OH and OD Reactions with BrO
J. Phys. Chem. A, Vol. 105, No. 25, 2001 6165
sampling mass spectrometry. The rate constant for reaction 1
was obtained from the numerical simulation of the observed
temporal profiles of BrO. The value found for k1 was (7.5 (
calculations and (ii) the value of the branching ratio for HBr
formation (1%), which is the low limit of the range proposed
by the models (1-3%). Finally, it is difficult to make definitive
conclusion from this work if the additional HBr source from
the OH + BrO reaction, although significant, will be sufficient
to explain the difference between current modeled and observed
stratospheric HBr concentrations, since the uncertainties on the
kinetic data obtained here and the existing numerical simulations
overlap.
-
11
3
-1 -1
4
.2) × 10 cm molecule
s
at T ) 300 K and 1 Torr total
pressure. This value is higher than that measured in the present
study by a factor of 2, although the two values overlap
considering the relatively high uncertainty range given in ref
8
. The reaction between OD and BrO radicals has been
investigated for the first time in the present study. Similar data
were obtained for the rate constants of reactions 1 and 3. A
negligible isotopic effect could be expected for the studied
reactions, since the OH(OD) bond is not involved in the
chemical transformation.
Acknowledgment. This study is a part of the project funded
by the European Commission within the ,Environment and
Climate. Program (Contract ENV - CT97 - 0576, “COBRA”).
To our knowledge, no experimental mechanistic study of the
References and Notes
OH + BrO reaction has been carried out previously. However,
this reaction has been the subject of a recent theoretical study
(1) De More, 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; NASA, JPL; California Institute of Technology: Pasadena, CA,
1997.
(2) Hills, A. J.; Howard, C. J. J. Chem. Phys. 1984, 81, 4458.
(3) Bedjanian, Y.; Riffault, V.; Le Bras, G. Int. J. Chem. Kinet.,
submitted for publication.
of Sumathi and Peyerimhoff.37 HOOBr was found to be the
most stable isomer (33.8 kcal mol- below the reactants) of
the adducts which can be formed from the OH and BrO
association. It was also shown that the barrier for the HBr
1
-1
formation from this adduct is very high (∼39 kcal mol above
the HOOBr intermediate). As a result, the HBr formation in
the reaction OH + BrO was predicted to be of importance only
at temperatures above 2000 K. This is in disagreement with
the present study where HBr (at least DBr in reaction 2)
formation in reaction 1 was unambiguously observed at T )
(
4) Bedjanian, Y.; Le Bras, G.; Poulet, G. Chem. Phys. Lett. 1997,
266, 233.
(5) Chipperfield, M. P.; Shallcross, D. E.; Lary, D. J. Geophys. Res.
Lett. 1997, 24, 3025.
6) Chartland, D. J.; McConnell, J. C. Geophys. Res. Lett. 1998, 25,
(
55.
2
98 K.
(7) Nolt, I. G.; Ade, P. A. R.; Alboni, F.; Carli, B.; Carlotti, M.; Cortesi,
U.; Epifani, M.; Griffin, M. J.; Hamilton, P. A.; Lee, C.; Lepri, G.;
Mencaraglia, F.; Murray, A. G.; Park, J. H.; Park, K.; Raspollini, P.; Ridolfi,
M.; Vanek, M. D. Geophys. Res. Lett. 1997, 24, 281.
The atmospheric implications of the present kinetic data can
be briefly discussed. The potential role of the reaction OH +
5
,6
BrO has already been investigated in two modeling studies,
where it was shown that even with a very low yield of HBr
(8) Bogan, D. J.; Thorn, R. P.; Nesbitt, F. L.; Stief, L. J. J. Phys. Chem.
1
996, 100, 14838.
9) Bedjanian, Y.; Le Bras, G.; Poulet, G. J. Phys. Chem. A 1999, 103,
017.
(10) Stevens, P. S.; Brune, W. H.; Anderson, J. G. J. Phys. Chem. 1989,
93, 4068.
(11) Persky, A.; Kornweitz, H. Int. J. Chem. Kinet. 1977, 29, 68.
12) Bedjanian, Y.; Le Bras, G.; Poulet, G. Int. J. Chem. Kinet. 1999,
(
5
6
(
1-2% and 2-3% ), reaction 1b should be the dominant source
7
of HBr at altitudes between 20 and 35 km. Moreover, this will
reconcile model calculations with the results of stratospheric
7
,38,39
HBr observations.
In both studies (refs 5 and 6), the rate
(
-
11
3
-1 -1
constant k1 ) 7.5 × 10
cm molecule
s
measured by
31, 698.
8
Bogan et al. was used in the calculations. That means that 1%
(13) Nicovich, J. M.; Wine, P. H. Int. J. Chem. Kinet. 1990, 22, 379.
(14) Edrei, R.; Persky, A. Chem. Phys. Lett. 1989, 157, 265.
(15) Walther, C. D.; Wagner, H. G. Ber. Bunsen-Ges. Phys. Chem. 1983,
7, 403.
yield of HBr corresponded to the partial rate constant k1b ) 7.5
-
13
3
-1 -1
×
10 cm molecule s . This value can be compared with
8
the experimental value obtained in the present study: k1b <1.0
(16) Glaschick-Schimpf, I.; Leiss, A.; Monkhouse, P. B.; Schurath, U.;
-
12
3
-1 -1
×
10 cm molecule s . As one can see, the experimental
Becker, K. H.; Fink, E. H. Chem. Phys. Lett. 1979, 67, 318.
(17) Wada, Y.; Takayanagi, T.; Umemoto, H.; Tsunashima, S.; Sato, S.
J. Chem. Phys. 1991, 94, 4896.
upper limit for of k1b does not contradict with that proposed by
5
Chipperfield et al. to account for the difference between the
(18) Bedjanian, Y.; Riffault, V.; Le Bras, G.; Poulet, G. J. Phys. Chem.
measured and calculated HBr profiles. A comparison can be
also made with the experimental data obtained for the OD +
BrO reaction. However, in this case, two assumptions should
be made: the branching ratio for HBr formation in OH + BrO
reaction is independent of temperature and similar to that for
the DBr-forming channel of the reaction OD + BrO. These
assumptions seem to be reasonable, considering that they hold
A 2001, 105, 3167.
(19) Kaufman, F. J. Phys. Chem. 1984, 88, 4909.
(20) Morrero, T. R.; Mason, E. A. J. Phys. Chem. Ref. Data 1972, 1, 3.
(21) Mallard, W. G.; Westley, F.; Herron, J. T.; Frizzell, D.; Hampson,
R. F. NIST Chemical Kinetics Data Base, ver. 17-2Q98; NIST Standard
Reference Data: Gaithersburg, MD, 1998.
(22) Bossard, A. R.; Singleton, D. L.; Paraskevopoulos, G. Int. J. Chem.
Kinet. 1988, 20, 609.
(
23) Gilles, M. K.; Burkholder, J. B.; Ravishankara, A. R. Int. J. Chem.
Kinet. 1999, 31, 417.
24) Bedjanian, Y.; Riffault, V.; Le Bras, G.; Poulet, G. J. Phys. Chem.
40,41
for the analogous reactions of ClO radicals with OH and OD.
Thus, the 1% yield of DBr measured in the present work and
(
-
11
3
the value of the total rate constant k1 ≈ 5 × 10
molecule
(at T ) 220-230 K) gives k1b ≈ 5 × 10
at altitudes of 20-35 km. This value is
lower than that used in the model calculations (k1b ) 7.5 ×
cm molecule s ) by a factor 1.5, although both values
cm
A 2001, 105, 573.
(25) Bedjanian, Y.; Riffault, V.; Le Bras, G.; Poulet, G. J. Photochem.
Photobiol., A: Chemistry 1999, 128, 15.
-
1
-1
-13
s
3
-1 -1
cm molecule
s
(26) Toohey, D. W.; Brune, W. H.; Anderson, J. G. J. Phys. Chem.
1987, 91, 1215.
-13
3
-1 -1 5
10
(27) Laverdet, G.; Le Bras, G.; Mellouki, A.; Poulet, G. Chem. Phys.
Lett. 1990, 172, 430.
overlap if the uncertainty of 50% on the branching ratio k2b/k2
is considered. In conclusion, the present work gives an
experimental evidence for the occurrence of HBr formation in
the OH + BrO reaction. However, the impact of this reaction
on the total HBr budget in the stratosphere seems to be less
important than currently predicted by the models due to (i) a
lower value for the total rate constant than that used in the
(28) Ravishankara, A. R.; Wine, P. H.; Langford, A. O. Chem. Phys.
Lett. 1979, 63, 479.
(29) Jourdain, J. L.; Le Bras, G.; Combourieu, J. Chem. Phys. Lett. 1981,
7
8, 483.
30) Cannon, B. D.; Robertshaw, J. S.; Smith, I. W. M.; Williams, M.
D. Chem. Phys. Lett. 1984, 105, 380.
31) Ravishankara, A. R.; Wine, P. H.; Wells, J. R. J. Chem. Phys. 1985,
83, 447.
(
(