Kinetics of the gas-phase reaction of BrNO2 with NO

Article abstract of DOI:10.1021/jp982812b

BrNO₂ was prepared in situ in a static reactor (v = 420 L) by photolyzing Br₂/NO₂/N₂ mixtures in the wavelength range 500-700 nm at temperatures between 263 and 294 K. After the lights were switched off, the excess NO was added, and IR and UV spectra were monitored simultaneously as a function of time. From the pseudo-first-order decay of the IR absorption of BrNO₂ in the presence of a large excess of NO, the second-order rate constant for reaction 4, BrNO₂ + NO → BrNO + NO₂, was determined to be k₄ = 2.3 × 10^-12 exp[(-17.8 ± 2.1) kJ mol^-1/RT] cm³molecule^-1s^-1 (2σ). The measured yields of BrNO were close to 100percent (98 ± 5percent). These results suggest that reaction 4 is unimportant as a loss process of BrNO₂ under most tropospheric conditions. Additional experiments on the thermal stability of BrNO₂ led to an upper limit of 4.0 × 10^-4 s^-1 for its thermal gas-phase decomposition rate constant at 298 K in 1 atm of synthetic air. Finally, the mechanism of the Br + NO₂ reaction and the thermochemistry of BrNO₂ and BrONO are discussed in light of the results of the present experiments and of previous work from the literature.

Full text of DOI:10.1021/jp982812b

8626
J. Phys. Chem. A 1998, 102, 8626-8631
Kinetics of the Gas-Phase Reaction of BrNO2 with NO
R. Bro1ske and F. Zabel*
Bergische UniVersita¨t-Gesamthochschule Wuppertal, Physikalische Chemie/FB 9,
D-42097 Wuppertal, Germany
ReceiVed: June 29, 1998; In Final Form: September 1, 1998
BrNO2 was prepared in situ in a static reactor (V ) 420 L) by photolyzing Br2/NO2/N2 mixtures in the
wavelength range 500-700 nm at temperatures between 263 and 294 K. After the lights were switched off,
the excess NO was added, and IR and UV spectra were monitored simultaneously as a function of time.
From the pseudo-first-order decay of the IR absorption of BrNO2 in the presence of a large excess of NO, the
second-order rate constant for reaction 4, BrNO2 + NO w BrNO + NO2, was determined to be k4 ) 2.3 ×
10-12 exp[(-17.8 ( 2.1) kJ mol-1/RT] cm3molecule-1s-1 (2σ). The measured yields of BrNO were close to
100% (98 ( 5%). These results suggest that reaction 4 is unimportant as a loss process of BrNO2 under
most tropospheric conditions. Additional experiments on the thermal stability of BrNO2 led to an upper
limit of 4.0 × 10-4 s-1 for its thermal gas-phase decomposition rate constant at 298 K in 1 atm of synthetic
air. Finally, the mechanism of the Br + NO2 reaction and the thermochemistry of BrNO2 and BrONO are
discussed in light of the results of the present experiments and of previous work from the literature.
1. Introduction
time until a stationary temperature of the photolysis lamps was
achieved. The gas mixtures were analyzed by long-path IR
absorption (optical path length 40 m) using an FT-IR spec-
trometer (NICOLET Magna 550). UV absorption spectra could
be measured simultaneously (optical path length 3 m) with a
diode array spectrometer, which consists of a diode array
detector (EG&G; model 1412) and a modified 25 cm double
monochromator (SPEX).
BrNO2 is presently being discussed as a product of the
reaction of N2O5 with sea salt aerosols (see, e.g., refs 1 and 2).
It is difficult to estimate its source strength in the marine
atmosphere because the reaction rates of volatile compounds
on sea salt are still very uncertain. To assess the bromine
balance in the marine troposphere, the rate parameters for the
most important loss processes of BrNO2 are also needed.
However, the present knowledge on the kinetic properties of
BrNO2 is very sparse. Previous work on the thermal gas-phase
BrNO2 was formed according to the mechanism
Br2 + hν w Br + Br
(1)
3,4
decomposition of BrNO2 is not unequivocal, but evidence
emerges now (ref 4, this work) that its thermal lifetime around
room temperature is on the order of an hour or more. In the
troposphere and under sunlight conditions, photolysis is esti-
mated to be faster than thermal deomposition on the basis of
the UV absorption cross sections from Scheffler et al.5 For the
conditions July 1, noon, 50 °N, no clouds, the photolysis lifetime
of BrNO2 is about 4 min. The reaction of BrNO2 with NO is
another possible atmospheric loss process of BrNO2. In the
present work, the rate constant of this reaction has been
measured as a function of temperature.
Br + NO2 (+M) w BrNO2 (+M)
(2a)
It was identified by comparison of the photolysis product spectra
with published low-temperature matrix and gas-phase IR spectra
of BrNO2.5,7-10 After a certain period of time that depended
on the initial NO2 concentration, the BrNO2 concentration
leveled off, probably owing to the loss reaction
BrNO2 + Br w Br2 + NO2
(3)
After stationary concentrations of BrNO2 were established, the
photolysis lamps were switched off. In the dark and with no
NO present, the lifetime of BrNO2 in the photoreactor was on
the order of 1 h. In the presence of NO (3.2 × 1013-1.4 ×
1014 molecule cm-3), BrNO2 decayed on the time scale of
minutes. The decay of BrNO2 and the simultaneous formation
of the product BrNO were monitored by their IR absorption.
IR absorption coefficients of BrNO2 and BrNO were determined
from their simultaneously measured IR and UV absorptions,
on the basis of Beer’s law and the UV absorption cross sections
of Scheffler et al.5 for BrNO2 and of Houel and van den Bergh11
for BrNO.
2. Experimental Section
The experiments were performed at total pressures close to
1 bar in a temperature-controlled 420 L photochemical reaction
chamber from DURAN glass, which was described elsewhere6
in more detail. BrNO2 was prepared in situ by photolyzing Br2/
NO2/N2 mixtures with fluorescent lamps radiating in the
wavelength range 500-700 nm (Phillips TLD36W/16). In the
first 1-2 min of photolysis, the light intensity increased with
* Present address: Institut fu¨r Physikalische Chemie der Universita¨t,
Universita¨t Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
10.1021/jp982812b CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/07/1998

Kinetics of the Gas-Phase Reaction BrNO2 + NO
J. Phys. Chem. A, Vol. 102, No. 44, 1998 8627
Figure 1. Concentration-time profiles for BrNO2 formation at different
initial concentrations of NO2, T ) 293 ( 1 K, p ) 988 ( 2 mbar, M
) N2; initial concentrations (in molecule cm-3): triangles, [Br2]0 )
7.1 × 1014, [NO2]0 ) 1.7 × 1014; squares, [Br2]0 ) 7.1 × 1014, [NO2]0
) 3.5 × 1014; circles, [Br2]0 ) 8.2 × 1014, [NO2]0 ) 7.1 × 1014.
Known amounts of NO2, Br2, and NO were introduced into
the reaction chamber using a glass bulb of calibrated volume
for NO and gastight syringes for NO2 and Br2. NO2 dimer
formation in the syringe was taken into account using the
equilibrium constants for NO2 + NO2 S N2O4 from Harwood
and Jones.12
Research grade Br2 (Aldrich, 99%) and NO2 (Messer
Griesheim, 1.8) were used as supplied. NO, which was present
as an impurity in NO2, was titrated with O3 in some of the
experiments. Pressures were measured using capacitance pres-
sure gauges. The temperature in the reaction chamber was
measured with a platinum resistance gauge. The temperature
was constant within (0.5 K during an experiment.
Figure 2. IR spectra during photolysis of a Br2/NO2/N2 mixture with
500-700 nm light, photolysis time, 475 s, T ) 292 K, p ) 989 mbar,
M ) N2, [Br2]0 ) 8.2 × 1014 molecule cm-3, [NO2]0 ) 7.1 × 1014
molecule cm-3, water vapor absorption lines have been subtracted from
both spectra: (a, top) product spectrum; (b, bottom) absorptions from
NO2, N2O5, and traces of BrNO subtracted from spectrum a, showing
3. Results and Discussion
the IR absorptions of BrNO2 centered at 1200, 1292, and 1667 cm-1
;
between 1580 and 1640 cm-1 there is residual absorption from the
nearly saturated NO2 band.
Stationary State in the Br2/NO2 Photolysis. When Br2/
NO2/N2 mixtures were photolyzed in the wavelength range
500-700 nm, stationary concentrations of BrNO2 were estab-
lished after certain periods of time (Figure 1). Traces of NO
in the NO2 sample led to a reduced yield of BrNO2 according
to the chain reaction
NO2, N2O5, and traces of BrNO and water is displayed in Figure
2b, showing the three IR absorption bands of the photolysis
product BrNO2 in the range 1100-1900 cm-1 5
Whereas
.
ClONO is formed in the photolysis of Cl2/NO2 mixtures as the
main primary product which is then converted to the more stable
isomer ClNO2,15,16 BrONO could not be identified as a product
in the present work. This is in line with previous experimental
findings of Yarwood and Niki17 and consistent with recent
theoretical work of Lee,18 who calculated BrNO2 to be more
BrNO2 + NO w BrNO + NO2
BrNO + Br w Br2 + NO
(4)
(5)
(k5,298K ) 3.7 × 10-10 13 or 5.2 × 10-12 14 cm3 molecule-1 s-1).
For this reason, NO was titrated with ozone in the BrNO2
lifetime experiments (next section) before the photolysis was
started, leading to larger stationary concentrations and longer
lifetimes of BrNO2. As a consequence, small amounts of excess
ozone led to the formation of N2O5 in these experiments;
however, there was no experimental evidence for any interfer-
ence of the equilibrium N2O5 S NO3 + NO2 with either the
rate of BrNO2 formation or its stationary concentration. The
equilibrium concentration of the reactive NO3 radical was
extremely low owing to the large excess of NO2. An IR
spectrum of the reaction mixture during photolysis is shown in
Figure 2a, and the residual after subtracting the absorptions from
stable than cis- and trans-BrONO by 27 and 43 kJ mol-1
,
respectively. Since the absorption cross sections of BrNO2 are
lower by a factor of 10-20 as compared to those of Br2 in the
500-540 nm range (photolysis threshold of Br2 ∼540 nm,
photolysis rate constant of Br2 in the photoreactor 1 × 10-4
s-1) and lower than 10-20 cm2 molecule-1 beyond this wave-
length, the photolysis of BrNO2 cannot be important in the
present reaction system.
Provided that reactions 2a and 3 are the only source and loss
processes, respectively, for BrNO2, the stationary concentration
of BrNO2 is given by

8628 J. Phys. Chem. A, Vol. 102, No. 44, 1998
[BrNO2]ss ) (k2a/k3)[NO2]0
Bro¨ske and Zabel
(I)
since the NO2 concentration is in excess and nearly constant
during photolysis. For this reason, [BrNO2]ss is expected to be
proportional to [NO2]0. It turns out, however, that [BrNO2]ss
is proportional to ([NO2]0)1.3, indicating that the mechanism of
reactions 1-3 is not complete (see below). The time constant
of the approach of the BrNO2 concentration to its stationary
state depends on the initial concentration of NO2 (see Figure 1
for comparison) and should also depend on the initial Br2
concentration, which, however, was only slightly varied in the
present work. With [Br2]0 ) 8.2 × 1014 molecule cm-3 and
[NO2]0 ) 7.1 × 1014 molecule cm-3, for example, a relatively
high stationary concentration of BrNO2 is formed after a
relatively long time. Contrary to this, at lower initial concentra-
tions of NO2 (e.g., [Br]0 ) 7.1 × 1014 molecule cm-3, [NO2]0
) 1.7 × 1014 molecule cm-3), the stationary concentration of
BrNO2 is much smaller, and the stationary state is established
in a much shorter time. This time behavior is rationalized by
the fact that the stationary Br atom concentration increases with
decreasing NO2 concentration such that the initial BrNO2
formation rate remains unaffected whereas the rate of reaction
3 increases, thus reducing the stationary BrNO2 concentration.
Semiquantitatively, the concentration-time profiles of BrNO2
in Figure 1 can be reproduced by computer simulation of a
simple mechanism consisting of reactions 1, 2a, and 3, i.e.,
assuming BrNO2 to be stable on the time scale of the
experiments, in agreement with the experimental findings (see
below). Using the kinetic simulation program LARKIN19 and
applying the measured photolysis rate rate constant for Br2, the
best fits of the concentration-time profiles of Figure 1 were
obtained by selecting the ratio k2a/k3 to be about 0.016. With
this ratio and k3,298K ) 2.4 × 10-11 cm3 molecule-1 s-1 20 from
Mellouki et al.,21 k2a ) 3.8 × 10-13 cm3 molecule-1 s-1 is
calculated, which is lower by a factor of 13 as compared to k2a
) 4.9 × 10-12 cm3 molecule-1 s-1 for M ) N2 derived from
the work of Kreutter et al.3 Assuming the k3 value of Mellouki
et al. to be correct, the present result on k2a/k3 is consistent with
the work of Kreutter et al. only if one assumes that BrONO is
the main primary product of reaction 2 and only a small fraction
(∼ 8%) leads to BrNO2:
Figure 3. Thermal decay rate constants of BrNO2 (BrONO): triangles,
from ref 3, p ) 267 mbar, M ) N2, isomer assumed to be BrNO2;
straight line, calculated from ref 3 for p ) 1 bar; crosses, ref 4, isomer
BrNO2, p ) 1000 mbar, M ) He, propene added in order to scavenge
Br atoms; full squares, this work, isomer BrNO2, p ) 988 ( 2 mbar,
M ) synthetic air, trans-2-butene added in order to scavenge Br atoms;
full circles, 293/300 K, 805 ( 5 mbar, M ) synthetic air, [Br2]0 ) 7.8
× 1014 molecule cm-3, [NO2]0 ) 1.6 × 1014 molecule cm-3, excess
CH3CHO (3.5 × 1015 molecule cm-3) added in order to scavenge Br
atoms.
The nonlinear increase of [BrNO2]ss with increasing [NO2]0
([BrNO2]ss
[NO2]01.3) is difficult to rationalize. The most
reasonable explanation is to assume a reaction between BrONO
and NO2
BrONO + NO2 w BrNO2 + NO2
(7)
i.e. an NO2-catalyzed isomerization of BrONO. A value of
about 2 × 10-16 cm3 molecule-1 s-1 for k7 is able to reproduce
the observed 1.3th power dependence of [BrNO2]ss on the initial
NO2 concentration as shown in Figure 1. The uncertainty about
the existence of reaction 7 does not affect the rate data on the
reaction of BrNO2 with NO since BrONO is thermally too
3
unstable (τ ) 1.2 s at 298 K, 1 bar of N2 ) to accumulate to
nonnegligible concentrations on the time scale of the present
experiments.
Behavior of BrNO2 in the Dark. After the lights were
switched off, BrNO2 disappeared according to a first-order rate
law with a time constant between ≈1 and several hours. This
is in line with the work of Frenzel et al.,4 who suggested that
Br + NO2 (+M) w BrNO2 (+M) (minor product) (2a)
Br + NO2 (+M) w BrONO (+M) (major product) (2b)
In the next section, evidence will be given to support this
assumption. This being true, BrONO must also be the isomer
primarily formed in the work of Mellouki et al.,21 and the rate
constant for reaction 3 determined by these authors has to be
assigned to the reaction
the room-temperature value of k-2a
,
BrNO2 (+M) w Br + NO2 (+M)
(-2a)
is smaller by several orders of magnitude than the value derived
from the work of Kreutter et al.3 (see Figure 3). To elucidate
these discrepancies, experiments similar to those of Frenzel et
al. (ref 4, 10 cm quartz cell) were performed in our 420 L
photoreactor, i.e., at considerably larger volume:surface ratios.
The measured first-order loss rate constants can be either larger
than k-2a owing to wall loss or smaller owing to regeneration
of Br atoms and thus of BrNO2 via reaction 2a. To avoid the
re-formation of BrNO2, excess trans-2-butene (8.6 × 1014
molecule cm-3) was added to the reaction mixture as a Br atom
scavenger in several experiments (k298(Br + trans-2-butene) )
9.3 × 10-12 cm3 molecule -1 s-1 22). Synthetic air was used as
a buffer gas in these experiments in order to scavenge the
Br + BrONO w Br2 + NO2
(6)
rather than reaction 3. On the basis of the preceding discussion,
it is concluded that both isomers BrNO2 and BrONO are
involved in the mechanism, and k2a/k2b, k3, and k-2a are
important but unknown parameters that are needed to fully
describe the Br + NO2 recombination/dissociation system. It is
difficult to unravel these rate parameters from the present
experiments, in particular since k6 is also not well-known and
the time resolution in the present work is poor at the beginning
of the photolysis phase owing to the time dependence of the
light intensity.

Kinetics of the Gas-Phase Reaction BrNO2 + NO
J. Phys. Chem. A, Vol. 102, No. 44, 1998 8629
olefin-Br addition product by addition of O2. There was no
difference in the lifetime of BrNO2 whether trans-2-butene was
present or not. Since the Br atoms scavenged by trans-2-butene
could have been reliberated in the course of the bromine atom
initiated degradation of trans-2-butene, acetaldehyde (3.5 × 1015
molecule cm-3) was added in place of trans-2-butene in three
experiments since Br atoms are irreversibly converted to HBr
by the reaction with acetaldehyde
k-2b,298K ) 1.2 s-1 can be derived for 298 K and M ) N2.3
Frenzel et al. analyzed their data on the thermal decay of BrNO2
at different temperatures as a superposition of homogeneous
and heterogeneous decomposition, characterized by different
activation energies. By fitting their data with a sum of
exponential terms, they arrived at an Arrhenius expression for
the homogeneous decomposition for 1 bar, M ) He, yielding
k-2a ) 4 × 10-5 s-1 at 298 K. From the falloff curves of
Kreutter et al. for the reverse reaction, k2a(N2)/k2a(He) )
k-2a(N2)/k-2a(He) ) 1.6 can be derived. Combining this ratio
with k-2a (M ) He) from Frenzel et al., k-2a (M ) N2) ) 6.4
× 10-5 s-1 results. The difference of the rate constants derived
above for k-2b and k-2a is considered to reflect the difference
in bond energies in BrONO and BrNO2. This conclusion relies
on the assumption that both reactions have a similar preexpo-
nential factor of the high-pressure rate constant and a comparable
falloff at atmospheric pressure. Setting ln(k-2b/k-2a) )
ln(1.2/6.4 × 10-5) ) -∆(∆H°r,298)/RT, ∆H°r,298(Br-NO2) -
∆H°r,298(Br-ONO) ) ∆H°f,298(BrONO) - ∆H°f,298(BrNO2) ∼
(24 ( 3) kJ mol-1 is derived. This value compares favorably
with the ab initio value of (27 ( 4) kJ mol-1 calculated by
Lee18 for the difference of the heats of formation of trans-
BrONO and BrNO2. From this difference of bond energies and
the above value for the heat of formation of BrONO,
∆H°f,298(BrNO2) ) (43 ( 6) kJ mol-1 is calculated. According
to the previous discussion, the findings of the experimental
studies of Kreutter et al., Frenzel et al., Mellouki et al., and the
present work on the Br + NO2 system are consistent with each
other and with the theoretical work of Lee provided that the
main product of the Br + NO2 recombination is the isomer
BrONO and that BrNO2 is thermally much more stable than
BrONO.
Br + CH3CHO w HBr + CH3CO
(8)
-1
(k8,298 ) 3.9 × 10-12 cm3 molecule
s-1 23). In these
experiments, synthetic air was used as a buffer gas in order to
scavenge the acetyl radicals formed in reaction 8 and to prevent
them from regeneration of Br atoms via the reaction
CH3CO + Br2 w CH3C(O)Br + Br
(9)
(k9,298 ) 1.08 × 10-10 cm3 molecule-1 s-1 24). In the presence
of synthetic air, the acetyl radicals react via the reactions
CH3CO + O2 (+M) w CH3C(O)O2 (+M)
(10)
CH3C(O)O2 + NO2 (+M) w CH3C(O)O2NO2 (+M) (11)
Since the first-order loss rate constants of BrNO2 at identical
reaction conditions varied within a factor of 2-3, these
experiments were conducted in such a way that the excess of
acetaldehyde was added in the middle of a long observation
period. There were indications for a slight increase of the first-
order loss rate constant after the addition of CH3CHO by a factor
of e2; however, this effect was difficult to quantify. Since it
was considered to be safe that the scavenging of Br atoms by
acetaldehyde is irreversible, these latter experiments were used
to derive a rigorous upper limit for k-2a at 298 K and
atmospheric pressure (M ) synthetic air) of k-2a e 4.0 × 10-4
s-1 (full circles in Figure 3).
The lifetimes of BrNO2 in our reaction chamber are larger
by 2 orders of magnitude as compared to the observation times
applied in the experiments for the reaction of BrNO2 with NO;
i.e., the loss of BrNO2 in the presence of NO is due to the
reaction of BrNO2 with NO and not to the reaction sequence
The results of the present work on the thermal lifetime of
BrNO2 are shown in Figure 3, together with the results of
Kreutter et al.3 and Frenzel et al.4 The first-order decay rate
constants of BrNO2 from the present work are consistent with
the results of Frenzel et al.4 and extend their temperature range
to lower temperatures. At present the origin of the discrepancies
between these low upper limits of k-2a and the high values
derived from the work of Kreutter et al. is not completely clear.
Probably, the BrONO isomer was primarily formed in the
reaction system of Kreutter et al.,3 as was already suggested by
Frenzel et al. BrONO was then thermally stable on the short
time scale of their experiments below 350 K, and regeneration
of Br atoms occurred above 350 K owing to thermal decom-
position of BrONO. The much longer lifetime of BrNO2 as
determined in the work of Frenzel et al. and the present work
can then be explained by the larger bond energy of BrNO2 as
compared to BrONO, which is also suggested by the theoretical
work of Lee.18 This being true, the value for the enthalpy of
formation of BrNO2 derived by Kreutter et al.3 and adopted by
the most recent JPL25 and IUPAC23 reviews should be assigned
to BrONO rather than BrNO2 (after reevaluation of the entropy
contribution). The most direct thermochemical result of Kreutter
et al., which is largely independent of the correct isomer
assignment, is the second-law value for the reaction enthalpy
of reaction 2 (∆H°r,298 ∼ ∆H°r,401 ) 78 ( 3 kJ mol-1, see Figure
5 of ref 3). If this value is assigned to the formation of BrONO
rather than BrNO2, ∆H°f,298(BrONO) ) (67 ( 3) kJ mol-1
results. From the rate parameters presented in their paper,
BrNO2 (+M) w Br + NO2 (+M)
Br + NO (+M) w BrNO (+M).
(-2a)
(12)
Another possible loss process of BrNO2 is the self-reaction
of BrNO2:
BrNO2 + BrNO2 w Br2 + 2NO2
(13)
Such a reaction is known for INO2 (k298K(INO2 + INO2 w I2
+ 2NO2) ) 4.7 × 10-15 cm3 molecule-1 s-1 23,26) but not for
ClNO2. However, reaction 13 must be of second order in
BrNO2, different from the first-order rate law observed in the
present work. In addition, the long lifetime of BrNO2 in the
dark shows that reaction 13 cannot be important on the time
scale of the present experiments.
Behavior of BrNO2 in the Presence of NO. The following
conclusions are drawn from the two previous sections:
(i) Probably, only a small fraction of the Br + NO2
recombination leads, at room temperature, to the isomer BrNO2.
Assigning the thermal decomposition rate constants of BrNO2
derived by Kreutter et al.3 to the isomer BrONO, BrONO would
be too unstable in the present reaction system to accumulate to
nonnegligible concentrations; i.e., the only important species
to react with NO is the isomer BrNO2.
(ii) BrNO2 is thermally stable under the conditions of the
present experiments, i.e., the loss of BrNO2 in the presence of
NO can be assigned to its bimolecular reaction with NO.

8630 J. Phys. Chem. A, Vol. 102, No. 44, 1998
Bro¨ske and Zabel
Figure 4. Decay of BrNO2 with time for different excess NO
concentrations, T ) 294 K, added NO (units of molecule cm-3): open
squares, no NO added; crosses, 1.78 × 1013, full squares, 3.85 × 1013,
triangles, 1.42 × 1014.
Figure 6. Concentration-time profiles of BrNO2 (triangles) and BrNO
(squares) in the dark; reaction conditions same as in Figure 5. During
the initial 10 s, the time behavior deviates from first order owing to
the finite mixing time of the added NO. In Figure 4, the time of the
second data point of each profile is set equal to zero.
Figure 5. Difference spectrum of a BrNO2/NO/N2 mixture in the dark
obtained by subtracting a spectrum before NO addition from a spectrum
after NO addition; T ) 283 K, p ) 988 mbar, M ) N2, reaction time
in the presence of NO ∼86 s; absorption at 1800 cm-1
: BrNO,
Figure 7. Pseudo-first-order rate constants of BrNO2 loss as a function
of [NO] at different temperatures.
absorptions at 1667 and 1292 cm-1: BrNO2.
The pseudo-first-order rate constants for BrNO2 loss, kfirst order
) k4[NO], are presented in Figure 7 as a function of [NO] at
294, 283, 273, and 263 K. The slopes of the straight lines
represent k4. An Arrhenius plot of k4 is shown in Figure 8.
The temperature dependence of k4 is represented by k4 ) 2.3
× 10-12 exp[(-17.8 ( 2.1) kJ mol-1/RT] cm3 molecule-1 s-1
(2σ). In Table 1, the Arrhenius parameters are compared with
those of related reactions. To the best of our knowledge, rate
constants for the reaction of NO with FNO2 and INO2 are not
available. However, for ClNO2 an Arrhenius expression with
a very similar preexponential factor but a higher activation
energy has been determined by Wilkins et al.28 and in this
laboratory.29 The latter work shows that the reaction of ClONO
with NO is faster than the reaction with ClNO2 by more than
an order of magnitude. Similarly, the rate constant for the
reaction of NO with BrONO
(iii) The increase of the stationary BrNO2 concentrations in
the photolysis experiments with the NO2 concentration according
to a power larger than 1 can possibly be explained by reaction
7. In the present study on the rate of reaction 4, however, this
reaction is unimportant owing to the absence of BrONO in the
dark.
In the presence of a large excess of NO, BrNO2 disappeared
according to a first-order rate law, the apparent first-order rate
constant being proportional to the NO concentration. This is
shown in Figure 4 for three different excess NO concentrations
and for [NO]0 ) 0 at 294 K. Simultaneous to the decay of
BrNO2, the formation of BrNO was observed by both its IR27
and UV11 absorptions. Absolute concentrations of BrNO2 and
BrNO were derived by using the UV absorption cross sections
of Scheffler et al.5 and Houel and van den Bergh,11 respectively.
BrNO was the only product apparent in the product IR spectrum
(Figure 5). The time constants for the decay of BrNO2 and the
formation of BrNO were the same (Figure 6).
BrONO + NO w BrNO + NO2
might be expected to be larger than k4.
(14)

Kinetics of the Gas-Phase Reaction BrNO2 + NO
J. Phys. Chem. A, Vol. 102, No. 44, 1998 8631
TABLE 1: Kinetic Data on Reactions XNO2 + NO w XNO + NO2 (X ) halogen)
∆H°r,298
k298
NO + ...
(kJ mol-1
)
(cm3 molecule-1 s-1
)
A (s-1
)
Ea (kJ mol-1
)
ref
FNO2
ClNO2
BrNO2
INO2
-17.85 (ref 23)
2.1 × 10-17
1.7 × 10-15
2.3 × 10-12
2.3 × 10-12
28.8
17.8 ( 2.1 (2σ)
28
this work
-18 ( 6 (ref 23)a
a See text for ∆H°f,298(BrNO2).
References and Notes
(1) Finlayson-Pitts, B.; Livingston, F.; Berko, H. Nature 1990, 343,
622.
(2) Behnke, W.; Scheer, V.; Zetzsch, C. J. Aerosol Sci. 1994, 25 (Suppl.
I), 277.
(3) Kreutter, K. D.; Nicovich, J. M.; Wine, P. H. J. Phys. Chem. 1991,
95, 4020; 1992, 96, 7146.
(4) Frenzel, A.; Scheer, V.; Sikorski, R.; George, Ch.; Behnke, W.;
Zetzsch, C. J. Phys. Chem. A 1998, 102, 1329.
(5) Scheffler, D.; Grothe, H.; Willner, H.; Frenzel, A.; Zetzsch, C.
Inorg. Chem. 1997, 36, 335.
(6) Barnes, I.; Becker, K. H.; Fink, E. H.; Reimer, A.; Zabel, F.; Niki,
H. Int. J. Chem. Kinet. 1983, 15, 631.
(7) Tevault, D. E. J. Phys. Chem. 1979, 83, 2217.
(8) Feuerhahn, M.; Minkwitz, R.; U. Engelhardt J. Mol. Spectrosc.
1979, 77, 429.
(9) Finlayson-Pitts, B. J.; Livingston, F. E.; Berko, H. N. J. Phys. Chem.
1989, 93, 4397.
(10) Frenzel, A.; Scheer, V.; Behnke, W.; Zetzsch, C. J. Phys. Chem. A
1996, 100, 16447.
(11) Houel, N.; van den Bergh, H. Int. J. Chem. Kinet. 1977, 9, 867.
(12) Harwood: M. H.; Jones, R. L. J. Geophys. Res. 1994, 99, 22955.
(13) Hippler, H.; Luu, S. H.; Teitelbaum, H.; Troe, J. Int. J. Chem. Kinet.
1978, 10, 155.
Figure 8. Arrhenius plot for k4.
(14) Grimley, A. J.; Houston, P. L. J. Chem. Phys. 1980, 72, 1471.
(15) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem.
Phys. Lett. 1978, 59, 78.
(16) Leu, M.-T. Int. J. Chem. Kinet. 1984, 16, 1311.
(17) Yarwood, G.; Niki, H. Private communication to K. D. Kreutter et
al. (1991), cited in ref 3.
(18) Lee, T. J. J. Phys. Chem. 1996, 100, 19847.
(19) Deuflhard, P.; Nowak, U. Ber. Bunsen-Ges. Phys. Chem. 1986, 90,
940.
(20) The mean value of the range of possible k3 values reported by
Mellouki et al. in ref 21.
4. Conclusion
Analysis of the kinetic data on BrNO2/BrONO from the
literature and the present work suggests that BrNO2 is more
stable than BrONO by about 23 kJ mol-1 and that the major
primary product in the recombination of Br atoms with NO2 is
BrONO. BrNO2 is thermally quite stable, with a thermal
lifetime of about 1 h or more at 298 K. This kinetic behavior
is analogeous to ClNO2/ClONO.
In the lower troposphere and for the presence of 1 ppb NO,
the lifetime of BrNO2 with respect to reaction with NO is 6.5
h; i.e., at low solar radiation and relatively high mixing ratios
of NO, the reaction with NO could be a nonnegligible loss
process of BrNO2. However, in those areas (the marine
troposphere) where the mixing ratio of BrNO2 can be expected
to be relatively high, the mixing ratio of NO is considerably
smaller than 1 ppb, and, for most tropospheric conditions,
thermal decomposition and, in particular, photolysis are probably
the major loss processes of BrNO2.
(21) Mellouki, A.; Laverdet, G.; Jourdain, J. L.; Poulet, G. Int. J. Chem.
Kinet. 1989, 21, 1161.
(22) Bierbach, A.; Barnes, I.; Becker, K. H. Int. J. Chem. Kinet. 1996,
28, 565.
(23) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr,
J. A.; Rossi, M. J.; Troe, J. J. Phys. Chem. Ref. Data, 1997, 26, 521.
(24) Nicovich, J. M.; Shackelford, C. J.; Wine, P. H. J. Photochem.
Photobiol. A. 1990, 51, 141.
(25) 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
Modeling; Evaluation No. 12, JPL Publication 97-4; Pasadena, CA, 1997.
(26) Van Den Bergh, H.; Troe, J. J. Chem. Phys. 1976, 64, 736.
(27) Finlayson-Pitts, B. J.; Johnson, S. N. Atmos. EnViron. 1988, 22,
1107.
Acknowledgment. The authors thank Prof. K. H. Becker
for his continuous support of this work. Financial support by
the EC (European Commission) under Contract EV5V-CT93-
0338 is gratefully acknowledged.
(28) Wilkins, Jr., R. A.; Dodge, M. C.; Hisatsune, I. C. J. Phys. Chem.
1974, 78, 2073.
(29) Bro¨ske, R. Unpublished results, 1998.