Kinetic study of the ClOO + NO reaction using cavity ring-down spectroscopy

Article abstract of DOI:10.1021/jp052688d

Cavity ring-down spectroscopy was used to study the reaction of ClOO with NO in 50-150 Torr total pressure of O₂/N₂ diluent at 205-243 K. A value of k(ClOO+NO) = (4.5 ± 0.9) × 10^-11 cm³ molecule^-1 s^-1 at 213 K was determined (quoted uncertainties are two standard deviations). The yield of NO₂ in the ClOO + NO reaction was 0.18 ± 0.02 at 213 K and 0.15 ± 0.02 at 223 K. An upper limit of k(ClOO+Cl₂) < 3.5 × 10 ^-14 cm³ molecule^-1 s^-1 was established at 213 K. Results are discussed with respect to the atmospheric chemistry of ClOO and other peroxy radicals.

Full text of DOI:10.1021/jp052688d

3546
J. Phys. Chem. A 2006, 110, 3546-3551
Kinetic Study of the ClOO + NO Reaction Using Cavity Ring-Down Spectroscopy
Shinichi Enami, Yosuke Hoshino, Yuki Ito, Satoshi Hashimoto, and Masahiro Kawasaki*
Department of Molecular Engineering and Graduate School of Global EnVironmental Studies, Kyoto
UniVersity, Kyoto 615-8510, Japan
Timothy J. Wallington
Scientific Research Laboratories, Ford Motor Company, Dearborn, Michigan 48121-2053
ReceiVed: May 23, 2005; In Final Form: December 12, 2005
Cavity ring-down spectroscopy was used to study the reaction of ClOO with NO in 50-150 Torr total pressure
of O2/N2 diluent at 205-243 K. A value of k(ClOO+NO) ) (4.5 ( 0.9) × 10-11 cm3 molecule-1 s-1 at 213
K was determined (quoted uncertainties are two standard deviations). The yield of NO2 in the ClOO + NO
reaction was 0.18 ( 0.02 at 213 K and 0.15 ( 0.02 at 223 K. An upper limit of k(ClOO+Cl2) < 3.5 × 10-14
cm3 molecule-1 s-1 was established at 213 K. Results are discussed with respect to the atmospheric chemistry
of ClOO and other peroxy radicals.
1. Introduction
ClOO + NO + M f ClONO2 + M
∆H ) -165 kJ mol-1 (2c)
ClOO is a weakly bound species (Cl-O bond strength, 19.5
kJ mol-1), which exists in rapid equilibrium with Cl atoms and
O2, Kp(T) ) k1/k-1 ) (1.04 ( 0.04) × 10-5 exp((2668 ( 25)/
We have used cavity ring-down spectroscopic techniques to
study (i) the kinetics of the ClOO + NO and ClOO + Cl2
reactions in 50-150 Torr of O2/N2 diluent at 205-243 K, and
(ii) the NO2 yield from the ClOO + NO reaction at 213-243
K. The present work had two goals. First, to provide kinetic
and mechanistic for the reaction of ClOO with NO that can be
used to shed light on the chemistry of other peroxy radicals.
Second, to extend the utility of the time-resolved cavity ring-
down technique by demonstrating its application to study ClOO
radical kinetics.
T) bar-1 1-7
.
Cl + O2 + M a ClOO + M
(1, -1)
In the troposphere, the [ClOO]/[Cl] ratio increases from
approximately 2% near the Earth’s surface to 8% at 12 km,
reflecting the dominant effect of decreased temperature (as
opposed to decreased [O2]) with increased altitude on the
equilibrium. For altitudes above 12 km the effects of increasing
temperature and decreasing [O2] reinforce each other resulting
in a steadily decreasing [ClOO]/[Cl] ratio with increasing
altitude ([ClOO]/[Cl] ≈ 1% at 25 km, ≈ 0.1% at 35 km).8,9
2. Experimental Section
2.1. Kinetic Study of ClOO + Cl2 and ClOO + NO
Reactions. Cavity ring-down spectroscopy (CRDS) was first
introduced by O’Keefe and Deacon13 and has been applied
widely in spectroscopic and kinetic studies.14,15 The technique
is based on monitoring the decay of a laser pulse trapped in an
optical cavity. The presence of absorbing species within the
cavity increases the decay rate (shortens the ring-down time)
of the intensity of the laser pulse within the cavity. Measurement
of the ring-down time provides information on the strength of
absorption within the cavity. Lin and co-workers16 reported the
first application of CRDS to kinetic studies by combining CRDS
with laser flash photolysis. Lin and co-workers derived kinetic
data for reactions of phenyl radicals by monitoring ring-down
time as a function of delay between photolysis and probe laser
pulses in a technique called “time-resolved CRDS”16,17 Over
the past decade, time-resolved CRDS has been used by several
research groups in kinetic studies of a variety of species, e.g.,
FCO,18 CHO,19 BrO,20 IO,21 C2H5,22 C2H5O2,22 and NO3.23
Peroxy radicals of the general formula RO2 (e.g., HO2,
CH3O2, C2H5O2, etc.) are key intermediates in the atmospheric
chemistry of all organic compounds. The reaction of RO2 with
NO is particularly important in the chemistry responsible for
the formation of urban air pollution. ClOO provides an
interesting example of a peroxy radical that is weakly bound
and unstable at room temperature. Information concerning the
kinetics and mechanisms of reactions of ClOO radicals provides
useful insight into the atmospheric chemistry of peroxy radicals
in general. Although the kinetics of the ClOO self-reaction and
its reaction with Cl atoms are reasonably well understood,10-12
there have been few studies of the kinetics and mechanisms of
reactions of ClOO radicals with other atmospherically relevant
species. For example, the ClOO + NO reaction has three
thermochemically allowed reaction channels, but the kinetics
and mechanism of this reaction have yet to be studied.
ClOO + NO f ClO + NO2
ClOO + NO f ClNO + O2
∆H ) -53 kJ mol-1 (2a)
Figure 1a shows a schematic diagram of the time-resolved
CRDS apparatus used in the present study. The system employs
pulsed photolysis and probe lasers. After the photolysis laser
beam traverses a Pyrex glass tube reactor (21 mm i.d.), the probe
laser beam is injected nearly collinear to the axis of the
photolysis laser through one of two high-reflectivity mirrors
(Research Electro-Optics, reflectivity > 0.999 at 266 nm). Light
∆H ) -136 kJ mol-1
(2b)
* To whom corresponding should be addressed. Fax number: + 81-75-
383-2573. E-mail address; kawasaki@moleng.kyoto-u.ac.jp.
10.1021/jp052688d CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/11/2006

Kinetic Study of the ClOO + NO Reaction
J. Phys. Chem. A, Vol. 110, No. 10, 2006 3547
The temperature of the gas flow region in the glass tube
reactor was controlled over the range 205-298 K. The differ-
ence between the sample gas temperatures at the entrance and
the exit of the gas flow region was <1 K. Typical conditions
for decay measurements were total flow rate 1000-4500 cm3
min-1 (STP), [Cl2] ) 1014-1016 molecule cm-3, and [NO] )
1014-1015 molecule cm-3. Fluctuations of the total pressure,
gas flow rates, and temperature in the reaction cell during an
experiment were less than 1%, 3%, and 1%, respectively. ClOO
decay profiles were measured 30-1000 µs after the photolysis
laser pulse. NO2 and ClNO are expected products of reaction
2. NO2 and ClNO have modest absorption cross sections in the
region 485-491 nm (σ ) 10-19-10-20 cm2 molecule-1). To
monitor the formation of NO2 and ClNO from reaction 2, it
was desirable to use higher concentrations of Cl2 (typically
1016-1017 molecule cm-3) than used in the kinetic experiments
to generate larger concentrations of ClOO.
To determine the initial Cl atom concentration, [Cl]0, the
production of ClO at 273 K was measured at 266 nm using
Cl2/O3/O2 mixtures with photolysis of Cl2 at 355 nm and [Cl2]
) (1-10) × 1015 molecule cm-3. Formation of ClOO is
negligible at 273 K and Cl atoms are converted quantitatively
into ClO radicals via
Cl + O3 f ClO + O2
(4)
O3 was produced by irradiating an oxygen gas flow (slightly
higher than 760 Torr) with a low-pressure Hg lamp (Hamamatsu
Photonics, L937-02). The initial Cl atom concentration, [Cl]0,
calibrated using σ(ClO) ) 5.35 × 10-18 cm2 molecule-1 at 266
nm,8 was observed to increase linearly with [Cl2]. For the low
radical concentration employed in the present work dimerization
of ClO is not a complication. For experiments at low temper-
atures, [Cl]0 was calculated using the calibration at 273 K with
an appropriate correction for the temperature dependence of the
Cl2 absorption cross section.8 The initial chlorine atom con-
centration in the kinetic experiments was 1012-1013 molecule
Figure 1. Schematic diagrams of CRDS systems used in the kinetic
(a) and spectroscopic (b) studies. Labeled components in (a): 1, digital
oscilloscope; 2, photomultiplier; 3, thermometer; 4, pressure gauge; 5,
band-pass filter; 6, high-reflective mirrors; 7, delay generator; 8, mass
flow controller.
leaking from the exit mirror was detected by a photomultiplier
tube (Hamamatsu Photonics, R212UH) through suitable narrow
band-pass filters (Edmund Optics) to reduce scattered light from
the 355 nm photolysis laser pulse. Decay profiles of the light
intensity were recorded using a digital oscilloscope (Tektronix,
TDL-714L, 8-bit digitizer) and transferred to a personal
computer. In the presence of an absorbing species, the light
intensity within the cavity is given by
cm-3
.
2.2. Spectroscopic Study for NO2 Detection. The CRDS
cell shown in Figure 1b was used to record low-temperature
absorption spectra of NO2 between 485 and 491 nm using a
dye laser (Lambda Physik, ScanMate, spectral resolution < 0.01
nm). The typical concentration of NO2 was 3.2 × 1014 molecule
cm-3 and the corresponding τ0 was 1.5 µs with mirrors (II-VI
Optics). As shown in Figure 1b, the two highly reflective mirrors
were set into the jacketed region of the cell to keep the gas
temperature uniform between the mirrors at T ) 213-243 K.
The cavity length was 550 mm.
I(t) ) I0 exp(-t/τ) ) I0 exp(-t/τ 0 - σncLRt/LC) (3)
where I0 and I(t) are the light intensities at time 0 and t, τ is the
cavity ring-down time with photolysis beam, τ0 is the cavity
ring-down time without photolysis laser light (typically 1.5 µs),
LR is the length of the reaction region (400 ( 10 mm), LC is
the cavity length (1040 mm), c is the velocity of light, and n
and σ are the concentration and the absorption cross section of
absorbing species. A value of σ(ClOO) ) 9.05 × 10-18 cm2
molecule-1 at 266 nm11 was used to calculate the absolute
concentration of ClOO. By varying the delay between the
photolysis and probe laser pulses, the concentration of ClOO
could be monitored as a function of time.
Allowance was made for NO2 dimerization using the reported
equilibrium constant in units of cm3 molecule-1 8
.
NO2 + NO2 a N2O4
Kc(T) ) 5.2 × 10-29 exp(6643/T)
All reagents were obtained from commercial sources and used
without further purification: Cl2 (high purity, Sumitomo Seika
Co.); NO (1.00% in N2 diluent, Takachiho Co.); NO2 (99.9%,
Teisan Co.); N2 (99.999%, Teisan Co.); O2 (>99.995%, Teisan
Co.).
The 355 nm output of a Nd3+:YAG laser (Lambda Physik,
LPY400) was used to dissociate Cl2 to generate Cl atoms which
then react with O2 to give ClOO. The photolysis laser was
operated with a typical power of 10 mJ/pulse at a repetition
rate of 2 Hz. Shot-to-shot fluctuation of the laser power was
less than 10%. Absorption by ClOO at 266 nm was monitored
(at various delay times with respect to the photolysis laser pulse)
with a Nd3+:YAG laser beam (Spectra-Physics, Lab 130).
3. Results
3.1. Measurement of k(ClOO+Cl2). Prior to the study of
k(ClOO+NO), a limited series of experiments was performed
using mixtures of (3-20) × 1015 molecule cm-3 of Cl2 and 4
× 1018 molecule cm-3 of O2 in 100 Torr total pressure of N2
diluent at 213 K to check for ClOO loss via reaction with Cl2.

3548 J. Phys. Chem. A, Vol. 110, No. 10, 2006
Enami et al.
In the experiments, [Cl]0 was maintained at a constant value
(within 5%) by adjusting the photolysis laser fluence. With [Cl]0
essentially constant, the loss of ClOO via self-reaction and
reaction with Cl should be unchanged and any change in the
decay of ClOO can be ascribed to reaction with Cl2. No change
in the decay of ClOO was observed as a function of the Cl2
concentration, from which we derive an upper limit of
k(ClOO+Cl2) < 3.5 × 10-14 cm3 molecule-1 s-1. This result
can be compared to the value of k(ClOO+Cl2) ) 3.4 × 10-12
cm3 molecule-1 s-1 reported by Baer et al.10 to explain the fast
decay of ClOO observed in their experiments at room temper-
ature. Experimental constraints in the work of Baer et al.10
precluded variation of [Cl2] to confirm that the observed ClOO
decay was indeed attributable to reaction with Cl2 and Baer et
al.10 viewed their value as a “tentative determination”. In the
present work, [Cl2]0 was varied with no impact on the ClOO
decay rate and we conclude that ClOO reacts slowly, if at all,
with Cl2 at 213 K. Our observations suggest that reaction with
Cl2 is not the explanation of the fast decay of ClOO observed
at room temperature by Baer et al.10
Figure 2. Typical ClOO decay profiles obtained using mixtures
containing 3.3 × 1015 of Cl2 and 4.0 × 1018 molecule cm-3 of O2 in
100 Torr of N2 diluent at 205 K with [NO] ) 0 (squares), 5.1 × 1014
(circles), or 1.4 × 1015 molecule cm-3 (filled triangles). The inset shows
the same data plotted using a logarithmic y-axis scale.
3.2. Measurement of k(ClOO+NO). Figure 2 shows typical
decay profiles of ClOO observed in the presence and absence
of NO in 100 Torr total pressure of N2 diluent with [O2] ) 4.5
× 1018 molecule cm-3 at 205 K. In the absence of NO, ClOO
radicals are lost via diffusion from the detection region, reaction
with Cl, and self-reaction. As seen from the insert in Figure 2,
there is a small residual absorption evident at reaction times
>0.5 ms which we attribute to absorption by stable products
(probably ClNO) formed in the system. For simplicity, in our
kinetic analysis, we treated the residual absorption as a
background offset and subtracted it from the entire signal trace.
In reality, the small absorption by stable products such as ClNO
will not be constant but will grow during the decay of ClOO.
To quantify the error introduced by our simplified analysis
method, the IBM chemical kinetics simulator with the chemical
mechanism described below was used to provide simulated
absorption traces. Applying our simplified method to treat the
small residual absorption led to <1% error in the values of k2
returned by analyzing the simulated data traces.
Figure 3. Plot of the pseudo-first-order component of the ClOO decay
versus [NO]. Experiments were conducted at 213 K using the following
reagent concentrations (molecule cm-3): O2, 4.0 × 1018; N2, 4.7 ×
1017; Cl2, 3.2 × 1014; NO, (2-60) × 1014. The photolysis laser fluence
was either 7 mJ/pulse (circles) or 17 mJ/pulse (squares).
The decay of [ClOO] can be analyzed as a sum of first and
second-order kinetics.1,20
The first complication is that in addition to ClOO radicals, Cl
atoms also react with NO. Loss of Cl atoms via reaction with
NO in the system will lead to an enhanced rate of loss of ClOO
as the equilibrium (1, -1) shifts to compensate for the lost Cl
atoms. To appreciate the magnitude of this effect on the results
shown in Figure 3, we must first calculate the fraction of Cl
atoms that are present as free Cl atoms compared to those
present as ClOO. For the conditions used to obtain the data in
ClOO + ClOO f products
(5)
(6)
2k5
2k5
1
1
)
+
exp(k1stt) -
(
)
k1st
k1st
{(
)
}
[ClOO]t
[ClOO]0
where [ClOO]0 is the initial ClOO concentration, [ClOO]t is
the concentration of ClOO at time t, and k1st is the pseudo-
first-order rate of loss of ClOO with respect to reaction with
NO. Figure 3 shows a plot of k1st versus the NO concentration.
As seen from Figure 3, variation of the photolysis laser pulse
energy from 7 to 17 mJ pulse-1 had no discernible effect on
k1st, indicating that the initial ClOO radical concentration does
not impact the kinetic analysis. The line through the data in
Figure 3 is a linear least-squares analysis and provides informa-
tion on the kinetics of the reaction of ClOO radicals with NO.
In a typical kinetic study, the slope of the regression line in
Figure 3 could be equated with the bimolecular rate constant
k2. Unfortunately, in the present work because of the rapid
equilibrium between ClOO and Cl and consequent presence of
significant concentrations of Cl, it is not possible to simply
equate the slope in Figure 3 to k2.
Figure 3 ([O2] ) 4.0 × 1018, [N2] ) 4.7 × 1017 molecule cm-3
,
T ) 213 K), using k1 ) 2.7 × 10-33(T/298)-1.5 cm6 molecule-2
s-1 and k-1 ) 2.8 × 10-10 exp(-1819/T) cm3 molecule-1 s-1
,
it can be calculated that at equilibrium [Cl]/([Cl] + [ClOO]) )
0.75. Using k(Cl+NO+M) ) 9.0 × 10-32(T/300)-1.6 cm6
molecule-2 s-1,8 it follows that for [NO] ) 3 × 1015 molecule
cm-3 (a representative [NO] used in our experiments, see Figure
3) the pseudo-first-order loss rate of Cl atoms with respect to
reaction with NO is 2100 s-1. This loss rate can be compared
to the observed pseudo-first-order loss rate for ClOO in the
presence of 3 × 1015 molecule cm-3 of NO of approximately
3 × 104 s-1. The loss of Cl atoms via reaction with NO makes
a small, but significant, contribution to the observed loss of
ClOO.
The presence of Cl atoms gives rise to two complications
that must be accounted for before a value of k2 can be derived.
The second complication is caused by the fact that in all
experiments the concentration of Cl atoms exceeds that of ClOO

Kinetic Study of the ClOO + NO Reaction
J. Phys. Chem. A, Vol. 110, No. 10, 2006 3549
TABLE 1: Values of k(ClOO+NO) Determined in the
Present Work (quoted uncertainties are two standard
deviations)
1011kClOO+NO
temp (K)
(cm3 molecule-1 s-1
)
205
213
223
233
243
4.3 ( 0.8
4.5 ( 0.9
4.2 ( 0.8
4.9 ( 1.0
5.5 ( 1.2
radicals. Loss of ClOO radicals via reaction 2 will be compen-
sated to some degree by reaction of Cl with O2, generating more
ClOO radicals in an attempt to restore the equilibrium. For a
system at equilibrium a linear least-squares fit to the measured
ClOO loss rates in Figure 3 will underestimate k2 by a factor
of ([ClOO] + [Cl])/[ClOO]. For the conditions relevant to Figure
3 given above ([ClOO] + [Cl])/[ClOO] ) 4.1. In reality, the
system is not quite at equilibrium and it is necessary to employ
a numerical method to compute the necessary correction factors.
We constructed a simple chemical mechanism consisting of
reactions (1, -1, and 2), simulated the time profile for ClOO
using the IBM Chemical Kinetics Simulator Software package,
fitted a first-order decay to the profile, and compared the
simulated pseudo-first-order decay, kfirst(sim), to that expected
in the absence of problems caused by the (1, -1) equilibrium,
k2[NO]. Correction factors, k2[NO]/kfirst(sim) obtained using this
method were similar to, although about 20-30% larger than,
those expected from the ratio ([ClOO] + [Cl])/[ClOO]. The fact
that the correction factors are slightly larger than expected from
the ratio ([ClOO] + [Cl])/[ClOO] is explained by the fact that
the system never quite reaches equilibrium with [Cl] always
exceeding, and [ClOO] always below, that expected at equi-
librium. It is an unfortunate, but unavoidable, fact that for
experiments conducted under conditions relevant to the lower
stratosphere (approximately 100 Torr, 215 K) the correction
factors to account for the equilibrium between Cl and ClOO
are substantial (varying from 3.8 for 205 K to 13.7 for data at
243 K).
Table 1 provides a listing of the rate constants k2 obtained
after correction for the two complications noted above. Quoted
uncertainties in Table 1 include statistical uncertainties in data
plots such as that shown in Figure 3 and systematic uncertainties
associated with the corrections applied (based upon the work
of Suma et al.1 we assume 15% uncertainty in k1/k-1). As seen
from Table 1, within the admittedly large uncertainties, there
is no discernible effect of temperature on k2 for the limited range
of temperature, 205-243 K. Variation of the total pressure over
the range 50-150 Torr of O2/N2 diluent at 213 K did not reveal
any effect of total pressure on k2.
3.3. Measurement of NO2 Yield in the ClOO + NO
Reaction. As discussed in the Introduction, NO2, ClNO, and
ClONO2 are thermochemically allowed products of the ClOO
+ NO reaction. Cavity ring-down spectroscopy was used to
quantify the NO2 yield using its characteristic absorption in the
485-491 nm region; ClONO2 does not absorb at wavelengths
above 440 nm,8 and absorption by ClNO is unstructured and
25 times weaker than that of NO2.24
Figure 4. Product spectra (thin lines) recorded 1.5 ms after the 355
nm irradiation of Cl2/O2/NO/N2 mixtures in experiments conducted at
either 213 K (a), (b) or 223 K (c). The thick lines are NO2 reference
spectra scaled to match the product absorption. The hashed area in panel
(a) is attributed to absorption by ClNO. The 355 nm photolysis laser
was operated at either 10 Hz (a) or 2 Hz (b), (c).
product experiments) scaled to match the product spectra (thin
lines). Comparing the thin and thick lines in Figure 4, one sees
that NO2 is formed in the system. Evidence for NO2 formation
was also observed in experiments performed at 233 and 243
K; however, the lower ClOO concentrations and corresponding
weak product absorption precluded a quantitative analysis.
Control experiments were performed to check for possible
formation of NO2 from irradiation of mixtures of NO and O2.
The 355 nm irradiation of a mixture of 4.3 × 1018 molecule
cm-3 of O2 and 1.6 × 1015 molecule cm-3 of NO at 213 K did
not give any discernible absorption at 485-491 nm.
The product spectrum in Figure 4a is more intense than those
in Figure 4b,c because, when a photolysis laser frequency of
10 Hz is used, the gas mixture in the cell is exposed to multiple
photolysis laser pulses and the product accumulates in the cell.
The spectra shown in Figure 4b,c were obtained with a 2 Hz
repetition rate, which provides sufficient time for the products
to leave the flow cell between photolysis laser pulses.
To measure the yield of NO2 in the system, the reference
spectra were scaled to provide the best fit to the product spectra,
as shown in Figure 4. Interestingly, it was found that the best
fits were obtained if we assumed the product spectra to be
composed of two spectral components: the structured spectrum
of NO2 and an unstructured absorption. The hashed area in
Figure 4a shows the unstructured absorption component, giving
the best fit for this experiment. It seems reasonable to assume
that the unstructured absorption component reflects the forma-
CRDS product spectra recorded 1.5 ms after the 355 nm
irradiation of Cl2/O2/NO/N2 mixtures are shown by the thin lines
in Figure 4. The spectra in Figure 4a,b were obtained at 213 K
with photolysis laser frequencies of 10 and 2 Hz, respectively.
The spectrum in Figure 4c was obtained at 223 K with a
photolysis laser frequency of 2 Hz. The thick lines in Figure 4
are NO2 reference spectra (at the same temperatures as the

3550 J. Phys. Chem. A, Vol. 110, No. 10, 2006
Enami et al.
TABLE 2: Reaction Mechanism Used in ClOO + NO
Reaction
rate constant
(cm6 molecule-2 s-1 or
reaction
cm3 molecule-1 s-1 or s-1
)
ref
Cl + O2 + M f ClOO + M
ClOO + M f Cl + O2 + M
ClOO + Cl f 2 ClO
2.7 × 10-33(T/298)-1.5
8
25
8
8
a
a
8
8
8
2.8 × 10-10 exp(-1819/T)
1.2 × 10-11
ClOO + Cl f Cl2 + O2
ClOO + NO f ClO + NO2
ClOO + NO f ClNO + O2
Cl + ClNO f Cl2 + NO
ClO + NO f Cl + NO2
Cl + NO + M f ClNO + M
2ClOO f Cl2 + 2O2
2.3 × 10-10
YNO2 × 4.5 × 10-11
(1 - YNO ) × 4.5 × 10-11
2
5.8 × 10-11 exp(100/T)
6.4 × 10-12 exp(290/T)
9.0 × 10-32(T/300)-1.6
1.6 × 10-11
10
b
ClOO f diffusion
4.0 × 103
Figure 5. Temporal profile of NO2 observed in an experiment at 213
K using the following reagent concentrations (molecule cm-3): O2,
4.0 × 1018; N2, 4.7 × 1017; Cl2, 1.9 × 1016; NO, 1.6 × 1015. The solid
circles are experimental results. The thick and thin curves are results
a YNO is the branching ratio for the NO2 formation in ClOO + NO
reaction2. b Loss of ClOO via diffusion from the viewing zone was
obtained from the y-axis intercept in the second-order plots (e.g., Figure
3).
of simulations using YNO ) 0.18 and 0.00, respectively.
2
tion of ClNO as a product in the system. It should be noted
that the absorption cross section of ClNO is much smaller than
that of NO2 by a factor of about 2524 and the relatively small
absorption shown in Figure 4a does not imply that ClNO is a
minor product.
We also note that in the calibration of the NO2 reference
spectra shown by the thick lines in Figure 4, allowance was
made for NO2 dimerization using the reported equilibrium
constant. However, when considering the product spectra (thin
lines in Figure 4), we can neglect NO2 dimerization because
conversion into N2O4 is calculated to remove <1% of the
product NO2 on the experimental time scale.
predicted NO2 formation with YNO ) 0 and represents NO2
2
generated from reaction 7 followed by reaction 8. Fortunately,
reaction 7 makes only a minor contribution to NO2 formation.
The thick curve in Figure 5 shows the simulated rise of NO2
using best-fit value of YNO at 213 K of 0.18 ( 0.01. In a similar
2
fashion, a value of YNO ) 0.15 ( 0.01 at 223 K was obtained
2
from data with the delay time fixed at 1.5 ms. The uncertainties
associated with YNO quoted above reflect statistical uncertainty
2
in the measurement of NO2. If we account for uncertainties in
the initial Cl atom concentration caused by fluctuations of laser
power, pressure, temperature, and gas flow rate, we arrive at
values of YNO ) 0.18 ( 0.02 at 213 K and YNO ) 0.15 ( 0.02
2
2
The equilibrium between ClOO and Cl and the consequent
presence of Cl atoms is a potential complication, which needs
to be considered in the determination of the branching ratio for
at 223 K.
4. Discussion
NO2 formation, YNO ) k2a/k2, from the observed NO2 formation.
2
The present work is the first application of CRDS to study
ClOO radical reaction kinetics and the first direct study of the
ClOO + NO reaction. The reaction proceeds with a rate constant
k(ClOO+NO) ) (4.5 ( 0.9) × 10-11 cm3 molecule-1 s-1 at
213 K. There is no discernible effect of temperature on the rate
of reaction over the range studied (205-243 K); see Table 1.
NO2 was observed as a reaction product and branching ratios
of k2a/k2 ) 0.18 ( 0.02 at 213 K and k2a/k2 ) 0.15 ( 0.02 at
223 K were determined. Evidence for the formation of ClNO
product was observed in the form of a small featureless residual
absorption at 485-491 nm. The magnitude of the residual
absorption is consistent with ClNO being a major product,
perhaps accounting for the balance of reaction, i.e., k2b/k2 ≈
0.8. However, in light of uncertainties associated with quanti-
fication of the residual absorption, this should be regarded as a
tentative conclusion.
It is of interest to compare our results for reaction 2 with the
database for reactions of other peroxy radicals with NO. It is
well established that reaction of HO2 and alkyl peroxy radicals
(e.g., CH3O2, C2H5O2) with NO proceed with a rate constant
of approximately 1 × 10-11 cm3 molecule-1 s-1 at the
temperatures employed in the present work.8 Acyl peroxy
radicals (e.g., CH3C(O)O2) react more rapidly with NO with
rate constants approaching 3 × 10-11 cm3 molecule-1 s-1 at
220 K. ClOO radicals are more reactive toward NO than any
other peroxy radical yet studied. The absence of any pronounced
dependence of k2 on temperature is consistent with the behavior
of HO2 and organic peroxy radicals whose reaction kinetics
display weak negative temperature dependencies consistent with
the reaction proceeding via formation of an intermediate adduct.
There are two source of ClO radicals in the system: reaction
of ClOO with Cl and reaction of ClOO with NO via channel
(2a). ClO radicals react rapidly with NO giving NO2 and
regenerating Cl atoms.
ClOO + NO f ClO + NO2
ClO + NO f Cl + NO2
Cl + ClOO f 2ClO
(2a)
(7)
(8)
To investigate the potential complications caused by such
secondary reactions in the system, the experimentally observed
rise profiles for NO2 were simulated using the mechanism given
in Table 2. Temporal profiles of NO2 were measured at 489
nm in experiments at 213 K using gas mixtures with [O2] )
4.0 × 1018, [N2] ) 4.7 × 1017, [Cl2] ) 1.9 × 1016, and [NO]
) 1.6 × 1015 molecule cm-3. The contribution of ClNO
absorption to these data was included. The chemistry occurring
in the system was simulated using the IBM Chemical Kinetics
Simulator Software. The initial Cl atom concentration was
calculated from [Cl2] and the photolysis laser power as described
in section 2.1. The only parameter varied in the fitting procedure
was the branching ratio, YNO ) k2a/k2, which was varied to fit
2
the limiting NO2 plateau at times >0.3 ms (see Figure 5). It is
gratifying that the fit shown as the curve through the data points
in Figure 5 reproduces both the limiting NO2 concentration and
the kinetic behavior; the observed rate of NO2 formation is
consistent with the value for k2 derived from observing the ClOO
loss. The thin curve close to the x-axis in Figure 5 is the

Kinetic Study of the ClOO + NO Reaction
J. Phys. Chem. A, Vol. 110, No. 10, 2006 3551
References and Notes
The dominant products of the reaction of HO2 and organic
peroxy radicals with NO are the corresponding oxy radical and
NO2 (e.g., CH3O2 + NO f CH3O + NO2). There is an
analogous reaction channel in the ClOO + NO reaction, but it
is a minor, k2a/k2 ) 0.18 ( 0.02 at 213 K, not dominant, channel.
The dominant channel in the ClOO + NO reaction appears to
give ClNO. The different behavior probably reflects either the
weak Cl-O bond in ClOO and ease with which ClNO could
be eliminated from the ClOONO adduct or the existence of a
direct abstraction channel in the ClOO + NO reaction in which
the incoming NO can abstract the Cl atom. Ab initio studies
are needed to distinguish between these possibilities.
(1) Suma, K.; Sumiyoshi, Y.; Endo, Y.; Enami, S.; Aloisio, S.;
Hashimoto, S.; Kawasaki, M.; Nishida, S.; Matsumi, Y. J. Phys. Chem. A
2004, 108, 8096.
(2) Suma, K.; Sumiyoshi, Y.; Endo, Y. J. Chem. Phys. 2004, 121, 8351.
(3) Gane, M. P.; Williams, N. A.; Sodeau, J. R. J. Chem. Faraday.
Trans. 1997, 93, 2747.
(4) Zhu, R. S.; Lin, M. C. J. Chem. Phys. 2003, 119, 2075.
(5) Li, Q.; Lu, S.; Xu, W.; Xie, Y.; Schaefer, H. S., III. J. Phys. Chem.
A 2002, 106, 12324.
(6) Thomsen, C. L.; Philpott, M. P.; Hayes, S. C. J. Chem. Phys. 2000,
112, 505.
(7) Finlayson-Pitts, B. J.; Pitts Jr., J. N. Chemistry of the Upper and
Lower Atmosphere: Theory, Experiments and Applications; Academic
Press: San Diego, CA, 2000.
It is of interest to compare reaction 2 with the analogous
reaction of FO2 radicals. The FO2 + NO reaction proceeds
slowly, k ) 3 × 10-13 cm3 molecule-1 s-1 at 220 K, with
kinetics that display a positive temperature dependence (Ea/R
) 690), giving FNO and O2 as products.8 There are two
significant differences between FO2 and ClOO that can explain
their different reactivities toward NO. First, the F-O bond
strength in FO2 is approximately 54 kJ mol-1 and is much
stronger than the Cl-O bond in ClOO, 19.5 kJ mol-1. Second,
the reaction channel to give XO + NO2 is endothermic for FO2
but exothermic for ClOO. Ab initio studies of the reactions of
FO2 and ClOO with NO would be useful in providing a more
fundamental understanding of these reactions.
Finally, it should be noted that given the reactive nature of
Cl atoms toward the large amounts of ozone in the stratosphere
(k(Cl+O3) ) 1 × 10-11 cm3 molecule-1 s-1 at 220 K, [O3] )
1012-1013 molecule cm-3 at 10-20 km) and the relatively small
fraction of Cl atoms that is tied up in the form of ClOO radicals
(see Introduction), reaction of ClOO with NO (or any other
species) does not play a significant role in atmospheric
chemistry.
(8) Sander, S. P.; Friedl, R. R.; Ravishankara, A. R.; Golden, D. M.;
Kolb, C. E.; Kurylo, M. J.; Huie, R. E.; Orkin, V. L.; Molina, M. J.;
Moortgat, G. K.; Finlayson-Pitts, B. J. Chemical Kinetics and Photochemical
Data for Use in Stratospheric Modeling; Evaluation 14, Jet Propulsion
Laboratory: Pasadena, CA, 2003.
(9) U.S. Standard Atmosphere, 1976, NOAA, NASA, USAF, Wash-
ington, DC, 1976.
(10) Baer, S.; Hippler, H.; Rahn, R.; Siefke, M.; Seitzinger, N.; Troe, J.
J. Chem. Phys. 1991, 95, 6463.
(11) Mauldin, R. L.; Burkholder, J. B.; Ravishankara. A. R. J. Phys.
Chem. 1992, 96, 2582.
(12) Nicovich, J. M.; Kreutter, K. D.; Shackelford, C. J.; Wine, P. J.
Chem. Phys. Lett. 1991, 179, 367.
(13) O’Keefe, A.; Deacon, D. A. G. ReV. Sci. Instrum. 1988, 59, 2544.
(14) Wheeler, M. D.; Newton, S. M.; Orr-Ewing, A. J.; Ashfold, M.
N. R. J. Chem. Soc., Faraday Trans. 1998, 94, 337.
(15) Berden, G.; Peeters, R.; Meijer, G. Int. ReV. Phys. Chem. 2000,
19, 565.
(16) Yu, T.; Lin M. C. Int. J. Chem. Kinet. 1993, 25, 875.
(17) Yu, T.; Lin M. C. J. Phys. Chem. 1995, 99, 8599.
(18) Ninomiya, Y.; Goto, M.; Hashimoto, S.; Kawasaki, M.; Wallington,
T. J. Int. J. Chem. Kinet. 2001, 33, 130.
(19) Ninomiya, Y.; Goto, M.; Hashimoto, S.; Kagawa, Y.; Yoshizawa,
K.; Kawasaki, M.; Wallington, T. J.; Hurley, M. D. J. Phys. Chem. A 2000,
104, 7556.
(20) Nakano, Y.; Goto, M.; Hashimoto, S.; Kawasaki, M.; Wallington,
T. J. J. Phys. Chem. A 2001, 105, 11045.
(21) Nakano, Y.; Enami, S.; Nakamichi, S.; Aloisio, S.; Hashimoto, S.;
Kawasaki, M., J. Phys. Chem. A. 2003, 107, 6381.
(22) Atkinson, D. B.; Hudgens, J. W. J. Phys. Chem. A 1997, 101, 3901.
(23) Brown, S. S.; Ravishankara, A. R.; Stark, H. J. J. Phys. Chem. A
2000, 104, 7044.
(24) Roehl, C. M.; Orlando, J. J.; Calvert, J. G. J. Photochem. Photobiol.
A: Chem. 1992, 69, l.
(25) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr,
J. A.; Rossi, M. J.; Troe, J. J. Chem. Phys. Ref. Data 1997, 26, 521.
Acknowledgment. We are grateful to Prof. S. Aloisio of
California State University for helpful discussion. This work is
supported partly by grants from the 21st Century COE project
of Kyoto University, and the Yazaki Foundation. S.E. thanks
the JSPS Research Fellowship for Young Scientists.