A kinetic study of gas-phase reaction of thiophene with NO3 by using absolute and relative methods

Article abstract of DOI:10.1016/S0009-2614(02)00494-3

The gas-phase rate coefficient for the reaction of thiophene with nitrate radical, NO₃, has been determined, using relative (gas chromatography as analytical tool) and absolute methods at 298 K. The absolute experiments were carried out using a fast-flow-discharge technique, with detection of the NO₃ radical by Laser Induced Fluorescence (LIF). The proposed rate coefficient at 298 K is, k = (3.16 ± 0.27) × 10^-14 cm³ molecule^-1 s^-1. The temperature dependence was studied by the absolute technique in the range 260-433 K, observing that in the interval of 260-277 K the rate coefficient decreases as the temperature increases and from 298 K onwards the rate coefficient increases with temperature.

Full text of DOI:10.1016/S0009-2614(02)00494-3

7
June 2002
Chemical Physics Letters 358 (2002) 401–406
www.elsevier.com/locate/cplett
A kinetic study of gas-phase reaction of thiophene with NO3
by using absolute and relative methods
*
B. Caba n~ as ,P. Mart ꢀı n,S. Salgado,M.T. Baeza,M.R. L oꢀ pez,E. Mart ꢀı nez
Facultad de Qu ꢀı micas, Universidad de Castilla-La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain
Received 21 December 2001; in final form 6 March 2002
Abstract
The gas-phase rate coefficient for the reaction of thiophene with nitrate radical,NO
relative (gas chromatography as analytical tool) and absolute methods at 298 K. The absolute experiments were carried
3
,has been determined,using
out using a fast-flow-discharge technique,with detection of the NO
À14
3
3
radical by Laser Induced Fluorescence (LIF). The
À1 À1
proposed rate coefficient at 298 K is, k ¼ð3:16 Æ0:27ÞÂ10
cm molecule
s . The temperature dependence was
studied by the absolute technique in the range 260–433 K,observing that in the interval of 260–277 K the rate coefficient
decreases as the temperature increases and from 298 K onwards the rate coefficient increases with temperature. Ó 2002
Elsevier Science B.V. All rights reserved.
1
. Introduction
of different studies [14–24]. However,the investi-
gation about the reactivity of these compounds,
Nitrate,(NO
3
) and hydroxyl (OH) radicals to-
gether with ozone (O ) are important oxidant
3
with the NO radical,is specially scarce,and more
kinetic data for the NO reaction with heterocycle
3
3
agents in the Earth’s atmosphere [1–6]. Their re-
actions are the principal degradation routes for
many organic compounds. The OH radical reac-
tion is normally the main sink for numerous
compounds are needed to get a better under-
standing on its role in the atmospheric chemistry.
Thiophene is a five-member heterocycle ring
that is emitted in processes of energy production
such as coal gasification,fuel oil,natural gas and
coal combustion and also it is detected over sea-
weed fields [25–28]. Thiophene is a sulphur-con-
taining compound and the study of its chemistry is
important to understand the whole role of the
sulphur compounds in the atmospheric global
sulphur cycle. The reaction of thiophene with OH
compounds during the daytime. However the NO
3
radical that undergoes photolysis by sunlight [7] is
mainly involved in night-time atmospheric chem-
istry. Many kinetic and mechanistic studies of the
oxidation of organics by OH,O 3 and NO have
3
been reported [4–6,8–14]. The reactions of some
3
heterocycles with OH and O have been the subject
and O
[
3
has been reported in different works
6,14,15,21], but the night-time reaction of the
*
3
NO radical with thiophene has been less investi-
Corresponding author.
E-mail address: beatriz.cabanas@uclm.es (B. Caba n~ as).
gated [4,5,29]. So far the available rate constant
0
009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614(02)00494-3

402
B. Caba ~n as et al. / Chemical Physics Letters 358 (2002) 401–406
has been obtained from relative technique and no
data on the temperature dependence have been
reported.
The rate coefficient, k,is defined by:
d½NO
dt
3
ꢀ
¼k½NO ꢀ½thiopheneꢀ:
3
ðIÞ
In this work,we report the first absolute kinetic
study of the reaction of NO
fast flow system by using LIF detection of the NO
radical at different temperatures between 260 and
33 K,together with the relative study of this re-
action at 298 K,using 1-butene as reference
compound.
3
with thiophene in a
As all the kinetic experiments were performed
under pseudo-first-order conditions,with a large
excess of reactants over NO for the analysis of the
experimental results of these reactions,the inte-
grated rate expression [32] used:
3
3
4
½
NO
3
ꢀ
0
0
ln
¼ðk½thiopheneꢀÞt ¼k t;
ðIIÞ
½NO
where
3
ꢀt
2
. Experimental
k0¼k½thiopheneꢀ:
The pseudo-first-order rate coefficient, k ,is ob-
tained as the slope of the straight line when
ðIIIÞ
2
.1. Absolute rate experiments
0
All the kinetic measurements of absolute rate
constants were carried out using a discharge-flow
tube reactor with LIF detection for the nitrate
radical. The experimental set-up has been des-
cribed elsewhere [30] and only the relevant details
are given here. All the experiments were carried
out at low pressure (about 1 Torr). The tempera-
ture flow tube was set between 260 and
lnð½NO
3
ꢀ=½NO
3
ꢀÞis plotted vs. time. The second-
0
t
order rate coefficient k is obtained as the slope of
0
the plot of k vs. [thiophene] according to Eq. (III).
2.2. Relative rate experiments
The relative rate experiments were carried out
in a ca. 60 l collapsible Teflon bag (Alltech). The
bag was housed in a dark cabinet,and operated at
atmospheric pressure ð720 Æ10 TorrÞ and
298 Æ1 K. Known quantities of each component
of the reaction mixture were metered using a vac-
uum line and volume–pressure measurement.
These components were introduced separately into
the reactor in a stream of bath gas. Concentrations
of thiophene,reference and internal-standard
compounds contained in the bag were measured
by sampling the bag to a gas chromatograph (SRI
8610C) equipped with flame ionisation detection
4
35ðÆ1ÞK.
Nitrate radicals were generated by the reaction,
F þHNO
into the flow tube through a fixed port. Fluorine
atoms were obtained by passing F –He mixtures
through a microwave discharge. The initial NO
concentrations were in the range 0:6–10Â
molecule cm .
Quantitative detection of NO
by monitoring the fluorescence emitted after the
3
!NO
3
þHF,and they were admitted
2
3
12
À3
1
0
3
was carried out
0
2
0
(
0–0) 2E
k ¼662:0 nm radiation from a dye laser (Quantel,
TDL90). Absolute concentrations of NO were
3
A transition of NO ,pumping with
2
3
(GC-FID) via a 5 cm sample loop. Separation of
3
determined before or after each kinetic run by
chemical titration with known amounts of te-
tramethylethene (TME) [31] The detection limit
individual compounds in the sample was achieved
using a stainless steel (SS) column packet Chro-
mosorb 102 80-100 MT 250. The reference com-
pound was 1-butene and the internal standard was
ethane. Typical initial concentrations of the inter-
nal standard and reference compounds in the bag
for the NO
around 10 molecule cm .
3
1
concentration in our LIF system is
À3
1
Thiophene was introduced through a sliding
injector,whose position could be varied up to 70
cm from the centre of the observation region. The
15
15
were ð3:4–3:7ÞÂ10 and ð5:9–6:5ÞÂ10 mole-
À3
cule cm ,respectively. The temperature for the
column was programmed from 313 to 493 K.
13
13
concentrations,ranging from 1 Â10 to 5 Â10
À3
molecule cm . Contact times between NO and
3
NO
decomposition of N O
3
radicals were generated by the thermal
,which was synthesized in
the reactant were in the range 0–80 ms.
2 5

B. Caba ~n as et al. / Chemical Physics Letters 358 (2002) 401–406
403
a separate set up by the reaction of O
NO according to reactions (1) and (2).
3
excess with
NO
NO
2
2
and reference,NO
and substrate,NO
3
radical and substrate,
3
radical and internal
2
standard,and NO
experiments were carried out in order to examine if
any possible nitrogen oxide compounds,i.e. NO
,NO ,would react with the gas chromato-
2
and internal standard. Some
O
3
þNO !NO þO
2
3
2
ð1Þ
ð2Þ
2
,
NO ¡N
2
þNO
3
2 5
O
N
2
O
5
3
The relative rate technique is based on monitoring
the relative decay rates of a series of organics,in-
graphic column packing to produce ‘ghost’ peaks.
cluding at least one organic whose NO
reaction rate constant must be reliably known,in
presence of NO radicals.
3
radical
3. Materials
3
The NO radical reacts with thiophene,with an
3
3.1. Absolute experiments
unknown rate coefficient k,and with a reference
organic compound,in our case 1-butene with a
Helium was used as carrier gas in the absolute
experiments. Its purity (better than 99.999%,
Carburos Met ꢀa licos C50) was improved by means
of an Oxisorb (Messer Griesheim) trap and a
molecular-sieve trap at room temperature.
Nitric acid vapour was obtained by bubbling
He through 2:1 concentrated sulphuric–nitric acid
mixtures (P.A. quality,Panreac) at 252 K. Mo-
lecular fluorine was supplied by Praxiar mixed
known rate coefficient,
À1 À1
k
cm molecule s [4]. The reactions are:
1-butene ¼ð1:2 Æ0:4ÞÂ
À14
3
1
0
NO
NO
3
þthiophene !products
ð3Þ
ð4Þ
3
þreference organic !products
The rate coefficient, k,is defined by Eq. (I). Pro-
viding that thiophene and the reference organic
only react with NO
3
radicals,the loss of organic
with helium (5% F ,95% He).
2
compounds is given by
ꢀ ½
½
thiopheneꢀ
ꢁ
3.2. Gas chromatography
0
ln
ÀDt
thiopheneꢀ
t
ꢀ ½
ꢁ
Nitrogen (C50) was supplied by Praxair,and
hydrogen was generated by a Hydrogen Generator
referenceꢀ0
referenceꢀt
where subscripts 0 and t refer to time 0 and t,
¼
k=kreference ln
ÀD
t
;
ðIVÞ
½
(Parker Chrom Gas,9150).
respectively,of an experiment for the thiophene
3.3. N
2
O
5
synthesis
t
and the reference organic compound and D is the
dilution factor at time t,due to the small
amounts of dilution occurring from the incre-
mental additions of N O to the reactant mix-
NO
2
(Praxair + 99%) and O
2
(Carburos
Met ꢀa licos + 99%) were used as received.
2
5
tures. Plots of lnð½thiopheneꢀ=½thiopheneꢀÞÀD Þ
3.4. Organics
0
t
t
vs. ðlnð½referenceꢀ=½referenceꢀÞÀD Þshould yield
t
0
t
straight lines with slope k=kreference
With this experimental technique,the initial
.
Thiophene (96%) was purified by freeze-pump-
throw cycles before use,ethane (99+%),and 1-
butene (99+%) are from Aldrich.
15
concentration of thiophene was 3:2 Â10 mole-
À3
cule cm . Typical incremental amounts of N
2
O
5
,
15
À3
in the range of 0:6–1:8 Â10 molecule cm per
addition,were added to the reactor during the
experiment.
4. Results and discussion
To verify that there were no reaction products
that would interfere with the chromatographic
analysis,separate experiments were carried out on
Obtained data from the absolute technique,are
shown in Fig. 1,according to Eq. (I),where the
pseudo-first-order loss of NO is plotted vs. time,
3
the following reactions: NO
3
radical and reference,
for the NO –thiophene reaction at 298 K. The
3

404
B. Caba ~n as et al. / Chemical Physics Letters 358 (2002) 401–406
action of reference, kreference,gives the rate coeffi-
cient. Fig. 3 shows the data obtained for the
reaction studied in the present work.
If we compare the results obtained for both
À14
3
À1 À1
relative ðk ¼4:01 Â10
cm molecule s Þand
À14 3
À1 À1
absolute
methods for the NO –thiophene reaction,it can be
ðk ¼3:16 Â10
cm molecule s Þ
3
seen that both methods lead to similar values,but
the absolute rate coefficient is lower than the ob-
tained by the relative method. This difference
could be due to the uncertainty in the rate coeffi-
cient for the reference compound,or to a possible
pressure dependence for such reaction. On the
other hand,our absolute value agrees with the
recommended one by Atkinson based on the rel-
ative method [4]. The concordance is an indicative
of the absence of secondary chemistry at the con-
ditions in which the absolute experiments were
done in this work.
The temperature dependence was investigated
in the flow tube over the range of temperature
260–433 K. A calibration of the temperature along
the flow tube was previously done as reported
Canosa et al. [33] and Martinez et al. [34]. The
calculated rate coefficients at different tempera-
tures are shown in Table 1. Fig. 4 is an Arrhenius
plot of the data. From this table and this figure it
can be observed that the reaction between thioph-
Fig. 1. First-order plot for the reaction of NO
at 298 K.
3
with thiophene
slope of this plot yields the pseudo-first-order rate
0
constant k ,based on Eq. (II). Fig. 2 shows the
0
variation of k vs. thiophene concentration at 260,
2
98 and 433 K. The second-order rate constants, k,
are obtained from the slopes of such straight lines,
according to Eq. (III).
By using the relative rate technique,the rate
coefficient for the studied reaction was obtained
from Eq. (IV). The data were fitted to a straight
line by the linear least-squares procedure. The
slope combined with the rate constant of the re-
3
ene and NO shows a complicated temperature
dependence. For low temperatures (260–277 K),
Fig. 3. Plots of Eq. (IV) for thiophene with 1-butene as the
reference organic. Errors calculated by the method of Brauers et
al. [35]. R is thiophene and S is 1-butene.
3
Fig. 2. Second-order plots for the reaction of NO with thi-
ophene at different temperatures. R is thiophene.

B. Caba ~n as et al. / Chemical Physics Letters 358 (2002) 401–406
405
Table 1
Summary of the measured rate coefficients for the reactions of
periments (see Table 1 and Fig. 4) where the rate
constant at 277 and 298 K are quite similars and
coincide at the inflexion zone in the figure. A
similar behaviour has been observed in kinetic
studies of OH radicals with aromatic compounds
NO
3
with thiophene at different temperatures
T
k
(10
À14
3
À1 À1
(K)
cm molecule s )
2
2
2
3
3
3
3
4
60
63
98
21
45
67
89
33
3.78 Æ0.20
3.39 Æ0.60
3.16 Æ0.27
2.40 Æ0.38
3.08 Æ1.17
3.53 Æ1.20
4.99 Æ1.70
11.0 Æ0.21
[
15,22].
This result could be due to two parallel mech-
anisms. One of them predominant in the low
temperature range,another one,more significant
at higher temperatures. However,these sugges-
tions have to be considered with caution due to the
lack of data about these reactions. Therefore more
kinetics and products information about the re-
actions of NO radical with thiophene and other
3
heterocycle compounds ought to be obtained to
elucidate the reaction mechanism.
12
À3
½
NO
3
ꢀ¼ð0:6 À3ÞÂ10 molecule cm , P
T
¼
1 to 1.2
Torr, t ¼0:006 to 0:08 s, v ¼4 to 9 m/s,error defined by stu-
dent t-test.
Acknowledgements
The authors gratefully thank the C.I.C.Y.T
Comision Interministerial de Ciencia y Tecno-
(
log ꢀı a) project PB97-0432,for supporting this
work.
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