Kinetics of reactions of Cl atoms with ethane, chloroethane, and 1,1-dichloroethane

Article abstract of DOI:10.1021/jp0275138

Reactions of Cl atoms with ethane, chloroethane, and 1,1-dichloroethane were studied experimentally with the discharge flow/resonance fluorescence technique over wide ranges of temperatures and at 2.3-9.2 torr. The rate octane of the reaction of Cl atoms with ethane obtained agreed with the results of earlier low-temperature measurements. Reactivity with respect to Cl atom attack decreased in the sequence of ethane, chloroethane, and dichloroethane. Rate constants of the reactions of Cl with chloroethane and 1,1-dichloroethane demonstrated different temperature dependences in the low-temperature (room temperature to 378 K) and the high-temperature (484-810 K) regions. The differences were due to the regeneration of Cl atoms at higher temperatures as a consequence of the fast thermal decompositions of radical products with a chlorine atom in the β position.

Full text of DOI:10.1021/jp0275138

J. Phys. Chem. A 2003, 107, 6565-6573
6565
Kinetics of Reactions of Cl Atoms with Ethane, Chloroethane, and 1,1-Dichloroethane
Mikhail G. Bryukov,† Irene R. Slagle, and Vadim D. Knyazev*
Research Center for Chemical Kinetics, Department of Chemistry, The Catholic UniVersity of America,
Washington, D.C. 20064
ReceiVed: NoVember 20, 2002; In Final Form: April 18, 2003
Reactions of Cl atoms with ethane (1), chloroethane (2), and 1,1-dichloroethane (3) were studied experimentally
with the discharge flow/resonance fluorescence technique over wide ranges of temperatures and at pressures
between 2.3 and 9.2 Torr. Results of earlier investigations of the reactions of Cl atoms with ethane and
chloroethanes are analyzed and compared with the results of the current work. The rate constants of the
reaction of Cl atoms with ethane obtained in the current study (k1 ) (4.91 × 10-12)T0.47 exp(-82 K/T) cm3
molecule-1 s-1, 299-1002 K) agree with the results of earlier determinations. Combination of the temperature
dependence of k1 obtained in the current work with the existing (mostly at lower temperatures) literature data
results in the expression k1 ) (7.23 × 10-13)T0.70 exp(+117 K/T) cm3 molecule-1 s-1 (203-1400 K). Rate
constants of the reactions of Cl atoms with chloroethane and 1,1-dichloroethane demonstrate different
temperature dependences in the low-temperature (room temperature to 378 K) and the high-temperature (484-
810 K) regions. The differences are due to the regeneration of Cl atoms at higher temperatures as a consequence
of the fast thermal decomposition of radical products with a chlorine atom in the â position. This information
enables quantitative differentiation between the site-specific reaction channels. The results of the current
study, combined with those of earlier relative-rate experiments, were used to derive expressions for the
temperature dependences of the rates of the site-specific abstraction channels: k2a(T) ) (2.76 ( 0.28) ×
10-11 exp(-455 ( 44 K/T) (296-810 K), k2b(T) ) (1.92 ( 0.47) × 10-11 exp(-789 ( 84 K/T) (296-378
K), k3a(T) ) (1.46 ( 0.21) × 10-11 exp(-733 ( 58 K/T) (293-810 K), and k3b(T) ) (2.98 ( 1.42) × 10-11
exp(-1686 ( 160 K/T) (293-378 K) cm3 molecule-1 s-1, where channels a and b signify abstraction of
hydrogen atoms in the R and â positions, respectively.
I. Introduction
The temperature behavior of the fast reaction 1 has been
studied by a number of groups (refs 19-26 and references
Reactions of Cl and H atoms with chlorinated hydrocarbons
(CHCs) are among the most important and sensitive reactions
in the combustion and incineration of CHCs.1-9 In CHC/O2 and
CHC/hydrocarbon/O2 flames, reactions of Cl and H atoms with
CHCs together with unimolecular decomposition are the major
channels of consumption of CHCs.1-5,7-14 As a part of a project
directed at the elucidation of the kinetics of these important
reactions, we recently reported15-18 the results of our experi-
mental and computational studies of the reactions of H and Cl
atoms with methane and chlorinated methanes, and the reactions
of H atoms with ethane and three chlorinated ethanes. In the
current work, we extend such kinetic studies to the reactions of
Cl atoms with ethane and two chlorinated ethanes (C2H5Cl and
CH3CHCl2) conducted over wide ranges of temperatures:
therein) with generally good agreement between the results of
different studies. Reviews of these data can be found, for
example, in refs 27 and 28. The current study extends the
temperature range of direct experimental studies of reaction 1
by sensitive flow techniques to 1002 K, almost to the high-
temperature range covered by shock-tube-based methods.26
Although reactions 2 and 3 have been studied before,22,29-37
earlier studies were confined to relatively low temperatures,
where research was stimulated by the importance of these
reactions for atmospheric chemistry; most of these studies
involved indirect determinations of rate constants. Only one
direct determination of the rate constants of reactions 2 and 3
as functions of temperature is available in the literature.35
In the current study, the kinetics of reactions 1-3 was
investigated by the discharge flow technique with resonance
fluorescence detection of Cl atoms. The excellent sensitivity of
the method (detection limit of <108 atoms cm-3) permits
experiments with very low initial Cl concentrations (e1011
atoms cm-3), thus ensuring the absence of any complications
due to potential fast secondary reactions. The rate constants of
reactions 1-3 were obtained as functions of temperature in
direct experiments. In reactions 2 and 3, different temperature
dependences were obtained in the low-temperature (296-378
K) and the high-temperature (484-810 K) regions. The differ-
ences are due to the regeneration of Cl atoms at higher
temperatures as a consequence of the fast thermal decomposition
of radical products with a Cl atom in the â position. This
information enables quantitative differentiation between the site-
specific reaction channels. In particular, the current study
Cl + C2H6 f HCl + C2H5 (299-1002 K)
Cl + C2H5Cl f HCl + CH3CHCl (296-810 K) (2a)
f HCl + CH2CH2Cl (2b)
Cl + CH3CHCl2 f HCl + CH3CCl2 (293-810 K) (3a)
f HCl + CH2CHCl2 (3b)
(1)
The numbers in parentheses indicate the experimental temper-
ature ranges of the current work.
* To whom correspondence should be addressed. E-mail: knyazev@
cua.edu.
† Current address: Biodynamics Institute, Louisiana State University,
711 Choppin Hall, Baton Rouge, LA 70803.
10.1021/jp0275138 CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/31/2003

6566 J. Phys. Chem. A, Vol. 107, No. 34, 2003
Bryukov et al.
provides the first direct determination of the rate constant of
the abstraction of a hydrogen atom in the R position in reactions
2 and 3. The information obtained on the site-specific and the
overall rate constants, combined with the results of earlier
relative-rate studies,29,32,34 was used to derive expressions for
the temperature dependences of the rates of the individual
reaction channels.
The paper is organized as follows: Section I (current) is an
introduction. The experimental method, procedures, and results
are reported in section II. The results are discussed in section
III. Brief conclusions are given in section IV.
II. Experimental Section
Rate constant measurements were conducted in a heatable
tubular flow reactor under pseudo-first-order conditions with a
large excess of molecular substrate. Cl atoms were detected by
resonance fluorescence, and their decay was measured as a
function of contact time over wide ranges of experimental
conditions.
II.1. Experimental Apparatus. Details of the experimental
apparatus have been described previously in connection with
our investigation of reactions of H atoms with methane and
chloromethanes;15 modifications introduced to enable studies
of Cl atom reactions have been presented in ref 18. Thus, only
a brief description of the apparatus is given here. Cl atoms were
generated in a sidearm of a heated tubular quartz reactor by a
2.45 GHz microwave discharge in a Cl2/He mixture. Chlorine
atoms formed in the discharge area were carried through the
reactor by a flow of helium, and their concentration was
monitored by resonance fluorescence in the detection zone
located downstream. The molecular substrate (C2H6-XClX) was
introduced through a quartz movable injector. Care was taken
to ensure that, under all experimental conditions used in the
current work, the plug flow approximation was valid. The only
exception to the plug flow approximation was the minor
contribution of axial and radial diffusion of Cl atoms; diffusional
corrections had to be introduced into the experimentally obtained
atom decay rates (vide infra). The uniformity of the temperature
profiles in the working zone of the reactor (15-30 cm in length)
was no worse than 0.5% of T.
Three reactors were used in these experiments: a quartz
reactor with an internal diameter (i.d.) of 1.93 cm, a quartz
reactor with i.d. ) 3.19 cm, and a Pyrex reactor with i.d. )
4.66 cm. The reactor surface, the surface of the movable injector,
and the inside of the discharge tube were treated to reduce
heterogeneous loss of Cl atoms by first soaking in a 5% aqueous
solution of ammonium bifluoride for 30 min with subsequent
rinsing with distilled water and then by the method of Sepehrad
et al.38 This procedure, essentially, results in a very thorough
cleaning of the glass surface. The resultant values of the first-
Figure 1. Examples of experimentally obtained ln(SCl) vs x depend-
ences. Data from experiments 9 on Cl + C2H6 and 15 on Cl +
CH3CHCl2 (see Tables 1 and 2). Numbers are the concentrations of
the C2H6-XClX substrate (1012 molecules cm-3).
titration with Br2 (the rate constant of the Cl + Br2 reaction is
g10-10 cm3 molecule-1 s-1).42,43 The sensitivity limit (defined
by unity signal-to-noise ratio) was <108 atoms cm-3
.
Molecular substrates (C2H6-XClX) were stored undiluted in
Pyrex reservoirs. Flows of these reagents into the reactor were
determined by measuring the pressure drop over time in a
calibrated volume. To ensure the absence of interference from
heterogeneous absorption and desorption processes on the walls
of the vacuum manifold, it was verified that the measured flows
were independent of the surface-to-volume ratio of the calibrated
volume. Flows of molecular chlorine to the atom-producing
discharge were measured in a similar way. The dissociation
efficiency of the discharge was in the 20-50% range, which
resulted in the concentrations of undissociated Cl2 in the reactor
being no more than a factor of 2 higher than the initial Cl atom
concentrations.
A flow of CF4 with concentrations in the (3.1-20) × 1014
molecules cm-3 range was added to the carrier gas through an
inlet located upstream from the reaction zone to ensure fast
equilibration44 of the ground and the excited spin-orbit states
of the Cl atom. Literature values of the room-temperature
quenching rate coefficient range from 0.23 × 10-10 to 1.5 ×
order wall loss rate constant, kw, were always below 20 s-1
.
Typical values of kw were ∼2-8 s-1. Reactors of different
diameters were used to rule out potential contributions of
heterogeneous reactions to the rate constant values obtained in
the experiments (vide infra).
Cl atom resonance fluorescence was induced by light from a
discharge flow resonance lamp39,40 and detected by a solar blind
photomultiplier (EMR model 542-G-09, peak of sensitivity in
the 115-170 nm spectral interval). CaF2 windows (cutoff 123
nm) and a gas optical filter (1 Torr of N2O, optical path length
3 cm)40,41 were used to block potential radiation from H and O
atoms (121.6 and 130.2-130.6 nm, respectively) while trans-
mitting the Cl resonance lines at 135-139 nm. The sensitivity
of the atom detection system to Cl atoms was determined by
10-10 cm3 molecule-1 s-1 45-47
Therefore, the large concentra-
.
tions of CF4 used in the current work keep the rate of spin-
orbit equilibration much faster than the rates of the reactions
under study.
II.2. Reaction Rate Measurements. All experiments to
measure the rate constants of reactions 1-3 were conducted
under conditions of an excess of molecular substrate. Initial
concentrations of Cl atoms in the detection zone were in the

Kinetics of Reactions of Cl Atoms with Ethanes
J. Phys. Chem. A, Vol. 107, No. 34, 2003 6567
TABLE 1: Conditions and Results of Experiments To Measure Rate Constants of the Reactions of Cl Atoms with Ethane and
Chloroethane (Reactions 1 and 2)
[C2H6-XClX]c
range/1012
((k′ - k′OBS)/
d
c
f
no.a
T/K
Pb/Torr
[CF4]c/1014
k0 /s-1
Ve/m s-1
[Cl]0 /1010
k′OBS))max
kig/10-11
Cl + C2H6 f HCl + C2H5 (1)
1
2
3
4*
5
6*
7
8*
9+
10+
11*
12
299
299
346
402
426
498
604
707
758
758
855
1002
3.78
2.47
3.64
2.45
4.73
2.45
5.99
4.02
4.08
4.11
4.00
9.22
1.55-5.88
0.66-4.62
1.18-5.00
0.14-2.59
0.71-4.24
0.12-2.4
0.62-2.52
0.34-3.12
0.285-1.75
0.33-1.81
0.32-3.87
0.60-4.19
13.5
0.1 ( 7.4
3 (16
32.7
2.8
8.9
3.9
1.8
3.2
3.7
3.2
5.4
1.9
0.86
6.1
2.4
0.045
0.033
0.036
0.054
0.029
0.049
0.018
0.046
0.079
0.080
0.056
0.028
5.23 ( 0.20
5.46 ( 0.56
6.14 ( 0.26
6.94 ( 0.60
6.88 ( 0.40
7.66 ( 0.63
8.27 ( 0.46
9.44 ( 0.58
10.03 ( 0.26
9.93 ( 0.68
10.68 ( 0.63
11.63 ( 0.60
7.5
5.2
24.5
35.8
14.1
39.0
17.5
35.5
27.9
14.5
14.5
33.3
57.3
-3.8 ( 7.6
3.6 ( 1.0
-6 ( 11
-0.1 ( 8.5
-7.2 ( 7.3
-6 ( 12
1.7 ( 2.6
-3.5 ( 7.4
0 ( 13
10.1
16.9
8.3
16.0
5.5
9.2
6.2
3.1
11.7
14 ( 14
Cl + C2H5Cl f HCl + C2H4Cl (2)
1
2
3
4
5*
6
7*
8
9
10
11
12*
13
296
320
346
378
378
484
494
523
557
604
713
758
810
4.29
2.85
3.60
3.94
4.03
7.26
4.03
5.88
4.32
5.91
7.52
4.06
7.24
10.9-36.9
4.25-21.1
4.14-28.9
3.24-27.8
2.88-12.1
5.28-20.2
1.54-14.0
4.73-30.0
2.55-14.1
2.61-16.0
4.6-23.8
12.0
9.6
-1.3 ( 9.4
5 ( 10
31.0
23.3
34.8
37.9
7.3
29.2
9.6
44.7
31.2
35.7
51.2
14.4
39.2
3.0
6.9
2.6
5.4
1.5
4.3
1.5
2.7
2.8
4.6
3.3
1.6
1.9
0.048
0.024
0.032
0.034
0.109
0.031
0.098
0.029
0.014
0.017
0.025
0.075
0.011
0.76 ( 0.04
0.82 ( 0.07
0.97 ( 0.04
1.05 ( 0.07
1.04 ( 0.08
1.11 ( 0.04
1.07 ( 0.09
1.08 ( 0.06
1.12 ( 0.11
1.27 ( 0.13
1.45 ( 0.10
1.68 ( 0.12
1.65 ( 0.12
11.0
13.0
19.2
9.0
0.6 ( 6.4
9 ( 11
4.8 ( 5.6
2.7 ( 5.2
6.8 ( 7.0
6 ( 11
5.6
16.7
14.3
6.6
0 ( 10
6 ( 13
16.0
7.7
17.5
10 ( 15
5.6 ( 6.7
-0.4 ( 7.9
1.32-9.78
2.99-10.9
a Experiment number. A quartz reactor with i.d. ) 3.19 cm was used in experiments marked with an asterisk, a Pyrex reactor with i.d. ) 4.66
cm was used in experiments marked with a plus sign, and a quartz reactor with i.d. ) 1.93 cm was used in unmarked experiments. b Reactor
pressure. c Units of concentrations are molecules cm-3 d Zero-abscissa intercept on the k′ vs [C2H6-XClX] dependence. e Average flow velocity.
.
f Maximum relative value of the diffusion correction. g Units of rate constants are cm3 molecule-1 s-1. Error limits represent statistical uncertainties
and are reported as 2σ. Maximum estimated systematic uncertainties are 8% of the rate constant value (see the text).
range (0.9-9) × 1010 atoms cm-3. Exact knowledge of the Cl
atom concentrations is not needed for the determination of rate
constants because all experiments were conducted under pseudo-
first-order conditions. The rate of heterogeneous loss of Cl atoms
on the walls of the reactor and the movable injector (0.5-17
s-1) was regularly measured using the method described in ref
18 (in the absence of molecular substrate).
the nonnegligible losses of Cl atoms on the surfaces of the
reactor and the movable injector and can acquire both positive
and negative values.15 The values of k0 obtained in the current
study were minor compared with the first term in eq II (see
Table 1), and uncertainties in k0 were generally comparable with
the k0 values. Examples of experimentally obtained k′ vs
[C2H6-XClX] dependences are presented in Figure 2.
The time of contact between the molecular substrate and Cl
atoms was varied by changing the position of the movable
injector. The effective first-order rate constant values, k′OBS, were
obtained from the dependences of the Cl atom fluorescence
signal, SCl, on the distance between the tip of the movable
injector and the detection zone. Examples of experimentally
obtained ln(SCl) vs x dependences are presented in Figure 1.
The observed values of k′OBS were corrected for axial and radial
diffusion of Cl atoms.48-50 The values of the Cl atom diffusion
coefficient in He were calculated using the
Gases used in the experiments were obtained from MG
Industries (He, >99.9999%; CF4, >99.9%), Aldrich (C2H5Cl,
>99.7%), SynQuest Laboratories, Inc. (CH3CHCl2, g99.6%),
and Matheson (C2H6, g99.999%; Cl2, 99.999%). The indicated
purity values are nominal, as specified by the manufacturer.
All gases except helium and CF4 were purified by vacuum
distillation prior to use. Helium was purified by passing through
liquid nitrogen cooled traps. To verify that no potential
impurities in the ethane or chlorinated ethanes could affect the
measured rate constants, these gases were analyzed for potential
contaminants by gas chromatography. After purification by
vacuum distillation, the purity of all chloromethanes exceeded
the specifications provided by the manufacturers. A discussion
of impurities and their potential influence on the experimental
results is given in the next subsection (II.3).
D ) 0.0237T1.75 Torr cm2 s-1
(I)
formula.51 This correction for atom diffusion never exceeded
11% of k′OBS. A more detailed description of the experimental
procedure and the diffusion correction is given in ref 18.
The bimolecular rate constants of reactions 1-3 were
obtained from the slopes of the linear dependences of k′ on the
concentration of substrate, [C2H6-XClX]:
II.3. Results. Conditions and results of experiments to
determine the values of the rate constants of reactions 1-3 are
presented in Tables 1 and 2. The rate constants demonstrate no
dependence on pressure, initial concentration of Cl atoms, bulk
flow velocity, or concentration of the CF4 spin-orbit quencher
within the experimental ranges. The fact that the rate constants
are independent of the initial Cl atom concentration indicates
the absence of any influence of potential secondary reactions
on the kinetics of Cl atoms, as can be expected due to the low
k′ ) ki[C2H6-XClX] + k0
(II)
Here, ki is the bimolecular rate constant of the reaction under
study (i ) 1-3) and k0 is the zero-abscissa intercept of the k′
vs [C2H6-XClX] dependence. The k0 intercept appears due to

6568 J. Phys. Chem. A, Vol. 107, No. 34, 2003
Bryukov et al.
(CH2CH2Cl and CH2CHCl2) is negligible, the experimentally
determined rate constants represent sums of the rates of Cl atom
attack on both the R and the â H atoms of the C2H5Cl and the
CH3CHCl2 molecules. Thus, the Arrhenius expressions obtained
in the low-temperature region
k2 ) (3.42 ( 0.95) × 10-11 exp(-447 ( 84 K/T) cm3
molecule-1 s-1 (296-378 K) (IV)
k3 ) (3.7 ( 2.3) × 10-11 exp(-1001 ( 161 K/T) cm3
molecule-1 s-1 (293-378 K) (V)
correspond to the total rates of reactions 2 and 3, k2 ) k2a
k2b and k3 ) k3a + k3b.
+
At temperatures above 373 K, rates of decomposition of
CH2CH2Cl and CH2CHCl2 (products of reaction channels 2b
and 3b, respectively) increase sharply with an increase in
temperature.56 Decomposition of these radicals results in
regeneration of Cl atoms, which, when the decomposition rates
are comparable with the rates of Cl + C2H6-XClX, should re-
sult in nonexponential Cl decay profiles. Fortunately, temper-
ature dependences of the radical decomposition rates are rather
sharp; at T g 480 K CH2CH2Cl and CH2CHCl2 should
decompose virtually instantaneously on the time scale of the
current experiments (characteristic time is less than 0.7 ms for
CH2CH2Cl56). Therefore, at sufficiently high temperatures,
reaction channels 2b and 3b result in regeneration of Cl atoms
and cannot be observed in experiments where determination of
rate constants is based on detection of Cl.
Thus, at temperatures above 480 K, the experimental rate
constants correspond to abstraction of H atoms in the R positions
only (channels 2a and 3a). These results can be represented by
the expressions
Figure 2. Examples of experimentally obtained k′ vs [C2H5Cl] (a) and
k′ vs [CH3CHCl2] (b) dependences. Experimental temperatures are
indicated on the plots.
values of [Cl]0 used ([Cl]0 ) (0.9-9) × 1010 atoms cm-3).
Potential regeneration of Cl atoms through the secondary R +
Cl2 f RCl + Cl processes (rate constants 2 × 10-11 cm3
molecule-1 s-1 or less22,52-55) are negligible due to the low Cl2
concentrations (<2 × 1011 molecules cm-3).
k2a ) (3.28 ( 0.82) × 10-11 exp(-560 ( 130 K/T) cm3
molecule-1 s-1 (484-810 K) (VI)
k3a ) (1.87 ( 0.41) × 10-11 exp(-905 ( 119 K/T) cm3
molecule-1 s-1 (484-810 K) (VII)
To ensure that the measured rates of Cl atom decay in the
presence of C2H6-XClX represent the homogeneous reactions
1-3, experiments were conducted with reactors of different
internal diameters (1.93-4.66 cm) possessing different surface-
to-volume ratios. The experimentally obtained values of the rate
constants were independent of the reactor used (Tables 1 and
2). This independence, as well as the linearity of the observed
k′ vs [C2H6-XClX] dependences, indicates the absence of any
significant effects of heterogeneous reactions on the values of
the rate constants.
The temperature interval between 378 and 484 K, where
decomposition of the CH2CH2Cl and the CH2CHCl2 radical
products is expected to occur on the time scales roughly similar
to those of the Cl decay due to reactions 2 and 3, was avoided
in the experiments because interference from Cl regeneration
cannot be quantitatively factored out in an easy and reliable
way. Here, evaluation of the expected rates of decomposition
of CH2CH2Cl and CH2CHCl2 was based on the results of ref
56.
In principle, for each of the reactions 2 and 3, it is possible
to describe the experimental temperature dependence of the rate
constants with a single modified Arrhenius expression covering
the whole experimental temperature range. Deviations of
individual data points from such expressions will be less than
12% for reaction 2 and less than 20% for reaction 3. However,
it is known56 that decomposition of the radical products of the
â-abstraction channels occurs at temperatures above 378 K.
Therefore, such a simple representation of the k2(T) and k3(T)
dependences would not be physically meaningful; instead, the
analysis described above is appropriate.
The rate constants of reactions 1-3 exhibit positive temper-
ature dependences (Figures 3-5). The experimental k1(T)
dependence obtained can be represented with the modified
Arrhenius expression
k1 ) (4.91 × 10-12)T0.47 exp(-82 K/T) cm3 molecule-1 s-1
(299-1002 K) (III)
In reactions 2 and 3, different temperature dependences were
obtained in the low-temperature (293-378 K) and the high-
temperature (484-810 K) regions. The differences are due to
the regeneration of Cl atoms at higher temperatures as a
consequence of the fast thermal decomposition of the radical
products with a Cl atom in the â position.56 At low tempera-
tures, where thermal decomposition of these radical products
The error limits of the experimentally obtained values reported
in this work (Tables 1 and 2) represent 2σ statistical (random)
uncertainty. The maximum estimated systematic uncertainty is

Kinetics of Reactions of Cl Atoms with Ethanes
J. Phys. Chem. A, Vol. 107, No. 34, 2003 6569
TABLE 2: Conditions and Results of Experiments To Measure Rate Constants of the Reactions of Cl Atoms with
1,1-Dichloroethane (Reaction 3)
[C2H4Cl2]c
range/1013
((k′ - k′OBS)/
k3h,i/10-12
(corrected)
d
c
f
no.a
T/K
Pb/Torr
[CF4]c/1014
k0 /s-1
Ve/m s-1
[Cl]0 /1010
k′OBS))max
k3g,i/10-12
1
2
3
4
5
6
7
8*
9
10
11
12
13
14*
15
293
297
320
320
346
378
378
378*
484
523
556
604
713
758*
810
2.28
3.52
2.86
2.84
3.63
3.94
3.97
4.03
7.15
5.89
4.37
5.97
7.55
4.08
7.24
2.60-16.7
2.16-18.1
1.98-11.1
1.30-9.63
0.81-10.7
0.918-8.0
2.47-12.7
0.664-3.99
0.527-6.74
0.422-9.62
0.637-4.92
1.36-6.93
1.03-6.79
0.77-2.94
1.29-4.72
11.3
9.5
7.8
20.0
4.7
8.6
10.2
15.2
7.4
7.5
11.9
8.7
8.2 ( 8.7
7.3 ( 9.3
4.8 ( 6.2
7.7 ( 8.4
-1.8 ( 7.0
6.9 ( 6.2
13 ( 14
1.0 ( 5.4
6.9 ( 9.0
13.5 ( 8.6
13 ( 11
21.4
32.2
24.2
22.6
35.9
37.3
37.3
7.3
27.9
44.1
31.7
34.9
50.1
14.5
39.4
1.2
3.9
2.2
3.0
8.0
4.5
2.1
2.2
3.6
2.0
2.1
1.3
4.4
2.4
1.5
0.032
0.036
0.022
0.021
0.027
0.026
0.037
0.099
0.028
0.032
0.018
0.026
0.029
0.074
0.021
1.42 ( 0.09
1.35 ( 0.08
1.54 ( 0.10
1.65 ( 0.15
2.24 ( 0.10
2.81 ( 0.13
2.61 ( 0.20
2.78 ( 0.21
3.10 ( 0.22
3.21 ( 0.17
3.93 ( 0.37
4.11 ( 0.31
5.55 ( 0.32
5.84 ( 0.37
6.13 ( 0.58
1.33 ( 0.09
1.26 ( 0.08
1.45 ( 0.10
1.56 ( 0.15
2.15 ( 0.10
2.72 ( 0.13
2.52 ( 0.20
2.69 ( 0.21
3.01 ( 0.22
3.12 ( 0.17
3.84 ( 0.37
4.02 ( 0.31
5.46 ( 0.32
5.75 ( 0.37
6.04 ( 0.58
5 ( 12
4 ( 12
-1.0 ( 6.5
8 ( 16
12.4
5.4
9.7
a Experiment number. A Pyrex reactor with i.d. ) 4.66 cm was used in experiments marked with an asterisk, and a quartz reactor with i.d. )
1.93 cm was used in unmarked experiments. b Reactor pressure. c Units of concentrations are molecules cm-3 d Zero-abscissa intercept on the k′ vs
[C2H4Cl2] dependence. e Average flow velocity. f Maximum relative value of the diffusion correction. g Values of k3 before correction for the effects
of C2H2Cl2 impurity. h Values of k3 corrected for the effects of C2H2Cl2 impurity (see the text). i Units of rate constants are cm3 molecule-1 s-1
.
.
Error limits represent statistical uncertainties and are reported as 2σ. Maximum estimated systematic uncertainties are 8% of the rate constant value
(see the text).
8% of the rate constant value. The error limits in expressions
IV-VII are given as 2σ and represent only the statistical
uncertainties of the fit.
To evaluate potential effects of impurities in the chlorinated
ethanes on the measured rate constants, these gases were
analyzed for contaminants by gas chromatography. A Shimadzu
GC-9A gas chromatograph and a Hewlett-Packard 5890 II Plus/
5989B GC/MS instrument were used in these analyses. It was
found that, after purification by vacuum distillation, the purity
of both chloroethanes (C2H5Cl, >99.98%; CH3CHCl2, 99.72%)
exceeded the specifications provided by the manufacturers. The
CH3CHCl2 sample obtained from SynQuest Laboratories, Inc.
was stabilized with 0.1% nitromethane. The GC/MS analysis
confirmed the 0.1% impurity of CH3NO2 and revealed the
presence of impurities of two isomers of dichloroethylene (cis-
1,2-C2H2Cl2, 0.075%; trans-1,2-C2H2Cl2, 0.020%) as well as
1,2-C2H4Cl2 (0.055%), 1,1,1-C2H3Cl3 (0.026%), and 1,1,2-
C2H3Cl3 (0.007%). Reactions of a Cl atom with CH3NO2, 1,2-
C2H4Cl2, 1,1,1-C2H3Cl3, and 1,1,2-C2H3Cl3 are too slow35,57-60
to affect the results of the current experimental study. However,
Results of direct studies where temperature dependences were obtained
corrections for the reactions of Cl with dichloroethylenes had
to be applied to the values of k3. The temperature-independent
value of the rate constant of the reaction
Figure 3. Temperature dependence of the rate constant of reaction 1.
are displayed: Manning and Kurylo,19 Lewis et al.,20 Dobis and
Benson,21 Pilgrim et al.,24 Takahashi et al.,26 and the current study.
Solid and dotted lines represent the modified Arrhenius fits to the results
of the current study (eq III) and to the combined data set shown in this
plot (eq VIII), respectively.
Cl + 1,2-C2H2Cl2 f products
(4)
refs 27 and 28. In the room-temperature study of Tyndall et
al.,25 a value of k1 ) 5.75 × 10-11 cm3 molecule-1 s-1 was
reported, in agreement with the results of the current and earlier
investigations. The experiments of Pilgrim et al.24 were per-
formed in the 292-800 K temperature range using the 193 nm
laser photolysis of CF2Cl2 to produce Cl atoms and long-path
infrared absorption to detect the HCl formed in reaction 1. The
results of ref 24 are somewhat lower than those obtained in the
current study, with the difference reaching ∼20% at T g 700
K. These differences fall within the combined experimental
uncertainties of the two studies. The recent study of Takahashi
et al.26 was conducted under shock tube conditions (T ) 1100-
1400 K) using atomic resonance spectroscopy for Cl atom
detection. The authors report a temperature-independent value
of k1 ) 1.1 × 10-10 cm3 molecule-1 s-1, in agreement with the
results of the current and the earlier studies.
(k4 ) 9.6 × 10-11 cm3 molecule-1 s-1 61) was used. The
corrections are 7% at the lowest temperature used and smaller
at higher temperatures. The corrected values of k3 are presented
in Table 2.
III. Discussion
III.1. Experimental Rate Constant Values. Reaction 1 has
been studied previously, with the results of direct and indirect
studies being in general agreement with each other (see, for
example, refs 19-26). Reviews of data published prior to 1997
(T e 604 K) can be found in refs 28 and 27. The rate constants
obtained in the current study agree with these earlier lower
temperature measurements (Figure 3; direct studies that reported
temperature dependences are displayed). Three direct studies24-26
of reaction 1 appeared since the publication of the reviews of

6570 J. Phys. Chem. A, Vol. 107, No. 34, 2003
Bryukov et al.
If the temperature dependence of k1 obtained in the current
work is combined with the values reported in the studies
included in Figure 3,19-21,24,26 the resultant combined set of data
can be represented with the following modified Arrhenius
expression:
k1 ) (7.23 × 10-13)T0.70 exp(+117 K/T) cm3
molecule-1 s-1 (203-1400 K) (VIII)
Only two direct determinations of the rate constant of reaction
2 are available in the literature.22,35 Wine and Semmes35 used
laser photolysis of Cl2 and resonance fluorescence to detect Cl
atoms in the 258-419 K temperature interval; reactions 2 and
3, as well as several other reactions of Cl atoms with chlorinated
ethanes, were studied. Kaiser et al.22 used laser photolysis of
Cl2 and time-resolved infrared spectroscopy to detect HCl
formed in reaction 2 at 298 K. The results of the current study
are in agreement with these two earlier investigations (Figure
4).
Figure 4. Temperature dependence of the rate constant of reaction 2.
Results of direct studies and indirect studies where temperature
dependences were reported are displayed: Wine and Semmes,35
Niedzielski et al.,29 Kaiser et al.,22 and the current study. Here, the
relative rate data of ref 29 were converted to the values of k2 using the
experimental temperature dependence of the Cl + CH4 reference
reaction reported by Bryukov et al.18 Thin solid lines represent the
Arrhenius fits of eqs IV and VI in the low- and the high-temperature
regions.
Tschuikow-Roux et al.31,33 and Niedzielski et al.29,30 studied
reaction 2 in the 280-368 K temperature interval using the
relative-rate technique with photolytic production of Cl atoms
and gas chromatographic analysis of final products. The results
of these studies agree with each other. Reference 29 presents
the most comprehensive set of data, which is displayed in Figure
4 along with the k2(T) values of refs 22 and 35 and those of the
current work. The room-temperature relative-rate study of ref
34 results in a k2 value (8.1 × 10-12 cm3 molecule-1 s-1; eq III
was used for the rate constant of the Cl + C2H6 reference
reaction) that is in agreement with the results of direct studies;
it is not included in Figure 4 to avoid plot congestion.
As was discussed in subsection II.3, different k2(T) temper-
ature dependences were obtained in the low-temperature (296-
378 K) and the high-temperature (484-810 K) regions. At low
temperatures, the experimentally determined rate constants
represent sums of the rates of Cl atom attack on both the R and
the â H atoms of C2H5Cl (k2 ) k2a + k2b). At high temperatures,
fast thermal decomposition of the CH2CH2Cl radical56
CH2CH2Cl f Cl + C2H4
(5)
results in the regeneration of Cl, and thus, reaction channel 2b
is not observed. Under these conditions, k2 ) k2a. Reaction 5
has been studied experimentally in ref 56 under conditions
(temperatures and pressures) similar to those used in the current
study; theoretical modeling performed in ref 56 provided a
convenient means for calculating the values of k5 and evaluating
the temperature range where the effect of reaction 5 changes
from negligible to complete regeneration of Cl atoms. This
intermediate temperature interval between 378 and 484 K, where
decomposition of CH2CH2Cl is expected to occur on time scales
roughly comparable to those of the Cl decay due to reaction 2,
was avoided in the experiments because of the difficulty in
quantitative interpretation of the nonexponential Cl decays.
The current study presents the first direct determination
of the rate constant of reaction channel 2a. This informa-
tion, combined with the results of the relative-rate study of
Niedzielski et al.,29 is used to derive the channel-specific k2a(T)
and k2b(T) rate constant dependences (vide infra).
Figure 5. Temperature dependence of the rate constant of reaction 3.
Results of direct studies and indirect studies where temperature
dependences were reported are displayed: Wine and Semmes,35
Tschuikow-Roux et al.,32 and the current study. Here, the relative-rate
data of ref 32 were converted to the values of k3 using the experimental
temperature dependence of the Cl + CH4 reference reaction reported
by Bryukov et al.18 Thin solid lines represent the Arrhenius fits of eqs
V and VII in the low- and the high-temperature regions.
agreement with those of the current study, although the
activation energy of ref 32 is somewhat lower than that of the
current work. The results of Wine and Semmes agree with the
room-temperature k3 value of the current study but are somewhat
lower at higher temperatures (30% difference at 378 K). In this
experimental investigation35 of reactions 2 and 3 and several
other reactions of Cl with chlorinated ethanes, the kinetics of
Cl decay was affected by the regeneration of chlorine atoms,
as reported by the authors. The following reactions of the Cl-
containing radical products were suggested in ref 35 as the cause
of this Cl regeneration: thermal decomposition, reaction with
Cl2, and reaction with the chlorinated ethane substrates (Cl
Reaction 3 has been studied before at low temperatures (e412
K) in direct laser photolysis/resonance fluorescence experiments
of Wine and Semmes35 and in the relative-rate investigation of
Tschuikow-Roux et al.32 The results of these two studies and
of the current work are displayed in Figure 5. As can be seen
from the plot, the k3 values of Tschuikow-Roux et al. are in

Kinetics of Reactions of Cl Atoms with Ethanes
J. Phys. Chem. A, Vol. 107, No. 34, 2003 6571
substitution). To suppress this regeneration, the authors added
large amounts of O2 to convert radical products such as C2H3Cl2
into relatively unreactive peroxy radicals. At temperatures above
400 K in the absence of added O2, Wine and Semmes observed
nonexponential Cl atom decays, which did not depend on Cl2
concentration; addition of O2 resulted in the disappearance of
the nonexponentiality. However, Cl decay rates were slower
than was expected from kinetic modeling using the above
secondary reactions. The authors of ref 35 concluded that ki-
netic observations at T > 400 K cannot be explained using the
kinetic scheme employed. This conclusion has negative im-
plication for the rate constants obtained at these temperatures;
it may rationalize the differences between the results of the
current experimental study and those of Wine and Semmes at
T ≈ 400 K.
The temperature dependence of the kinetics of reaction 3 is
similar to that of reaction 2. At low temperatures (296-378
K), the observed Cl atom decays represent sums of the rates of
Cl atom attack on both the R and the â H atoms of CH3CHCl2
(k3 ) k3a + k3b). At high temperatures (484-810 K), fast thermal
decomposition of the CH2CHCl2 radical results in the regenera-
tion of Cl, and thus, reaction channel 3b is not observed. Under
these conditions, k3 ) k3a. The intermediate temperature interval
between 378 and 484 K, where decomposition of CH2CHCl2 is
expected to occur on time scales roughly comparable to those
of the Cl decay due to reaction 3, was avoided. It was assumed
that the temperature and pressure dependence of the rate constant
of thermal decomposition of CH2CHCl2 is similar to that of
CH2CH2Cl.56
III.2. Site-Specific Abstraction Rate Constants. The ratio
of the rates of abstraction of a hydrogen atom in the R and â
positions in reaction 2 was studied by two groups using relative-
rate techniques.29-31,33,34 The room-temperature values of the
k2a/k2b ratio obtained in different studies are in agreement with
each other. The temperature dependence of this ratio was studied
by Tschuikow-Roux et al.31,33 and Niedzielski et al.29,30 over
the 280-368 K temperature interval; these studies used the same
experimental method (photolytic production of Cl atoms and
gas chromatographic analysis of final products), and their results
coincide. The most comprehensive set of data was presented in
ref 29 (281-367 K), which reported the
Figure 6. Temperature dependences of the rate constants of reactions
2a and 2b, R- and â-abstraction of hydrogen from C2H5Cl by a Cl
atom: filled circles, experimental k2a values obtained in the current
study in the high-temperature region; open circles and squares, values
of k2a and k2b, respectively, calculated from the experimental k2 values
determined in the low-temperature region and the R-to-â ratio of the
site-specific rate constants reported by Niedzielski et al.,29 eq IX; wide
solid lines, results of the relative-rate study of Niedzielski et al.29
converted to the values of k2a and k2b using the experimental temperature
dependence of the reference reaction (Cl + CH4) reported in ref 18;
thin solid lines, Arrhenius fits of eqs XI and XII; dashed line, k2a(T)
dependence of eq XV.
thermal decomposition of the radical products of the â-abstrac-
tion channels), provide the k2a(T) and k3a(T) dependences over
the 296-810 and 293-810 K temperature ranges, respectively.
The values of the â-abstraction channel rate constants are
obtained only in the low-temperature range, T e 378 K. The
resultant temperature dependences of the site-specific rate
constants are displayed in Figures 6 and 7. They can be
reproduced with the following Arrhenius equations:
k2a/k2b ) 1.1 exp(429 K/T)
(IX)
temperature dependence. The R- and â-abstraction rates for
reaction 3 were studied by Tschuikow-Roux et al.32 relative to
those of the Cl + CH4 reaction. These authors found a
k2a(T) ) (2.76 ( 0.28) × 10-11 exp(-455 × 44 K/T)
(296-810 K) (XI)
k3a/k3b ) 0.873 exp(765 K/T)
(X)
k2b(T) ) (1.92 ( 0.47) × 10-11 exp(-789 ( 84 K/T)
(296-378 K) (XII)
temperature dependence of the R-to-â-abstraction ratio (T )
280-368 K), in agreement with the earlier study of Cillien et
al.37
k3a(T) ) (1.46 ( 0.21) × 10-11 exp(-733 ( 58 K/T)
(293-810 K) (XIII)
The results of these relative-rate studies can be combined
with those of the current determination of the overall rate
constants to derive information on the site-specific rates of
hydrogen abstraction. Here, the rate constants of reactions 2
and 3 obtained in the current work in the low-temperature region
(where the observed rate constants represent the sums of both
the R- and the â-abstraction channels) were converted into the
values of site-specific rate constants using eqs IX and X. The
resultant values of the R-abstraction rate constants, combined
with the k2 and k3 dependences obtained in the current work in
the high-temperature regions (where only the R-abstraction
reaction is observed due to the regeneration of Cl atoms via
k3b(T) ) (2.98 ( 1.42) × 10-11 exp(-1686 ( 160 K/T)
(293-378 K) (XIV)
(rate constant units are cm3 molecule-1 s-1). The error limits
in expressions XI-XIV are given as 2σ and represent only the
statistical uncertainties of the fit. The k2a(T) dependence sug-
gests some curvature; however, the existence of this curvature
cannot be unequivocally asserted because of the scatter of the
data points. An alternative modified Arrhenius expression

6572 J. Phys. Chem. A, Vol. 107, No. 34, 2003
Bryukov et al.
reaction energy barriers and the lowest vibrational frequencies
of the transition states were adjusted to achieve approximate
agreement of the calculated rate constants with the experimental
data. However, the reaction models resulted in curvatures of
the ln(k) vs T-1 dependences that are much larger than the
experimental ones; thus, the transition-state-theory approach
failed. A similar problem was observed in our recent study of
the reaction of a Cl atom with chloromethane:18 the calculated
curvature of the Arrhenius plot significantly exceeded the
experimental one. It was suggested18 that variational effects and
the effects of the dependence of the zero-point vibrational energy
on the reaction coordinate65,66 may be responsible for the failure
of nonvariational transition-state theory.
It is interesting to note that the optimized energies of the
transition states for reactions 2a and 3a obtained in the above
computational exercise were approximately 2 kJ mol-1 below
the energy of the reactants. Nevertheless, the calculated tem-
perature dependences of the rate constants are positive despite
the negative barriers because of the increase with temperature
of the ratio of the partition functions of the transition state and
the reactants. This strong temperature dependence of the
partition function ratio is due to the presence in the transition
states of low-frequency vibrational modes (Cl‚‚‚H‚‚‚R bends
in the 90-154 cm-1 range), which are very close to the classical
limit at all experimental temperatures.
The results of the current study confirm the qualitative
observations of the previous studies:32,35 that Cl substitution,
in both the R and the â positions, increases the experimental
activation energies for the abstraction of hydrogen by a chlorine
atom. However, one should be very cautious in attempts to
interpret these observations in terms of the effects of Cl
substitution on the C-H bond energies and on the abstraction
energy barriers. The above description of the failed attempt to
model reactions 2a and 3a with transition-state theory demon-
strates the differences between the observed positive activation
energies and underlying negative values of the energy barriers.
The decrease of the rate constants in the sequence of ethane,
chloroethane, and dichloroethane can be rationalized on the basis
of the transition-state theory expression for the rate constant.
The ratio of the rotational partition function of the transition
state to that of the reactants decreases in this sequence, which
qualitatively explains the observed reactivity trend. The most
likely mechanism of the R-abstraction reactions 2a and 3a, in
view of the slightly negative barriers, is the formation of a
loosely bound intermediate complex followed by a transition
state, the energy of which is below the energy of the reactants
(refs 67-69 provide examples of other reactions where similar
mechanisms have been observed or invoked to explain experi-
mental behavior).
Figure 7. Temperature dependences of the rate constants of reactions
3a and 3b, R- and â-abstraction of hydrogen from CH3CHCl2 by a Cl
atom: filled circles, experimental k3a values obtained in the current
study in the high-temperature region; open circles and squares, values
of k3a and k3b, respectively, calculated from the experimental k3 values
determined in the low-temperature region and the R-to-â ratio of the
site-specific rate constants reported by Tschuikow-Roux et al.,32 eq X;
wide solid lines, results of the relative-rate study of Tschuikow-Roux
et al.32 converted to the values of k3a and k3b using the experimental
temperature dependence of the reference reaction (Cl + CH4) reported
in ref 18; thin solid lines, Arrhenius fits of eqs XIII and XIV.
accounting for the curvature can be used:
k2a(T) ) (8.12 × 10-14)T0.81 exp(-77 K/T) cm3
molecule-1 s-1 (296-810 K) (XV)
The k(T) dependences resulting from the relative-rate studies
of Niedzielski et al.29 and Tschuikow-Roux et al.32 are also
displayed in Figures 6 and 7. Here, the temperature dependence
of the rate constants of the reference reaction (Cl + CH4) used
in these studies was taken from our recent experimental study18
(eq X of ref 18). As one can see from the plots, the results of
the current study are in general agreement with those of
Niedzielski et al.29 and Tschuikow-Roux et al.;32 this is not
unexpected because these earlier relative-rate studies were used
here to derive the site-specific rate data. The activation energies
of the k3a(T) and the k3b(T) dependences of Tschuikow-Roux et
al. are somewhat lower than that those of eqs XIII and XIV,
which reflects the differences between the results of the current
study and those of ref 32 observed in Figure 5 (the overall k3(T)
dependences).
IV. Conclusions
Reactions of Cl atoms with ethane, chloroethane, and 1,1-
dichloroethane have been studied experimentally with the
discharge flow/resonance fluorescence technique over wide
ranges of temperatures and at pressures between 2.3 and 9.2
Torr. The resultant rate constants can be represented with the
modified Arrhenius expressions III-VII.
An attempt at using transition-state theory to reproduce the
temperature dependences of the R-abstraction rate constants (k2a
and k3a) was performed. The properties (rotational constants and
vibrational frequencies) of the reactants and the initial ap-
proximation to the properties of the transition state of reactions
2a and 2b were obtained in quantum chemical calculations
performed at the BH&HLYP/6-311G(d,p) level.62-64 The
Gaussian 9864 program was used in the calculations. The
The rate constants k1 of the reaction of Cl atoms with ethane
obtained in the current study agree with the results of earlier
low-temperature measurements. Combination of the temperature
dependence of k1 obtained in the current work at temperatures
ranging from ambient to 1002 K with the literature data results
in the k1(T) dependence described by eq VIII.
Reactivity with respect to Cl atom attack decreases in the
sequence of ethane, chloroethane, and dichloroethane. This trend

Kinetics of Reactions of Cl Atoms with Ethanes
J. Phys. Chem. A, Vol. 107, No. 34, 2003 6573
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can be qualitatively inferred from the decrease (in this series)
in the ratio of the rotational partition function of the transition
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Cl with chloroethane and 1,1-dichloroethane demonstrate dif-
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Acknowledgment. This research was supported by the
National Science Foundation, Combustion and Thermal Plasmas
Program, under Grant No. CTS-9807136. We thank Dr. L. J.
Stief for helpful advice and the loan of equipment, Dr. V. L.
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