Rate Coefficients for the Thermal Decomposition of BrONO2 and the Heat of Formation of BrONO2

Article abstract of DOI:10.1021/jp9620274

Rate coefficients (k_-7) for the thermal decomposition of bromine nitrate, BrONO2 + M -> BrO + NO2 + M, have been obtained at temperatures between 320 and 340 K and pressures between 100 and 1000 Torr.These data are combined with recommended values for the reverse reaction to obtain an equilibrium constant for the reaction pair, K_P,7 = 5.44E-9 exp(14192/T) atm^-1, and a heat of reaction for the thermal dissociation of 28.2 +/- 1.5 kcal/mol at 298 K.This reaction enthalpy is used in conjunction with literature data to arrive at a consistent set of ΔH⁰_f(298 K) data for BrONO2 (10.1 +/- 2.0 kcal/mol), BrO (30.4 +/- 2.0 kcal/mol), HOBr (-14.1 +/- 2.0 kcal/mol), and Br2O (27.3 +/- 2.0 kcal/mol).Additional measurements were made to determine the rate coefficient for Br atom reaction with BrONO2 (k₁₁) relative to the rate coefficient for its reaction with CH3CHO (k₁₂) at 298 K: k₁₁/k₁₂ = 12.5 +/- 0.6.This relative rate measurement yields a rate coefficient of (4.9 +/- 1.5)E-11 cm³ molecule^-1 s^-1 for k₁₁, using the currently recommended value for k₁₂.Approximate rate constant for reaction of NO (reaction 17) and BrNO (reaction 19) with BrONO2 were also obtained: k₁₇ = 3E-19 cm³ molecule^-1 s^-1, k₁₉ > 1E-16 cm³ molecule^-1 s^-1.

Full text of DOI:10.1021/jp9620274

19398
J. Phys. Chem. 1996, 100, 19398-19405
Rate Coefficients for the Thermal Decomposition of BrONO2 and the Heat of Formation of
BrONO2
John J. Orlando* and Geoffrey S. Tyndall
Atmospheric Chemistry DiVision, National Center for Atmospheric Research, Boulder, Colorado 80303
ReceiVed: July 8, 1996; In Final Form: October 4, 1996X
Rate coefficients (k-7) for the thermal decomposition of bromine nitrate, BrONO2 + M f BrO + NO2 + M,
have been obtained at temperatures between 320 and 340 K and pressures between 100 and 1000 Torr. These
data are combined with recommended values for the reverse reaction to obtain an equilibrium constant for
the reaction pair, KP,7 ) 5.44 × 10-9 exp(14192/T) atm-1, and a heat of reaction for the thermal dissociation
of 28.2 ( 1.5 kcal/mol at 298 K. This reaction enthalpy is used in conjunction with literature data to arrive
at a consistent set of ∆Hf°(298 K) data for BrONO2 (10.1 ( 2.0 kcal/mol), BrO (30.4 ( 2.0 kcal/mol), HOBr
(-14.1 ( 2.0 kcal/mol), and Br2O (27.3 ( 2.0 kcal/mol). Additional measurements were made to determine
the rate coefficient for Br atom reaction with BrONO2 (k11) relative to the rate coefficient for its reaction with
CH3CHO (k12) at 298 K: k11/k12 ) 12.5 ( 0.6. This relative rate measurement yields a rate coefficient of
(4.9 ( 1.5) × 10-11 cm3 molecule-1 s-1 for k11, using the currently recommended value for k12. Approximate
rate constants for reaction of NO (reaction 17) and BrNO (reaction 19) with BrONO2 were also obtained: k17
) 3 × 10-19 cm3 molecule-1 s-1, k19 > 1 × 10-16 cm3 molecule-1 s-1.
Introduction
the inorganic bromine budget in the lower stratosphere. Rate
coefficients for its formation via reaction 7 have been obtained
over a range of temperatures and pressures,6-8 and the currently
recommended values9 are obtained from the analysis of Thorn
et al.8 Stratospheric loss of BrONO2 is expected to occur largely
via photolysis10,11 during the day, although the products of its
photochemistry are not known. Heterogeneous loss of BrONO2
on stratospheric aerosol is also rapid and may contribute to its
loss during conditions of low sun.12 While these data allow
reasonably accurate determinations of its contribution to strato-
spheric chemistry, our understanding of the basic thermochem-
istry and reactivity of BrONO2 is lacking.
The role of bromine in the destruction of ozone in the earth’s
stratosphere is now well characterized.1-5 Free bromine atoms
are released from the destruction of source compounds such as
CH3Br and the halons and participate in catalytic ozone
depleting cycles such as the following:
Br + O3 f BrO + O2
OH + O3 f HO2 + O2
HO2 + BrO f HOBr + O2
(1)
(2)
(3)
(4)
To the best of our knowledge, the BrO-NO2 bond strength
has not yet been experimentally determined. Early estimates13
were based on the assumption that the BrO-NO2 and ClO-
NO2 bond strengths were equal. Current recommendations9 give
a ∆Hr°(298 K) for thermal decomposition of ClONO2 of 26.8
kcal/mol, based largely on the work of Anderson and Fahey.14
However, the low-pressure limiting rate coefficient, k0, for
BrONO2 formation is significantly larger than for the corre-
sponding ClONO2 formation reaction,8,9 possibly indicating a
stronger bond in BrONO2 than in ClONO2. Also, measured
values of k0 are only slightly less than theoretical estimates7,8,13
of the strong-collision k0 for BrONO2 formation (and to a lesser
extent for ClONO2 formation), implying an unrealistically high
collision efficiency. This inconsistency could be rationalized
either by again assuming a stronger BrO-NO2 bond than is
the case for ClO-NO2 or by invoking the formation of other
isomers in the BrO + NO2 recombination reaction. More
recently, ab initio calculations15 have been performed which
yield a BrO-NO2 bond strength (35 kcal/mol) that is consider-
ably larger than the accepted value for ClO-NO2. While such
a strong bond would explain the discrepancy between measured
and calculated k0 values, the ab initio calculations significantly
overestimate the ClO-NO2 bond strength (by about 6 kcal/
mol), and though the trend obtained in the calculations of Rayez
et al.15 is indicative of a stronger BrO-NO2 bond, it is unlikely
to be as strong as the ab initio value.
HOBr + hν f OH + Br
net: 2O3 f 3O2
Br + O3 f BrO + O2
Cl + O3 f ClO + O2
BrO + ClO f Br + Cl + O2
net: 2O3 f 3O2
(1)
(5)
(6)
Br + O3 f BrO + O2
BrO + NO2 f BrONO2
BrONO2 + hν f Br + NO3
NO3 + hν f NO + O2
NO + O3 f NO2 + O2
net: 2O3 f 3O2
(1)
(7)
(8)
(9)
(10)
Bromine nitrate is thought to be an important component of
X Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)02027-8 CCC: $12.00 © 1996 American Chemical Society

Thermal Decomposition of BrONO2
J. Phys. Chem., Vol. 100, No. 50, 1996 19399
In this paper, we report rate coefficients for thermal decom-
position of BrONO2,
1014 molecules/cm3), NO2 (≈1015 molecules/cm3), O2 (50 Torr),
and N2 (650 Torr). Photolyses were performed with a Xe arc
lamp, equipped with a Corning 3-74 filter (λ > 410 nm) to
limit photolysis of NO2.
BrONO2 + M f BrO + NO2 + M
(-7)
B. Thermal Decomposition of Bromine Nitrate. The thermal
decomposition of BrONO2 was studied by admitting BrONO2,
(4-10) × 1013 molecules/cm3, in the cell with NO, (1-2) ×
1014 molecules/cm3, and N2 (100-1000 Torr) and monitoring
the temporal profile of BrONO2, NO, and NO2 via IR
spectroscopy. Spectra were recorded at intervals of 25-45 s
(depending on the rate of BrONO2 decay) and consisted of 5-10
coadded scans. The heterogeneous loss of BrONO2 was
measured before and/or after each measurement of the thermal
dissociation under the same conditions of temperature and
pressure. These experiments were conducted by placing
BrONO2, ≈(4-10) × 1013 molecules cm-3, and NO2, ≈2 ×
1015 molecules cm-3, in the cell with N2.
at T ) 320-340 K and P ) 100-1000 Torr. These data are
combined with currently recommended rate constants for
reaction 7 to determine the equilibrium constant, K7 ) k7/k-7
,
as a function of temperature and hence determine the BrO-
NO2 bond strength. The data are used to derive self-consistent
∆Hf° data for BrO, BrONO2, HOBr, and Br2O. During the
course of these experiments, upper limits for the rate coefficients
for unimolecular decay to Br + NO3 and for isomer formation
in the BrO + NO2 + M reaction were obtained. Also, a rate
coefficient for Br atom reaction with BrONO2 (measured relative
to Br + CH3CHO) is reported, along with approximate rate
coefficients for reaction of NO and BrNO with BrONO2.
3. Materials. BrONO2 was synthesized via the reaction of
BrCl with ClONO2, according to the method of Spencer and
Rowland.10 ClONO2 was prepared21,22 by combining Cl2O
(obtained from the reaction of Cl2 with HgO powder) with
excess N2O5 (prepared from reaction of O3 with NO)18 at dry
ice temperature and allowing the mixture to warm overnight to
273 K. The ClONO2 was removed from the less volatile N2O5
by vacuum distillation at 195 K. BrCl was prepared by mixing
Br2 with excess Cl2.11 The BrCl was then condensed onto the
ClONO2 at dry ice temperature and allowed to stand for 1-2
days. This resulted in the conversion of a small fraction (<2%)
of the ClONO2 to BrONO2. ClONO2, BrCl, and Cl2 were
removed from the BrONO2 by vacuum distillation. For this
procedure, the ClONO2/BrONO2/BrCl/Cl2 mixture was held at
dry ice temperature and was pumped on through a liquid
nitrogen trap. The more volatile Cl2, ClONO2, and BrCl were
removed from the BrONO2 and recondensed in the liquid
nitrogen trap. This recondensed mixture was then allowed to
stand for another 1-2 days at dry ice temperature, and the
distillation process was repeated to obtain another BrONO2
sample. The only impurity detected in the BrONO2 via infrared
spectroscopy was HNO3, which was present at levels of 2-15%.
No systematic variations in measured BrONO2 decay rates with
HNO3 levels were noted; its presence as an impurity is not
expected to interfere with any of the measurements reported
here.
Experimental Section
1. Fourier Transform Spectrometer System. All experi-
ments were conducted in a 2 m long, 47 L stainless steel
chamber,16,17 interfaced to a BOMEM DA3.01 Fourier transform
infrared spectrometer. The chamber is equipped with Hanst-
type multipass optics which provided an IR analysis beam path
length of 32.6 m. Water from a NESLAB Model EX-250HT
bath was circulated around the cell to control the temperature.
The temperature, as monitored by eight thermocouples along
the length of the cell, was constant to (0.4 K. Fourier transform
spectra, obtained from the coaddition of 5-50 scans, were
typically recorded over the range 800-3900 cm-1 at a spectral
resolution of 1 cm-1. Components of the gas mixturessBrONO2,
CH3CHO (Aldrich, 99%), NO (Linde), NO2 (synthesized from
addition of excess O2 to NO), and Br2 (Aldrich, 99.5%+)swere
added to the cell via expansion from smaller calibrated volumes
and flushed into the cell with O2 (U.S. Welding, UHP Grade)
or N2 (boil-off from liquid N2 Dewar). Calibration of IR spectra
was achieved as follows: NO, NO2, and CH3CHO from
comparison with standard spectra; N2O5 using the absorption
cross sections of Cantrell et al.,18 CH3C(O)O2NO2 (PAN) using
the absorption cross sections of Tsalkani and Toupance,19 and
BrONO2 using absorption cross sections of Burkholder et al.11
Burkholder et al. report absorption cross sections for the band
near 800 cm-1; absolute cross sections for the 1280 cm-1 band
were obtained in the current work from the simultaneous
measurement of both bands. To accomplish this, BrONO2
spectra were recorded in a 15 cm long Pyrex absorption cell
which was housed in the sample compartment, which enabled
spectra near 800 cm-1 to be recorded with higher signal-to-
noise ratios than is possible in the long path cell.
4. Chemical Modeling. Modeling of the experimental
system was conducted using the ACUCHEM software.23 Rate
coefficients were obtained from recent evaluations9,20 when
available; sources of other rate coefficients used in the model
are noted in the text.
Results and Discussion
2. Experimental Procedures. A. Rate Coefficient for
Reaction of Br with BrONO2. Because the presence of Br atoms
was anticipated in the thermal decomposition of BrONO2 in
the presence of NO, a reliable rate constant for Br with BrONO2
Rate Coefficient for Reaction of Br with BrONO2. As
described above, the rate coefficient for reaction 11 was
determined relative to that for reaction 12 via photolysis of Br2
in the presence of mixtures of CH3CHO, BrONO2, NO2, O2,
and N2. The relevant chemical reactions expected to occur in
this system are as follows:
Br + BrONO2 f Br2 + NO3
(11)
was required to assess the contribution of this reaction in the
study of the BrONO2 thermolysis. The rate coefficient, k11, was
obtained relative to the known reaction rate coefficient for
reaction of Br with CH3CHO,
Br2 + hν f Br + Br
(13)
(11)
(12)
(14)
(15)
Br + BrONO2 f Br2 + NO3
Br + CH3CHO f HBr + CH3CO
NO3 + NO2 + M f N2O5 + M
CH3CO + O2 + M f CH3COO2 + M
Br + CH3CHO f HBr + CH3CO
(12)
k12 ) 3.9 × 10-12 cm3 molecule-1 s-1 20
BrONO2 ((3-5) × 1013 molecules/cm3), CH3CHO ((5-8) ×
.
For these experiments,
Br2 (≈1015 molecules/cm3) was photolyzed in the presence of

19400 J. Phys. Chem., Vol. 100, No. 50, 1996
Orlando and Tyndall
CH3COO2 + NO2 + M f CH3COO2NO2 (PAN) + M
(16)
Reaction of Br with BrONO2 then results in essentially
stoichiometric production of N2O5 while reaction of Br with
CH3CHO leads to 1:1 formation of PAN. Successive irradia-
tions were conducted for periods of 1-2 s, with IR spectra
recorded after each irradiation. The rate coefficient for reaction
11 relative to reaction 12 was then obtained as follows:
d[BrONO2]
-
) k11[Br][BrONO2]
) k12[Br][CH3CHO]
dt
d[CH3CHO]
-
dt
d ln[BrONO2] k11
)
Figure 1. Relative loss of BrONO2 and CH3CHO observed in the
photolysis of Br2/CH3CHO/BrONO2/NO2/O2 mixtures. Solid line
represents a linear least-squares fit to the data.
k12
d ln[CH3CHO]
Because it was found that k11 > k12, only small changes in
CH3CHO were observed. Hence, a more accurate means of
determining the loss of CH3CHO was through quantification
of [PAN]: [CH3CHO]t ) [CH3CHO]0 - [PAN]t. At larger
conversions, when CH3CHO decay was quantifiable, this value
was used directly for analysis. Small corrections to the BrONO2
loss were made to account for its loss via heterogeneous
processes, which was measured before each experiment.
Relative BrONO2 and CH3CHO loss rates are plotted against
each other in Figure 1. These results are a compilation of five
separate fills of the cell, with 2-5 irradiations performed per
fill. A nonweighted linear least-squares fit of the data, as shown
in Figure 1, yields the ratio k11/k12 ) 12.5 ( 0.6. The negative
intercept obtained in the fit is likely the result of imprecisions
in the points obtained at low conversion, due to uncertainties
in the correction for BrONO2 wall loss and in the determination
of the small CH3CHO loss through quantification of PAN.
Recent literature values for k12 at 298 K range from 3.5 × 10-12
followed by reaction of NO3 with NO,
NO3 + NO f 2NO2
(18)
to produce NO2. Experiments conducted in a more controlled
fashion (with less NO) failed to reveal BrNO as a product and
were suggestive of a reaction of BrNO with BrONO2 leading
to NO2 formation,
BrNO + BrONO2 f Br2 + 2NO2
(19)
The results of all experiments can be explained if reaction 17
proceeds with a rate coefficient of about 3 × 10-19 cm3
molecule-1 s-1 and reaction 19 proceeds with a rate coefficient
g10-16 cm3 molecule-1 s-1. Analogous reactions of ClONO2
with NO, k ) 2.1 × 10-12 exp(-5966/T) cm3 molecule-1 s-1
,
and with ClNO, k ) 2 × 10-20 cm3 molecule-1 s-1 at 343 K,
have been reported by Knauth.28
to 4.45 × 10-12 molecules cm-3 s-1 24-26
,
with a currently
Thermal Decay Rates of BrONO2. BrONO2 loss in the
presence of NO was found to obey first-order kinetics, with
observed loss rates of 10-3-10-2 s-1 for the range of conditions
studied here (320 K < T < 340 K, 100-1000 Torr total
pressure). Experiments were conducted in the presence of NO
as described in the experimental section, in order to convert
BrO to Br and hence stop the re-formation of BrONO2 via
reaction 7:
recommended value of 3.9 × 10-12 cm3 molecule-1 s-1 20
.
Using this recommended rate coefficient yields a value of (4.9
( 1.5) × 10-11 cm3 molecules-1 s-1 for k11, where the estimated
uncertainty (1σ) includes precision ((5%), systematic uncer-
tainties ((15%) in the relative rate determination (including
uncertainties in the IR band strength of PAN and CH3CHO),
and the estimated uncertainty in the reference rate coefficient
((10%). Soller et al.27 have recently reported k11 ) 2.08 ×
10-11 exp(320/T), in agreement with our room temperature
value.
BrONO2 f BrO + NO2
BrO + NO f Br + NO2
(-7)
Reaction of BrONO2 with NO and BrNO. During the
course of this work, other reactions of BrONO2 were discovered.
It was initially thought that the optimum conditions for
conducting the thermal decomposition experiments would be
in a large excess of NO over BrONO2. Under these conditions,
the NO2 to NO concentration ratio would remain low, and re-
formation of BrONO2 via reaction 7 would not occur. Also,
the large NO concentration would provide a sink for Br atoms
and remove the uncertainty associated with the occurrence of
reaction 11, for which no rate coefficient data were originally
available. However, addition of large quantities of NO (of order
10 Torr) to BrONO2 samples resulted in rapid consumption of
the BrONO2, with concomitant formation of NO2 and BrNO.
The observations were consistent with a relatively slow reaction
of BrONO2 with NO,
(20)
Concentrations of NO were kept below 2 × 1014 molecules cm-3
to minimize the occurrence of reaction 17. The Br atoms
produced in reaction 20 would then be expected to react with
BrONO2 (reaction 11), with NO, NO2, BrNO, and BrNO2:
Br + NO + M f BrNO + M
Br + NO2 + M f BrNO2 + M
(21)
(22)
Br + BrNO f Br2 + NO
Br + BrNO2 f Br2 + NO2
(23)
(24)
Under the conditions employed in these experiments, the
predominant fate of the Br-atoms is reaction with BrONO2
BrONO2 + NO f BrNO + NO3
(17)

Thermal Decomposition of BrONO2
J. Phys. Chem., Vol. 100, No. 50, 1996 19401
(reaction 11, k11 ) 4.9 × 10-11 cm3 molecule-1 s-1, see above).
The NO3 produced in reaction (11) would rapidly react with
NO,
TABLE 1: Kinetic Data Used and Results Obtained in the
Third Law Analysis of BrONO2 Dissociation Rates
a
b
temp press.
(K) (Torr)
k-7
(s-1
k7
(cm3 s-1
Kp
-∆Hr°
)
)
(atm-1
)
ln(Kp) (kcal/mol)
340.2 700 6.00E-03c 1.73E-12 6.23E+09 22.552 -28.11
333.9 700 1.90E-03 1.85E-12 2.14E+10 23.788 -28.41
325.2 700 9.30E-04 2.03E-12 4.93E+10 24.622 -28.21
328.0 700 1.40E-03 1.97E-12 3.15E+10 24.174 -28.16
328.4 700 1.30E-03 1.96E-12 3.37E+10 24.242 -28.24
331.8 700 2.20E-03 1.89E-12 1.90E+10 23.669 -28.15
338.0 700 3.40E-03 1.77E-12 1.13E+10 23.150 -28.33
336.0 700 3.10E-03 1.81E-12 1.28E+10 23.270 -28.24
335.4 700 3.90E-03 1.82E-12 1.02E+10 23.048 -28.04
336.0 700 2.35E-03 1.81E-12 1.68E+10 23.547 -28.43
336.0 125 1.25E-03 7.00E-13 1.22E+10 23.229 -28.21
339.7 700 4.00E-03 1.74E-12 9.41E+09 22.965 -28.35
339.6 700 4.00E-03 1.74E-12 9.41E+09 22.965 -28.34
339.7 700 3.70E-03 1.74E-12 1.02E+10 23.043 -28.40
336.0 1000 3.20E-03 2.22E-12 1.52E+10 23.443 -28.36
335.9 100 8.80E-04 6.34E-13 1.58E+10 23.481 -28.37
336.0 300 1.40E-03 1.18E-12 1.84E+10 23.637 -28.49
320.0 700 4.70E-04 2.15E-12 1.05E+11 25.378 -28.24
323.0 700 5.70E-04 2.08E-12 8.30E+10 25.142 -28.35
324.2 700 7.00E-04 2.05E-12 6.64E+10 24.919 -28.31
322.1 700 6.10E-04 2.10E-12 7.85E+10 25.087 -28.23
NO3 + NO f 2NO2
(18)
Hence, thermal dissociation of a BrONO2 molecule actually
leads to the consumption of a second BrONO2. To a first
approximation, the overall stoichiometry (given by the sum of
reactions -7, 20, 11, and 18) should then be as follows:
2BrONO2 + 2NO f Br2 + 4NO2
The infrared spectra confirmed that this stoichiometry was
obeyed. On average, 1.9 ( 0.3 molecules of NO2 were
produced and 0.9 ( 0.2 molecules of NO consumed per
BrONO2 lost. No evidence for production of BrNO or BrNO2
was found in any of the experiments conducted, supporting the
hypothesis that Br atoms produced in this system were reacting
with BrONO2 and hence that about two BrONO2 molecules are
lost per thermal dissociation event.
BrONO2 was also susceptible to loss at the cell walls, a
process that needed to be accounted for in the analysis of the
thermal dissociation data. As described in the Experimental
Section, experiments were conducted in the presence of NO2
to determine the rate of this process. Under these conditions,
thermal dissociation to BrO and NO2 in the presence of NO2 is
quickly followed by re-formation of BrONO2 via reaction 7 and
hence leads to no BrONO2 loss. Hence, loss of BrONO2 is
most likely dominated by nonreversible loss at the walls of the
cell. Decay of the BrONO2 was found to be considerably slower
(≈10-4 s-1) than for experiments conducted in the presence of
NO.
a Values measured in this work. b Determined from parameters given
in ref 9, for the pressure and temperature at which k-7 was measured.
c Read as 6.00 × 10-3
.
Rate constants for reaction -7 for each experiment were
obtained from a box model simulation of the reaction system,
in order to accurately account for the contributions of reactions
7, 11, and 17 to the BrONO2 temporal profile. For these
simulations, the heterogeneous loss rate of BrONO2 was fixed
at the value measured in the presence of NO2, and the rate
coefficient for k-7 was adjusted in the model to obtain the best
match to the observed BrONO2 temporal profile. For typical
conditions, the observed loss of BrONO2 was 1.8-1.9 times
the value of k-7 extracted from the model, again showing the
importance of reaction 11 in determining the BrONO2 temporal
profile. Modeling tests showed that the extracted values of k-7
were not particularly sensitive to the value of k11sdecreasing
this rate coefficient by an order of magnitude changed the value
of k-7 by less than 15%.
The values of k-7 obtained from the simulations at each
temperature and pressure studied are given in Table 1. The
data recorded at 700 Torr pressure are plotted in Arrhenius form
in Figure 2. Linear least-squares analysis yields the following
best fit line (also shown in Figure 2): ln(k-7) ) 30.96-12360/
T. Of course, this relationship is only applicable at 700 Torr
total pressure as the reaction is in the falloff region between
first- and second-order kinetics at this pressure.
Figure 2. Plot of the dissociation rate of BrONO2 as a function of
inverse temperature, for all measurements made near 700 Torr. Solid
line represents a linear least-squares fit to the data.
These BrONO2 dissociation rate coefficients are considerably
slower than those for the analogous dissociation of ClONO2.14,29,30
As a check of our experimental system, and as a check of
previously measured values of ClONO2 thermal dissocia-
tion,14,29,30 this process was investigated briefly as a function
of pressure at an arbitrarily selected temperature of 323 K in a
method similar to that used to study reaction -7. Shown in
Figure 4 are data for dissociation of ClONO2 at pressures
ranging from 100 to 800 Torr at 323 K. Also shown in Figure
4 are the predicted dissociation rate constants, obtained from
the combination of the equilibrium constant recommended by
Anderson and Fahey14 with the formation rate constants
recommended by the NASA Panel.9 Since the measurements
agreed within their uncertainty with the calculated values, no
further experiments were performed. However, it is clear that
the dissociation of ClONO2 is considerably faster than that of
BrONO2 (about 10-2 s-1 at 323 K, 700 Torr for ClONO2,
compared to 3 × 10-3 at 336 K, 700 Torr for BrONO2). As
will be presented in detail later, this is largely due to a stronger
XO-NO2 bond in BrONO2 than in ClONO2.
The pressure dependence of k-7 was investigated at 336 K
at pressures between 100 and 1000 Torr (see Figure 3). The
line shown is obtained by dividing the rate coefficient data for
k7 (obtained from the parameters of refs 8 and 9) by the
equilibrium constant KP,7 derived below (KP,7 ) 5.44 × 10-9
exp(14192/T) atm-1). The agreement with the falloff is
excellent, indicating that the pressure dependence observed in
this work agrees well with that measured for the formation
reaction 7.

19402 J. Phys. Chem., Vol. 100, No. 50, 1996
Orlando and Tyndall
Figure 3. Dissociation rate of BrONO2 as a function of pressure, for
measurements made at 336 ( 0.2 K. (b) Measured dissociation rates.
The solid line is obtained by dividing k7 data from ref 8 by the
equilibrium constant, K7, obtained in this work.
Figure 5. Equilibrium constant (K7) plotted as a function of inverse
temperature. The linear least-squares fit (second law analysis) gives
∆Hr° of 27.1 ( 2.6 kcal/mol. Third law analysis of the same data yields
∆Hr° ) 28.2 ( 1.5 kcal/mol.
Thus, even if all the loss assigned to heterogeneous processes
is actually due to formation of some species other than BrONO2,
the rate coefficient for this other reaction can at most be a factor
of 4 less than that for reaction 7 under the conditions of
temperature and pressure studied herein. This is almost certainly
an upper limit, since no evidence for other products is obtained
from the IR spectra. This type of experiment is the first to show
that BrONO2 is indeed the major product of BrO/NO2 recom-
bination (at least at the temperatures and pressures employed
here).
Determination of the ∆Hr°(298 K) and ∆Sr°(298 K) for
BrONO2 Thermal Dissociation. In order to properly assess
the thermodynamics involved in the reaction system, analysis
was performed using both the “second law” and “third law”
methods. Both of these analyses require the calculation of
equilibrium constants KP,7 ) k7/k-7(RT)-1
. These values
(obtained using the measured k-7 and values for k7 from the
recommended parameters)9,20 are presented in Table 1. In the
second law analysis, the well-known relationship
Figure 4. Dissociation rate of ClONO2 as a function of pressure at
323 K. (b) Measured dissociation rates. The solid lines is obtained
by dividing the equilibrium constant for ClONO2 S ClO + NO2 from
ref 14 by ClO + NO2 + M rate coefficient data from ref 9.
∆Hr 1
∆Sr
-ln(KP,7) )
-
(
)
(
)
R
T
R
Decomposition of BrONO2 to Br and NO3 is possible,
is employed. Figure 5 shows a plot of ln KP,7 as a function of
inverse temperature. The slope of this plot, as obtained by
nonweighted linear least-squares analysis, gives a value of ∆Hr°
for the decomposition reaction at a temperature of 335 K (the
mean of the temperature range studied here) of 27.1 ( 2.6 kcal/
mol (2σ precision error only). Correction of this value to 298
K is negligible (<0.1 kcal/mol), and hence the reaction enthalpy
at 298 K is adequately represented by the value obtained from
the fit. The intercept of this second-order plot yields an entropy
change, ∆Sr°, for this reaction of 34.3 cal K-1 mol-1. Com-
parison of these values with those obtained by the third law
method will be made below.
BrONO2 f Br + NO3
(25)
although it is unlikely due to the fact that it is more endothermic
than reaction -7. Experiments conducted in the presence of
NO2 (to determine the wall loss) can also be analyzed to obtain
an upper limit for this process. Under conditions employed in
these experiments, NO3 produced via reaction 25 would be
converted via reaction 14 to N2O5; the Br atom would react at
least in part with BrONO2 via reaction 11, generating another
NO3 and hence another N2O5. No N2O5 was observed by IR
spectroscopy in any of these experiments, indicating that
decomposition to Br + NO3 (reaction 25) occurs at a rate that
is at least 25 times slower than decomposition via reaction -7.
These same experiments conducted in the presence of NO2
can be used to consider the possibility of formation of products
other than BrONO2 in reaction 7. In these experiments,
BrONO2 decomposition and re-formation occurs a number of
times before the heterogeneous reaction results in nonreversible
BrONO2 loss. For typical experiments conducted here, the
thermal dissociation occurred at a rate that was about 4-5 times
faster than the loss rate assigned to the heterogeneous reaction.
In the “third law” analysis, each individual determination
of KP,7 in Table 1 is used to determine ∆Hr°(298 K) as
follows:14,31
∆Hr°(298 K) ) -RT ln[KP,7(T)] -
[-TS°(T) + [H°(T) - H°(298 K)]]
∑
where the summation is over products minus reactants. Values
of S°(T) and [H°(T) - H°(298 K)] for NO2 were obtained from

Thermal Decomposition of BrONO2
J. Phys. Chem., Vol. 100, No. 50, 1996 19403
TABLE 2: Thermodynamic and Kinetic Data for the Halogen Nitrates (All Data at 298 K)
∆Hf° for XONO2
XO-NO2 bond strength
X-O bond strength
X-ONO2 bond strength
k0(formation)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(cm6 molec-2 s-1
)
âc
X ) F
X ) Cl
X ) Br
X ) I
3.1 ( 2.0a
31 ( 5b
52.6 ( 5b
33.5 ( 2.0b
2.6 × 10-31 c
1.8 × 10-31 c
5.2 × 10-31 c
5.9 × 10-31 c
0.4d
0.6d
0.7f
5.5c
26.8c
64.1c
41.0c
10.1 ( 1.5e
<9.5g
28.2 ( 1.5f
>25.4g
55.9 ( 1.5e
58 ( 2i
34.2 ( 1.5e
>33.6g
(7 ( 2)h
(28 ( 2)h
(36.5 ( 2)h
a From ab initio calculation of ref 44. b Using ∆Hf° value of ref 44, with ∆Hf° values for O, F, FO, NO2, and NO3 from ref 9. c From ref 9.
d From ref 15. e Using BrONO2 + H2O S HOBr + HNO3 equilibrium constant measurement of ref 34, BrO-NO2 bond strength from this work,
an average ∆Hf° for HOBr from refs 38, 39, and 42, and ∆Hf° values for O, Br, NO2, and NO3 from ref 9 (see text for details). f This work. g Using
upper limit to ∆Hf° for IONO2 from kinetics study of ref 45, estimated ∆Hf° for IO of 27 kcal/mol, and ∆Hf° values for I, I2, NO2, and NO3 from
ref 9. h Based on estimated I-ONO2 bond strength (see text), estimated ∆Hf° for IO of 27 kcal/mol, and ∆Hf° values for I, NO2, and NO3 from ref
9. i From an estimated value for ∆Hf° for IO of 27 kcal/mol, which is the middle of the range of values reported in refs 9, 20, and 46-50 and ∆Hf°
values for I and O from ref 9.
interpolation of the JANAF tables,31 while values for BrO and
BrONO2 were calculated from known (or estimated) spectro-
scopic parameters, using methods outlined by Chase et al.31
Spectroscopic parameters for BrO were obtained from Orlando
et al.,32 while those for BrONO2 were obtained from Spencer
and Rowland,10 Sander et al.,6 and Patrick and Golden.13 The
average value for ∆Hr°(298 K) from the third law method is
28.2 ( 0.2 kcal/mol (2σ precision error only).
Heat of Formation for BrONO2. There is considerable
uncertainty in the ∆Hf° values for BrO, HOBr, and BrONO2,
all of which are of importance in atmospheric chemistry.
Experiments (such as the one conducted here) often determine
the difference between the heats of formation of the various
species, making absolute values difficult to obtain. Nonetheless,
it appears that sufficient information is now available from
which a consistent set of ∆Hf° values for BrO, HOBr, and
BrONO2 can be obtained.
Thus, the ∆Hr°(298 K) values for BrONO2 dissociation
obtained by second law and third law analysis agree within the
uncertainties of the two analyses. Because of the larger
uncertainty associated with obtaining ∆Hr° from the second law
analysis over such a narrow temperature range, the value
obtained from the third law analysis is recommended.31 How-
ever, additional uncertainties (other than the precision error)
exist in this third law value which will now be discussed. First,
we estimate a possible (20% systematic error in the measured
values of k-7 associated with potentially unforeseen secondary
chemistry not properly accounted for in the simulations.
Second, values of k7 used in the calculation of K7 have a (15%
uncertainty associated with them. These two factors contribute
a combined uncertainty of (0.3 kcal/mol to the ∆Hr° value.
Finally, the two lowest frequency BrONO2 vibrations have not
been observed and are obtained from estimates.6 Variation of
these frequencies by a factor of 2 from the estimated values
leads to an uncertainty of (1.0 kcal/mol in the ∆Hr° value.
Combining these uncertainties, we report a ∆Hr°(298 K) value
of 28.2 ( 1.5 kcal/mol. From the third law analysis, the value
of ∆Sr° for reaction -7 is found to be 37.8 cal K-1 mol-1. Since
these ∆Hr° and ∆Sr° are essentially independent of temperature
over the range studied, an equilibrium constant of KP,7 ) 5.44
× 10-9 exp(14192/T) atm-1 can be obtained.
The simplest approach to the determination of ∆Hf°(298 K)
for BrONO2 from the experiments conducted here is to combine
the measured heat of reaction -7 with known values of
∆Hf°(298 K) for BrO and NO2. While the value for NO2 is
well-known, 7.9 ( 0.2 kcal/mol,9 less confidence can be applied
to the BrO value. The IUPAC panel,20 based on spectroscopic
work of Durie and Ramsay,33 recommend ∆Hf°(298 K) ) 30
kcal/mol for BrO. However, in their most recent evaluation,
the NASA Panel9 recommend 26 ( 5 kcal/mol. Because there
is no direct experimental evidence to contradict the Durie and
Ramsay33 estimate, we choose at this point to use the value of
30 kcal/mol obtained from their analysis. This leads to a value
of 9.7 ( 2.0 kcal/mol for the ∆Hf°(298 K) of BrONO2. Owing
to the stronger BrO-NO2 bond determined here compared to
previous estimates, the ∆Hf° value is somewhat lower than
previous recommendations.9,20
An alternative approach is provided by the recent work of
Hanson et al.,34 who studied the equilibrium process
BrONO2 + H2O T HOBr + HONO2
(26, -26)
and report ∆Hr°(298 K) for the forward reaction 26 to be 1.3
( 0.7 kcal/mol. The ∆Hf° value for HOBr has been the subject
of numerous recent investigations;35-42 the most direct measure-
ments38,39,42 yield an average value of -14.1 kcal/mol. Use of
this value for HOBr and the ∆Hr° value for reaction 26 of 1.3
( 0.7 kcal/mol yields a ∆Hf°(298 K) for BrONO2 of 10.1 (
0.8 kcal/mol, in agreement with the value determined above.
This independent value of ∆Hf°(298 K) for BrONO2, combined
with the BrO-NO2 bond strength determined here, can be used
to determine a ∆Hf°(298 K) value for BrO of 30.4 ( 2 kcal/
mol, thus confirming the value reported by Durie and Ramsay.33
Although the two methods outlined above for determination of
the heats of formation of the various bromine species are entirely
self-consistent, the ∆Hf° of HOBr would seem to be better
characterized than that of BrO, and the latter data are recom-
mended. Finally, a study of the Br2O/H2O/HOBr equilibrium
system35 allows a value of 27.3 ( 2 kcal/mol to be obtained
for ∆Hf°(298 K) of Br2O, consistent with recently reported
values of 25.6 ( 1.740 and 29.1 ( 1.0 kcal/mol.43
To the best of our knowledge, this is the first experimentally
determined value for the BrO-NO2 bond strength in BrONO2.
Patrick and Golden13 reported a value for this bond strength of
26.7 kcal/mol, but this was estimated by analogy to ClONO2.
More recently, Rayez and Destriau15 conducted both ab initio
and semiempirical calculations to determine the XO-NO2 bond
strengths for X ) F, Cl, Br, and I. They report an ab initio
value of 34.2 kcal/mol for BrO-NO2 at 0 K (which extrapolates
to 35 kcal/mol at 298 K), significantly higher than our value. It
is worth noting, however, that while their ab initio value agrees
with experiment for FONO2, the ab initio value for ClONO2 is
some 5 kcal/mol higher than experiment. Thus, it is apparent
that the trend determined in the ab initio calculations is
semiquantitatively correct but that the absolute values of the
ClO-NO2 and BrO-NO2 bond strength are overestimated by
the ab initio method. The semiempirical value for BrONO2
decomposition obtained by Rayez and Destriau is considerably
larger than either their ab initio value or our experimentally
determined value.
Evaluation of Related Thermodynamic and Kinetic Data.
Table 2 gives heats of formation for the halogen nitrates, as

19404 J. Phys. Chem., Vol. 100, No. 50, 1996
Orlando and Tyndall
well as X-O, X-ONO2, and XO-NO2 bond strengths and low-
pressure rate coefficients (k0) for formation of the halogen
nitrates. It is seen that the ratio of the X-O bond strengths in
XONO2 and XO (for X ) F, Cl, Br) is constant, 0.63 ( 0.02.
This is similar to the correlation reported by Zhang et al.51 for
X-O and X-OH bond strengths. If it is assumed that this
correlation extends to iodine, an estimate of the I-ONO2 bond
strength of 36.5 ( 2.0 kcal/mol can be made. This leads to a
value of 7 ( 2 kcal/mol for ∆Hf°(298 K) of IONO2, assuming
∆Hf°(298 K) for IO is 27 kcal/mol, the midrange of recently
reported values.7,20,46-50 This is consistent with the best
available estimate for ∆Hf° for IONO2 of <9.5 kcal/mol, which
is obtained from the observation45 of a rapid reaction between
I2 and NO3 (see Table 2).
Thanks are due to Lee Mauldin and Chris Cantrell for helpful
comments on the manuscript. We are indebted to Jim Burkhold-
er (NOAA Aeronomy Laboratory) for helpful hints in the
preparation and handling of BrONO2 and to Jim Burkholder,
Dave Hanson, A. R. Ravishankara, and Ned Lovejoy (all of
the NOAA Aeronomy Lab) for helpful discussions regarding
BrONO2, HOBr, and BrO thermochemistry and for communica-
tion of results prior to publication. NCAR is partially supported
by the NSF.
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