Kinetics of the Reaction of OH with HI between 246 and 353 K

Article abstract of DOI:10.1021/jp984321x

The laser-flash photolysis technique combined with resonance fluorescence detection of OH was used to investigate the kinetics of the title reaction at temperatures between 246 and 353 K and at a total pressure of 75 ± 1 Torr Ar. Under these conditions the rate coefficient shows a negative temperature dependence which is described by k₄(246-353 K) = 7.0_-0.4^+1.9 × 10^-11(T/298)^-1.5±0.5 cm³ s^-1. Both the large rate coefficient and trends in the reactivity of OH with HCl, HBr, and HI are suggestive of a reaction mechanism that proceeds without a significant activation barrier. The room temperature (294 K) rate constant is k₄ = 6.5_-0.4⁺² × 10^-11 cm³ s^-1, which is significantly larger than previous measurements. Reasons for this difference and the implications of the new rate coefficients for the lifetime of HI in the atmosphere are discussed.

Full text of DOI:10.1021/jp984321x

2712
J. Phys. Chem. A 1999, 103, 2712-2719
Kinetics of the Reaction of OH with HI between 246 and 353 K
†
P. Campuzano-Jost and J. N. Crowley*
Max-Planck-Institut f u¨ r Chemie DiVision of Atmospheric Chemistry, Postfach 3060, 55020 Mainz, Germany
ReceiVed: NoVember 1, 1998; In Final Form: January 28, 1999
The laser-flash photolysis technique combined with resonance fluorescence detection of OH was used to
investigate the kinetics of the title reaction at temperatures between 246 and 353 K and at a total pressure of
7
5 ( 1 Torr Ar. Under these conditions the rate coefficient shows a negative temperature dependence which
+1.9
-11
-1.5(0.5
3
-1
is described by k
4
(246-353 K) ) 7.0- × 10 (T/298)
cm s . Both the large rate coefficient and
0.4
trends in the reactivity of OH with HCl, HBr, and HI are suggestive of a reaction mechanism that proceeds
without a significant activation barrier. The room temperature (294 K) rate constant is k
cm s , which is significantly larger than previous measurements. Reasons for this difference and the
implications of the new rate coefficients for the lifetime of HI in the atmosphere are discussed.
+
2
0.4
-11
4
) 6.5- × 10
3
-1
1. Introduction
order to resolve the discrepancies in the literature and to extend
the temperature range to cover those relevant to atmospheric
conditions.
1
-5
Iodine chemistry is believed to play a role in tropospheric,
6
and, potentially, also stratospheric ozone loss. CH3I, which is
produced by marine algae, has generally been considered to
1
2
. Experimental Procedure
.1. General Setup. The salient features of the present
be the major source of iodine in the atmosphere, although marine
sources of photolabile iodo-carbons such as CH2I2, C2H5I, and
CH2ICl have recently been identified,7 and which may be of
a greater combined source strength than that of CH3I. As the
release of I atoms from molecules such as CH2I2 is facile and
occurs close to the surface, the iodine chemistry of the lower
troposphere may be even more important than previously
assumed. Similar to bromine, I atoms are generally unreactive
toward hydrocarbons and thus preferentially react with O3.
2
,8
13
experimental setup have been given elsewhere and are
recounted only briefly here. The reaction took place in a
thermostated, Teflon-coated stainless steel reaction vessel of ca.
1
50 mL volume. OH radicals were usually generated from
suitable precursors by the pulsed (20 ns), unfocused light from
an excimer laser. Two 8 mm irises placed before the cell define
the size and shape of the laser pulse. Resonantly scattered
photons from OH (excited by the emission of a microwave-
powered resonance lamp) were detected by a photomultiplier
that was screened by a 309 ( 5 nm interference filter. The
3
Because the major inorganic reservoir of iodine, HOI, is very
short lived with respect to photodissociation, the presence of
reactive iodine species can efficiently catalyze O3 destruction:
9
9
-3
detection limit (S/N ) 1) is estimated as ≈1 × 10 OH cm
I + O f IO + O2
(1)
(2)
(3)
3
for an integration time of about 0.5 s. The temperature of the
gas at the point of intersection of the laser and resonance lamp
IO + HO f HOI + O2
2
(which defines the reaction volume) was determined using a
HOI + hν f I + OH
Teflon coated iron-constantan thermocouple which could be
inserted into the reaction volume. The pressure was monitored
using a 100 Torr capacitance manometer (MKS Baratron) with
a quoted accuracy of 0.1%, and gas flows were controlled by
calibrated mass-flow controllers (Tylan). The experiments were
carried out using a large excess concentration of HI over OH,
so that the OH decay is described by
(
net) HO + O f OH + 2 O
2
3
2
The least photolabile inorganic iodine reservoir is HI, which
is formed in the reaction of I atoms with HO2, and which is
second in abundance to HOI in the lowest 2 km above the
3
ocean. The major gas-phase loss process of HI is expected to
be reaction with the OH radical, which returns I atoms to the
gas phase:
[
OH] ) [OH] exp - {(k [HI] + c)t}
(i)
t
0
4
OH + HI f H O + I
(4)
2
where [OH]t is the OH concentration at time ) t after the laser
pulse, k4 the bimolecular rate coefficient for the reaction of OH
with HI, and c the summed rate constant for other first-order
loss processes such as reaction of OH with its photolytic
precursor and diffusion out of the reaction zone. The first-order
decay rate constant k1st was obtained by unweighted, nonlinear
least-squares fitting of the OH decays and is related to the
desired rate coefficient, k4 as shown in eq ii.
To date there have been three kinetic studies10,11,12 of the
reaction between OH and HI, all of which were conducted at
room temperature only (≈298 K) and yield rate constants of
-
11
-11
(
1
1.3 ( 0.5) × 10 , (2.7 ( 0.2) × 10 , and (3.3 ( 0.2) ×
-11
3 -1
0
cm s , respectively. The aims of the present study were
to measure the kinetics of this reaction at room temperature in
†
Present address: Rosenstiel School of Marine and Atmospheric
kfirst ) k [HI] + c
(ii)
Sciences, 4600 Rickenbacker Causeway, Miami, FL 33149.
4
1
0.1021/jp984321x CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/23/1999

Reaction of OH with HI
.2. OH Source. The choice of photolytic OH precursor for
kinetic experiments with HI was limited by its high reactivity
and propensity to form aerosol. A number of OH sources were
tested, including reactions 5, 6, 7 + 8, and 9 + 10 + 8.
J. Phys. Chem. A, Vol. 103, No. 15, 1999 2713
2
which was dependent on the amount of NO2 added, and on the
mixing time for the HI and NO2 flows prior to the photolysis
cell. At long mixing times (t > 1s) and with concenrations of
1
6
-3
14
Ar (20 Torr) and H2 (≈2 × 10 cm ) and NO2 (4 × 10
-
3
cm ) a stray light signal of ≈ 20000 cps was observed. The
3
HNO + hν (248 nm) f OH + NO2
(5)
(6)
addition of ≈2 × 10 cm HI resulted in a signal of > 100000
cps. (These stray light values are not directly comparable to
those quoted in section 2.2.1 due to a changed experimental
setup). A flattening off of the stray-light signal with increasing
13
-3
H O + hν (248 nm) f 2 OH
2
2
HI + hν (248 nm) f H + I
(7)
(8)
[
NO2] indicated that HI was probably almost completely
consumed in a reaction with NO2 at high NO2 and long mixing
times. We also observed a broad absorption centered at ≈520
nm when the same concentrations of NO2 and HI were flowed
into the optical absorption cell (mixing time ) 20 s) and which
was absent for separate flows of HI and NO2 in Ar. The λmax
of the absorption feature at 520 nm rules out that it is I2, which
absorbs with a maximum closer to 490 nm.
H + NO f OH + NO
2
1
NO + 2hν (439.4 nm) f O( D) + NO
(9)
2
1
O( D) + H f OH + H
(10)
2
2
.2.1. HNO3 + hν (248 nm). A very large increase in the
These observations are indicative of a thermal, dark reaction
scattered light from the resonance lamp could be observed when
between NO and HI in our experiment, which results in the
2
15
-3
13
-3
flows of HNO3 (≈1 × 10 cm ) and HI (≈2 × 10 cm ) in
formation of a product that reacts very rapidly with OH, and
7
5 Torr Ar were mixed and introduced into the reaction vessel.
which has a high scattering cross section at ≈309 nm.
This increase in scattered light in the absence of any photolytic
generation of OH was attributed to aerosol formation due to
reaction between these molecules. Typical scattered light signals
of 700 cps (cps ) counts per second) obtained for separate flows
of HNO3/Ar or HI/Ar increased to 7000-8000 cps when the
flows were mixed. Efforts were made to reduce the mixing time
of these two gas flows and to decrease the HNO3 as far as
2.2.3. H O + hν (248 nm). Finally, the photolysis of H O
2
2
2
2
at 248 nm proved to be the most suitable source of OH. Due to
a larger absorption cross section at 248 nm than HNO and a
3
16
quantum yield of 2 for OH formation relatively low concentra-
14
-3
tions of H O (3 × 10 cm ) were sufficient to generate useful
2
2
OH signals. The amount of H O was estimated from the decay
2
2
rates of OH in the absence of HI, the known rate constant for
13
possible. Even by reducing the HNO3 concentration to 7 × 10
cm (with [HI] ) 2 × 10 cm ) and the contact time to ≈
.2 s, the measured scattered light signal was still substantial
at ≈ 2000 cps. Optical absorption experiments with higher
HNO3] and [HI] showed that the HI was quantitatively removed
OH + H O , and the typical rate constant for OH loss due to
2
2
-
3
13
-3
-1
diffusion in the reactor (≈100 s at 75 Torr).
A flowing mixture of H O with ≈2 × 10 HI cm-3 caused
13
0
2
2
an approximate 25% increase in the scattered light from the
resonance lamp, when compared to that from flowing pure Ar.
The size of the scattered light signal was however found to be
unchanged upon variation of either the H O concentration
[
by reaction with HNO3 and that condensates were observed on
the walls of the absorption cell. The above observations led us
to reject HNO3 as suitable OH source.
2
2
1
4
15
(between 1 × 10 and 1 × 10 ) or the contact time (between
0.5 and 4 s) and similar in magnitude to that observed when
HI/Ar flows were added to the cell with no H O present.
2
.2.2. HI + hν (248 nm) + NO2 and NO2 + 2hν + H2/H2O.
As HI absorbs relatively strongly at 248 nm (see below),
reactions 7 + 8 represent a further source of OH if NO2 is added
to the HI/Ar mixture. The concentration of OH thus generated
was found to depend linearly on the HI concentration as
expected and (once corrected for the background signal) its
decay was exponential. However, the expected linear depen-
dence of the pseudo-first-order rate constant kfirst versus the
concentration of HI was not obtained. We have no firm
explanation for this, but note that the photolysis of HI at 248
nm to give H and I has ≈ 2 eV of excess energy, presumably
mostly available as translational energy in H, which may have
perturbed both the formation and reactivity of the OH product
formed in reaction 8. There is evidence for an altered reactivity
2
2
To avoid perturbation of the OH decay via reaction 12, the
OH concentration (calculated using the laser fluence at 248 nm
11
and the H O concentration) was kept low at [OH] ≈ 5 × 10
2
2
0
-
3
cm .
OH + H O f HO + H O
(11)
(12)
2
2
2
2
OH + HO f H O + O
2
2
2
The above discussion of various OH sources reveals that the
addition of either NO2 or HNO3 to a HI/Ar mixture results in
a complex chemistry that via dark, thermal reactions leads to
aerosol generation, formation of other gas-phase products and
loss of HI from the gas phase. Although these observations are
not directly relevant to the kinetics of the reaction of OH with
HI, they may provide an explanation for some of the discrep-
ancies found in the literature data for this reaction. We shall
return to this point in the discussion, where we compare our
data to that obtained in both flash photolysis and discharge flow
systems.
1
4
of vibrationally excited OH toward HBr and possibly HCl.
Alternatively, the addition of NO2 also enables the use of an
15
OH source that has recently been developed in this laboratory,
the two-photon dissociation of NO2 in the presence of H2
reactions 9, 10, and 8). The use of this source generated
exponential decays of OH, and a linear relationship was
(
1
3
-3
observed between kfirst and [HI] for [HI] < 3 × 10 cm .
However, several observations ruled out the use of this method
of OH generation. First we observed a strong dependence of
the rate constant on the initial concentrations of NO2. Room-
2.3. Flows and Concentrations. Extraction of the bimolecu-
lar rate coefficient for reaction of OH with HI (k4) using eq ii
requires the absolute concentration of HI in the reaction cell to
be known. Gaseous HI (prepared as described below) was
accurately diluted in Ar in darkened 13 L glass bulbs using 1
and 1000 Torr capacitance manometers to determine the nominal
-
10
temperature rate constants of between 1.2 × 10
and ≈5 ×
-
11
3 -1
1
0
cm s could be obtained, with the lowest rate constants
measured at low [NO2].
Also, as in the HNO3/HI experiments, an enhanced scattered
light signal from the resonance lamp was observed, the size of
-
4
mixing ratio, which was varied between 1.3 and 2.5 × 10 . A

2714 J. Phys. Chem. A, Vol. 103, No. 15, 1999
Campuzano-Jost and Crowley
flow of HI/Ar was introduced into the reaction cell via a Kel-F
needle valve which was used to vary the HI/Ar flow rate. The
use of mass flow controllers and metal or Teflon needle valves
proved to be nonviable for HI/Ar mixtures due to the rapid
decomposition of the HI on these surfaces. Three mass flow
controllers of range 10, 100, and 1000 cm3 (STD) min-1
would not be encountered in a kinetic experiment in which time
is allowed for conditioning of glass surfaces before data is taken.
Thus, although the 20-25% discrepancy may be regarded as
an upper limit to a potential systematic overestimation of the
HI concentration, in the final analysis we add an asymmetric
error of +25% to our final rate constant for OH + HI at each
temperature.
()sccm) were used to control separate flows of Ar. The flows
through the 1000 and 100 sccm controllers were directed into
the photolysis cell; the flow through the 10 sccm controller
passed through a bubbler containing concentrated H2O2 before
entering the photolysis cell. Initially a flow of H2O2/Ar (total
flow rate ≈ 550 sccm) was established, which, as evidenced
by observations of a constant OH decay rate and signal height
upon photolysis of H2O2 at 248 nm, took ≈1 h to stabilize.
Following this conditioning period, the total flow rate and
the pressure in the reaction cell (100 Torr capacitance manom-
eter) were noted. A flow of HI/Ar was added to the H2O2/Ar
flow just before the photolysis cell and the increase in pressure
noted. The total pressure was then adjusted to its original value
by reducing the flow through the 100 sccm Ar controller. The
change in flow through this controller then provides the
equivalent flow of HI /Ar through the needle valve. This was
combined with the total flow rate, the pressure in the photolysis
cell, and the HI/Ar mixing ratio to yield the HI concentration.
The base pressure in the absence of the HI/Ar flow was
redetermined at regular intervals to check for changes in, for
example, the pumping efficiency which would invalidate this
method of calculating [HI]. For experiments at 75 Torr, each
All experiments were carried out at a total pressure of 75 (
1 Torr, and a total flow rate of ≈550 sccm, the temperature
was varied between 246 and 363 K. The H2O2 concentration
and laser fluence (per pulse at 10 Hz) were usually maintained
1
4
-3
-2
at 3 × 10 cm and 8 mJ cm , respectively, resulting in an
11
-3
initial OH concentration of ≈5 × 10 cm . The HI concentra-
1
2
13
-3
tion was varied between 5 × 10 and 4.5 × 10 cm . Each
HI/Ar mixture was used to determine a rate coefficient at room
temperature and at one other temperature. This helped to
eliminate systematic errors related to the mixing ratio of HI in
the storage bulb that otherwise could have masked the true
variation of rate coefficient with temperature.
2.4. Chemicals and Purification. Anhydrous HNO3(l) was
prepared by the reaction of KNO3 with concentrated H2SO4 and
vacuum distilled into a cold trap at liquid N2 temperature, before
storage in the dark at -40 °C. H2O2 was purchased as a 70%
(wht) solution in H2O (Solvay Interox). The H2O2 was precon-
centrated by passing Ar through it for several days. HI was
prepared by drying a 55% aqueous solution (Fluka) on P4O10
at 235 K, and was purified of I2 and phosphor-iodides by
repeated vacuum distillation at 163 K. NO2 (purchased as N2O4,
Merck, 99.5%), N2O (Hoechst, 99.5%), and Ar (Linde, 99.999%)
were used without further purification.
2.5. UV Spectrum of HI. As part of the present study, the
UV spectrum of HI was determined. Initially it was anticipated
that accurate optical absorption measurements of HI concentra-
tions could supplement the determination via partial flow and
pressure. However, the low concentrations of HI used resulted
in optical densities that were too low to be considered reliable.
The use of multireflection optics was not possible due to the
rapid decomposition of HI on the metal mirror supports. In
addition, the rather large volume (3 L) of our optical absorption
cell meant that very long conditioning periods were required
for the low HI concentrations. However, the UV spectrum of
HI also enables an assessment of its loss due to photodissociation
in the atmosphere, and so experiments were carried out to
measure its absorption cross sections between 200 and 340 nm
at room temperature. For these experiments, a quartz absorption
cell of 132 cm optical path length was used. A D2 lamp was
used as source of the analysis light, which was dispersed and
detected by a 0.5 m monochromator with a grating blazed at
1
2
-3
concentration step (around 5 × 10 HI cm ) resulted in a
pressure increase of ≈0.45 Torr compared to the maximum
resolvable pressure change of 0.01 Torr. Drifts of greater than
0
.2 Torr in the base pressure was the criterion to eliminate data.
13
-3
At [HI] ) 5 × 10 cm this is just 4% of the total change in
pressure due to the HI/Ar flow but is ≈ 25% for [HI] ) 1 ×
13
-3
1
0
cm .
Before being mixed with the H2O2/Ar flows, the HI /Ar flow
was passed through a glass spiral immersed in a cold bath at
110 °C. The vapor pressure of HI at this temperature is >5
-
Torr as verified in the present experiments on the UV spectrum
of HI, whereas that of I2 is at least 6 orders of magnitude lower.
This precautionary measure should efficiently remove any I2
impurity that may have been formed in a thermal self-reaction
of HI in the darkened storage bulb or due to conditioning of
the glass and Teflon tubing. As I2 reacts extremely rapidly with
-
10
3
-1
16
OH (k13 ) 1.8 × 10
cm s at 298 K ) the presence of I2
could result in an overestimation of the rate constant.
OH + I f HOI + I
(13)
2
2
00 nm with 300 or 600 lines/mm and a 1024 element
As HI has a high affinity for surfaces, an additional check of
the manometrically determined HI/Ar mixing ratio in the
darkened storage bulb was undertaken. Following a kinetic
measurement at 294 K, a well-defined fraction of the remaining
contents of a HI/Ar bulb were drawn through an aqueous
solution (100 mL) of 0.1 M KOH The aqueous solution was
then analyzed for I- using a freshly calibrated ion chromato-
graph (Dionex AS 11), and the results were compared with the
expected value calculated from the nominal gas-phase mixing
ratio in the glass bulb. This diagnostic test was carried out on
two separate occasions and yielded on both counts a HI mixing
ratio that was ≈20-25% lower than that obtained manometri-
cally. Although it remains unclear what the source of the
discrepancy might be, we note that losses of HI on the
unconditioned glass tubing between the storage bulb and the
KOH bubbler would result in a lower mixing ratio. Such effects
photodiode array, respectively. To prevent formation of a
adsorbed film of HI on the Suprasil quartz windows, they were
heated to ≈50 °C. This was found necessary to obtain
reproducible results in the long-wavelength tail of the HI
spectrum. Pure HI was introduced into the absorption cell from
2
its solution at ≈170 K which removed any I impurity. The
absence of any I was confirmed by absorption measurements
2
at 500 nm. Reproducible results were only obtained once the
absorption cell had been preconditioned by leaving ≈5 Torr of
HI in the cell for several minutes. Beer-Lambert linearity could
be confirmed for seven different wavelengths between 205 and
320 nm and optical densities of up to 1.5.
3
. Results
3.1. UV Absorption Spectrum of HI. The final absorption
spectrum of HI is shown in Figure 1, along with previous

Reaction of OH with HI
J. Phys. Chem. A, Vol. 103, No. 15, 1999 2715
Figure 1. The UV-vis absorption cross sections of HI at 298 K. The
Figure 3. Plot of k1st versus concentration of HI at three different
temperatures and at 75 Torr Ar total pressure. The bimolecular rate
coefficient each temperature is derived from the slopes (straight line
fits to eq ii). The vertical error bars represent 2 σ error in k1st for each
circles are the data of Huebert et al.1
7
[HI] and the horizontal error bars are 4% error in the estimated [HI]
concentrations, which were based on observations of pressure variation
in the cell during the experiment.
TABLE 1: Rate Data for OH + HI at 294 K
c
no. of
[HI]
k
4
-11
( 2σ
4 4
k /k -
HI bulb decaysb (1012 cm-3)
k
b
1st (s-1
)
(10 cm s-1) (av)
3
I
II
III
IV
V
VI
VII
VIII
IX
7
7
7
7
6
6
6
6
6
5.69-45.5
5.19-42.5
5.10-40.8
4.53-34.0
5.28-31.7 1096-2725 6.35 ( 0.19
5.54-24.9 1011-2197 6.60 ( 0.20
8.43-46.4 1329-3557 6.12 ( 0.23
8.31-47.8 1356-4586 7.13 ( 0.43
8.94-53.7 1416-4552 7.32 ( 0.34
6.49-49.8 1442-4593 7.25 ( 0.33
6.53 ( 0.44
654-3171 5.45 ( 0.12
872-2850 5.78 ( 0.14
980-2869 6.53 ( 0.19
980-2787 6.76 ( 0.26
0.83
0.88
1.00
1.04
0.97
1.01
0.94
1.09
1.12
1.11
a
a
X
7
65
all data
a
Ion chromatographic tests of the HI mixing ratio were carried out
on these mixtures. b Not counting decays in the absence of HI. 2σ
statistical errors including 4% error in [HI] due to pressure variation
in the photolysis cell during an experiment.
c
dependent on [HI] (see eq ii). The room-temperature data (294
K) are summarized in Table 1.
Figure 2. Exponential decays of OH in the presence of excess
As can be seen, a large number of decays (65) were obtained
at 294 K, yielding an average rate constant of (6.53 ( 0.44) ×
concentrations of HI at 294 K and a total pressure of 75 Torr Ar. The
11
-3
initial OH concentration was ≈5 × 10 cm . cps ) counts per second.
-
11
-3
1
0
cm . We note that the statistical errors are considerably
determinations of the cross sections by Huebert et al.17 As can
be seen, there is generally good agreement in those wavelength
regions where the spectra overlap. The HI spectrum consists of
broad absorption features with a well-defined maximum at ≈220
nm. To the long-wavelength side of the maximum, the absorp-
tion falls approximately exponentially into the actinic region
smaller than the fluctuations in the rate constant from one HI
mixture to the next, for which 294 K rate constants varied from
-
11
-3
(5.5-7.3) × 10
cm .
3.3. Kinetics of OH + HI Between 246 and 353 K. As
already mentioned, kinetic data, gathered at temperatures other
than 294 K, were subject to a normalization procedure designed
to eliminate apparent changes in rate coefficient with temper-
ature that were in fact due to fluctuations in the mixing ratio of
HI in its storage bulb from one experiment to the next. In Table
1 are listed the room-temperature rate constants obtained using
different HI storage bulbs and, in the last column, the values of
the ratio of the 294 K rate constant to the average value for all
(
λ > 300 nm).
3
.2. Kinetics of OH + HI at Room Temperature (294 K).
Figure 2 displays OH decays at 294 K obtained in various
concentrations of HI. HI was always in large excess over OH,
and as expected from eq i, OH decays are exponential over at
least three half-lives. The slopes of such plots yield k1st, which,
as shown for data at 264, 294, and 354 K (Figure 3), is linearly
-
11
3
-1
data at 294 K, which is 6.53 × 10
cm s . The inverse of

2716 J. Phys. Chem. A, Vol. 103, No. 15, 1999
Campuzano-Jost and Crowley
TABLE 2: Rate Data for OH + HI at Various Temperatures
no. of decaysa
[HI] (1012 cm-3
)
k
1st (s-1
)
k
( 2σ (10 cm s-1)
b
-11
3
k
4
(norm) (10 cm s-1)
c
-11
3
temp K
HI bulb
4
246
264
272
283
294
315
334
353
VI (0.99)
V (1.03)
II (1.14)
IV (0.96)
all data
VIII (0.92)
VII (1.06)
IX (0.89)
8
8
8
6
65
8
3.98-31.8
4.42-35.3
5.64-45.1
6.29-37.7
Table 1
5.88-49.0
5.59-42.9
5.60-42.0
681-3205
1005-4153
1033-4801
1692-3718
Table 1
1166-4050
1236-3260
987-3135
7.96 ( 0.25
9.41 ( 0.24
9.23 ( 0.20
6.75 ( 0.26
Table 1
6.68 ( 0.35
6.14 ( 0.24
5.75 ( 0.24
7.88 ( 0.51
9.69 ( 0.54
10.5 ( 0.50
6.48 ( 0.54
6.53 ( 0.44
6.14 ( 0.62
6.51 ( 0.43
5.12 ( 0.54
8
7
a
Excluding decays in the absence of HI. b 2σ statistical errors only. c Normalized data have been corrected for the fluctuations in the 294 K rate
data for each bulb. The final errors on k include errors in k at 294 K (the normalization temperature).
4
4
this factor (listed in parentheses in the second column of Table
given H2O2/HI combination was observed. This observation
tends to rule out that H2O2 and HI are interacting significantly
within the contact time to influence the OH kinetics by formation
2) was then used to normalize the data at the other temperatures.
4. Discussion
of I or loss of HI. After 3 h of experiments at low temperatures,
2
the OH decay rate in the absence of HI was usually remeasured
4
.1. Kinetics of OH with HI at 294 K. As mentioned above,
and agreed with that obtained before HI was introduced into
the cell. This observation rules out any formation, storage and
slow degassing of I2 from the cold reactor walls, which would
enhance the rate of removal of OH above that expected from
reaction with H2O2 alone.
Under our experimental conditions, not only H2O2, but also
HI is dissociated by the 248 nm excimer laser pulse. Using the
absorption cross sections of HI in Figure 1, we calculate that at
the 10 individual measurements of the 294 K rate constant were
carried out with different samples of HI diluted in Ar. The
statistical errors (2σ) on each rate constant determination are
less than 6%, whereas the reproducibility is considerably worse,
-
11
3
with rate constant varying between (5.5 and 7.3) × 10
cm
-
1
s . A number of potential systematic errors in our experimental
procedure are discussed below.
The reaction between OH and I2 is very fast (k298 K ) 1.8 ×
1
3
-
10
3
-1 16
the high end of the HI concentration range ([HI] ) 5 × 10
1
0
cm s ) and the presence of I2 in our HI sample could
-3 11 -3
cm ) ≈ 2.5 × 10 H and I atoms cm are generated along
lead to an overestimation of the rate constant. The cold trap at
110 °C on the HI/Ar flow line, located a few centimeters
1
1
-3
with the ≈5 × 10 OH cm . At lower HI concentrations the
ratio of OH to H or I is much larger. A potential perturbation
of the OH kinetics arises if H atoms can react with H2O2 to
regenerate OH and also HO2:
-
before the mixing point with the main Ar and H2O2/Ar flows
should however have quantitatively removed any I2 present in
the sample. This could be confirmed by analyzing the HI/Ar
mixture by mass spectrometry. The ratio of signals at m/e )
+
+
+
H + H O f OH + HO
2
(15)
(16)
1
27 (I ) and 128 (HI ) to m/e ) 254 (I2 ) exceeded 200 despite
2
2
the potential for HI decomposition on metal components of the
mass spectrometer. In the kinetics experiments, if any HI had
decomposed to form I2 as shown below:
H + HO f 2 OH
2
However, the rate constant for H + H2O2 is very low (k15 ) 4
-
14
3
-1
2
HI f I + H2
(14)
× 10
cm s ) and results in an initial OH reformation rate
2
-
1
14
-3
of just 12 s for [H2O2] ) 3 × 10 cm compared to decay
-
1
and I2 (but not H2) is removed in the trap, then the loss of two
HI molecules results in the formation of one molecule of H2,
which is relatively unreactive to OH (k298 K ) 6.7 × 10 cm
rate of up to 4000 s . Alternatively, the reaction of H atoms
with O2 impurity (which may be formed in the thermal
decomposition of H2O2) can form HO2, which may react as
above to form OH. By analogy with OH + Br, the reaction of
I atoms with OH is expected to be too slow to perturb the OH
decays significantly. In addition to the above considerations, a
detailed analysis of the influence of these reactions, and others
resulting from the simultaneous formation of OH, I, and H in
roughly equal concentrations, was made by numerical simula-
-
15
3
-1 16
s ). The ensuing reduction in the HI-mixing ratio would result
in an underestimation of the rate constant. Similarly, the mixing
ratio of HI in the photolysis cell would be overestimated if HI
were lost in the cold trap, once again leading to an underestima-
tion of the rate coefficient. After a day’s experiments a small
amount of a pale brown deposit was observed in the cold trap,
which evaporated rather quickly when warmed to room tem-
perature. As a consequence of the method of preparation of HI,
a potential impurity in the HI is PH4I. However, this has a
negligible vapor pressure below -40 °C, and thus should be
removed by the distillation. A potentially more severe systematic
error in our measurements is the formation of I2 after the HI
sample has passed through the -110 °C cold trap, possibly
catalyzed by the presence of H2O2 and surfaces. Experiments
were therefore carried out, in which the contact time between
the H2O2/Ar flow and the HI/Ar flow was varied by changing
the point of mixing of the two gas flows. Note that for HNO3
and HI the same experiment in which the contact time was
shortened resulted in a considerable reduction in the formation
of aerosol (see experimental section). For H2O2/HI neither a
change in the scattered light intensity from the resonance lamp
nor a change in the pseudo-first-order decay rate of OH for a
1
8
tion of the reaction scheme outlined in Table 3. The results
of the simulation (Figure 4) show that in the “worst case”
scenario, high laser fluence and high [HI] (see figure caption
for details), the OH profile is perturbed only slightly from the
expected exponential decay. A sensitivity analysis showed that
in the absence of I2 impurity, the modeled decay rate was
reduced by ≈1.5% when compared to that expected on the basis
of reaction of OH with H2O2 and HI alone. The addition of a
1% I2 impurity is expected to increase the decay rate by ≈2.5%
over that obtained from reaction with HI and H2O2 alone. When
this I2 impurity was modeled with complete chemistry, the
increase in decay rate was modified to just +1%.
Thus, under our normal experimental conditions, we conclude
that none of the modeled secondary chemistry was able to
influence the OH decays significantly. To confirm this experi-
mentally, OH decays were measured in which the laser fluence

Reaction of OH with HI
J. Phys. Chem. A, Vol. 103, No. 15, 1999 2717
TABLE 3: Chemical Scheme Used in Numerical Simulations
results, even at high pressures of HI. Our ion chromatographic
spot checks on two samples indicated a deviation of ≈ -20%
and -25% from that expected. As the experimental procedure
was the same for the preparation of each HI storage bulb we
assume that any variation in HI is statistical, and therefore take
an average of the 10 determinations at 294 K to derive a rate
k(298 K,
1
00 Torr)
3
-1
reaction
[cm s
]
ref
H
2
O
2
+ hν f 2 OH
16 (cross sections)
this work (cross sections)
16
this work
HI + hν f H + I
-
-
-
-
12
11
10
10
-11
3
-1
OH + H
OH + HI f I + H
OH + HO f O + H
OH + I f HOI + I
OH + HOI f H O + OI
HO + HO f H + O
OH + OH f H
I + I f I
I + HO f HI + O
H + HI f H + I
H + H
I + H
H + HO
H + I f HI + I
O
2 2
f HO
2
+ H
2
O
1.7 × 10
6.0 × 10
1.1 × 10
1.8 × 10
constant of k4 ) (6.53 ( 0.44) × 10
cm s . Considering
2
O
the ≈25% deviation in IC measurement as a potential systematic
2
2
2
O
16
16
estimated
+2
-0.4
-11
3 -1
error we derive a final value of k4 ) 6.5
294 K.
× 10 cm s at
2
-
10
a
2
2 × 10
-
12
12
13
13
2
2
2
O
2
2
1.7 × 10
2.0 × 10
3.0 × 10
3.8 × 10
16
16
27
16
We now compare our 294 K rate constant to other determina-
-
-
-
O
2 2
10
tions. Takacs and Glass used the discharge flow method with
2
EPR detection of OH to measure the kinetics of the reaction at
298 K. As discussed by these authors, a low sensitivity to OH
in their experiment resulted in poor spatial resolution in the flow
tube, resulting in inaccuracies in the rate constant, which they
2
2
-
11
2
2 × 10
28, 29
30
estimated
-
14
2
O
2
f OH + H
2
O
4.0 × 10
-
16
b
2
O
2
f HI + HO
2
< 5 × 10
-
11
10
13
13
2
f OH + OH
7.2 × 10
6.2 × 10
2.0 × 10
4.0 × 10
31
29
16, note c
32
-11
3
-1
measured as (1.3 ( 0.5) × 10
cm s . As noted by Mac
-
-
-
2
1
1
Leod et al., the same experimental setup was used by Takacs
and Glass to measure a rate constant for OH + HBr which has
since been shown to be too low by a factor of 2. Another
discharge flow/EPR study of this reaction was undertaken by
H + O
H + OH f H
2
f HO
2
2
O
a
Estimated upper limit. b By comparison to Br + H
. c The main
. Initial concentrations of
OH, H atoms, and I atoms were calculated from the laser fluence and
the H and HI concentrations, respectively. The H concentration
2
O
2
2 2 2
source of O was the decomposition of H O
1
2
Lancar et al., who, with improved OH sensitivity, derived a
-
11
3
-1
rate constant of (3.3 ( 0.2) × 10
cm s . These authors
O
2 2
2 2
O
was calculated via the OH decay rate in the absence of HI (see text).
occasionally employed a cold trap at 250 K to remove any I2
impurity from their HI /He flow but noted that it had no effect
on the rate constant.
At the same time, and in the same research group, experiments
were carried out using the laser-photolysis resonance fluores-
1
1
cence technique (Mac Leod et al. ), which yielded a rate
-
11
3 -1
constant of (2.7 ( 0.2) × 10 cm s at 298 K. We note that
1
1
in the experiments of Mac Leod et al. the 248 nm photolysis
of ≈7 × 10 HNO3 cm was used to generate (2-4) × 10
1
4
-3
11
-
3
cm OH. As we discussed earlier, the mixing of HNO3 and
HI in our experiments under similar conditions resulted in
aerosol formation, as shown by enhanced scattered light from
the resonance lamp, and for higher HNO3 in total removal of
the HI from the gas phase. From Figure 2 of Mac Leod et al.11
it is apparent that the signal from OH (laser on) is only a factor
of 3 greater than the background signal when HNO3 and HI
flows were present, but the laser was disabled. As the
background signal in the absence of HI was not reported in
this work we cannot infer if the relatively high scattered light
signal is enhanced due to aerosol formation. In our experiment,
in the absence of aerosol, the ratio of OH signal to background
1
1
-3
signal was ≈20 for [OH]i ) 5 × 10 cm . Any loss of HI via
reaction with HNO3 would result in an underestimated rate
11
constant for OH + HI. Further, Mac Leod et al. draw attention
to an anomaly in their second-order plot of kfirst versus [HI].
The measured loss rate of OH in the absence of HI was 150 (
Figure 4. Calculated concentration profiles for OH, and other important
1
4
-3
13
species in the photolysis of H
2
O
2
(3 × 10 cm ), and HI (5 × 10
-
3
-2
-1
cm ). The 248 nm laser fluence (per pulse) was 10 mJ cm , T ) 294
K, the total pressure was 75 Torr Ar. The chemistry modeled in the
calculation is listed in Table 3. The solid lines are derived from chemical
modeling, the dotted line is the calculated, nonperturbed exponential
30 s , compared to an extrapolated intercept on the ordinate
-1
of 284 ( 42 s in the presence of HI, indicating some curvature
in the data, and possibly that the rate constant is underestimated.
The difference between the room-temperature rate constant
of Mac Leod et al. and the present study are thus probably
explained by the use of different OH precursors. Using the same
conditions as Mac Leod et al., we were unable to gather useful
data for the reaction of OH + HI. Explanations for the different
rate constant between the present result and the discharge flow
study of Lancar et al. are more difficult to find. We note
however, that the OH source used was the reaction of H-atoms
with NO2. We have shown that the mixing of HI and NO2 can
also be problematic and lead to aerosol formation (i.e., to loss
of HI). However, under our conditions, the mixing time for the
separate HI/Ar and NO2/Ar flows in the inlet to the photolysis
2 2
decay of OH due to reaction with HI and H O only.
was increased by up to 30 mJ cm-2 which resulted in an increase
by about a factor of 3-4 in the initial radical concentrations
(including H and I). Even under these conditions, no dependence
of reaction rates on the laser energy could be observed.
The preferred explanation for the scatter in the rate constant
at 294 K is a systematic error in the HI mixing ratio. The
accurate preparation of diluted samples of HI in Ar with HI
partial pressures of only 200 mTorr is difficult due to its affinity
for surfaces. As noted in the UV absorption experiments,
conditioned glass surfaces were necessary to get reproducible

2718 J. Phys. Chem. A, Vol. 103, No. 15, 1999
Campuzano-Jost and Crowley
Figure 6. Temperature dependence of the rate coefficient for OH +
HI. Error bars are 2σ. The vertical line at 294 K covers the total scatter
Figure 5. Room-temperature rate constant for OH + HX (this work
12
11
10
-11
-11
3
-1
at 294 K, Lancar, Mac Leod, and Takacs at 298 K) versus binding
energy of H-X, where X ) Cl, Br, and I. The OH + HCl and OH +
HBr data are taken from the most recent recommendation (JPL).16
in k(294 K) between 5.45 × 10
and 7.32 × 10
cm s . Also
12
11
shown are the data for OH + HI from Lancar, Mac Leod, and
10
Takacs.
cell was about 2 s, whereas the mixing time in the discharge
flow experiment was of the order of 0.1 s or less, and our
observations are not necessarily applicable. We also note that
the scheme for OH generation used by these authors produces
temperature dependence is more likely related to a temperature-
dependent preexponential factor. Due to the low bond energy
of H-I and its high polarizability there is not expected to be a
-n
significant activation barrier to reaction with OH, and the T
19
vibrationally excited OH, which is inefficiently quenched by
temperature dependence of the rate coefficient may be expected.
1
Torr of He used as buffer gas.20 It is very possible that
-(1.5 ( 0.5)
The present result of a T
temperature dependence
vibrational excitation in OH may manifest itself as a reduction
in the rate coefficient for reaction with HI (see discussion).
In Figure 5 we plot the room-temperature rate constants from
this work and those of Takacs and Glass, Mac Leod et al., and
Lancar et al. versus the bond energy of HI, and also data for
HBr and HCl. As can be seen the rate constant correlates
reasonably well with the bond energy of HX, indicating that
the reaction proceeds (mainly) via H atom abstraction.
for OH + HI may be compared to the reaction of OH with
HBr, which has been shown to have no significant activation
21
barrier between 249 and 416 K but a rate coefficient that
increases monotonically with decreasing temperature between
(-0.86 ( 0.1)
2
95 and 23 K is described by a T
temperature
22
dependence and which is decreased when OH (V ) 0) is
14
vibrationally excited, OH (V ) 1). For OH + HCl a positive
-1
16
activation barrier of 2.9 kJ mol is observed continuing the
trend of increasingly positive T dependencies for HI, HBr, and
HCl.
4
.2. Temperature-Dependent Rate Coefficients. Figure 6
displays the data at all temperatures. Only the average value
for 294 K is shown along with a vertical line that encompasses
the spread of results at this temperature. Despite the scatter in
the data the rate coefficient clearly displays a slight negative
temperature dependence. We emphasize again that due to the
normalization procedure, whereby each rate coefficient is
measured relative to one at room temperature, the slight increase
in rate coefficient with decreasing temperature is not due to an
experimental artifact associated with mixing ratio errors in HI.
Also, even without the normalization, the trend in rate coefficient
with temperature is negative.
We can also compare our results with the rapid reaction of
-10
3
-1 23,24
Cl atoms with HI (k295 ≈ 1.5 × 10
cm s )
which
displays a complex temperature dependence with the rate
coefficient decreasing with increasing temperature above ≈ 300
23
K and displaying the reverse behavior at t < 300 K. In
addition, the ratio of rate coefficient for Cl atoms with HI and
DI also showed a pronounced temperature dependence and an
increase in the translational energy of the Cl atom reduced the
rate coefficient. These results were interpreted in terms of a
complex reaction mechanism in which the Cl atoms is first
attracted to the I end of HI prior to a H rotation to give the HCl
The rate coefficient is given by
2
5
and I products. Vibrational excitation in OH may lead to
similar effects. Our present results do not enable us to draw
conclusions regarding the detailed mechanism of the reaction
between OH and HI, and we note that experiments using
deuterated analogues of HI and OH would be useful in this
respect.
k4 (246-353 K) ) 7.0-0.4 × 10-11(T/298)-1.5(0.5 cm s
+
1.9
3 -1
A negative temperature dependence is frequently observed when
a reaction proceeds via formation of an association complex,
which is more readily stabilized at low temperatures and higher
pressures. In such a case, neither a large rate coefficient for a
radical-molecule reaction nor the observed good correlation
between the room temperature rate constant and the bond energy
are expected. For the reaction of OH with HI, the negative
4.3. Atmospheric Lifetime of HI. In this section we examine
relative efficiencies of the various gas-phase loss processes for
HI in the atmosphere. Assuming an OH concentration of 5 ×
6
-3
10 cm , representative for the sunlit troposphere and lower

Reaction of OH with HI
J. Phys. Chem. A, Vol. 103, No. 15, 1999 2719
stratosphere at mid-latitudes, we calculate the loss rate of HI
(8) Carpenter, L. J.; Sturges, W. T.; Penkett, S. A.; Liss, P. S.; Alicke,
B.; Hebestreit, K.; Platt, U. J. Geophys. Res. 1998. Manuscript submitted
for publication.
-
4
-1
of ≈3 × 10 s , resulting in a lifetime of close to 1 h with
respect to removal by OH. To assess the contribution of
photolysis of HI to its overall loss in the atmosphere, J value
calculations2 were performed using the UV spectrum presented
in Figure 1 and assuming a quantum yield of unity. The J value
(9) Bauer, D.; Ingham, T.; Carl, S. A.; Moortgat, G. K.; Crowley, J.
N. J. Phys. Chem. 1998, 102, 2857.
(10) Takacs, G. A.; Glass, G. P. J. Phys. Chem. 1973, 77, 1948.
6
(
11) MacLeod, H.; Balestra, C.; Jourdian, J. L.; Laverdet, G.; LeBras,
G. Int. J. Chem. Kinet. 1990, 22, 1167.
12) Lancar, I. T.; Mellouki, A.; Poulet, G. Chem. Phys. Lett. 1991,
177, 554.
(13) Crowley, J. N.; Campuzano-Jost, P.; Moortgat, G. K. J. Phys. Chem.
996, 100, 3601.
14) Cannon, B. D.; Robertshaw, J. S.; Smith, I. W. M.; Williams, M.
D. Chem. Phys. Lett. 1984, 105.
15) Carl, S. A.; Crowley, J. N. J. Phys. Chem. 1998, 102, 8131.
-
5 -1
was found to be 1 × 10
s
at the surface, increasing to 2 ×
(
-
5
1
0
at an altitude of 10-20 km and zenith angles of 40°. This
is clearly too slow to compete with reaction with OH, which
represents the dominant loss process for HI throughout the
atmosphere.
1
(
(
5
. Conclusions
(16) 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. Chemical Kinetics and Photochemical data for use in stratospheric
modelling. JPL Publication 97-4; Jet Propulsion Laboratory: Pasadena,
Rate coefficients for the reaction of OH with HI were obtained
at 75 Torr and at temperatures between 246 and 253 K. The
rate coefficient displays a negative temperature dependence in
this range, k4 (246-353 K) ) 7.0
1997.
(17) Huebert, B. J.; Martin, R. M. J. Phys. Chem. 1968, 72, 3046.
+
1.9
0.4
-11
-1.5(0.5
× 10 (T/298)
-
(18) Curtis, A. R.; Sweetenham, W. P. FACSIMILE release H user’s
3
-1
cm s , and is a factor of ≈2 greater than previous determina-
manual. AERE report R11771; HMSO, London, 1987).
(19) Silver, J. A.; Dimpfl, W. L.; Brophy, J. H.; Kinsey, J. L. J. Chem.
Phys. 1976, 65, 1811.
6
tions at room temperature. At OH concentrations of ≈5 × 10
-
3
cm this expression results in a lifetime for HI of ≈1 h. As
part of this study the UV absorption cross sections of HI were
also determined which allowed the calculation of the lifetime
of atmospheric HI with respect to photodissociation as ≈18 h.
(20) Rensberger, K. J.; Jeffries, J. B.; Crosley, D. R. J. Chem. Phys.
1989, 90, 2174.
(21) Ravishankara, A. R.; Wine, P. H.; Langford, A. O. Chem. Phys.
Lett. 1979, 63, 479.
(
22) Sims, I. R.; Smith, I. W. M.; Clary, D. C.; Bocherel, P.; Rowe, B.
R. J. Chem. Phys. 1994, 101, 1748.
23) Mei, C.-C.; Moore, C. B. J. Chem. Phys. 1977, 67, 3936.
(24) Dolson, D. A.; Leone, S. R. J. Chem. Phys. 1982, 77, 4009.
Acknowledgment. We thank the European community for
financial support (Grant ENV4-CT95-0013, LEXIS), and Jochen
Landgraf for the J value calculations.
(
(25) Mei, C.-C.; Moore, C. B. J. Chem. Phys. 1979, 70, 1759.
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