the first four reaction steps K11-K14 in Scheme II can
2
be applied for reactions of acidic bromate with mH Q. Steps
K15 and K16 suggest how phenol and its derivatives could be
involved in the reactions with acidic bromate and are not
applicable here since the substance mH Q has two phenolic
2
groups. Our previous investigation on the bromate-1,2-hydro-
quinone oscillator has illustrated that eliminating step K11 from
the OKN model does not influence the oscillatory behavior
qualitatively. Therefore, the reaction steps employed in the
following calculations also excluded step K11 from the OKN
model. More specifically, the present model, as listed in Table
1
1
, consists of 13 reaction steps K1-K10, and K12-K14, and
-
-
1 variables, BrO
, HAr(OH)O*, HArO
Q, HAr(OH)O* is mH
BrAr(OH) is brominted mH Q.
.2. The Photochemical Step. The chemical principle behind
the photocontrolled chemical oscillator is the photoreduction
of mQ, which is otherwise a final product in the bromate-mH
reaction. The photochemistry of light-induced reduction of mQ
to mH Q in aqueous solution is a rather complicated process
3
, Br , BrO
2
*, HBrO
2
, HOBr, Br*, Br
2
,
HAr(OH)
is mH
2
2
, and BrHQ, where HAr(OH)
2
2
2
2
Q radical, HArO is mQ, and
2
2
4
2
Q
2
involving multiple reaction steps. For simplicity, the following
schematic step is employed to represent the complicated
photoreduction of mQ:
mQ + H O + hν f fmH Q (rate constant, kP1)
2
2
(P1)
The reaction rate of the above schematic step is represented
by a first-order reaction with respect to mQ with a rate constant
Figure 4. Time series obtained under conditions “a” and “b” labeled
in Figure 3. Note that the scale used in panel b is smaller than that in
panel a in order to make those oscillations visible.
kP1 that was adjusted arbitrarily to reflect the variation of
illumination intensity. A stoichiometric factor f is employed in
the present model to represent the complicated photoreduction
2
of mQ to mH Q. According to previous studies on the
6
over acid is more critical than their absolute concentrations in
the onset of oscillatory behavior in the illuminated bromate-mQ
reaction.
mechanism of photoreduction of mQ to mH Q, step P1 can be
2
roughly divided into the following three processes:
Two typical time series achieved under the conditions near
the upper and lower boundaries in Figure 3 are presented in
Figure 4. Our study indicates that the number of peaks increases
gradually as the reaction conditions are moved toward the center
of the parameter window. Near the lower boundary, the system
exhibits large amplitude oscillations. In contrast, under the
reaction conditions near the upper boundary of the phase
diagram, the system exhibits small amplitude oscillations. As
the reaction conditions are adjusted toward the upper boundary,
the oscillation amplitude decreases gradually, but there is no
significant decrease in the total number of peaks. It is interesting
mQ + hV f mQ* (rate constant, k1)
mQ* f mQ (rate constant, k2)
(P1A)
(P1B)
mQ* + H O f mH Q (rate constant, k3)
2
2
(P1C)
where mQ* is the excited state of mQ. Step P1A is the excitation
of mQ, step P1B is the quenching of mQ*, and step P1C is the

1
3350 J. Phys. Chem. A, Vol. 114, No. 51, 2010
Amemiya and Wang
TABLE 1: Reaction Mechanism for Photocontrolled Bromate-Benzoquinone Systems
rate constant
no.
reaction
forward
reverse
ref.
Scheme I
) 2.1 [H ] M s-1
+
2
-1
k
k
r1 ) 1 × 10 M s-1
4
-1
a
BrO3- + Br + 2H T HBrO + HOBr
-
+
(K1)
(K2)
(K3)
(K4)
(K5)
k
k
k
k
k
1
2
) 2 × 10 [H ] M s-1
9
+
-1
a
-
r
+
2
3
4
5
HBrO + B + H f 2HOBr
2
) 1 × 10 [H ] M s-1
4
+
-1
r3 ) 2 × 10 M s-1
7
-1
a
-
+
*
BrO3 + HBrO + H T 2BrO + H O
2
2
2
) 1.4 × 10 M s-1
3
-1
a, b
a
*
BrO2 + HAr(OH) f HBrO + HAr(OH)O*
2
2
) 4 × 10 M s-1
7
-1
-
+
2
HBrO f BrO + HOBr + H
2 3
5
-1
r6 ) 2 × 10 M s-1
2
-1
a, b
a, b
(
(
K6)
K7)
HOBr + HAr(OH)O* T Br* + HArO + H O
k
k
6
7
) 2 × 10 M s-1
k
k
2
2
) 1 × 10 M s-1
4
-1
-
+
Br* + HAr(OH)O* f Br + HArO + H
2
) 9.5 × 10 [H ] M s-1
9
+
-1
r8 ) 1.1 × 10 s
2
-1
a
-
+
(
(
(
K8)
k
k
k
8
HOBr + Br + H T Br + H O
2
2
2
-1 -1
a, b
a, b
-
+
K9)
9
) 8 × 10 M
s
Br + HAr(OH) f BrAr(OH) + Br + H
HOBr + HAr(OH) f BrAr(OH) + H O
2
2
2
-
1
-1
K10)
10 ) 1 M
s
2
2
2
Scheme II
12 ) 1 × 10 M s-1
3
-1
a, b
*
-
+
(
K12)
k
k
Br + HAr(OH) f Br + HAr(OH)O* + H
2
13 ) 2.2 × 10 M-1 s-1
-
2
k
k
r13 ) 4 × 10 M s-1
4
-1
a, b
a, b
(
(
K13)
K14)
HAr(OH) + HArO T 2HAr(OH)O*
2HAr(OH)O* f Ar (OH)
2
2
14 ) 2 × 10 M s-1
4
-1
2
4
Photochemical Reaction Step
(
P1)
HArO + H O + hν f fHAr(OH)
2
k
P1 (M- s-1
1
)
c
2
2
a
Orban, Koros, and Noyes, 1979 (ref 24). b Adustable parameter chosen to give a good fit to data. c Proposed in this study. In this scheme,
is mH Q, HAr(OH)O* is the radical obtained by hydrogen atom abstraction, HArO is the related benzoquinone, BrAr(OH) is the
brominated derivative, and Ar (OH) is the coupling product.
HAr(OH)
2
2
2
2
2
4
production of mH
metric factor f is found to be equivalent to 1/(1 + (k /k )) under
2 3
the steady-state approximation for mQ*.
Combination of the generic OKN model for the reaction of
bromate and hydroquinones with the photoreduction model for
mQ forms the complete model for the bromate-mQ photore-
action that consists of 14 steps as listed in Table 1. This model
2
Q. From the above three steps, a stoichio-
similar between the calculations and experiments. We also tried
to simulate the experimentally obtained phase diagram in the
bromate and acidic concentration space as shown in Figure 3,
and found that the present model produces oscillations in a much
wider range of bromate and acidic concentrations. For instance,
oscillations could be obtained with initial concentrations of
[NaBrO ] ) 0.06 M, [H SO ] ) 4 M, and [mQ] ) 0.011 M in
the calculation (not shown). Such a discrepancy may result from
the selection of rate constants. We carefully adjusted the values
of those rate constants for steps K6-K14, which involve organic
species, to give good agreement between experimental results
and numerical calculations, yet the discrepancies could not be
completely eliminated.
3
2
4
also resembles the model used to simulate the photoreaction of
bromate and Q,1
2,13
except here a stoichiometric factor is
introduced to account for the subtle influence of light.
.3. Numerical Results. The calculation was carried out by
4
integrating a set of differential equations resulting from the
application of the law of mass action to the reactions with the
rate constant as listed in Table 1. Figure 5 shows oscillatory
In the simulation, rate constant kP1 reflects the intensity of
light, where a larger kP1 value corresponds to more intense
illumination. No oscillations were exhibited at light intensities
-
behavior in [Br ] obtained from the model. The initial concen-
trations of [NaBrO
3
] ) 0.06 M, [H
2 4
SO ] ) 1.6 M, and [mQ] )
0
.011 M used in the calculation (Figure 5a) were the same as
-
5
-1
lower than 7.5 × 10 s , which agrees with the experimental
result. Similar to what was observed experimentally, increasing
light intensity in the simulation shortens the induction time.
However, continuous increase of light intensity in the model
eventually leads to oscillations without any induction time. Such
a discrepancy arises from the fact that light only affects the
those used in the experiment as shown in Figure 1. We can
find that the present model reproduces the spontaneous oscil-
lations as observed in the experiment semiquantitatively; the
periods of the oscillations were about 30 and 200 s in the
experiment and in the calculation, respectively. Increase in
the initial concentration of [mQ] ) 0.025 M gives increase in
the induction time as shown in Figure 5b. The calculation shows
that variables BrO
H)O*, and BrAr(OH)
whereas only one variable, namely Q, did not oscillate in the
present configuration (not shown).
2
production of mH Q in the model, while the experimental
-
results, in particular the opposite influences of mQ and light
intensity on induction time, has suggested that light is involved
in several photochemical processes in this chemical oscillator.
3
, HBrO
2 2 2
, BrO *, HOBr, Br*, Br , HAr(O-
2
also exhibited periodic oscillations,
In the above calculations, the stoichiometric factor f is crucial
to reproduce the experimentally obtained results semiquantita-
tively. If this factor f is close to unity, the agreement becomes
far from quantitative; either the IT is very long (more than
10 000 s) or the NP is very large (more than 100), or both. The
dependence of NP and IT on the initial concentration of [mQ]
as shown in Figure 2 could not be reproduced neither, even
Figure 6 shows the calculated number of peaks (NP) and
induction time (IT) with initial concentrations of [NaBrO
.06 M, [H SO ] ) 1.6 M, and [mQ] ) 0.011 M. Agreement
of these results with those obtained in the experiments (Figure
) is good; the maximal behavior of NP and monotonic increase
in IT as a function of the initial concentration of [mQ] are quite
3
] )
0
2
4
2

Chemical Oscillator Based on Photoreduction of mQ
J. Phys. Chem. A, Vol. 114, No. 51, 2010 13351
Figure 6. Calculated influence of mQ concentration on the
bromate-mQ photoreaction behavior. Other conditions are the same
as those in Figure 5.
-
Figure 5. The variation of Br concentration calculated with the model
-
4
listed in Table 1 at (a) [mQ] ) 0.011 M and (b) [mQ] ) 0.025 M. The
When the amount of mQ added was large, such as 8 × 10
-
3
-1 -1
rate constant kP1 was 3 × 10 (M
s
), and the stoichiometric factor
M, there was a short induction time before the reoccurrence of
spontaneous oscillations. This perturbation experiments strongly
suggest that “higher” mQ concentration may also be responsible
for the long induction time. This is somehow different from
what has been reported in the bromate-Q photoreaction.
The qualitative reproduction of spontaneous oscillations with
the proposed model suggests that the core of this photochemical
-
f was 0.015. Other initial concentrations were [BrO ] ) 0.06 M,
3
-
-9
[
H
2
SO
4
] ) 1.6 M, [Br ] ) 1.0 × 10 M, and zero for all other
intermediate species.
though different sets of rate constants as listed in Table 1 were
tested in the calculation.
2
oscillator is still the autocatalytic production of HBrO , despite
5
. Discussion
the critical role of the photoreduction. To examine whether the
long induction time also arises from the desired accumulation
of certain intermediates, especially brominated substances,
bromide was added to the system to generate bromine and
subsequently the brominated quinone species; no significant
decrease in the induction time was observed. Again, this is
This study reports a new photochemical oscillator, in which
the photoreduction of mQ is vital in sustaining the chain reaction
process. Chemical oscillations stopped immediately upon turning
off and resumed rapidly after turning back the illumination.
According to an earlier investigation on the photolysis of mQ
in aqueous solution, mH
1
3
6
different from the bromate-Q photochemical oscillator.
2
Q is produced through the following
, and only
Calculations show that the mQ concentration starts to decrease
exponentially upon illumination, and then remains at a very low
steady-state value, suggesting that the disappearance of spon-
taneous oscillations in experiments is due to the consumption
stoichiometric relationship: where R represents CH
3
-
of mQ. Oscillations in [Br ] or other species took place during
a period just before mQ exhibits the steady state. Accumulation
of mQBr occurs in the same way as seen in the bromate-Q
modeling,1 in which the calculation shows that the concentra-
tion of mQBr finally exhibits a steady value that is the total
concentrations of the organic species used in the calculation.
For the moment, effects of the methyl group on the rate of steps
K4, K6, K7, and K9-K14 are not available. Further study of
the bromate-Q and bromate-mQ systems may clarify the
reaction mechanism of benzoquione photochemical oscillators.
2,13
2
one mH Q molecule is produced through the consumption of
two mQ. Therefore, the consumption of mQ is expected to be
responsible for the limited number of oscillation peaks. The
above conclusion is confirmed in this study: under the conditions
used in Figure 4b, after the oscillation has stopped, adding 2.2
-
4
×
10 M mQ to the reaction mixture revived oscillations
Acknowledgment. J.W. would like to acknowledge the
financial support from the Natural Sciences and Engineering
immediately, where more than six peaks have been observed.

1
3352 J. Phys. Chem. A, Vol. 114, No. 51, 2010
Amemiya and Wang
Research Council (NSERC), Canada, and an invitation fellow-
ship from the Japan Society for the Promotion of Science (JSPS).
(13) Zhao, B.; Wang, J. J. Photochem. Photobiol. A 2007, 192, 204–
2
10.
(
14) Wang, J.; K a´ d a´ r, S.; Jung, P.; Showalter, K. Phys. ReV. Lett. 1999,
8
2, 855–858.
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