Identification of BrONO as the Major Product in the Gas-Phase Reaction of Br with NO2

Article abstract of DOI:10.1021/jp993713g

Products of the gas-phase reaction of Br atoms with NO₂ have been quantitatively determined at temperatures between 215 and 300 K in an environmental chamber interfaced to an FT-IR spectrometer. The major product of the reaction (yield > 75%) was found to be the cis isomer of BrONO, which was identified and quantified by means of its N=O stretching fundamental at 1660 cm^-1; this represents the first gas-phase detection of this species. Although rapid thermal decomposition back to Br and NO₂ precludes its detection at room temperature (lifetime < 1 s), BrONO was detected at temperatures at and below 263 K. Isomerization of the BrONO to BrNO₂ was found to be an important fate of BrONO at low temperatures. The rate coefficient for this process was found to increase with decreasing pressure, indicative of a heterogeneous process. At 700 Torr, this isomerization rate was (0.013 ± 0.003) s^-1, independent of temperature over the range 218-243 K. Evidence was also obtained for rapid reactions between Br atoms and both BrONO and BrNO₂ (10^-10 > k > 10^-11 cm³ molecule^-1 s^-1).

Full text of DOI:10.1021/jp993713g

2048
J. Phys. Chem. A 2000, 104, 2048-2053
Identification of BrONO as the Major Product in the Gas-Phase Reaction of Br with NO2
John J. Orlando*
Atmospheric Chemistry DiVision, National Center for Atmospheric Research, Boulder, Colorado 80303
James B. Burkholder
Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80303
ReceiVed: October 18, 1999; In Final Form: January 11, 2000
Products of the gas-phase reaction of Br atoms with NO2 have been quantitatively determined at temperatures
between 215 and 300 K in an environmental chamber interfaced to an FT-IR spectrometer. The major product
of the reaction (yield > 75%) was found to be the cis isomer of BrONO, which was identified and quantified
by means of its NdO stretching fundamental at 1660 cm-1; this represents the first gas-phase detection of
this species. Although rapid thermal decomposition back to Br and NO2 precludes its detection at room
temperature (lifetime < 1 s), BrONO was detected at temperatures at and below 263 K. Isomerization of the
BrONO to BrNO2 was found to be an important fate of BrONO at low temperatures. The rate coefficient for
this process was found to increase with decreasing pressure, indicative of a heterogeneous process. At 700
Torr, this isomerization rate was (0.013 ( 0.003) s-1, independent of temperature over the range 218-243
K. Evidence was also obtained for rapid reactions between Br atoms and both BrONO and BrNO2 (10-10
>
k > 10-11 cm3 molecule-1 s-1).
Introduction
and stratosphere.26
Termolecular reactions between halogen atoms and NO2 have
received a great deal of attention,1-18 both out of fundamental
interest and because of their potential importance in the
atmosphere. The reaction of Br atoms with NO2 has been studied
in cryogenic matrixes1,2 and in the gas phase.3-5 By analogy to
the reactions of other halogens with NO2, the products of the
reaction are expected to be BrONO (cis and/or trans isomers)
and/or BrNO2:
N2O5(g) + HBr(s) f BrNO2(g) + HNO3(s)
(3)
Computational studies21 have shown that BrNO2 is more
stable (Br-N bond strength at 298 K of 22.5 kcal/mol) than
either the cis- or trans-BrONO isomer (Br-O bond strengths
at 298 K of 16.1 and 12.2 kcal/mol, respectively), but that cis-
BrONO is sufficiently thermally stable to exist in the atmo-
sphere. Furthermore, the thermal lifetime of BrNO2 has been
shown5,23 to be of order 1 h at room temperature, considerably
longer than the predicted27 lifetime of the Br + NO2 reaction
product observed (indirectly) by Kreutter et al.4 Thus, it is now
believed5,23 that, although BrONO has yet to be observed in
the gas phase, it is the major product of reaction 1 and has
eluded detection largely due to its thermal instability (lifetime
likely of order 1 s or less near room temperature).27 By analogy
to the ClONO/ClNO2 system,9,10 the isomerization of BrONO
to BrNO2, reaction 4, may also play a role in the inability to
detect this species:
Br + NO2 + M f BrONO + M
f BrNO2 + M
(1a)
(1b)
In the study of Tevault,1 BrNO2 was the major product
identified from the co-condensation of Br atoms and NO2 in an
Ar matrix, while a similar study by Feuerhahn et al.2 appears
to have led to the production of both BrNO2 and trans-BrONO.
Early gas-phase studies3,4 focused on the rate coefficient of the
Br + NO2 reaction, and reaction products were not determined.
However, Kreutter et al.4 observed that the reaction product
(believed at the time to be BrNO2, but now thought to be
BrONO, see below) was thermally unstable, and that the reverse
of reaction 1 occurred with a time constant of order a few
milliseconds near 400 K.
BrONO + M f BrNO2 + M
(4)
In this work, products of the reaction of Br with NO2 were
studied by infrared spectroscopy over the range 215-300 K in
a static reaction chamber. BrONO was indeed observed at
temperatures below ambient, and is shown to be the major
product of reaction 1. In addition, rate coefficients for its
isomerization and thermal decomposition and for the reaction
of Br atoms wtih both BrONO and BrNO2 are estimated over
the temperature range studied.
Further interest5,19-25 in the thermodynamics, spectroscopy,
and chemistry of BrNO2 has been stimulated, at least in part,
by the observation that this species, in addition to being a
product of reaction 1, is also a product of some heterogeneous
reactions of importance in the Earth’s troposphere19
N2O5(g) + NaBr(s) f BrNO2(g) + NaNO3(s)
(2)
Experimental Section
Experiments were conducted in a temperature-regulated
stainless steel reaction chamber, described in detail previ-
*Corresponding author.
10.1021/jp993713g CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/09/2000

BrONO from the Gas-Phase Reaction of Br with NO2
J. Phys. Chem. A, Vol. 104, No. 10, 2000 2049
ously.28,29 The chamber is 2 m long, with a volume of 47 L,
and is interfaced via a set of Hanst-type optics to a Bomem
DA3.01 Fourier Transform Spectrometer. The analyzing path
length for these measurements was 32.6 m, and spectra were
recorded over the range 800-3900 cm-1 with a resolution of 1
cm-1. Spectra were obtained from the co-addition of 8-50
interferograms and required 10-60 s total acquisition time. The
temperature of the cell was controlled by flowing chilled ethanol
from a circulating bath through a jacket surrounding the cell.
Experiments involved the cw photolysis of static mixtures
of Br2 (typically about 2.5 × 1014 molecule cm-3, but varied
from (1-10) × 1014 molecule cm-3) and NO2 (5-70 × 1014
molecule cm-3) in N2 (700 Torr). At low temperatures, N2O4
was also present in some experiments ([N2O4] e 7 × 1013
molecule cm-3); its presence had no effect on the measurements.
Br2 and NO2 were swept into the photolysis chamber from
calibrated volumes; pressure measurements in the calibrated
volumes were used to determine the initial [Br2] and [NO2].
The photolysis light source was a cw Xe arc lamp, equipped
with a water-cooled filter to provide radiation at λ > 410 nm
such that Br2 was efficiently photolyzed (with a first-order rate
coefficient of (2.2 ( 0.4) × 10-3 s-1), but photolysis of NO2
did not occur to any measurable extent (photolysis rate
coefficient <2 × 10-5 s-1). The Br2 photolysis rate was
measured periodically by monitoring the decay of HCHO in a
Br2/HCHO/O2/N2 photolysis experiment.
The Br2 used in these experiments was purchased from
Aldrich, and purified by several freeze-pump-thaw cycles
before use. NO2 was synthesized from the addition of excess
O2 to NO (Linde, UHP), followed by removal of the O2 by
pumping at liquid nitrogen temperature. N2 was obtained from
the boil-off from a liquid N2 dewar (U.S. Welding).
Figure 1. (a) Spectrum recorded following the photolysis of a mixture
of Br2 (2.6 × 1014 molecule cm-3), NO2 (9.2 × 1013 molecule cm-3),
and N2 (700 Torr) at 228 K. The NO2 absorption centered at 1616
cm-1 has been subtracted for clarity. (b) Spectrum recorded following
the dark decay of the gas mixture shown in (a). (c) Residual spectrum
obtained by subtraction of the BrNO2 features from the spectrum of
(a). This residual spectrum is assigned to cis-BrONO, see text for details.
Results and Discussion
Experimental Results. Experiments involved monitoring the
temporal profile of NO2 and reaction products both during and
following the photolysis of mixtures of Br2 (1-10 × 1014
molecule cm-3) and NO2 (0.5-10 × 1014 molecule cm-3) in
700 Torr N2. Initial experiments were conducted at relatively
low temperature (228 K) and high total pressure (1 atm), since
it was thought that these conditions would be optimum for
detection of BrONO. Experiments were also conducted at 218,
248, 263, and 298 K. The results discussed below apply to the
228 K experiments unless otherwise specified.
state value, which was maintained for many minutes. A product
spectrum obtained at 228 K from the photolysis of Br2 (2.6 ×
1014 molecule cm-3) in the presence of NO2 (9.2 × 1013
molecule cm-3) in 700 Torr N2, after steady-state is achieved
is shown in Figure 1a (the strong NO2 absorption centered at
1616 cm-1 has been subtracted for clarity). Absorptions of
BrNO2 are expected at 1292-1294 and 1667 cm-1 19-23
and
,
At 228 K, thermal dissociation of both BrONO and BrNO2
clearly this species is present in the product spectrum, as
expected.5 However, close inspection of the feature around 1665
cm-1 shows it to be slightly different in location and shape from
previously published BrNO2 spectra, and the possibility of the
presence of another absorber must be considered. The most
likely possibility is the cis isomer of BrONO, which has not
previously been observed in the gas phase, but for which an
NdO stretching frequency of 1664 cm-1 has been calculated.21
This species has also recently been observed in a matrix-isolation
study.25
BrONO + M f Br + NO2 + M
BrNO2 + M f Br + NO2 + M
(-1a)
(-1b)
should be negligibly slow,4,5,27 and the chemistry should be
controlled by the formation of BrONO and BrNO2 (reaction
1), the possible isomerization of BrONO to BrNO2 (reaction
4), and by the destruction of the BrONO and BrNO2 species
by reaction with Br atoms:3,5
A possible means of confirming the presence of cis-BrONO
is via the observation of its isomerization to the more stable21
BrNO2 (reaction 4), a process which is expected on the basis
of similar observations in the ClONO/ClNO2 system.9,10 Thus,
following the acquisition of spectra such as that shown in Figure
1a, the photolysis lamp was blocked and the behavior of the
gas mixture was monitored for a few minutes. The spectrum
shown in Figure 1b, obtained approximately 5 min after
cessation of photolysis of the same gas mixture shown in Figure
1a, shows an increase in absorption in the 1294 cm-1 band and
Br + BrNO2 f Br2 + NO2
Br + BrONO f Br2 + NO2
(5)
(6)
Photolysis of the Br2/NO2/N2 mixtures resulted in the
appearance of two new absorption features, one centered at 1294
cm-1 and the other occurring in the region around 1665 cm-1
.
The intensity of these absorption features increased over the
first minute or two of photolysis and then reached a steady-

2050 J. Phys. Chem. A, Vol. 104, No. 10, 2000
Orlando and Burkholder
a change in shape and intensity of the band(s) near 1665 cm-1
.
This spectrum now corresponds identically with that of
BrNO2,19-23 and clearly indicates that (at least) two species were
present initially. Subtraction of the contribution of BrNO2 from
the spectrum of Figure 1a (using the spectrum of Figure 1b,
from an examination of the 1294 cm-1 band) yields the spectrum
shown in Figure 1c. Based on a reasonable analysis of the
chemistry occurring in the system and the close correspondence
of the observed band center with that predicted,21 we assign
this residual absorption feature, centered at 1660 cm-1, to cis-
BrONO. Further evidence for the assignment of the spectrum
of Figure 1c to cis-BrONO is obtained from the work of
Scheffler et al.,22,25 who photolyzed BrNO2 in a matrix and
reported the conversion of the BrNO2 to BrONO. Although only
trans-BrONO was thought to be present initially,22 further
work25 proved the existence of cis-BrONO as well, and the
existence of the ν1 fundamental near 1650 cm-1 was confirmed.
There is no evidence for the presence of the trans-BrONO
species (NdO stretching frequency 1725 cm-1) in any of our
gas-phase spectra. Lee21 has calculated a difference in energy
between the cis and trans isomers of 3.7 kcal/mol, meaning that
the cis isomer would be favored by a factor of 500 at
equilibrium. Thus, although the trans isomer may also be formed
initially in reaction 1, it seems reasonable to assume that rapid
interconversion of the two isomers is possible, thus explaining
the absence of the trans isomer in the observed spectra. Similar
energy differences have been calculated for the cis- and trans-
ClONO isomers,12 and only the cis species is observed in this
case as well.9,10
The data of Figure 1 can be used to estimate the IR absorption
cross sections for both BrNO2 and cis-BrONO, by means of
mass balance arguments (i.e., by assuming that the loss of NO2
in a given Br2/NO2/N2 photolysis experiment is always equal
to the sum of the BrNO2 and BrONO present), and by assuming
a 1:1 conversion of BrONO to BrNO2 following the cessation
of photolysis. The determination of the absorption cross sections
for BrNO2 can be made by equating the final BrNO2 concentra-
tion after the dark decay period (i.e., after total conversion of
BrONO to BrNO2) with the change in the NO2 concentration
from the beginning to the end of the experiment. Multiple
experiments, conducted over a range of Br2 and NO2 concentra-
tions at 228 K, yielded absorption cross sections of 2.0, 3.0,
and 2.3 × 10-18 cm2 molecule-1 for the peaks of the P, Q, and
R branches near 1294 cm-1, and 1.5 and 1.7 × 10-18 cm2
molecule-1 for the peaks of the P and R branches near 1667
cm-1, with uncertainties (including precision and possible
systematic uncertainties) for all values of (20%. Integrated band
intensities were also obtained, and were found to be (4.3 (
0.6) × 10-17 cm2 molecule-1 cm-1 and (3.7 ( 0.5) × 10-17
cm2 molecule-1 cm-1 for the 1296 and 1667 cm-1 bands,
respectively.
Figure 2. Time profile of the concentrations of NO2 (solid circles),
BrNO2 (inverted triangles), and BrONO (open circles) following the
photolysis (0-380 s) and subsequent dark decay (380-620 s) of a
mixture of Br2 (2.6 × 1014 molecule cm-3), NO2 (9.2 × 1013 molecule
cm-3), and N2 (700 Torr) at 228 K. Solid lines represent model
calculations, using the rate coefficients given in the text.
Absorption cross sections for BrONO were then obtained
from an analysis of the dark decay period in the 228 K
experiments, in which the isomerization of the BrONO to BrNO2
occurred. Assuming 1:1 interconversion, an absorption cross
section of (1.7 ( 0.6) × 10-18 cm2 molecule-1 at the peak of
the P and R branches near 1660 cm-1 is obtained for BrONO
with an integrated band intensity of (3.7 ( 1.3) × 10-17 cm2
molecule-1. Strictly, these values should be treated as upper
limits, since it is possible that the BrONO is not stoichiomet-
rically converted to BrNO2. However, no significant change in
[NO2] was noted during the dark decay period in almost all
experiments carried out at 228 K and no other species were
detected, indicating the likelihood of near-stoichiometric conver-
sion. Furthermore, the agreement between our BrNO2 absorption
cross sections and previous values also argues against significant
loss of BrONO to other than BrNO2. Thermal dissociation can
be ruled out as a loss process for BrONO at 228 K from a
consideration of the available data on its thermal properties.4,27
Although no previous observations are available for comparison,
the cross section value reported here for the BrONO absorption
feature is broadly consistent with the findings of Lee,21 who
calculated the integrated band strength of the 1660 cm-1 band
in cis-BrONO to be 4.2 × 10-17 cm2 molecule-1. In addition,
the corresponding bands in ClNO2 and ClONO appear to be of
very similar intensities to each other,9,10 and to their bromine-
containing counterparts.
With the determination of the absorption cross sections for
both the BrNO2 and BrONO species, full concentration versus
time profiles for these species during the light and dark phase
of an experiment can be obtained, as shown in Figure 2. A
number of conclusions can be drawn from these data. Most
importantly, it is apparent that BrONO is the major product of
reaction 1, as can be seen from the product yields observed at
the onset of photolysis. In the first data point of Figure 2, the
concentration of BrONO exceeds that of BrNO2 by a factor of
2. Because of the rapid reaction of both species with Br atoms
(with k6 probably greater than k5, see later) and because of the
isomerization of BrONO to BrNO2, our experiments do not
provide sufficient time resolution to directly measure the initial
[BrONO]/[BrNO2] ratio. Additional experiments were con-
ducted, at temperatures of 218, 228, and 248 K, in which spectra
Gas-phase IR absorption cross sections for BrNO2 have
previously been reported by Frenzel et al.20 and Scheffler et
al.,22 who generated BrNO2 from the reaction of gas-phase
ClNO2 with Br- ions. Our measured peak absorption cross
sections appear to be about 10% higher than the room-
temperature absorption cross sections reported by Frenzel et al.,20
and about 20% higher than the low resolution spectrum of
Scheffler et al.22 The higher values obtained in our work are
likely due, at least in part, to the lower temperature employed
herein compared with the previous work. Integrated absorption
intensities are not reported in these previous studies for
comparison. The integrated absorption intensities reported for
both absorption bands in the ab initio study of Lee21 are 50%
higher than the values obtained in our work.

BrONO from the Gas-Phase Reaction of Br with NO2
J. Phys. Chem. A, Vol. 104, No. 10, 2000 2051
CH2O/O2/N2 mixtures), and reactions 1, 4, 5, and 6. The total
k1 was obtained from previously published data,3,4 and the value
of k4 was fixed to the 700 Torr value of 0.013 s-1 determined
above. The branching ratio for k1, and the values for k5 and k6
were then varied to achieve the best fit to a number of
experiments obtained over a range of temperatures and [NO2]o.
Averages of these fitted rate coefficient data are as follows:
k1a/k1 ) 0.85 ( 0.08, k6 ) (7 ( 3) × 10-11 cm3 molecule-1
s-1, and k5 ) (3 ( 2) × 10-11 cm3 molecule-1 s-1. A fit to a
single experiment using these rate coefficients is shown in Figure
2. Previous estimates3,5 of k5 and k6 are in reasonable accord
with our results. In the flow tube study of Mellouki et al.3, the
product of reaction 1, probably BrONO, was found to react with
Br with a rate coefficient of order 10-11-10-10, while Broske
and Zabel used a value for k5 of 2.4 × 10-11 cm3 molecule-1
s-1 to model their BrNO2 temporal profiles in room-temperature
Br2/NO2 static photolysis experiments. It should be stressed that
the rate coefficients and branching values determined above are
estimates only, since their values are somewhat dependent on
each other. More accurate values for the branching ratio to
reaction 1 will require experiments conducted with a better time
resolution than is possible with the FT-IR system used here.
Direct measurements of k5 would also be useful in better
quantifying the rate parameters describing this chemical system.
Figure 3. Sample plots of ln(BrONO absorbance) vs time in the dark
under a variety of conditions, to obtain the first-order rate of BrONO
isomerization to BrNO2. Filled triangles: 216 K, 700 Torr total pressure,
[NO2]o ) 3.6 × 1014 molecule cm-3, k4 ) 0.012 s-1; open circles:
228 K, 700 Torr total pressure, [NO2]o ) 1.2 × 1014 molecule cm-3
,
k4 ) 0.016 s -1; filled circles: 228 K, 700 Torr total pressure, [NO2]o
) 7.5 × 1014 molecule cm-3, k4 ) 0.014 s-1
.
were acquired as early as possible (over the first 10 s) following
the start of the photolysis. In all cases, [BrONO] exceeded
[BrNO2], usually by a factor of 3 or 4, very similar to
observations made in studies of the reaction of Cl with NO2.9,10,30
Thus, BrONO must account for at least 75% of the reaction
product at these temperatures. However, modeling studies (using
the measured Br2 photolysis rate and estimated values for k4,
k5, and k6, see details below) indicate that some BrNO2 (probably
about 5-20%) is very likely formed directly in reaction 1, since
the isomerization of BrONO to BrNO2 is too slow to account
for all of the BrNO2 observed early in the photolysis period.
This point will be discussed in more detail later.
Data such as those shown in Figure 2 can also be used to
determine approximate rate coefficients for reactions 4-6.
Values for k4 were determined by monitoring the conversion
of BrONO to BrNO2 during the dark period over a range of
conditions (see Figure 3). At 228 K, the isomerization rate
coefficient, k4, was found to increase with decreasing pressure,
from a value of 0.011 ( 0.003 s-1 at 1000 Torr to 0.023 (
0.003 s-1 at 150 Torr. This inverse pressure dependence provides
strong evidence for the influence of the cell walls in the
isomerization process. The isomerization rate was found to be
independent of the NO2 concentration (varied from 6 × 1013 to
60 × 1013 molecule cm-3), and N2O4 concentration (from < 1
× 1013 to 7 × 1013 molecule cm-3). Isomerization rates at 218
K were similar to those found at 228 K. Studies at higher
temperature do not provide clean measurements of k4 due to
the increased importance of thermal dissociation of BrONO
(reaction -1a), but no large change in the isomerization rate
coefficient was evident at 248 K. Isomerization of ClONO to
ClNO2 has also been reported to be influenced by heterogeneous
processes.9,10
At higher temperatures (248, 263, and 298 K), the steady-
state [BrONO] was significantly reduced compared to lower
temperature measurements under similar conditions, and, in fact,
no BrONO was observed at all at 298 K. This observation is
very likely the result of the increasing importance of thermal
decomposition (reaction -1a) of the BrONO with increasing
temperature. Modeling studies (with rate parameters fixed at
the central values reported above) indicated that the thermal
decomposition of BrONO was occurring with a rate coefficient
k-1a ≈ 0.02 s-1 at 248 K, k-1a ≈ 0.1 s-1 at 263 K, and k-1a
g
2 s-1 at 298 K. Again, these k-1a values should be considered
only as estimates, given that they depend on the actual values
of k5, k6, and k1a/k1b, which are quite uncertain and could vary
with temperature, but are quite consistent with values obtained
from an extrapolation27 of the data of Kreutter et al.4
The lack of the observation of BrONO at room temperature
is consistent with a previous study5 of this reaction system.
However, the observation of BrNO2 in our room temperature
experiments and in those conducted previously5 provides the
strongest evidence for its direct production (albeit in low yield)
from reaction 1. With isomerization of BrONO considerably
slower than its thermal decomposition (by about a factor of 100,
based on the Kreutter et al.4 thermal decomposition data), BrNO2
formation via this process is quite inefficient and, without direct
formation of BrNO2 from reaction 1, steady-state levels of
BrNO2 would be smaller than observed. We estimate that at
least a 6% branch to BrNO2 formation is required to explain
the observed BrNO2 levels, with a most likely value of about
15%. This point was originally made by Broske and Zabel,5
who used the rate of approach to steady-state in the photolysis
of Br2/NO2 mixtures at 298 K to estimate k1a/k1b. With
reasonable assumptions about the values for k5 and k6 and a
knowledge of their Br2 photolysis rate, they showed that the
approach to steady-state was about 10 times slower than
expected, based on measured values3,4 of the total k1. They then
reasoned that BrONO thermal decomposition was sufficiently
rapid that they were only observing k1b, which then must account
for about 10% of the total reaction.
Rate coefficients for reactions 5 and 6 could then be estimated
by examining the steady-state levels of BrNO2 and BrONO,
using a box model (run with the Acuchem software package31)
to simulate the reaction system. For the low-temperature
experiments (T ) 218 and 228 K), thermal decomposition of
BrONO and BrNO2, reactions (-1a) and (-1b), are both
sufficiently slow to be ignored. Thus, the model consisted of
only the photolysis of Br2 (k ) 2.2 × 10-3 s-1, measured by
monitoring the disappearance of CH2O in the photolysis of Br2/
Reevaluation of Thermodynamic and Theoretical Aspects
of k1. Based on the evidence available at that time, Kreutter et

2052 J. Phys. Chem. A, Vol. 104, No. 10, 2000
Orlando and Burkholder
al.4 assumed that BrNO2 was the major product of reaction 1;
this fact was then used in both a “third law” analysis of the
equilibrium constant for reactions 1 and -1 and in RRKM-
based calculations of the strong-collision rate coefficient for
reaction 1, koSC. With our present knowledge that BrONO is
the major product of reaction 1, and with the availability of
high-level ab initio thermochemical and spectroscopic data for
BrNO2 and the cis and trans isomers of BrONO,21 we thought
it worthwhile to re-consider the calculations of Kreutter et al.4
Kreutter et al.4 conducted both a “second law” and “third
law” analysis of their data on the equilibrium system involving
reactions 1 and -1, and found that the derived ∆H and ∆S
values were only in fair agreement. With the assumption that
cis-BrONO is the major product of reaction 1, and using a value
for S(BrONO) calculated using the spectroscopic data of Lee,21
a reevaluation of the third law calculation of Kreutter et al. leads
to values of ∆S(401 K) ) -30.5 cal/mole/K and ∆H(401 K)
) -20.07 kcal/mol, in somewhat better agreement with their
original second law data. Averaging the original second law
values with the revised third law data gives ∆S(401 K) ) -28.5
cal/mol/K and ∆H(401 K) ) -19.35 kcal/mol. In fact, because
the calculated entropies for BrNO2 and cis-BrONO are quite
similar using current21 spectroscopic and structural information,
the third law values are not strongly dependent on the products
of reaction 1.
events where Br chemistry is known to play a role36), the
BrONO loss will be controlled either by its isomerization to
BrNO2 (if this process occurs in the gas phase) or by photolysis.
Preliminary results37 suggest a photolysis time constant of <30
s for BrONO. The enhanced thermal stability of BrONO means
that the effective rate coefficient for reaction 1 will be equal to
the measured4 values at these lower temperatures. The dominant
atmospheric loss process for BrNO2 under most atmospheric
conditions is likely photolysis (lifetime ≈ 5 min for typical
tropospheric actinic fluxes22). However, reaction with Br atoms
could also be of importance during surface O3 depletion events,
where Br atom levels of order 107 molecule cm-3 have been
inferred.38 In fact, reactions 1, 4, and 5 could provide a
mechanism for Br atom loss during these events.
Conclusions
The species BrONO has been detected for the first time in
the gas phase, and has been shown to be the major product
(yield g 75%) obtained from the reaction of Br with NO2 over
the temperature range studied (218-300 K). Only the cis isomer
of BrONO is observed (as characterized by the observation of
its NdO stretching fundamental at 1660 cm-1), consistent with
the relative stability of the cis and trans isomers.21 The loss of
BrONO at low temperatures is controlled by its isomerization
to BrNO2, a reaction which occurs (at least in part) at the cell
walls. Evidence for rapid reactions of both BrONO and BrNO2
with Br atoms (10-10 > k > 10-11 cm3 molecule-1 s-1) has
also been presented.
Finally, Kreutter et al.4 calculated the low-pressure limiting
value for reaction 1 in the “strong collision” limit, koSC, using
methods developed by Troe.32-34 Typically, ko exceeds
SC
measured values of ko in N2 by about a factor of 3.34 Kreutter
et al. showed that the calculated koSC for reaction 1, 2.0 × 10-31
cm6 molecule-2 s-1, was in fact considerably smaller than the
measured value, 4.6 × 10-31 cm6 molecule-2 s-1, and suggested
the possibility of BrONO formation or the involvement of
excited states of BrNO2 in the reaction. We have recalculated
Acknowledgment. The National Center for Atmospheric
Research is operated by the University Corporation for Atmo-
spheric Research, under the sponsorship of the National Science
Foundation. This work was partially supported by the NASA
Upper Atmospheric Research Program, through grants to both
NCAR and NOAA. The authors are indebted to Dr. Charles E.
Miller (Haverford College) for helpful conversations and for
the communication of unpublished data. Thanks are also due
to Geoffrey S. Tyndall for helpful discussions and to Lee
Mauldin and Barry Lefer for carefully reading of the manuscript.
SC
the ko for reaction 1, including the formation of BrNO2 and
cis- and trans-BrONO. For these calculations, the Lennard-
Jones parameters of Kreutter et al. were used in conjunction
with vibrational frequencies, rotational constants, and critical
SC
energies from Lee.21 Values of ko for BrNO2, cis-BrONO,
and trans-BrONO (all in units of 10-31 cm6 molecule-2 s-1) of
SC
References and Notes
2.1, 1.3, and 0.6 are obtained, which gives total ko of 4.0 ×
10-31 cm6 molecule-2 s-1, still below the measured value of
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4.6 × 10-31 cm6 molecule-2 s-1 and well below the expected
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≈
SC
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Atmospheric Significance. The BrONO and BrNO2 species
likely play only a minor role in atmospheric chemistry, since
under most conditions they will photolyze rapidly,22 and thus
act as only temporary reservoirs for bromine. Nonetheless, it is
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At temperatures near 300 K (for example, in the low- to mid-
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decomposition (lifetime < 1 s).4,27 In fact, because of the
instability of BrONO at these temperatures and because it is
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for reaction 1 will be reduced by a factor of 4 or more from the
measured4 and currently recommended35 value. In colder regions
(in the lower stratosphere, or in polar surface ozone depletion
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