Formation of iodine monoxide radical from the reaction of CH2I with O2

Article abstract of DOI:10.1021/jp0481815

The rate constants of IO radical formation from the reaction of CH ₂I with O₂ were determined in the pressure range of 5-80 Torr with N₂ diluent at 278-313 K, using cavity ring-down spectroscopy. The room temperature rate

Full text of DOI:10.1021/jp0481815

J. Phys. Chem. A 2004, 108, 6347-6350
6347
Formation of Iodine Monoxide Radical from the Reaction of CH2I with O2
Shinichi Enami, Junya Ueda, Masashi Goto, Yukio Nakano, Simone Aloisio,
Satoshi Hashimoto, and Masahiro Kawasaki*
Department of Molecular Engineering and Graduate School of Global EnVironmental Studies,
Kyoto UniVersity, Kyoto 615-8510, Japan
ReceiVed: April 26, 2004
The rate constants of IO radical formation from the reaction of CH2I with O2 were determined in the pressure
range of 5-80 Torr with N2 diluent at 278-313 K, using cavity ring-down spectroscopy. The room temperature
rate constant is (4.0 ( 0.4) × 10-13 cm3 molecule-1s-1 at 30 Torr total pressure. No significant dependences
on temperature and total pressure were observed. The yield of IO from CH2I + O2 was estimated to be unity
in 100 Torr total pressure of N2 diluent.
Introduction
CH2I + O2 f IO + HCHO
(5)
The atmospheric chemistry of iodine monoxide radicals has
attracted attention for its potential effect on the oxidizing
capacity of the troposphere in the NOx and HOx budget and
toward dimethyl sulfide (DMS).1-3 DMS accounts for 75% of
natural sulfur emission. According to a recent kinetic measure-
ment, the reaction rates of IO/BrO radicals with DMS are much
faster than those previously reported, and hence the importance
of those reactions in the marine boundary layer is comparable
with that of the OH radical.2 Iodine atoms from the photodis-
sociation of CH2I2, CH2IBr, and CH2ICl are considered to be
the main source of IO in the marine boundary layer.4,5 CH2I2
is, for example, photodissociated by the solar flux with a lifetime
of a few minutes.6,7 The IO radical is produced by the reaction
of an I atom with ozone:
Although they suggested the possibility of formation of IO from
CH2I + O2, there has been no direct evidence of the IO
formation. If IO radicals are produced from CH2I produced in
reactions 1 and 3 followed by reaction 5 with an appreciable
yield, the IO formation mechanism in the atmosphere should
be altered. In the present work, we have investigated the rate
constants and the IO yield for the reaction of CH2I with O2 by
monitoring the absorption of IO radicals with cavity ring-down
spectroscopy (CRDS).2,14
Experimental Section
After the CRDS technique was introduced by O’Keefe and
Deacon, it has been widely applied to spectroscopy and chemical
reactions.15-18 Applicability of the CRDS apparatus to kinetic
study was discussed.19,20 The CRDS apparatus used in the
present study has been described elsewhere.2 The system
employs photolysis and probe pulsed lasers. After the photolysis
laser beam traverses the glass tube reactor, 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 of 0.9994 at 435 nm), which make up the
ring-down cavity of 1.04 m. The length of the reaction region
is 0.40 m. Light leaking from the end mirror is detected by a
photomultiplier tube through a narrow band-pass filter. The
decay of the light intensity is recorded using a digital oscil-
loscope and transferred to a personal computer.
CH2I2 + hν f CH2I + I
I + O3 f IO + O2
(1)
(2)
Another reactive iodine compound in the atmosphere is CH3I
originating from marine algae species.8 In the daytime atmo-
sphere, although the lifetime of CH3I is controlled almost
entirely by photodissociation, OH- and Cl-initiated attack could
account for 10-20% of the removal of CH3I.9
CH3I + OH f CH2I + H2O
(3)
Under atmospheric conditions it has been believed that the CH2I
radical produced via reactions 1 and 3 reacts with O2 to generate
CH2IO2 in analogy to other halogenated alkyl radicals:10-12
CH2I2 is injected into the cell using a bubble tube with N2
buffer gas, the concentration of which is monitored at 253.7
nm before the entrance, typically 1 × 1014 molecules cm-3
.
The 266 nm output of a Nd3+:YAG laser is used to dissociate
CH2I2 to give an I atom and a CH2I radical. The dissociation
yield of CH2I2 by the 266 nm photolysis laser is estimated to
be below a few percent from its absorption cross section and
the laser intensity. The IO radical concentration is monitored
at 435.63 nm, the band head of the A2Π3/2 r X2Π3/2 (3,0)
transition, with a dye laser (Spectra Physics, PDL-3, spectral
resolution < 0.01 nm). The IO absorption cross section was
previously measured to be 5.9 × 10-17 cm2 molecule-1 at this
CH2I + O2 + M f CH2IO2 + M
(4)
However, Masaki et al. found that the rate constants of the
CH2I + O2 reaction did not depend on the total pressures in
the range of 2-15 Torr and considered the following two-body
process:13
* To whom correspondence should be addressed: e-mail kawasaki@
moleng.kyoto-u.ac.jp; Fax + 81-75-383-2573.
10.1021/jp0481815 CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/02/2004

6348 J. Phys. Chem. A, Vol. 108, No. 30, 2004
Enami et al.
Figure 2. Second-order plots for the reaction of CH2I radicals with
O2 at 30 Torr of N2 diluent at 298 K. The solid line is a linear least-
squares fit.
concentrations of the radicals are low, the radical-radical
reactions do not affect the determination of the rate constant
k5. For example, CH2I radicals are not consumed by the reaction
with I atoms:21
Figure 1. A typical rise profile of the IO radical monitored at 435.63
nm after the initial pulse (time ) 0 µs) of the 266 nm output of an
Nd3+:YAG laser. The inset shows the A r X (3, 0) band of IO.
CH2I + I + M f CH2I2 + M
(6)
wavelength.2,14 The signal baseline is taken at 435.51 nm, a
region in which there is no IO absorption. The IO concentration
profile is measured between 30 and 2500 µs after the photolysis
laser pulse. A large excess amount of O2, 1015-1016 molecules
cm-3, is used to maintain the pseudo-first-order reaction
conditions. CH2IBr and CH2ICl are also used as alternative
sources of CH2I to confirm the measured rate constants. To
estimate the branching ratio of the IO radical formation from
CH2I + O2, the reactions of CH3I and CF3I with O atoms in
the presence of O2 are performed. In this experiment, the 266
nm photolysis of O3 generates O(3P) atoms under N2 diluent.
The reaction cell consists of a Pyrex glass tube (21 mm i.d.).
The temperature of the gas flow region is controlled over 278-
313 K. The difference between the temperature of the sample
gas at the entrance and exit of the flow region is <0.4 K. The
total flow rate is adjusted so that the gas in the cell is completely
replaced under 2 Hz laser operation.
The rate constants are also measured using CH2ICl and CH2-
IBr as precursors of the CH2I radical, which yield the same
rate constant as for CH2I2. On the basis of these results, any
secondary reactions are not significant in the present experiment.
Masaki et al. determined k5 ) (1.6 ( 0.2) × 10-12 cm3
molecule-1 s-1 from the decay profile of the CH2I signal using
a combination of pulsed laser photolysis and photoionization
mass spectrometry, which is 4 times larger than our value.13
Since their initial concentrations of CH2I were low, the radical-
radical reactions would not enhance the decay rate in their
experiment. Thus, the reason for the discrepancy between their
rate constant and ours is not clear. As will be described below,
the evolution curve of IO from the reaction of CH3I + O(3P) in
the presence of O2 can be reproduced by our rate constant, while
not by their rate constant.
The rate constants, k5, measured for the range of 278-313
K at 30 Torr are found to be essentially temperature-
independent: k5 in units of 10-13 cm3 molecule-1 s-1 are 3.6
( 0.3 at 278 K, 4.0 ( 0.2 at 288 K, 4.0 ( 0.4 at 298 K, and
3.7 ( 0.5 at 313 K. Furthermore, k5 at room temperature does
not show any pressure dependence for the range of 5-80 Torr
with N2 diluent. These results suggest that the two-body reaction
mechanism is dominant in CH2I + O2, or possibly the CH2IO2
formation path reaches already its high-pressure limit with a
very limited yield. As will be discussed below, the formation
yield of IO from CH2I + O2 is estimated to be unity. Thus,
CH2IO2 formation can be neglected under our experimental
conditions. Sehested et al. reported, however, the formation of
CH2IO2 under 1000 mbar total pressure of SF6 diluent.12 They
measured the absorption of CH2IO2 at 220-400 nm 2 µs after
the electron pulse initiation with spectral resolution of 0.8 nm.
The UV absorption at that region could also be due to IO and/
or HCHO from reaction 5 because IO and HCHO have the UV
absorption in the wavelength region similar to the reported CH2-
IO2 spectrum. In the present experiment, our spectral resolution
is less than 0.01 nm, which enables us to separate completely
the IO spectrum from other species including CH2IO2 and
HCHO. In addition, the heats of reaction for CH2X + O2 f
XO + HCHO are -30.6, -36.0, and -48.0 kcal mol-1 for X
) Cl, Br, and I, respectively.22 Since the iodine channel is highly
exothermic, CH2IO2 is not stabilized even under the high-
pressure condition while the formation of CH2ClO2 and CH2-
BrO2 was reported.10,11 In general, the IO formation from
reaction 5 is considered to occur via the four-centered inter-
mediate. Because CH2I has a low ionization potential (<8.6
Results and Discussion
Reaction Kinetics of CH2I with O2. The rate constants for
reaction 5 are determined by the rise time profiles of the IO
signal intensity (Figure 1). The monitored IO absorption
spectrum is shown in the inset. Under conditions with a large
excess O2 concentration over that of CH2I, the rise profile
followed pseudo-first-order kinetics. The formation of the IO
radicals are analyzed using the following equations:
[IO]t ) [CH2I]0{1 - exp(-k′t)}
k′ ) k5[O2] + kd
(I)
(II)
where [IO]t is the concentration of IO radicals at time t and
[CH2I]0 is the concentration of CH2I radicals at time t ) 0. k5
and k′ are the second-order and pseudo-first-order rate constants
for reaction 5, respectively. kd is mainly the rate constant for
diffusion out of the photolysis volume. Figure 1 shows a typical
rise profile of the IO concentration with O2 of 1.1 × 1015
molecules cm-3 at 298 K, which is fitted to eq I. Figure 2 shows
a plot of k′ vs [O2] at room temperature in 30 Torr total pressure
of N2 diluent. The second-order rate constant k5 is obtained from
a linear least-squares analysis of the data; k5 ) (4.0 ( 0.4) ×
10-13 cm3 molecule-1 s-1. In this run, kd is 720 s-1. This value
is in reasonable agreement with our previous experiments, e.g.,
kd ) 500-1000 s-1 for 20-100 Torr total pressure.2
When the concentration of CH2I2 is increased by 3 times, no
change of the rate constant k5 is observed. Since the initial

Formation of IO Radical from CH2I + O2
J. Phys. Chem. A, Vol. 108, No. 30, 2004 6349
O(3P) atoms react with CF3I through the following reaction
channels:
CF3I + O(3P) f CF3 + IO
f I + F + CF2O
(8a)
(8b)
(8c)
(8d)
(8e)
f I + CF3O
f IF + CF2O
f CF3IO
Figure 3. Rise profiles of IO radicals produced from the reactions of
O(3P) with CH3I (filled circle) and CF3I (open square) in the presence
of O2. [CH3I] ) [CF3I] ) 8 × 1014, [O2] ) 1 × 1016, [O3] ) 4 × 1012
molecules cm-3. The solid curve is a simulated one. See text for details.
The vertical arrows show the asymptotic values.
The reported product yields at 100 Torr with N2 diluent are
Y(IO)8a ) 0.83 ( 0.09, Y(CF3O)8c < 0.01, and Y(IF)8d < 0.01.24
The IO radicals are generated exclusively through reaction 8a
under our experimental conditions. Figure 3 shows the rise
profiles of IO radicals. The concentration of IO generated from
O(3P) + CH3I, [IO]CH I, is compared with [IO]CF I from O(3P)
eV)13 and hence the electron of the I atom could flow into the
binding O2, the O-O bond is weakened followed by the IO
formation via reaction 5.
Furthermore, to check the IO formation from reaction 5, we
test 355 nm photolysis of CH3I/Cl2/O2/N2 mixture at room
temperature; [CH3I] ) 5.1 × 1014, [Cl2] ) 1.1 × 1015, and [O2]
) 3.9 × 1016 molecules cm-3 in 30 Torr total pressure of N2
diluent. The photoproduced Cl atom abstracts an H atom from
CH3I to produce a CH2I radical that reacts with O2. The
formation of IO is confirmed by observing the IO (3,0) band
with our CRDS apparatus.23
3
3
+ CF3I. The ratio of two asymptotic values in Figure 3 is
[IO]CH I/[IO]CF I ) 1.15 ( 0.14. Hence, the IO production yield
3
3
from the reaction channels (7a and 7b) is estimated to be 0.95
( 0.20. Subtracting the reported yield of reaction 7a for direct
IO production Y(IO)7a ) 0.44 ( 0.04 from the present IO
production yield of 0.95 ( 0.20, the IO yield from reaction 5
through reaction 7b is estimated to be Y(IO)5-7b ) 0.51 ( 0.20,
which should be the same as the CH2I yield in reaction 7b.
Gilles et al.24 reported a much smaller value, Y(CH2I)7b ) 0.16
( 0.05. When the relatively large errors in two different
experiments are taken into account, it is safe to say that the
yield efficiency of the IO radical from reaction 5 is unity. From
the reaction of CH2I + O2, formation of HCOOH + I or CHIO
+ OH is energetically possible. However, these reactions do
not occur.
Estimation of the Yield of the IO Radical from the
Reaction of CH2I with O2. To measure the formation yield of
IO from of CH2I + O2, two different experiments are performed.
First, the concentration of IO radicals produced from the reaction
of O(3P) atoms with CH3I is measured. This reaction is initiated
by the 266 nm photolysis of CH3I/O2/O3/N2 mixtures; [CH3I]
) 8.1 × 1014, [O2] ) 1.1 × 1016, and [O3] ) 3.9 × 1012
molecules cm-3 in 100 Torr total pressure of N2 diluent.
Although O(1D) atoms are produced from the O3 photolysis,
they are efficiently quenched by N2 to O(3P) before reacting
with CH3I.
The evolution curve for IO from reaction 5 in Figure 3
reproduces the experimental data, which is simulated with the
present rate constant for k5, the reported rate constant k7 ) 1.71
× 10-11 23
and the estimated one k7b ) k5Y(IO)5-7b ) 6.0 ×
,
10-12 cm3 molecule-1 s-1. The rate constant for O(3P) + O2 is
The following reactions were proposed by Gilles et al.:24
adopted to be kO+O ) 1.96 × 10-15 cm3 molecule-1 s-1 22
. The
2
absolute concentration of O(3P) atoms is estimated from the
maximum concentration of IO.
CH3I + O(3P) f CH3 + IO
f CH2I + OH
(7a)
(7b)
Searching for another possible reaction channel of the IO
formation in the present experimental conditions, we test the
266 nm photodissociation of mixture gases of 9 × 1014
molecules cm-3 of CH3I or CF3I and 3 × 1015 molecules cm-3
of O2 in 100 Torr of N2 diluent without O3. IO radicals are not
detected. These results suggest that the following reactions do
not produce IO radicals:
f H + I + HCHO
f I + CH3O
(7c)
(7d)
f HI + HCHO
(7e)
They reported that product yields at 100 Torr with N2 diluent
are Y(IO)7a ) 0.44 ( 0.04, Y(CH2I)7b ) 0.16 ( 0.05, Y(H)7c
CH3O2 + I f IO + CH3O
CF3O2 + I f IO + CF3O
(9)
)
0.07 ( 0.02, Y(CH3O)7d < 0.03, and Y(HI)7e < 0.05. Under
our experimental conditions the contribution of IO formation
from I + O3 is estimated to be less than 1% because of the low
O3 concentration.25 The reaction channels (7c and 7d) do not
contribute to the IO radical formation under the present low O3
concentration. Thus, the IO radicals are generated exclusively
through reaction channels (7a and 7b) and the subsequent
reaction 5 with O2. Figure 3 shows the rise profile of the IO
(10)
Acknowledgment. The authors thank Dr. A. J. Orr-Ewing
of the University of Bristol for stimulating discussion. This work
is supported by a grant-in-aid in the priority research field
“Radical Chain Reactions” from the Ministry of Education,
Japan. S.A. thanks the Japan Society for Promotion of Science
for the Japan-US fellowship.
radicals, which goes up to (5.4 ( 0.2) × 1011 molecules cm-3
.
As a reference experiment, the concentration of IO radicals
produced from the reaction of O(3P) atoms with CF3I is
measured. This reaction is initiated by the 266 nm photolysis
of CF3I/O2/O3/N2 mixtures ([CF3I] ) 8.0 × 1014, [O2] ) 1.1 ×
1016, [O3] ) 4.0 × 1012 molecules cm-3 in 100 Torr total
pressure of N2 diluent). With these experimental conditions,
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