Rate coefficients for the OH + CF3I reaction between 271 and 370 K

Article abstract of DOI:10.1021/jp001827i

The rate coefficient, k₁, for the reaction OH + CF₃I → products was measured under pseudo-first-order conditions in hydroxyl radical, OH. OH temporal profiles were monitored by laser-induced fluorescence (LIF), and CF₃I concentrations were determined by UV/Visible absorption. We determined k₁ (T) to be (2.10 ± 0.80) × 10^-11 exp[-(2000 ± 140)/T] cm³ molecule^-1 s^-1, over the temperature range 271 to 370 K. The quoted uncertainties are 2σ (95% confidence limits, σ_A = Aσ_lnA). Previous measurements of k₁(T) are compared with our values, and possible reasons for the discrepancies are discussed. The heat of formation of HOI is deduced to be less than -16 kcal mole^-1, if the products of reaction 1 are mostly HOI and CF₃. These measurements support the earlier conclusion that the reaction of OH with CF₃I plays a negligibly small role in the atmospheric removal of CF₃I.

Full text of DOI:10.1021/jp001827i

J. Phys. Chem. A 2000, 104, 8945-8950
8945
Rate Coefficients for the OH + CF3I Reaction between 271 and 370 K
Mary K. Gilles,* R. K. Talukdar, and A. R. Ravishankara†
Aeronomy Laboratory, National Oceanic and Atmospheric Administration, 325 Broadway,
Boulder, Colorado 80305 and CooperatiVe Institute for Research in EnVironmental Sciences,
UniVersity of Colorado, Boulder, Colorado 80309
ReceiVed: May 18, 2000; In Final Form: July 20, 2000
The rate coefficient, k
conditions in hydroxyl radical, OH. OH temporal profiles were monitored by laser-induced fluorescence (LIF),
and CF I concentrations were determined by UV/Visible absorption. We determined k (T) to be (2.10 (
.80) × 10 exp[-(2000 ( 140)/T] cm molecule s , over the temperature range 271 to 370 K. The
quoted uncertainties are 2σ (95% confidence limits, σ ) AσlnA). Previous measurements of k (T) are compared
with our values, and possible reasons for the discrepancies are discussed. The heat of formation of HOI is
1 3
, for the reaction OH + CF I f products was measured under pseudo-first-order
3
1
-
11
3
-1 -1
0
A
1
-
1
3
deduced to be less than -16 kcal mole , if the products of reaction 1 are mostly HOI and CF . These
measurements support the earlier conclusion that the reaction of OH with CF
in the atmospheric removal of CF I.
3
I plays a negligibly small role
3
6
4
Introduction
the studies of Berry et al. and Garraway and Donovan and
that CF3I absorbs strongly at wavelengths <175 nm as well as
Since CF3I was first proposed for use as a fire suppressant,
its atmospheric chemistry has received increased attention.
Laboratory studies of photolysis rates and reactions with
tropospheric reactive species have been used to determine its
-
19
2
-1
between 240 and 320 nm (σ > 10
substantial photolysis of CF3I must have occurred in both
experiments.
cm molecule ),
5
Brown et al. produced OH from the reaction of NO2 with H
atmospheric lifetime, ozone depletion potential, and global
warming potential.1-3 Since OH is a major oxidant in the
atoms from a microwave discharge and monitored OH radical
concentrations using resonance fluorescence. Hence, they did
not photolyze CF3I. Their reported k1 is a factor of 2 smaller
troposphere, the rate coefficient for the reaction of OH with
CF3I
6
than that of Berry et al. and nearly a factor of 4 smaller than
that of Garraway and Donovan. However, Brown et al.5
observed nonexponential OH loss profiles and speculated that
a chain reaction was responsible for the increase in OH loss
rate at longer reaction times in their experiments. Only Berry
4
OH + CF I f products; k1
(1)
3
is of interest.
The measured values for the rate coefficient, k1, are highly
6
et al. have reported the temperature dependence of k1. Thus, it
4
5
disparate. Garraway and Donovan, Brown et al., and Berry et
appears that k1 (298 K) has not been measured in the absence
of secondary reactions and only a single study has explored
the temperature dependence of k1. Therefore, we have measured
k1 using a different OH precursor and briefly investigated the
effect of secondary chemistry in previous measurements of k1.
6
-13
al. reported, respectively, k1 (295 K) ) (1.2 ( 0.2) × 10
3
-1 -1
-14
-14
3
cm molecule s , k1 (300 K) ) (3.1 ( 0.5) × 10
cm
-
1
-1
3
molecule s , and k1 (292 K) ) (5.9 ( 1.4) × 10
cm
-
1
-1
4
molecule
s . Garraway and Donovan employed flash
photolysis of a mixture of O3, H2O, and CF3I and monitored
the OH reactant concentration by time-resolved absorption at
Experimental Section
3
08.15 nm. They measured k1 under pseudo-first-order condi-
tions in [OH], i.e., an excess of CF3I, and reported that k1 was
not a function of photolysis lamp intensity. Berry et al.
In our experiments, OH was produced by pulsed photolysis
6
in the presence of excess CF I. OH temporal profiles were
3
employed flash photolysis (λ > 120 nm) of a mixture of H2O
vapor and CF3I and monitored OH concentration via resonance
fluorescence detection. In these experiments, the value of k1
measured was a function of photolysis energy. Therefore, they
measured k1 at several flash lamp energies and obtained their
value of k1 from the intercept of a plot of measured k1 vs
photolysis energy. This type of linear extrapolation is based on
the assumption that secondary chemistry of OH with CF3I
photolysis products is responsible for the increase in k1 with
increasing flash energy. Given the CF3I concentrations used in
monitored by laser-induced fluorescence. The pseudo-first-order
rate coefficients for the loss of OH, k′, were measured as a
1
function of CF I concentration, and k was derived from a
3
1
weighted linear least-squares fit of k′ vs [CF I]. The concen-
3
1
tration of CF I was determined via UV absorption. By photo-
3
3
1
1
3
lyzing HONO at 351 nm, we minimize CF I photolysis to obtain
a more accurate value for k between 271 and 370 K. Further,
we report that the value for k measured using 248 nm laser
photolysis depends on photolysis laser fluence because CF I
7
absorbs significantly at this wavelength. The OH detection and
UV absorption apparatuses are described in previous publica-
tions. Details specific to the present study are presented here.
Determination of CF3I Concentration. The concentration
of CF3I was determined by UV/Visible absorption using Beer’s
*
To whom correspondence should be addressed. NOAA/ERL, R/AL2,
8
3
25 Broadway, Boulder, CO 80305, USA. E-mail: mgilles@al.noaa.gov)
†
Also affiliated with the Department of Chemistry and Biochemistry,
University of Colorado, Boulder, CO 80309, USA.
1
0.1021/jp001827i CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/13/2000

8946 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Gilles et al.
9
law over the wavelength range (225-450 nm) using a deuterium
lamp and a diode array spectrometer. The entire length of the
absorption cell was maintained at a constant temperature by
using recessed windows that constrained the absorption volume
to a uniform temperature region. The absorption cross section
[CF3I] (σO /σCF I) ) ∼100 at this wavelength). The CF3I
3
3
concentration in the reaction cell was corrected for the pressure
difference (usually 25-35%) between the absorption cell and
the reactor. At the beginning of the experiment, the ozone
concentration was determined from its absorption at 253.7 nm
in the 50 cm cell. During subsequent experiments, the ozone
was flowed directly into the reaction cell. [H2O] was calculated
from its vapor pressure, the measured flow rates of He through
the H2O bubbler, and of other gases, and the reaction cell
pressure was measured by a capacitance manometer.
-
19
2
-1
of CF3I (σ266.6 nm (298 K) ) 6.44 × 10
cm molecule )
shows a small temperature dependence; therefore, the temper-
1
ature-dependent absorption cross sections of Solomon et al.
were used to determine CF3I concentrations.
OH Production. The primary OH source for these experi-
ments was HONO photolysis at 351 nm. Since CF3I photolyzes
readily in the UV, 351 nm was preferred over 248 nm (σ248 nm/
248 nm Photolysis of H2O2. A single experiment using H2O2
photolysis at 248 nm was attempted, but due to the low
1
-1
-2
σ351 nm ∼ 1500) for photolysis. One test for secondary chemistry
absorption cross section of H2O2, a large (∼8 mJ pulse cm )
photolysis fluence was needed. This resulted in concentrations
of CF3I photolysis products that were much larger than [OH]0,
for example, [CF3]/[OH]0 > 15. It was immediately apparent
that the measured rate coefficient was much larger (greater than
a factor of 5) than those done with ozone or HONO photolysis;
H2O2 photolysis was not used further as an OH source.
Materials. He (99.997%) and O2 (99.99%) buffer gases were
used as supplied. CF3I (99%) was kept in an ice water bath to
suppress any possible I2 contamination. HONO was produced
by the dropwise addition of NaNO2 (0.1 M) to H2SO4 (40 wt
is to employ multiple radical sources. This proved difficult in
the present study since the low value of k1 required the use of
relatively high concentrations of CF3I. Photolysis of an O3/H2O
mixture at 248 nm and H2O2 photolysis at 248 nm were used
as alternative OH sources. These are discussed in more detail
in the following sections.
351 nm Photolysis of HONO. In the majority of experiments,
OH was produced by pulsed laser photolysis of HONO at 351
nm (XeF excimer laser).
HONO + hν f OH + NO
(2)
%
, 20 wt %, and 10 wt % solutions were used). A small He
flow, (1-6) sccm , over this solution was further diluted with
-
1
13
In the concentration range used, (4.8-8.5) × 10 molecule
-
1
He, (80-140) sccm , and CF3I prior to entering the reaction
cell. NaNO2 (assay 97%) and H2SO4 (assay 70.7%) were used
as supplied by the vendors and mixed with distilled water to
make the solutions for the HONO production. (We should note
that on one occasion we reused the H2SO4 solution from a
previous day for HONO generation: OH regeneration at longer
times was observed. In all subsequent experiments, fresh
reagents were used for HONO production, and they were
changed every few hours. In all of the data reported here there
was no indication of OH regeneration.) Ozone was prepared
-
3
cm , with a typical path length of 15.1 cm, absorption due to
HONO would be <0.0002, and hence, it could not be directly
measured via UV/Visible absorption. Therefore, the concentra-
tion of HONO was estimated from the known rate coefficient
3
-1 -1 9
(
k3 (298 K) ) 4.5 × 10-12 cm molecule s ) for the reaction
OH + HONO f products
(3)
and the measured OH loss rate coefficient in the absence of
CF3I. Assuming that reaction with HONO was the only loss
process for OH (i.e., neglecting loss of OH due to reactions
with impurities in the bath gas or in the HONO gas mixture
and the loss due to flow out of the detection region and
diffusion) an upper limit for [HONO] was calculated. This
estimate was sufficient to calculate a limit for the initial OH
by passing ultrahigh purity O through a commercial ozonizer
and stored on a silica gel trap kept in a dry ice/ethanol bath.
Several Torr of ozone was added to a preconditioned blackened
2
12-L Pyrex bulb and diluted with He. Deionized H O was kept
2
in a bubbler held at room temperature through which He was
1
1
-3
concentration ([OH]0, < 2 × 10 molecule cm ) from the
flowed.
measured photolysis laser fluence, the known absorption cross
-
20
2
-1
section of HONO (σ351 nm ) 21.2 × 10
cm molecule ),
Results
and the assumed unit quantum yield for OH production. The
calculated [OH]0 ensured that our experiments were carried out
under pseudo-first-order conditions.9
The temporal profile of OH in the presence of CF3I is
governed by the equation:
2
48 nm Photolysis of an O3/H2O Mixture. One alternative
ln S ) -k′t + constant
(I)
t
1
OH source was ozone photolysis at 248 nm
where St is the OH fluorescence signal at time t and k′ ) k1 +
1
1
O + hν f O( D) + O
(4)
kloss. kloss is the first-order rate coefficient for the loss of OH
due to its reaction with the OH precursor (HONO or O3) and
impurities in the bath gas as well as flow out of the detection
region. The detection region is defined as the volume produced
by the intersection of the photolysis and probe laser beams. The
temporal profiles of OH were recorded and fit to eq I using a
weighted linear least-squares method to obtain k′. k′ values
3
2
followed by the reaction
1
O( D) + H O f 2 OH
(5)
2
In these experiments, [O3] was ∼1.5 × 10 molecule cm-3,
14
1
1
-1
-2
photolysis fluences were <0.4 mJ pulse cm , and [H2O] was
were measured as a function of CF3I concentration and a
1
5
-3
∼
3.4 × 10 molecule cm , resulting in [OH]0 of (2.6-4.9)
weighted linear least-squares fit of k′ vs [CF3I] data yielded k1
1
11
-3
×
10 molecule cm . The first-order loss of OH in the absence
as the slope and kloss as the intercept.
of CF3I was due to its reactions with O3 (k (298 K) ) 6.8 ×
351 nm Photolysis of HONO. As seen in Figure 1, the
temporal profiles of OH obtained using HONO photolysis, both
in the absence and in the presence of CF3I, were exponential.
The ratio of [CF3I]/[OH]0 was generally >5000 in these
-
14
3
-1 -1 9
1
0
cm molecule s ) and impurities and due to flow and
diffusion out of the detection region. For these experiments,
CF3I was measured using a separate 50-cm-long absorption
cell prior to entering the reaction cell because any slight
fluctuations in [O3] would interfere with the determination of
11
experiments with the estimated [OH]0 between (0.6-1.3) × 10
-
3
16
molecule cm and [CF3I] from (1.0-11) × 10 molecule

Rate Coefficients for the OH + CF3I Reaction
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8947
TABLE 1: Summary of k
1
Measurements at 296 K Using HONO Photolysis at 351 nm to produce OHa
pressure
v
probe fluence
photolysis fluence
[HONO]
[OH]
0
l
3
[CF I]
no of measurements
k
1
( 2σc
2
2
2
2
2
2
2
1
2
2
2
2
4.3
5.5
9.5
3.9
4.3
4.3
4.3
7.5
0.3
5.2
5.3
5.0
20
20
22
18
20
19
19
12
12
10
20
21
0.6
0.6
0.6
0.6
0.6
0.8
1.2
0.9
0.8
0.5
2.3
0.3
5.5
4.8-5.8
6.7-8.5
5.2
1.1
1.0
1.0
0.6
0.9
0.9
0.9
1.2
1.1
1.3
1.5
0.8
13.2
13.2
13.2
13.2
13.2
25.8
25.8
25.8
25.8
25.8
25.8
15.1
1.07-9.24
1.91-11.4
1.0-10
12
15
12
7
2.21 ( 0.44
2.09 ( 0.16
2.11 ( 0.22
2.11 ( 0.34
1.87 ( 0.32
2.39 ( 0.18
2.31 ( 0.37
b
2.61 ( 0.42
2.46 ( 0.25
2.78 ( 0.82
2.62 ( 0.13
2.35 ( 0.54
3.3
5.5
3.3
5.4
2.2-11
4.4
5.0
1.35-10.8
1.86-8.02
1.86-8.02
7.2
1.97-7.28
2.19-8.53
1.45-6.00
1.04-6.92
8
4.7
5.0
6
4.7
5.3
6
1.3-7.0
6.0
5
5.5
5.6
8
5.3
6.6
6
5.9
7.0
5
4.2
4.9
15
average
a
Units are pressure: Torr; linear velocity, v: cm s ; fluence: mJ pulse cm-2; [HONO]: 10 molecule cm-3; [OH]
-1
-1
13
: 1011 molecule cm-3
0
;
1
6
-3
-14
3
-1 -1 b
l, absorption pathlength: cm; [CF
varied at a fixed CF
fit.
3
I]: 10 molecule cm ; k
1
: 10
cm molecule
s . In this experiment, the photolysis laser fluence was
c
3
I concentration. There was no significant change in the observed OH first-order loss rate coefficient. σ is precision from the
TABLE 2: Measured Value of k
1
as a Function of Temperature and Experimental Conditions Useda
T (K)
Pressure
25
probe fluence
photolysis fluence
3.9
[HONO]
5.5
[OH]
8.0
0
[CF
3
I]
no of measurements
k
1
( 2σc
271
296
315
337
354
370
0.23
2.49-8.37
12
105
11
12
15
1.19 ( 0.19
2.35 ( 0.54
3.40 ( 0.27
5.67 ( 0.31
7.21 ( 0.48
9.12 ( 0.22
b
25
25
26
26
0.25
0.23
0.30
0.38
2.8
3.0
4.6
5.5
6.4
6.0
5.5
4.9
6.4
6.6
9.2
9.7
1.56-10.3
0.55-6.59
1.25-4.41
0.26-3.12
13
a
Units are pressure: Torr; fluence: mJ pulse cm-2; [HONO]: 1013 molecule cm ; [OH]
-1
-3
: 1010 molecule cm-3; [CF
I]: 1016 molecule cm-3
;
0
3
-
14
3
-1 -1
b
c
k
1
: 10 cm molecule
s . This is the average value for all 296 K experiments. σ is precision from the fit.
The probe laser fluence was varied since previous attempts to
measure k1 in this laboratory using HNO3 photolysis at 193 nm
to produce OH has shown a dependence on this parameter. In
addition, the linear gas flow velocity was changed by a factor
-
1
of 2, 10-22 cm s . With these linear flow velocities at
pressures of 17.5 to 29.5 Torr, the gas mixture within the
detection volume was replenished several times between laser
pulses (10 Hz). None of these variations affected the measured
value of k1. Typical first-order OH loss rate coefficients in the
-1
absence of CF3I were 230-340 s . In many experiments, these
loss rate coefficients were measured both before and after
determining a first-order rate coefficient in the presence of a
known [CF3I]. This was done because variations in [HONO]
would affect the value measured for k′ at a given [CF3I].
1
These loss rate coefficients were dependent upon He flow
through the solution used to generate HONO (increasing or
decreasing [HONO]), pressure, total flow rate, and temperature.
First-order OH loss rate coefficients measured in the presence
Figure 1. Examples of the decay of OH signal in the presence of CF
The two decays with [CF I] ) 0 were measured at the beginning and
in the middle of the experiments.
3
I.
3
-1
of CF3I ranged from ∼500-1800 s at 271 K and from ∼500-
500 s at 370 K. Generally, the first-order rate coefficient
for OH loss due to diffusion and reaction with HONO was
20% of the measured value of k′ at average CF3I concentra-
-3
cm . The measured values of k1 (296 K) along with a summary
of the experimental conditions are presented in Table 1.
Since previous measurements of k1 have reported either
nonexponential loss of OH, or a dependence of measured k1 on
fluence, we were particularly concerned about secondary
chemistry that could influence the determination of k1. There-
fore, we carried out numerous tests using the HONO source
for OH production. To ensure the accuracy of the determination
of the CF3I concentration at 296 K, several different absorption
path lengths (13.2, 15.1, and 25.8 cm) were used. To verify
whether photolysis of CF3I affected the observed OH loss rate
-
1
3
<
1
tions. The variety of tests performed in the 351 nm experiments,
none of which produced a change in the measured value of k1,
gave us confidence in our reported value of k1 (296 K) of (2.35
-
14
3
-1 -1
(
0.54) × 10
cm molecule s .
The values of k1 measured between 271 and 370 K are
presented in Table 2. These were fit to an Arrhenius expression
using a least-squares analysis of ln k1 (T) versus 1/T data to
-
1
-11
coefficient, both the probe laser fluence, 0.24-2.3 mJ pulse
obtain k1(T) ) (2.10 ( 0.80) × 10
exp[-(2000 ( 140)/T]
-
2
-1
3
-1 -1
cm , and the photolysis laser fluence, 2.8-5.9 mJ pulse
cm molecule s . Here, the uncertainty in E/R is twice the
standard deviation of the slope and the uncertainty in A is 2σA
) 2AσlnA. Figure 2 displays the Arrhenius plot for these
experiments along with previous determinations of k1. The
uncertainty in k1 due to any fluctuations in [HONO] at average
CF3I concentrations was <5% at every temperature except 271
-2
cm , were varied. In another experiment, [CF3I] was fixed and
-
1
the photolysis laser fluence was varied from 1.3-7 mJ pulse
-
2
cm : no change in the OH loss rate coefficient was observed.
Hence, we believe that photolysis of NO2 present in the HONO
sample did not influence the measured OH loss rate coefficients.

8948 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Gilles et al.
TABLE 3: Measured Values of k
1
(296 K) Using Photolysis
a
of O
3
/H
2
O Mixture at 248 nm to Produce OH
no. of
fluence
k
1
3 0 3 2
measurements [CF I] [OH] [O ] [H O]
0
.02
.09
.21
.37
2.85 ( 0.32
3.46 ( 0.54
4.25 ( 0.49
5.52 ( 0.82
5
6
5
5
1.17-9.57 0.7 3.15 3.4
1.39-6.14 2.6 1.65 3.4
1.16-7.59 4.9 1.48 3.4
1.02-7.63 3.5 1.36 3.4
0
0
0
intercept 2.67 ( 0.15
a
Units are fluence: mJ pulse cm-2; k : 10-14 cm molecule s-1
-1
3
-1
1
10
;
I]: 1015 molecule cm ; [OH]
-3
: 10 molecule cm ; [O
-3
14
[
CF
3
0
3
]: 10
-
3
15
-3
molecule cm ; [H
precision of the slope in plots of k′ vs [CF I].
1 3
2
O]: 10 molecule cm . Uncertainties are 2σ
Discussion
Three previously reported values for k1, and the results from
this study, are summarized in Table 4. Measuring k1 is
particularly difficult because it is relatively small and CF3I is
thermally and photolytically unstable. Because k1 is small, higher
concentrations of CF3I are required to measure a significant
increase in the OH loss rate coefficient. The larger [CF3I]
coupled with its susceptibility to photolysis leads to high radical
concentrations when OH is generated by photolysis, particularly
at wavelengths <300 nm. Thus, one would conclude that the
pulsed photolysis method might not be ideal for this measure-
ment. However, the earlier flowtube experiments also observed
nonexponential decays at longer times. In the following sections,
we discuss possible secondary chemistry in experiments where
OH was produced photolytically. We also discuss the effect of
small I2 impurities in these experiments.
1
Figure 2. Plots of k (T) vs 1000/T from this study, and those of Berry
et al. Brown et al., and Garraway and Donovan. The temperature
6
5
4
1
dependence for this work from the least-squares fit of ln k versus 1/T
-11
is given by k
molecule
1
-1
(T) ) (2.10 ( 0.80) × 10 exp[-(2000 ( 140)/T] cm3
-
1
s , (2σ measurement precision).
Possible products from CF3I photolysis are CF3, and I or I*,
2
I ( P1/2). CF3I photolysis resulting in an excited state iodine atom
1
0,11
occurs primarily at λ < 300 nm.
reactions,
However, the obvious
I (or I*) + OH f IO + H
f HI + O
(6a)
(6b)
Figure 3. A plot of k
photolyzed at 248 nm, followed by reaction with H
production. The intercept yields a value for k
of (2.67 ( 0.15) × 10
1
versus photolysis laser fluence when O
3
was
2
O for OH
are endothermic given the currently accepted values for the
-14
1
9
3
-1 -1
enthalpies of formation for IO and HI. Addition of O2 would
cm molecule
s .
deactivate I* produced by CF3I photolysis through the near
resonant electronic-electronic energy transfer process
K, where it was <8%. The largest uncertainty in the CF3I
concentration is from the uncertainty in its absorption cross
section at 266.6 nm, which is estimated to be <5%. (Uncertainty
due to its UV/VIS absorbance is estimated to be less than 3%.)
Adding these uncertainties and combining them in quadrature
with 2σ precision of the fit gives an uncertainty of k1 of ∼20%
at 271 and 296 K and ∼10% at higher temperatures, all at the
2
3
2
1
I ( P ) + O ( Σ) f I ( P ) + O ( ∆)
(7)
1
/2
2
3/2
2
-11
3
-1 -1 12
where k7 ) 4 × 10
cm molecule s . O2 also reacts
with CF3
CF + O + M f CF O + M
(8)
3
2
3
2
9
5% confidence level.
-
12
3
-1 -1 13
(
k8 296 K, 25 Torr) ) ∼3 × 10
cm molecule s . By
2
48 nm Photolysis of an O3/H2O Mixture. At a fixed ozone
sequestering CF3 radicals, we should minimize the possible
reaction of
concentration, increasing the photolysis laser fluence from 0.02
-
1
-2
to 0.2 mJ pulse cm increased the measured value of k1 by
nearly a factor of 2. Typical first-order OH loss rate coefficients
CF + OH + M f CF OH + M
(9)
3
3
-
1
measured in the absence of CF3I were ∼135 s (those in the
presence of CF3I were from 180 to 450 s ). A plot of k1 vs
48 nm photolysis laser fluence is shown in Figure 3 and
-
1
To the best of our knowledge the rate coefficient for reaction 9
has not been published. Nevertheless, we expect the rate
coefficient for OH reaction with CF3O2 to be smaller (or at least
2
experimental conditions are given in Table 3. The linear
extrapolation of the rate coefficient to a photolysis laser fluence
different) than that with CF . In the 351 nm experiments we
3
-
14
3
-1
of zero yielded a value of k1 ) ∼2.7 × 10
cm molecule
added 5 Torr of O , and the measured value of k was not
2
1
-1
s
as the intercept. We should note that altering the photolysis
significantly different than those measured in the absence of
laser fluence varies not only the fraction of CF3I dissociated
but also [OH]0 in these experiments. Although less precise than
the value obtained using HONO photolysis, this intercept is
consistent with that value of k1.
O . Hence, we believe these reactions did not influence our 351
nm experiments. In addition, varying the radical concentration
in the 351 nm experiments by a factor of 2 (by varying the
photolysis laser fluence) did not affect the measured value for
2

Rate Coefficients for the OH + CF3I Reaction
TABLE 4: Comparison of Measured k
with the Previously Reported Valuesa
J. Phys. Chem. A, Vol. 104, No. 39, 2000 8949
1
k
1
T (K)
Pressure
temp range (K)
A
Ea
method
ref
1
2 ( 2
295
300
292
296
dis flow, res fluor (OH)
dis flow, res fluor (OH)
4
5
6
3
5
.1 ( 0.5
1.8-5.0
0.045-0.069
17.5-29.5
-
12
.9 ( 1.4
281-443
271-350
(5.8 ( 2.3) × 10
2
2.7 ( 0.3 flash photolysis (H O) res fluor (OH)
-
11
2
.35 ( 0.54
(2.10 ( 0.80) × 10
4.0 ( 0.3 laser photolysis (HONO),
this work
laser induced fluorescence (OH)
a
: 10-14 m molecule s-1; A: cm molecule s-1; Ea: kcal mole-1. Uncertainties are 2σ precision only for the work of Brown
3
-1
3
-1
Units are k
1
5
6
4
et al. and this work, 1σ precision for Berry et al. and unspecified for Garraway and Donovan.
3
k1. It is important to note that typical CF3 concentrations,
all O( P) before a significant fraction of CF3 (produced by
3
calculated from [CF3I], photolysis laser fluences, and its known
reaction 12 or photolysis) could react with O( P). Since we
1
1
absorption cross section, were in the range (0.6-3) × 10
observed k1 to vary with photolysis fluence when photolyzing
both H2O2 and O3/H2O sources at 248 nm, we believe that
reaction 9 contributed at least partially to the measured value
of k1, but we cannot rule out some influence from reaction 11
in the O3/H2O experiments.
-
3
molecule cm for the 351 nm experiments.
In our 248 nm experiments (O3/H2O), significantly larger
fractions of CF3I were photolyzed, i.e., [CF3]0 ranged from (0.3-
1
2
-3
7
.0) × 10 molecule cm even though lower photolysis laser
6
fluences were used. Unfortunately, O2 could not be added to
Berry et al. used flash photolysis (λ > 120 nm) of water
these experiments since it quenches9 O( D) to O( P) and
prevents OH formation. Another reason to use low ozone
concentrations and photolysis laser fluences in these experiments
was to minimize the reaction
1
3
vapor to produce OH in the presence of excess CF3I in Ar buffer
gas. They observed the measured k1 to increase with flash lamp
energy and extrapolated the measured value to zero energy to
obtain the true value of k1. Water absorbs strongly at λ < 190
nm, and the flashlamp radiation extends to longer wavelengths
where σ(CF3I) . σ(H2O). It is probable that significantly more
CF3I was photolyzed than water because the [H2O]/[CF3I] ratio
in their experiments on the average was only ∼3.5. If [CF3]0 is
(10)
3
I + O f IO + O2
3
-1 -1 9
[
k10 (298 K) ) 1.2 × 10-12 cm molecule s ]. Although
there are no reported values for the rate coefficient for the
reaction,
9
about equal to [OH]0, reaction could contribute to an increase
in the OH loss rate coefficient, if the rate coefficient for the
-
12
association reaction of CF3 radicals with OH is ∼1 × 10
OH + IO f products
(11)
3
-1 -1
cm molecule at 25 Torr. If [CF3]0 is greater than [OH]0,
s
the IO product from reaction 10 could react rapidly with OH
and influence the measured value for OH loss rate coefficient.
The ClO + OH and BrO + OH reactions have rate coefficients
a smaller rate coefficient for the reaction of CF3 radicals with
OH could influence the value measured by them.
Thus, there are many reactions that could have contributed
to the measured dependence of k1 on laser or flash lamp fluence.
In our 248 nm experiments, we observe a dependence upon
laser fluence; however, since we are unable to completely isolate
specific reactions, we cannot identify which reactions are
responsible for the fluence dependence.
Another possible reason for the measured k1 to increase with
photolysis laser fluence is the production of H atoms from
photolysis of water
-
11
3
-1 -1 9,14-16
on the order of 10
cm molecule s .
One test for
the influence of reactions 10 and 11 was to vary [O3] from (1.5-
1
4
-3
4
.2) × 10 molecule cm at a fixed [CF3I] such that the IO
production rate is altered. The photolysis laser fluence was
varied to maintain a constant [OH]0. The measured OH loss
rate coefficient was not affected by varying the ozone concen-
tration over this range.
3
Because photolysis of O3 at 248 nm also produces O( P) with
a quantum yield of 0.1, the reaction
H O + hν f OH + H
(16)
2
3
O( P) + CF I f CF + IO
(12)
3
3
-11
3
-1
which reacts quickly [k (298 K) ∼ 1.35 × 10 cm molecule
-
1 6,19
[
k12 (298 K) ) 4.3 × 10-12 cm molecule s ], followed
3
-1 -1 17
s ]
with CF3I
by reaction 11 could also influence the OH loss rate. In an
H + CF I f HI + CF
(17)
3
3
3
experiment where O( P) was produced in the presence of CF3I,
Watson et al.18 observed IF and postulated it to be formed via
15
Given an average concentration of CF3I of ∼1.6 × 10
the reactions
-3
molecule cm , <140 µs are necessary for complete removal
of H atoms (i.e., 3 1/e lifetimes for reaction 17). The rate
coefficient for the reaction,
3
O( P) + CF f F CO + F
(13)
3
2
F + CF I f IF + CF3
(14)
HI + OH f H O + I
(18)
3
2
1
9
-10
3
-1
Reaction 14 is thought to be rapid, k14 ∼10 cm molecule
-11
cm molecule s-1)9 is over a thousand times
3
-1
(
k18 ) 3 × 10
-1
-11
3
-1 -1 20
s , and k13 (294 K) ) 3.1 × 10
cm molecule s . If
larger than the value for k1 reported in this paper and more than
the IF molecule were formed in our experiment it would
probably react with OH, since both channels
00 times larger than the value of k1 reported by Berry et al.6
5
This implies that [CF3I]/[OH]0 must be >2500 for reactions 17
and 18 to have less than a 20% influence on their measured
value of k1. If only reaction 18 contributed the increase in
measured k1, the slope of their measured value of k1 with
photolysis energy should be larger than what we observe, but
the intercept should be the same as our measured value, as long
as the concentration of radicals that react with OH increased
IF + OH f HOF + I
f HOI + F
(15a)
(15b)
are likely to be exothermic. However, given the value of k12
and CF3I concentrations used, reaction 12 should have consumed

8950 J. Phys. Chem. A, Vol. 104, No. 39, 2000
Gilles et al.
6
linearly with photolysis energy. However, Berry et al. obtained
reaction 1 are HOI and CF3. The enthalpy of formation of -16.2
-
14
3
-1 -1
3
-1
a value of (5.9 × 10
cm molecule s ; 292 K) cm
molecule s , which is higher than the intercept we obtained
kcal mole for HOI is significantly larger than the value of
-
1
-1
-1
-8 to -9 kcal mole recommended by Ruscic and Berkowitz,
-
14
3
23
-1
24
in our ozone photolysis experiments (∼2.7 × 10
cm
and of -11.7 kcal mole , given by Glukhovtsev et al.
-1 -1
molecule s ; 296 K) or the value we measured using HONO
photolysis. The difference could be due to nonlinear dependence
of measured k1 on photolysis energy and/or additional OH loss
processes. In any case, due to the absence of secondary
chemistry, we believe that the 351 nm photolysis of HONO for
OH production is a more reliable method of determining k1. It
However, it is in reasonable agreement with the -14.3 ( 1.6
recommended by Hassanzadeh and excellent agreement with
the value of -16.6 ( 1.3 of Berry et al.
2
5
6
The atmospheric lifetime of iodine containing species has
1-3
been discussed in detail in other publications. In general, the
photolysis rates of iodine compounds are rapid, resulting in
4
should be noted that Garraway and Donovan should have seen
a dependence on photolysis energy based on our above analysis;
they did not report such dependence.
Contamination of the CF3I Sample. Another potential problem
in determining k1 is the possibility of I2 contamination. Because
of the large rate coefficient for the reaction of I2 with OH, 2.1
atmospheric lifetimes on the order of days. CF I is expected to
3
1
have a tropospheric lifetime of less than 2 days. Previous
1
-12
3
calculations assumed a rate constant of 1.2 × 10
cm
-
1
-1
molecule
s
for reaction 1 and showed that reaction with
OH could lower the calculated atmospheric lifetime of CF I by
3
< 25%. The much lower value of k reported here will further
1
-
10
3
-1 -1 21
×
10
cm molecule s , a 0.01% contamination of I2 in
lower the significance of CF I loss due to reaction with OH, as
3
CF3I will lead to an OH loss rate coefficient due to reaction
with I2 that is approximately the same as that due to reaction
with CF3I. In our experiments, CF3I (99%) was kept in an ice
water bath to suppress any possible I2 contamination. To test
for the presence of I2, a 100-cm cell was filled with the CF3I
sample and its absorption at 508 nm (Cd lamp) was monitored.
There was no change in absorbance at this wavelength upon
addition of 300 Torr of the CF3I sample. Given an absorption
compared to its photolytic loss.
Acknowledgment. This work was funded in part by the
Upper Atmospheric Research Program of NASA.
References and Notes
(1) Solomon, S.; Burkholder, J. B.; Ravishankara, A. R.; Garcia, R.
R. J. Geophys. Res. 1994, 99, 20929.
-
18
2
-1
cross section of 2.5 × 10
cm molecule for I2 at 508 nm
(2) Solomon, S.; Garcia, R. R.; Ravishankara, A. R. J. Geophys. Res.
994, 99, 20491.
1
and assuming we should have been able to observe an
absorbance of at least 3%, we place a limit of <0.002% for the
contamination from I2 in CF3I. In addition, the regulator on the
CF3I cylinder was flushed out at the beginning and end of each
set of experiments and the main He flow was added directly
after the CF3I regulator to continually flush out this line. This
was done because a slight trace of pink was observed in the
Teflon line upon prolonged use in preliminary experiments
where the He main flow was not added directly after the CF3I
gas regulator. Also, slightly larger values for k1 were measured,
presumably due to I2 formation. However, we should note that
an I2 contamination of 0.02% of the CF3I concentration would
be sufficient to account for the difference in the value of k1
(
3) Rattigan, O. V.; Shallcross, D. E.; Cox, R. A. J. Chem. Soc.,
Faraday Trans. 1997, 93, 2839.
(4) Garraway, J.; Donovan, R. J. J. Chem. Soc. Chem. Commun. 1979,
108.
1
(
5) Brown, A. C.; Canosa-Mas, C. E.; Wayne, R. P. Atmos. EnViron.
1
990, 24A, 361.
(6) Berry, R.; Yuan, J.; Misra, A.; Marshall, P. J. Phys. Chem. A. 1998,
102, 5182.
(
7) Vaghjiani, G. L.; Ravishankara, A. R. J. Phys. Chem. 1989, 93,
1
948.
(
8) Gilles, M. K.; Turnipseed, A. A.; Burkholder, J. B.; Ravishankara,
A. R.; Solomon, S. J. Phys. Chem. A 1997, 101, 5526.
9) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
(
Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E., Molina,
M. J., Eds.; Chemical Kinetics and Photochemical Data for Use in
Stratospheric Modeling; Jet Propulsion Laboratory, 1997.
(10) Hwang, H. J.; El-Sayed, M. A. J. Phys. Chem. 1992, 96, 8728.
11) Smedley, J. E.; Leone, S. R. J. Chem. Phys. 1983, 79, 2687.
12) Burde, D. H.; Yang, T. T.; McFarlane, R. A. Chem. Phys. Lett.
6
measured by Berry et al. and that reported here. Also, the
(
(
presence of such an impurity would account for differences in
6
the activation energies measured by Berry et al. and us.
1
1
993, 205, 69.
Reaction 1 could have several product channels, e.g.,
(13) Kaiser, E. W.; Wallington, T. J.; Hurley, M. D. Int. J. Chem. Kinet.
995, 27, 205.
(
14) Kegly-Owen, C.; Gilles, M. K.; Burkholder, J. B.; Ravishankara,
A. R. J. Phys. Chem. A. 1999, 103, 5040.
15) Bogan, D. J.; Thorn, R. P.; Nesbitt, F. L.; Stief, L. J. J. Phys. Chem.
1996, 100, 14383.
16) Gilles, M. K.; McCabe, D. C.; Burkholder, J. B.; Ravishankara, A.
R., to be published.
17) Gilles, M. K.; Turnipseed, A. A.; Talukdar, R. K.; Rudich, Y.;
OH + CF I f CF + HOI
(1a)
(1b)
3
3
(
f CF H + IO
3
(
However, only the HOI product has been observed.22 The
enthalpy of formation for HOI has been estimated from
(
Villalta, P. W.; Huey, L. G.; Burkholder, J. B.; Ravishankara, A. R. J. Phys.
Chem. 1996, 100, 14005.
6
experimental and theoretical studies. Berry et al. reported an
(
(
18) Watson, T. A.; Addison, M.; Wittig, C. Chem. Phys. 1983, 78, 57.
19) Morris, R. A.; Donohue, K.; McFaddden, D. L. J. Phys. Chem.
-
1
activation energy of 2.7 kcal mol , while we report a slightly
-
1
larger value: 4.0 kcal mol . Using ∆fH° (298 K) (OH) ) 9.3
1989, 93, 1358.
-
1
-1
(20) Ryan, K. R.; Plumb, I. C. J. Phys. Chem. 1982, 86, 4678.
kcal mole , ∆fH° (298 K) (CF3I) ) -140.9 kcal mole , and
(21) Gilles, M. K.; Burkholder, J. B.; Ravishankara, A. R. Int. J. Chem.
-
1
∆
fH° (298 K) (CF3) ) -112 kcal mole , we arrive at ∆rxnH°
Kinet. 1998, 31, 417.
-
1
)
∆fH° (298 K) (HOI) + 19.6 kcal mole . Combined with
(22) Monks, P. S.; Stief, L. J.; Tardy, D. C.; Liebman, J. F.; Zhang, Z.;
Kuo, S. C.; Klemm, R. B. J. Phys. Chem. 1995, 99, 16566.
(23) Ruscic, B.; Berkowitz, J. J. Chem. Phys. 1994, 101, 7795.
-
1
∆
rxnH° ) Ea - RT ) 3.4 kcal mole , for a bimolecular
-1
reaction, yields ∆fH° (298 K) (HOI) ) -16.2 kcal mole . This
is true if the products of reaction 1 are solely, or mostly, HOI
and CF3. It is important to ensure that the main products of
(
24) Glukhovtsev, M. N.; Pross, A.; Radom, L. J. Phys. Chem. 1996,
1
00, 3498.
(25) Hassanzadeh, P.; Irikura, K. K. J. Phys. Chem. A. 1997, 101, 1580.