Kinetics of the reactions F + H2S and F + D2S at 298 K

Article abstract of DOI:10.1016/S0009-2614(98)01154-3

The rate constants for the reactions F + H₂S and F + D₂S at 298 K were determined by a competitive method using the reaction F + CH₄ as a reference reaction. The ratios of rate constants k_{F + H2S}/k_{F + CH4} and k_{F + D2S}/k_{F + CH4} were determined by a discharge-flow-mass spectrometric technique. By combining the values for these ratios with the value for k_{F + CH4} at 298 K, the values k_{F + H2S} = (1.46 ± 0.12) × 10^-10 cm³ molecule^-1 s^-1 and k_{F + D2S} = (1.42 ± 0.12) × 10^-10 cm³ molecule^-1 s^-1 were obtained. The kinetic isotope effect k_{F + H2S}/k_{F + D2S} was determined directly and the value 1.04 ± 0.02 was obtained. The results of this study are compared with previous literature data.

Full text of DOI:10.1016/S0009-2614(98)01154-3

18 December 1998
Ž
.
Chemical Physics Letters 298 1998 390–394
Kinetics of the reactions FqH2S and FqD2S at 298 K
)
Avigdor Persky
Department of Chemistry, Bar-Ilan UniÕersity, Ramat Gan 52900, Israel
Received 10 August 1998; in final form 5 October 1998
Abstract
The rate constants for the reactions FqH2S and FqD2S at 298 K were determined by a competitive method using the
reaction FqCH4 as a reference reaction. The ratios of rate constants kF SrkFqCH and kFqD SrkFqCH were
qH 2
4
2
4
determined by a discharge-flow-mass spectrometric technique. By combining the values for these ratios with the value for
y
10
y10
Ž .
at 298 K, the values kFqH S s 1.46"0.12 =10
1 1
Ž
.
kF
cm3 moleculey1 sy1 and kFqD S s 1.42"0.12 =10
qCH 4
2
2
cm3 moleculey sy were obtained. The kinetic isotope effect kF
1.04"0.02 was obtained. The results of this study are compared with previous literature data. q 1998 Elsevier Science B.V.
SrkFqD was determined directly and the value
qH 2
2 S
All rights reserved.
1. Introduction
the results accumulated over the years for the FqH2
reaction. This reaction has been investigated by more
than 20 research groups, but a significant part of the
Hydrogen atom abstraction reactions by fluorine
atoms, which are usually rapid and very exothermic,
are of much interest in molecular dynamics. The
interest in such reactions is mainly due to the possi-
bility of designing chemical lasers based on them
and of using experimental data for testing basic
theories in molecular dynamics. In spite of the con-
siderable importance of such reactions, accurate ki-
netic data are relatively scarce as compared, for
example, to similar reactions with chlorine atoms.
This fact can probably be attributed to experimental
difficulties resulting from the high reactivity of fluo-
rine atoms and to the occurrence of undesirable
w
x
results is considered to be inaccurate 1–6 . The
results of only a few of the research groups have
been taken into account in kinetic data evaluations,
for recommending the most acceptable kinetic data
w
x
for this reaction 4–6 .
The title reactions
FqH2 S™HFqHS,
D H0o sy45.3"2.0 kcal moly1
1
Ž .
and
FqD2 S™DFqDS,
w
x
secondary reactions 1–3 . The experimental difficul-
ties and resulting inaccuracies of part of the reported
experimental data can be illustrated by examining
D H0o sy45.5"2.0 kcal moly1
2
Ž .
o
Ž
are very rapid and highly exothermic the D H0
values indicated above were taken from Table 1 of
)
w x.
w x
Ref. 7 . Agrawalla and Setser 7 found that vibra-
E-mail: persky@mail.biu.ac.il
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 98 01154-3

(
)
A. PerskyrChemical Physics Letters 298 1998 390–394
391
Ž
.
Ž
.
Ž
.
tional levels up to HF Õs4 and DF Õs6 are
spectrometer Balzers Model QMG 511 . As before
Ž . Ž .
w
x
populated for reactions 1 and 2 , respectively. The
absolute rate constant for reaction 1 at 298 K has
been determined only by one research group 8 .
They carried out the experiments in a fast-flow
system and followed the decay of H2 S with a mass
spectrometer under pseudo-first-order conditions with
10–12 , the walls of the discharge tube and reaction
Ž .
cell, which were made of quartz, were treated by a
10% HF solution to minimize the heterogeneous
recombination of F atoms. The flow rates of the
reagents into the reaction cell were controlled by
calibrated capillaries and were of the order of 0.005–
0.015 mmol sy1 for H2 S, 0.012–0.025 mmol sy1 for
CH4, and 0.003–0.015 mmol sy1 for CF4. The flow
rates of the Ar and He were of the order of 50–100
mmol sy1. The total pressure in the reaction cell was
1–1.5 Torr. The Ar, He, CF4, H2 S and CH4 were
obtained from Matheson and had the stated mini-
mum purities of 99.999%, 99.999%, 99.7%, 99.5%
and 99.97%, respectively. The D2 S was obtained
from Merck, Sharp and Dohme and had a stated
minimum isotopic purity of 97.0%.
The concentrations of the H2 S, D2 S and CH4
were followed by measuring the mass spectrometric
signals due to the ions H2 Sq, D2 Sq and CHq4 ,
which have the mass to charge ratios mres34, 36
and 16, respectively.
Under the conditions where reactions 1 and 3
compete, the ratio of rate constants kFqH SrkFqCH
is given by the expression
w x
w
Ž
x
w x
H2 S < F . They obtained the value kFqH S s
0
0
2
y10
.
Ž
1.28"0.04 =10
cm3 moleculey1 sy1 the er-
.
ror limits represent the standard deviation"s . Two
research groups determined relative rate constants at
room temperature. Agrawalla and Setser 7 carried
out experiments in a fast-flow reactor and measured
relative emission intensities from the products of the
competing reactions, for a constant concentration of
F atoms. From these measurements they obtained the
w x
value kFqH SrkFqCH s1.9"0.2, where kFqCH is
the rate con2stant for th4 e reaction
4
FqCH4 ™HFqCH3 .
3
Ž .
A much higher value for this ratio, 3.2"0.3, was
obtained by Williams and Rowland, who studied
competitive reactions involving thermalized 18 F
Ž .
Ž .
w x
atoms 9 . A value of 1.4"0.2 was obtained by
2
4
Agrawalla and Setser for kFqD SrkFqCH
.
4
2
Ž .
In the present study, reaction 3 was used as a
reference reaction in the determination of kFqH
kFqH
ln wH Sx rwH Sx
Ž .
2 S
2 S
2
2
0
s
I
Ž .
and kFqD S. We also directly determined the kinetic
2
kFqCH
ln wCH x rwCH x
Ž .
4
4
0
2
isotope effect kFqH SrkFqD S. The results of the
2
2
present study are compared with the earlier reported
results.
w
x
w
x
where H2 S and CH4 are the concentrations of
these reagents at the end of the reaction cell when
Ž
the reactions with F atoms take place microwave
.
w
x
w
x0
discharge turned on , and H2 S and CH4 are
their concentrations when no F atoms are present
0
2. Experimental section
Ž
.
microwave discharge turned off . In every experi-
ment the background signals due to ions with mre
The experimental apparatus and procedure used in
this study were similar to those used in our earlier
Ž
s34 in the presence of CF4 and CH4 and the
w
x
w x
studies of the reactions FqHBr 10 , FqDBr 11 ,
.
Ž
absence of H2 S , and ions with mres16 in the
w
x
and FqCH4 12 , and therefore they will be de-
.
presence of CF4 and H2 S and the absence of CH4 ,
scribed here only briefly.
with and without discharge, were subtracted from the
signals measured when all the reagents were present.
In the experiments where reactions 2 and 3
compete, the ratio of rate constants for these reac-
tions is given by
Fluorine atoms were produced by a microwave
discharge in a dilute mixture of CF4 in Ar or He, and
flowed into the reaction cell through one of its inlets.
Ž .
Ž .
Ž
.
The F atoms reacted with a mixture of H2 S or D2 S
and CH4, also diluted by Ar or He, which flowed
into the reaction cell through a second inlet. The
reaction mixture was continuously sampled through
a small orifice and analyzed by a quadrupole mass
kFqD
w
x w
x
x
ln D2 S r D2 S
Ž
.
.
2 S
s
II
Ž .
w
x0 w
kFqCH
ln CH4 r CH4
Ž
0
4

(
)
392
A. PerskyrChemical Physics Letters 298 1998 390–394
Ž .
Ž .
and in the experiments where reactions 1 and 2
compete, the kinetic isotope effect is given by
kFqH
w
x w
x
x
ln H2 S r H2 S
Ž
.
.
2 S
2 S
x0 w
s
III
Ž
.
w
kFqD
ln D2 S r D2 S
Ž
0
As in the experiments to determine kFqH SrkFqCH
,
2
also in the last two cases the appropriate backgroun4d
signals were taken into account.
The carrier gas in the experiments to determine
kFqH SrkFqCH was either Ar or He. All the experi-
2
4
ments which involved D2 S where carried out only in
the presence of He. Ar could not be used because of
the interference of signals with mres36, due to the
isotope 36Ar.
All the experiments were carried out at 298 K.
Žw
x
w
x.
Žw
x
w
x.
Fig. 2. Plot of ln D2 S 0 r D2 S versus ln CH4 0 r CH4 for
3. Results and discussion
the reaction of F atoms with a mixture of D2 S and CH4 at 298 K.
3.1. RelatiÕe rate constants
calculated by the least squares method. As can be
seen, no significant scatter of the points around the
line is observed, and the line has a near-zero inter-
cept. This behavior indicates that the calculated rate
constant is independent of the extent of reaction and
proves that secondary reactions, which might inter-
fere with the determination, are not important under
the conditions of our experiments. It should also be
noted that changes of flow rates of the reagents over
significant ranges did not affect the results to any
significant extent and that the results obtained with
He as a carrier gas were the same, within the experi-
mental errors, as those obtained with Ar as a carrier
gas.
The ratio of rate constants kFqH SrkFqCH was
4
determined from several sets of exper2iments in which
the extent of reaction was varied over a wide range,
by varying the concentration of F atoms. A plot of
Žw
x
w
x.
Žw
x
w
x.
ln H2 S r H2 S versus ln CH4 0r CH4 is pre-
0
sented in Fig. 1. The straight line in this figure was
The ratio of rate constants obtained from the slope
Ž .
of the line in Fig. 1, according to Eq. I , is
kFqH SrkFqCH s2.35"0.05, where the error lim-
2
4
Ž
.
its represent twice the standard deviation "2s .
This value is in between the value 1.9"0.2 reported
w x
by Agrawalla and Setser 7 and the value 3.1"0.3
w x
obtained by Williams and Rowland 9 , but is closer
to the lower value of the two.
The ratio of rate constants kFqD rkFqCH was
Žw
x2 S w
x.
4
determined from the plot of ln D2 S r D2 S versus
ln CH4 0r CH4 , which is shown in Fig. 2. The
points in this figure represent several sets of experi-
ments carried out over several days. Again, as in
0
Žw
x
w
x.
Žw
x
w
x.
Žw
x
w
x.
Fig. 1. Plot of ln H2 S 0 r H2 S versus ln CH4 0 r CH4 for
the reaction of F atoms with a mixture of H2 S and CH4 at 298 K.

(
)
A. PerskyrChemical Physics Letters 298 1998 390–394
393
Fig. 1, all the points lie on the straight line, calcu-
lated by the least squares method, or very near it,
and the line has a near-zero intercept. This proves
that, also in this case, secondary reactions are not
important. The ratio of rate constants obtained from
have a significant effect on the rate of the reaction.
Such behavior could be expected for the FqH2 S
reaction, which is very rapid and probably has a very
low barrier, much lower than the zero-point energy
of either H2 S or D2 S. Similarly, near-unity kinetic
isotope effects have been determined for the rapid
Ž .
Fig. 2, according to Eq. II , is kFqD SrkFqCH
s
2
4
Ž .
2.29"0.05. This value is much higher than the
reactions of F atoms with NH3 and ND3 1.1"0.2
w
x
value of 1.4"0.2 obtained by Agrawalla and Setser
13 and for the reactions of F atoms with HBr and
. w
w x
7 .
Ž
x
DBr 1.07"0.12 11 . These values are much lower
Ž
.
The value obtained by us for kFqD SrkFqCH is
than the reduced mass effect 1.4 predicted by the
conventional transition state theory for reactions
without a barrier, which indicates that this theory is
not applicable for such reactions.
2
4
only slightly lower than the value obtained for
kFqH SrkFqCH and this means that the kinetic iso-
2
4
Ž
tope effect kFqH SrkFqD is near unity 1.03"
2 S
2
.
0.03 . The kinetic isotope effect was also determined
directly from experiments in which F atoms reacted
with a mixture of H2 S and D2 S. The results of
the measurements are shown in Fig. 3 where
3.2. Absolute rate constants
In order to calculate absolute values for kFqH
2 S
Žw
x
w
x.
Žw x
ln H2 S r H2 S
is plotted versus ln D2 S
r D2 S . Again a straight line with a near-zero inter-
and kFqD from the relative values determined in
0
0
2 S
w
x.
this study, the absolute value of kFqCH is needed.
4
Ž
.
w
x
cept is obtained from which, according to Eq. III ,
In a recent study 12 we determined the ratio
the ratio kFqH SrkFqD S s1.04"0.02 is calcu-
kFqCH rkFqD over a wide temperature range and
2
2
by combining2the results with the earlier results for
4
lated. This value is in excellent agreement with the
value 1.03"0.03 calculated from the individual ra-
tios kFqH SrkFqCH and kFqD SrkFqCH , and is
w
x
x
the kinetic isotope effect kFqH rkFqD 14 and with
2
2 w
recommended kinetic data for kFqH 4,5 , we ob-
2
4
2
4
tained an Arrhenius expression for 2 kFqCH . More
significantly lower than the value 1.4"0.2 reported
4
w x
by Agrawalla and Setser 7 .
recently, a new critical evaluation of the available
The near-unity value obtained for the kinetic iso-
tope effect means that isotopic substitution does not
kinetic data for the FqH2 reaction has been carried
w x
out by us 6 . This evaluation led to a somewhat
different Arrhenius expression for kFqH than the
2
one recommended earlier. Using the updated Arrhe-
nius expression for kFqH , the following expression
2
is obtained for kFqCH
4
kFqCH s 1.28"0.15 =10y10
Ž
.
4
=exp y 215"60 rT
Ž
.
cm3 moleculey1 sy1
.
IV
Ž
.
Ž
The value obtained for 298 K is kFqCH s 6.20"
4
y11
cm3 moleculey1 sy1. This value is
.
0.50 =10
w
x
about 7% lower than the value reported earlier 12 .
Using this value for kFqCH , the following values are
4
y10
Ž
.
obtained: kFqH S s 1.46 " 0.12 = 10
cm3
2
moleculey1 sy1 and kFqD S s 1.42"0.12 cm3
Ž
.
2
moleculey1 sy1
.
The value obtained in the present study for kFqH
2 S
Ž
.
is in good agreement with the value 1.28"0.04 =
10y10 cm3 moleculey1 sy1 obtained by Schonle et
¨
Žw
x
w
x.
Žw
x
w
x.
Fig. 3. Plot of ln H2 S 0 r H2 S versus ln D2 S 0 r D2 S for
w x
al. 8 . Similar techniques were employed in the two
the reaction of F atoms with a mixture of H2 S and D2 S at 298 K.

(
)
394
A. PerskyrChemical Physics Letters 298 1998 390–394
Table 1
between the two studies is the fact that while in Ref.
w x
be determined, this was not needed in our study. It
should also be noted that a small number of experi-
Relative and absolute rate constants for the reactions FqH2 S and
FqD2 S at 298 K
8 the absolute concentration of the F atoms had to
Ratio or absolute rate constant Reference
w x
kFqH S rkFqCH
1.9"0.2
7
w x
9
w x
ments were carried out in Ref. 8 under conditions
2
4
4
3.2"0.3
of a large excess of H2 S over F atoms. The value
2.35"0.05a
1.4"0.2
this work
obtained for the rate constant under these conditions
w x
kFqD S rkFqCH
7
2
y10
cm3 moleculey1
2.29"0.05a
this work
Ž
.
is kFqH S s 1.7"0.4 =10
1.28"0.04b,c,d
8
sy1, but 2the authors consider this value to be less
reliable than their other value. The value obtained by
w x
kFqH
S
2
b,c,e
Ž
.
w x
1.7"0.4
8
1.46"0.12a,c
1.42"0.12a,c
1.4"0.2
this work
this work
w x
us is in between the two values obtained in Ref. 8 .
kFqD
kFqH SS rkFqD
7
2
All the results obtained in the present study are
summarized in Table 1, where they are compared
with the earlier available data.
w x
S
2
2
1.04"0.02a
this work
a Error limits are 2s.
b Error limits are 1s.
c Units: 10y10 cm3 moleculey1 sy1
.
d Experimental conditions: large excess of F atoms over H2 S.
e Experimental conditions: large excess of H2 S over F. These
results are considered by the authors to be less reliable than their
results under large excess of F atoms.
References
w x
Ž .
R. Foon, M. Kaufman, Prog. Reaction Kinet. 8 1975 81.
1
w x
2
Ž .
W.E. Jones, E.G. Skolnik, Chem. Rev. 76 1976 563.
w x
Ž
.
3
J.B. Anderson, Adv. Chem. Phys. 41 1980 229.
w x
4
R. Atkinson, D.L. Baulch, R.A. Cox, R.F. Hampson, J.A.
Ž
.
studies. In both of them a discharge-flow system was
used with mass spectrometric detection of the
reagents. However, the conditions under which the
experiments were carried out were very different.
Kerr, J. Troe, J. Phys. Chem. Ref. Data 21 1992 1125.
W.B. DeMore, S.P. Sander, D.M. Golden, R.F. Hampson,
M.J. Kurylo, C.J. Howard, A.R. Ravishankara, C.E. Kolb,
M.J. Molina, Chemical Kinetics and Photochemical Data for
Use in Stratospheric Modeling, Evaluation Number 11 of the
w x
5
w x
Whereas in Ref. 8 experiments were carried out
Ž
.
NASA Panel for Data Evaluation, JPL Publ., 94–26 1994 .
w x
Ž
.
under pseudo-first-order conditions with the concen-
tration of F atoms in large excess over the concentra-
tion of H2 S, our experiments were carried out under
second-order conditions, with the sum of concentra-
tions of H2 S or D2 S and CH4 being larger than the
concentration of the F atoms. Another difference
6
w x
7
A. Persky, H. Kornweitz, Int. J. Chem. Kinet. 29 1997 67.
B.S. Agrawalla, D.W. Setser, J. Phys. Chem. 90 1986
Ž
.
2450.
w x
8
G. Schonle, M.M. Rahman, R.N. Schindler, Ber. Bunsenges.
¨
Ž
.
Phys. Chem. 91 1987 66.
Ž
.
w x
Ž .
9
x
R.L. Williams, F.S. Rowland, J. Phys. Chem. 77 1973 301.
w
Ž
.
10 R. Edrei, A. Persky, Chem. Phys. Lett. 157 1989 265.