Kinetic and products of the BrO+CH3SH reaction: Temperature and pressure dependence

Article abstract of DOI:10.1016/S0009-2614(02)00561-4

The kinetics and the mechanism of the reaction BrO + CH₃SH → P (1) have been studied using the mass spectrometric discharge-flow method over the temperature range 259-333 K and at low total pressure between 0.5 and 3 Torr. The temperature depen

Full text of DOI:10.1016/S0009-2614(02)00561-4

17 May 2002
Chemical Physics Letters 357 (2002) 471–476
www.elsevier.com/locate/cplett
Kinetic and products of the BrO þ CH3SH
reaction: temperature and pressure dependence
ꢀ
ꢀ
Alfonso Aranda, Yolanda Dıaz de Mera, Diana Rodrıguez,
*
ꢀ
Sagrario Salgado, Ernesto Martınez
Departamendo de Quꢀımica Fisica, Facultad de Ciencias Quꢀımicas, Universidad de Castilla-La Mancha,
Avda. Camilo Joseꢀ Cela, no.10., Campus Universitario s/n, 13071 Ciudad Real, Spain
Received 11 February 2002; in final form 29 March 2002
Abstract
The kinetics and the mechanism of the reaction BrO þ CH3SH ! P (1) have been studied using the mass spec-
trometric discharge-flow method over the temperature range 259–333 K and at low total pressure between 0.5 and 3
Torr. The temperature dependence of the reaction rate constant has been determined under pseudo-first-order kinetic
conditions in excess of CH3SH over BrO radicals, k ¼ ð2:2 Æ 1:9Þ Â 10À15 exp½ð827 Æ 255Þ=TŠ cm3 moleculeÀ1
s
À1. The
reaction has been found to be pressure dependent in the pressure range studied. The experimental evidences suggest an
addition mechanism. The studied reaction constitutes an efficient atmospheric sink for the reduced sulfur compound,
CH3SH. Ó 2002 Published by Elsevier Science B.V.
1. Introduction
Methyl mercaptan (CH3SH) is among the ma-
jor trace reduced sulfur compounds emitted into
the atmosphere. Natural biogenic emissions are
the primary sources for this reduced compound [2].
The dominant fate of reduced sulfur compounds in
the atmosphere is chemical transformation. The
primary oxidants responsible for the initiation of
atmospheric oxidation for CH3SH in the tropo-
Gas phase sulfur compounds represent only a
small fraction of the Earth’s atmospheric compo-
sition, comprising less that 1 ppm by volume in air
[1]. However, the chemistry of these minor con-
stituents has a significant impact on the atmo-
sphere and the biosphere: acid rain, visibility
reduction and climate modification. An under-
standing of the natural sulfur cycle is thus required
in order to establish a base line with which an-
thropogenic perturbations can be compared.
sphere are the reaction with OH radicals (k298 K
¼
3:27 Â 10À11 cm3 moleculeÀ1 sÀ1 [3]) and the reac-
tion with NO3 under night-time conditions
(k ¼ 9:21 Â 10À13 cm3 moleculeÀ1 sÀ1 [3]. Under
specific conditions, like in the marine boundary
layer, initiation by different halogen compounds
may also be important.
* Corresponding author. Fax: +34-26-29-53-18.
E-mail address: emartine@qifi-cr.uclm.es (E. Martınez).
The principal known gas phase sources for
tropospheric inorganic halogen species, including
ꢀ
0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S0009-2614(02)00561-4

472
A. Aranda et al. /Chemical Physics Letters 357 (2002) 471–476
the oxides, are the organic halogen compounds.
The oceans provide an important natural source of
these compounds such as the methyl halides
(CH3Cl; CH3Br; CH3I). Once in the atmosphere,
these molecules are broken down by direct pho-
tolysis or by attack by photochemically generated
radicals [4]. Halogen atoms mainly react with
ozone to form halogen oxides (ClO, BrO, IO).
Known heterogeneous sources of halogen atoms
are the reactions of sea-salt particles with nitrogen
oxides and the photolysis of ozone in the presence
of sea-salt particles in the ocean surface layer [5].
Several reactions of CH3SH with halogen
compounds have been studied. For the reactions
with F, Cl and Br, the obtained values for the rate
constant at 298 K are 2:4 Â 10À10 [6], 1:97 Â 10À10
Fig. 1. Experimental set-up.
volving DMS and DMSO reactions with chlorine
atoms [11,12]. Only details relevant to the present
study are given. The main reactor consisted of a
pyrex tube (100 cm length and 2.7 cm i.d.) with a
jacket for the thermostated liquid circulation. A
system of two injectors was used in order to se-
quentially generate Br atoms (injector 1) and BrO
radicals (injector 2). The walls of the reactor as
well as the inner surfaces of the two injectors were
coated with halocarbon wax. Both radical and
molecular species were sampled from the reactor
and analyzed by a quadrupole mass spectrometer
(VG Smart IQþ) with electron impact ion source.
BrO radicals were generated in two steps. First, Br
atoms were produced by passing a Br2=He mixture
through a microwave discharge and added to the
He flow, inside injector 1. Secondly, BrO radicals
were generated inside the movable injector 2 by
reaction (2) before being introduced into the main
reactor:
[7] and 2:53 Â 10À12 [8] cm3 moleculeÀ1
s
À1, re-
spectively, which are fast enough as to be consid-
ered significant in comparison with the OH and
NO3 reactions.
Although the total amount of chlorinated
compounds is higher than species containing Br or
I, the inorganic reservoirs, HOX, XONO2, HX,
are rapidly photolysed when X ¼ Br, I. So most of
bromine and iodine are in their active form X or
XO. In relation to the halogen oxides XO (X ¼ I,
Br and Cl), up to now, only the reaction
CH3SH þ IO has been studied [9]. Halogen oxides
in tropospheric air have been measured with the
DOAS technique. For BrO, levels range from 5 to
60 Â 108 molecule cmÀ3 [10] showing that their
reaction towards methyl mercaptan ought to be
measured in order to evaluate the potential con-
tribution to the removal of CH3SH. So, in this
work we report the first absolute kinetic study of
the reaction BrO þ CH3SH (1), its temperature
(259–333 K) and pressure (0.5–3 Torr) depen-
dence, the observation of the products of reaction
with the possible mechanism and its atmospheric
implication.
Br þ O3 ! BrO þ O2
k2 ¼ 1:2 Â 10À12 cm3 moleculeÀ1 sÀ1
ð2Þ
(cf. [13]) with ½O3Š ꢀ ½BrŠ to ensure complete
consumption of the Br atoms, in order to avoid the
interference of Br þ CH3SH reaction (k ¼ 2:5 Â
10À12 cm3 moleculeÀ1 sÀ1 [13]). O3 was generated
in an ozonizer (Ozogas, 5 g/h) and collected in a
liquid N2 trap before being stored diluted in he-
lium. The O3 was used freshly prepared within an
experimental session to avoid the decomposition
of O3 in O2. Typical concentrations of Br2 and O3
in the reactor were ð0:7–2Þ Â 1012 and ð0:6–1Þ Â
1014 molecule cmÀ3, respectively. These chemicals
were followed by mass spectrometry at m=e ¼ 162
2. Experimental
The kinetic experiments were carried out using
the discharge flow-mass spectrometry technique
(DF-MS). The experimental set-up, shown in Fig.
1, was similar to that used in previous studies in-

A. Aranda et al. /Chemical Physics Letters 357 (2002) 471–476
473
and m=e ¼ 48, respectively, whereas BrO radicals
was checked at m=e ¼ 97.
Only reaction (5) involves the net elimination of
BrO radicals. The formation of Br atoms in pres-
ence of an excess of O3 implies the regeneration of
BrO. In most cases, for the relatively low BrO
concentrations used in this work, the homoge-
neous losses were negligible. The obtained rate
constants for the wall losses were also low,
kw ¼ ð1–3Þ sÀ1. No reaction of CH3SH with Br2 or
O3 (precursors of BrO radicals) was observed
within the contact time used in the experiments
(40–50 ms).
All the experimental kinetics were carried out
under pseudo-first-order conditions with CH3SH
in great excess over BrO with a ratio ½CH3SHŠ=
½BrOŠ from 200 to 2000. The kinetic expression
applying to such experiments was
The absolute concentration of BrO radicals
could be measured after or before a kinetic run by
titration with NO:
BrO þ NO ! Br þ NO2
ð3Þ
k3 ¼ 2:1 Â 10À11 cm3 moleculeÀ1 sÀ1
(cf. [13]) with subsequent mass spectrometric
quantitative detection of the NO2 at m=e ¼ 46.
This NO2 signal was calibrated using known NO2
concentrations from a storage balloon. Typical
concentration for BrO radicals was about 5 Â 1011
molecule cmÀ3
, excepting for experiments at
T ¼ 298 K and P ¼ 3 Torr, where concentration
was 2 Â 1012 molecule cmÀ3. The detection limit
for BrO was 5 Â 109 molecule cmÀ3. CH3SH con-
centrations ranged from 0.1 to 1 Â 1015
lnð½BrOŠ0=½BrOŠtÞ ¼ ðk0 þ kwÞt;
where k0 ¼ k1½CH3SHŠ:
ð6Þ
molecule cmÀ3
.
Typical logarithmic decays of the intensity of
BrO signal as a function of reaction time are
shown in Fig. 2. A slight curvature could be con-
jectured for times of reaction greater than ap-
proximately 30 ms. This could be due to secondary
reactions of BrO with the products of reaction (1).
However this curvature could not be completely
confirmed since it was not systematically observed
in all the kinetic runs. Furthermore the calculated
k0 using times up to 30 ms generally differs in less
than 5% from the calculation with times up to 50
ms and so it must not significantly affect the ex-
perimental pseudo-first-order constant. The bi-
molecular rate coefficient of interest, k1ðP; TÞ, is
The purities of the gases used were as follows:
O2, 99.999% (Praxair); NO, P99.5% (Praxair);
NO2, P99.5% (Praxair); He, 99.999% (Praxair),
which was passed through a liquid nitrogen trap;
Br2, >99.5% (Aldrich) and CH3SH, >99.5%
(Aldrich), which were purified by trap to trap
distillation.
3. Results
The experiments were carried out within the
temperature, pressure and mean flow velocity
ranges (259–333) K, (0.5–3.0) Torr and (650–1400)
cm sÀ1, respectively. In the absence of CH3SH,
BrO was expected to be lost through wall hetero-
geneous reactions and homogeneous recombina-
tion:
BrO þ BrO ! Br þ Br þ O2
ð4Þ
k4 ¼ 5:31  10À12 expðÀ211=TÞ cm3 moleculeÀ1 sÀ1
(cf. [14])
BrO þ BrO ! Br2 þ O2
ð5Þ
k5 ¼ 1:13 Â 10À14 expð983=TÞ cm3 moleculeÀ1 sÀ1
(cf. [14]).
Fig. 2. Examples of pseudo-first-order plots for reaction
BrO þ CH3SH with ½CH3SHŠ ð1014 molecule cmÀ3Þ ¼ 0:71 (r),
5.34 (j), 8.65 (N) at 298 K and P ¼ 1 Torr. Diluent gas: He.

474
A. Aranda et al. /Chemical Physics Letters 357 (2002) 471–476
evaluated from the slope of k0 versus [CH3SH] plot
applying weighted least squares fittings. Some ex-
amples of these plots are given in Fig. 3 showing
that the results are consistent with Eq. (6). In some
kinetic runs at 3 Torr, the BrO concentration was
slightly higher (up to 2 Â 1012 molecule cmÀ3) and
BrO self-recombination reaction was also taken
into account using the FACSIMILE program. The
values obtained in these cases for k0 were from 2 to
4 sÀ1 lower than applying Eq. (6) directly (ne-
glecting the homogeneous recombination). The
obtained results for k1 in all the experimental
conditions are shown in Table 1, where errors are
one standard deviation, beingÀ1 k1ð298 KÞ ¼
Fig. 4. Arrhenius plot for the BrO þ CH3SH reaction at 1 Torr
of total pressure in He.
ð3:21 Æ 0:15Þ Â 10À14 cm3 molecule sÀ1
.
Experiments were carried out at four different
temperatures and 1 Torr of total pressure of he-
lium. The reaction rate was found to increase with
decreasing temperature. The data obtained are
shown plotted in Arrhenius form in Fig. 4. The
Fig. 5. Rate coefficients for the BrO þ CH3SH reaction as a
function of pressure. Diluent gas: He.
following Arrhenius expression, derived from that
figure, was obtained:
k1 ¼ ð2:2 Æ 1:9Þ Â 10À15
 exp½ð827 Æ 255Þ=T Š cm3 moleculeÀ1 sÀ1
:
Fig. 3. Second-order plot for the BrO þ CH3SH reaction at
different temperatures. T ðKÞ ¼ 259 (j), 298 (r), 333 (N) at
P ¼ 1 Torr. Diluent gas: He.
ð7Þ
Concerning the pressure range studied (from 0.5
to 3 Torr), we have found a linear pressure de-
pendence for k1. The rate coefficient increased when
pressure was increased as it is shown in Fig. 5.
Table 1
Summary of second-order rate constants for BrO þ CH3SH
reaction at different temperatures and pressures. Errors are r
T
(K)
P
(Torr)
k1
ðcm3 moleculeÀ1 sÀ1
Þ
4. Discussion
259
278
298
1.0
1.0
0.5
1.0
3.0
1.0
ð5:71 Æ 0:44Þ Â 10À14
ð5:25 Æ 0:70Þ Â 10À14
ð2:69 Æ 0:53Þ Â 10À14
ð3:21 Æ 0:15Þ Â 10À14
ð4:85 Æ 0:36Þ Â 10À14
ð2:93 Æ 0:20Þ Â 10À14
Reactions of CH3SH with halogen atoms have
been studied by several authors [7,8] testing the
temperature and pressure dependence. For methyl
mercaptan reaction with halogen oxides only the
study of CH3SH þ IO reaction has been reported
333

A. Aranda et al. /Chemical Physics Letters 357 (2002) 471–476
475
k9 ¼ 4 Â 10À11 cm3 moleculeÀ1 sÀ1
[9]. In that work the absolute rate constant was
measured only at T ¼ 298 K ðk ¼ ð6:6 Æ 1:3Þ Â
10À16 cm3 moleculeÀ1 sÀ1Þ with detection of HOI
as product by mass spectrometry. This Letter re-
ports the first absolute measurement for the kinetic
rate constant of reaction BrO þ CH3SH and its
study with temperature and pressure changes.
In this work some additional experiments in-
creasing the concentration of the reactants were
conducted to identify the products and interme-
diates of reaction (1). In these experiments
½CH3SHŠ ranged from 1  1014 to 1  1015
molecule cmÀ3 and [BrO] from 2 Â 1012 to 5 Â 1012
molecule cmÀ3. The following peaks were observed
by mass spectrometry, m=e ¼ 63, 94, 96, 98, 126,
128. The signals at m=e ¼ 96, 98 are attributed to
HOBr (with different isotopic Br) which is consis-
tent with the observation of HOI as a product in
the CH3SH þ IO reaction. It was verified that
HOBr produced in reaction (1) did not contribute
to the BrOþ peak (m=e ¼ 97) by observing a
complete disappearance of the peak m=e ¼ 97
when high enough concentrations of CH3SH were
used to totally consume BrO. Such a contribution
of HOBr to the BrOþ peak would have led to
underestimate k1. No direct calibration of HOBr
could be performed and, so, the branching ratio
could not be obtained.
(cf. [17])
CH3S þ Br2 ! CH3SBr þ Br
ð9Þ
k10 ¼ ð1:7 Æ 0:5Þ Â 10À10 cm3 moleculeÀ1 sÀ1
(cf. [8])
CH3S þ O3 ! P ðCH3SOÞ
k11 ¼ 5:29 Â 10À12 cm3 moleculeÀ1 sÀ1
(cf. [13]).
The addition mechanism proposed in this work
is supported by the results obtained for similar
reactions. OH has been shown to react with
CH3SH through the formation of a weakly bound
complex [3,18]. Furthermore, DMS–BrO reaction
also proceeds through the formation of a long
lived addition intermediate [16].
Concerning the atmospheric implications in the
removal of CH3SH by BrO, the high [BrO] trop-
ospheric concentrations and the measured rate
constant of this reaction show reaction (1) as a
potentially important sink in the marine atmo-
sphere. Although BrO–CH3SH rate constant is
three orders of magnitude lower than the
OH–CH3SH rate constant, the measured BrO
concentration is in the range ð5–60Þ Â 108
molecule cmÀ3, much higher thanÀt3he [OH] con-
centration (9:7 Â 105 molecule cm [19]). From
the previous data, the room temperature gas phase
lifetime of methyl mercaptan may be calculated as
8.7 h for OH radicals, and from 15.6 to 1.3 h for
BrO. Thus, it may be concluded that CH3SH
would be scavenged efficiently by BrO and so our
results should be taken into account in the
chemical box models to evaluate the impact of the
removal of CH3SH in the marine boundary
layer.
ð10Þ
The relative low value of the pre-exponential
factor, the negative temperature dependence ob-
served for k1, and the increase of this constant with
higher pressures clearly suggest the formation of
an association complex [15,16] which can then
decompose either back to reactants or to give
products
ꢁ
BrO þ CH3SH ¢ ½CH3SðOBrÞHŠ
! CH3S þ HOBr
ð1Þ
In the experiments designed to identify prod-
ucts, where concentrations are considerably higher
than in the kinetic runs, CH3S radicals may un-
Furthermore our low pressure measurement
(p < 3:0 Torr) may underestimate the real atmo-
spheric rate constant for BrO–CH3SH since ther-
malization of the association complex may be
inefficient. So the potential importance of reaction
(1) under atmospheric conditions could even be
larger and kinetic and mechanistic data are also
needed.
dergo
self-recombination
giving
DMDS
(m=e ¼ 94), reaction with Br2 giving CH3SBr
(m=e ¼ 126, 128), and with ozone in excess origi-
nating CH3SO (m=e ¼ 63), what would explain all
the minor mass spectrometric peaks observed:
CH3S þ CH3S ! CH3SSCH3
ð8Þ

476
A. Aranda et al. /Chemical Physics Letters 357 (2002) 471–476
[10] M. Hausmann, U. Platt, J. Geophys. Res. 99 (1994) 25399.
References
ꢀ
ꢀ
[11] E. Martınez, A. Aranda, Y. Dıaz de Mera, D. Rodrıguez,
M.R. Lopez, J. Albaladejo. Environ. Sci. Technol. 36
ꢀ
ꢀ
[1] G.S. Tyndall, A.R. Ravishankara, Int. J. Chem. Kinet. 23
(1991) 483.
(2002) 1226.
ꢀ
[12] Y. Dıaz de Mera, A. Aranda, D. Rodrıguez, M.R. Lopez,
B. Cabanas, E. Martınez, J. Phys. Chem. A, to be
ꢀ
ꢀ
[2] S.P. Urbanski, R.H. Wine, in: Z.B. Alfassi (Ed.), S-
Centered Radicals, Wiley, Chichester, 1999, p. 97.
[3] R. Atkinson, D.L. Baulch, R.A. Cox, R.F. Hampson Jr.,
J.A. Kerr, M.J. Rossi, J. Troe, J. Phys. Chem. Ref. Data 26
(1997) 1329.
~
ꢀ
published.
[13] W.B. De More, S.P. Sander, D.M. Golden, R.F. Hamp-
son, M.J. Kurylo, C.J. Howard, A.R. Ravishankara, M.J.
Molina, C.E. Kolb, Chemical Kinetics and Photochemical
Data for Use in Stratospheric Model, JPL, California
Institute of Technology, Pasadena, CA, 1997.
[14] M.H. Harwood, D.M. Rowley, R.A. Cox, R.L. Jones, J.
Phys. Chem. A 102 (1998) 1790.
[4] R.P. Wayne, G. Poulet, P. Biggs, J.P. Burrows, R.A. Cox,
P.J. Crutzen, G.D. Hayman, M.E. Jenkin, G. Le Bras,
G.K. Moortgat, U. Platt, R.N. Schindler, in: R.P. Wayne
(Ed.), Halogen Oxides: Radicals, Sources and Reservoirs in
the Laboratory and in the Atmosphere, Atmospheric
Environment, Pergamon Press, Oxford, 1995.
[5] K.W. Oum, M.J. Lakin, D.O. DeHaan, T. Brauers, B.J.
Finlayson-Pitts, Science 279 (1998) 5347.
[15] A. Fontijn, R. Zellner, in: A. Fontijn, M.A.A. Clyne
(Eds.), Reactions of Small Transient Species, Academic
Press, London, 1983, p. 41.
[16] T. Ingham, D. Bauer, R. Sander, P.J. Crutzen, J.N.
Crowley, J. Phys. Chem. A 103 (1999) 7199.
[6] M.M. Rahman, E. Becker, U. Wille, R.N. Schindler, Ber.
Bunsenges. Phys. Chem. 96 (1992) 783.
[17] C. Anastasi, M. Broomfield, O.J. Nielsen, P. Pagsberg,
Chem. Phys. Lett. 182 (1991) 643.
[7] J.M. Nicovich, S. Wang, P.H. Wine, Int. J. Chem. Kinet.
27 (1995) 359.
[18] A.J. Hynes, P.H. Wine, J. Phys. Chem. 91 (1987) 3672.
[19] R. Prinn, R. Weiss, B. Miller, J. Huang, F. Alyea, D.
Cunnold, P. Fraser, D. Hartley, D. Simmons, Science 269
(1995) 187.
[8] J.M. Nicovich, K.D. Kreutter, C.A. Van Dijk, P.H. Wine,
J. Phys. Chem. 96 (1992) 2518.
[9] F. Maguin, A. Mellouki, G. Laverdet, G. Poulet, G. Le
Bras, Int. J. Chem. Kinet. 23 (1991) 237.