Kinetics of the reactions of CH2Cl, CH3CHCl, and CH3CCl2 Radicals with Cl2 in the Temperature Range 191-363 K

Article abstract of DOI:10.1021/jp909419v

The kinetics of three chlorinated free radical reactions with Cl ₂ have been studied in direct time-resolved measurements. Radicals were produced in low initial concentrations by pulsed laser photolysis at 193 nm, and the subsequent decays of the radical concentrations were measured under pseudo-first-order conditions using photoionization mass spectrometer (PIMS). The bimolecular rate coefficients of the CH₃CHCl + Cl₂ reaction obtained from the current measurements exhibit negative temperature dependence and can be expressed by the equation k(CH₃CHCl + Cl ₂) =(3.02 ± 0.14) × 10^-12)(T/300 K) ^-1.89±0.19 cm³ molecule^-1 s^-1 (1.7-5.4 Torr, 191-363 K). For the CH₃CCl₂ + Cl ₂ reaction the current results could be fitted with the equation k(CH₃CCl₂ + Cl₂) =(1.23 ± 0.02) × 10^-13)(T/300 K)^-0.26±0.10 cm³ molecule^-1 s^-1 (3.9-5.1 Torr, 240-363 K). The measured rate coefficients for the CH₂Cl + Cl₂ reaction plotted as a function of temperature show a minimum at about T ) 240 K: first decreasing with increasing temperature and then, above the limit, increasing with temperature. The determined reaction rate coefficients can be expressed as k(CH₂Cl + Cl₂) = (2.11 ± 1.29) × 10 ^-14) exp(773 ±183 K/T)(T/300 K)^3.26±0.67 cm³ molecule^-1 s^-1 (4.0-5.6 Torr, 201-363 K). The rate coefficients for the CH₃CCl₂ + Cl₂ and CH₂Cl + Cl₂ reactions can be combined with previous results to obtain: k_combined(CH₃CCl₂ + Cl ₂) =(4.72 kcombined 1.66) × 10^-15) exp(971 ± 106 K/T)(T/300 K)^{3.07 ±0.23} cm³ molecule ^-1 s^-1 (3.1-7.4 Torr, 240-873 K) and k _combined(CH₂Cl + Cl₂) =(5.18 ± 1.06) ± 10^-14) exp(525 ±63 K/T)(T/300 K) ^2.52±0.13 cm³ molecule^-1 s^-1 (1.8-5.6 Torr, 201-873 K). All the uncertainties given refer only to the 1θ statistical uncertainties obtained from the fitting, and the estimated overall uncertainty in the determined bimolecular rate coefficients is about ± 15%. Copyright

Full text of DOI:10.1021/jp909419v

J. Phys. Chem. A 2010, 114, 4805–4810
4805
Kinetics of the Reactions of CH Cl, CH CHCl, and CH CCl Radicals with Cl in the
2
†
3
3
2
2
Temperature Range 191-363 K
‡
Matti P. Rissanen, Arkke J. Eskola, and Raimo S. Timonen*
Laboratory of Physical Chemistry, UniVersity of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1),
Helsinki FIN-00014, Finland
ReceiVed: September 30, 2009; ReVised Manuscript ReceiVed: December 22, 2009
2
The kinetics of three chlorinated free radical reactions with Cl have been studied in direct time-resolved
measurements. Radicals were produced in low initial concentrations by pulsed laser photolysis at 193 nm,
and the subsequent decays of the radical concentrations were measured under pseudo-first-order conditions
using photoionization mass spectrometer (PIMS). The bimolecular rate coefficients of the CH
reaction obtained from the current measurements exhibit negative temperature dependence and can be expressed
3
CHCl + Cl
2
-
12
-1.89(0.19
3
-1 -1
by the equation k(CH
Torr, 191-363 K). For the CH
k(CH CCl + Cl
) ) ((1.23 ( 0.02) × 10 )(T/300 K)
K). The measured rate coefficients for the CH Cl + Cl
a minimum at about T ) 240 K: first decreasing with increasing temperature and then, above the limit,
increasing with temperature. The determined reaction rate coefficients can be expressed as k(CH Cl + Cl
(4.0-5.6 Torr, 201-363
reactions can be combined with previous
3
CHCl + Cl
2
) ) ((3.02 ( 0.14) × 10 )(T/300 K)
cm molecule
s
(1.7-5.4
3
CCl + Cl reaction the current results could be fitted with the equation
2
2
-
13
-0.26(0.10
3
-1 -1
3
2
2
cm molecule
s
(3.9-5.1 Torr, 240-363
2
2
reaction plotted as a function of temperature show
2
2
)
-14
3.26(0.67
3
-1 -1
)
((2.11 ( 1.29) × 10 ) exp(773 ( 183 K/T)(T/300 K)
K). The rate coefficients for the CH CCl + Cl and CH
CCl + Cl
cm molecule
Cl + Cl
s
3
2
2
2
2
-
15
3.07(0.23
results to obtain: kcombined(CH
3
2
2
) ) ((4.72 ( 1.66) × 10 ) exp(971 ( 106 K/T)(T/300 K)
3
-1 -1
-14
cm molecule
s
(3.1-7.4 Torr, 240-873 K) and kcombined(CH
2
2
Cl + Cl ) ) ((5.18 ( 1.06) × 10 ) exp(525
2
.52(0.13
3
-1 -1
(
63 K/T)(T/300 K)
cm molecule
s
(1.8-5.6 Torr, 201-873 K). All the uncertainties given refer
only to the 1σ statistical uncertainties obtained from the fitting, and the estimated overall uncertainty in the
determined bimolecular rate coefficients is about (15%.
Introduction
CH CHCl + Cl f products
(2)
(3)
3
2
Chlorinated alkyl free radicals are common intermediates in
1
,2
CH CCl + Cl f products
3 2 2
combustion and atmospheric chemistry. They are frequently
involved through the use of chlorinated solvents and reagents
in industrial synthesis and also in smaller scale processes.3
They are important intermediates in the incineration of hazard-
-6
These reactions have all been studied previously, but the
temperature range (T ) 191-295 K) measured in this work
has not been an object of previous kinetic studies for the title
reactions.
ous wastes,1,7 can enhance soot formation in fuel-rich oxidation,
8
9
and play a role in stratospheric ozone depletion.
2
Chlorine (Cl ) is one of the most produced materials in the
Johnston et al.13 and Bell et al.14 have studied the CH
2
Cl +
2
reaction by means of transition state theory (TST) and BEBO
world, and its common uses in industry are numerous. It is used,
for example, in bleaching paper and cloths, cleaning waters,
manufacturing pesticides, drugs, rubber and plastics, and so
Cl
theory, respectively. Seetula et al. measured the CH
2
Cl + Cl
CCl + Cl
2
15,16
(
(
T ) 295-873 K, p ) 1.8-5.3 Torr)
and CH
3
2
2
1
0,11
on.
The high reactivity of Cl
2
prevents its occurrence as a
15
T ) 414-873 K, p ) 3.1-7.4 Torr) reaction rate coefficients,
free molecule in the universe, and in our environment it readily
using similar LP-PIMS setup with a different temperature
controlling system than is used in the present work, and also
studied the reaction transition states with ab initio methods.
12
forms chlorinated radicals through sunlight photolysis. Because
chlorinated radicals and chlorine molecules are present together
in many different environments, the rates and the mechanisms
of their mutual reactions are of industrial as well as of scientific
interest. Here we present the results of direct kinetic measure-
ments of the rate coefficients of three free radical reactions with
molecular chlorine
1
5
This work was performed to extend the temperature ranges in
which CH Cl + Cl and CH CCl + Cl reactions have been
2
2
3
2
2
measured to lower temperatures, which have not been previously
attained.
Kinetics of the CH
measured directly by Knyazev et al. using LP-PIMS at low
3
CHCl + Cl
2
reaction at 298 K has been
17
18,19
pressures (2-11 Torr). Kaiser et al.
CH CHCl + Cl reaction with respect to the CH
reaction using relative rate method and initiating the reactions
using 351 nm photolysis of Cl in a reaction cell (T ) 298 K,
700 Torr of N ) and employing absorption spectroscopy for
product analysis. Dobis and Benson measured the kinetics of
have studied the
CH Cl + Cl f products
(1)
2
2
3
2
3
CHCl + O
2
†
Part of the special section “30th Free Radical Symposium”.
Corresponding author, raimo.timonen@helsinki.fi.
Current address: School of Chemistry, University of Leeds, Leeds LS2
2
*
‡
2
20
9
JT, U.K.
1
0.1021/jp909419v  2010 American Chemical Society
Published on Web 02/05/2010

4
806 J. Phys. Chem. A, Vol. 114, No. 14, 2010
Rissanen et al.
CH
3
2
CHCl + Cl reaction under very low pressure conditions
(
VLPR-method) at room temperature.
Experimental Section
Details of the experimental apparatus and the procedures used
to measure the bimolecular reaction rate coefficients of the
studied radical-molecule reactions have been described previ-
ously,21 and only an overview is given here. The reactions were
studied in a temperature-controlled stainless steel flow tube
reactor (8 mm i.d., coated with Halocarbon Wax, Supelco)
coupled to a photoionization mass spectrometer. Reactions were
initiated by producing the radicals homogeneously along the
reactor with unfocused exciplex laser (ASX-750) photolysis of
suitable precursors at 193 nm. The gas flow velocity inside the
-1
reactor was about 5 ms which ensured that the repetitive
photolysis at 5 Hz always photolyzed a fresh gas mixture. In
every measurement the molecular reactant was in large excess
over initial radical concentration ([Cl
2
0
] . [R] ) resulting in
pseudo-first-order decay kinetics with respect to radical
concentration.
The change in the radical concentration as a function of time
was continuously measured by taking a portion of the gas
mixture through a small hole in the wall of the reactor. The
sampled gas was formed into a beam by a conical skimmer
Figure 1. A plot of the measured pseudo-first-order rate coefficients
as a function of reactant concentration used to obtain the bimolecular
reaction rate coefficient of the CH
2
Cl + Cl
2
reaction under the
17
experimental conditions: T ) 267 K, [He] ) 1.28 × 10 molecules
-
3
-2
cm , 8 mm reactor with HW coating, laser fluence ≈ 30 mJ cm
Also included in insets are the CH Cl radical decay and the corre-
sponding CH Cl (m/z ) 88) product formation signals, respectively.
They were measured under the conditions of the hollow square in the
.
2
and ionized with a radiation from a resonance gas lamp (Cl
with CaF windows, 8.9-9.1 eV). The ion of the radical was
2
2
2
2
1
4
-3
plot: [Cl
exponential lines in the radical and product signals give the pseudo-
first-order rate coefficients under these experimental conditions: k
2
] ) 4.02 × 10 molecules cm . The fitted first-order
mass selected in a quadropole mass filter (Extrel, C-50/150-
QC/19 mm rods) and detected with an electron multiplier. The
resulting single ion counts were recorded with a multichannel
scaler (EG&G Ortec MCS plus). Typically 1500-20000 laser
pulses were needed for averaging before an exponential function
d
′
-
1
-1
(CH Cl) ) 113 ( 7 s and k ′(CH Cl ) ) 107 ( 6 s
2
f
2
2
.
to avoid interference from second-order radical reactions. No
dependences of the rate coefficients on initial radical concentra-
tions were observed in these experiments.
At the beginning (and in the end) of measurements the decay
rate of the produced radical concentration was measured without
added reactant. The radical loss rate obtained this way corre-
sponds to all the other losses of the radical in the system, except
for the loss in the reaction under study, and was termed as the
{
[R]
to obtain the pseudo-first-order decay rate coefficient (k′) as the
initial radical concentration ([R] ) decreases to concentration
R] at time t.
Exciplex laser photolysis at 193 nm was used to produce the
radicals for the kinetic measurements. The chloromethyl radicals
were produced in the photolysis of CH ClBr
t
) [R]
0
exp(-k′t)} was fit to the measured radical signal
0
[
t
2
wall rate coefficient (k
w
2
′). After this the reactant gas (Cl ) was
CH ClBr + hν(193 nm) f CH Cl + Br
(4)
2
2
introduced to the carrier gas flow in different concentrations,
which were determined by measuring the pressure change in a
known volume, and the respective pseudo-first-order rate
coefficients of the radical loss were measured. When the
measured first-order radical decay rate coefficients (k′) were
f other products
The 1-chloroethyl radicals were generated in the photolysis
of 1,1-dichloroethane. The primary dissociation channel is the
C-Cl bond fission
plotted as a function of used reactant concentration [Cl
the bimolecular reaction rate coefficient k(R + Cl ) could be
obtained from the slope of the plot according to the k′ ) kwall
+ k(R + Cl )[Cl ] equation. An example of this procedure is
shown in Figure 1 for the CH Cl + Cl reaction with the
2
],
2
CH CHCl + hν(193 nm) f CH CHCl + Cl
(5)
3
2
3
2
2
f other products
2
2
measured radical decay and product formation signals in the
insets.
Products of the studied reactions were sought by applying
different ionization energies by changing the light-emitting gas
and the window material of the resonance gas lamp. Lamps
and windows employed to ionize and detect radicals and
The dichloroethyl radicals were produced from methyl
chloroform
CH CCl + hν(193 nm) f CH CCl + Cl
(6)
3
3
3
2
f other products
products were: a Cl lamp with CaF
CH Cl, CH CHCl, and CH CCl ; a H lamp with MgF
(10.2 eV) for CHCl, CH Cl, CHCl , CCl , CH Cl , CH
CH CHCl, CH CHCl , CH CCl ; and a Ne lamp with a CHS
(collimated hole structure) plate (16.7 and 16.9 eV) for Cl, HCl,
Cl , CCl , CH Cl , CH CHCl, CH CCl , CH CCl , and
CH CCl . All the experiments to study the kinetics of the
2
windows (8.9-9.1 eV) for
windows
CCl,
2
3
3
2
2
Initial radical concentrations in the measurements were
calculated with the known absorption cross section (CH ClBr
), laser fluence, and the concentration of the
2
2
2
2
2
2
2
3
3
2
3
2
2
and CH
precursor, or it was estimated from the precursor decomposi-
tion signal (CH CHCl ). Calculated initial radical concentrations
were between 0.5 and 6 × 10 molecules cm in all
3 3
CCl
21
2
2
2
2
2
2
2
2
3
3
2
3
3
1
1
-3
reactions were performed using a chlorine lamp for ionization.
However, some radical signals were measured with a hydrogen
11
-3
measurements. Mostly it was close to 1 × 10 molecules cm

Kinetics of the R + Cl2 Reactions
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4807
TABLE 1: Results and Conditions of the Experiments Used to Measure Reactions R + Cl
2
2
f Products (R ) CH Cl,
a
CH
3
CHCl, and CH
3 2
CCl )
[He]/1017 molecules cm-3
[Cl
]/1013 molecules cm-3
k /10 cm molecule s-1
b
-13
3
-1
k′/s-1
k
wall/s-1
T/K
2
R ) CH
2
Cl, (CH
2
Cl + Cl
2
f products)
Current data:
k ) ((2.11 ( 1.29) × 10 14) exp((773 ( 183) K/T)(T/300 K)3.26(0.67 cm molecule s-1
-
3
-1
Current and previously measured15 data combined:
-
14) exp((525 ( 63) K/T)(T/300 K)2.52(0.13 cm molecule s-1
3
-1
201
221
241
254
267
298
298
336
363
1.36k ) ((5.18 ( 1.06) × 10
10.9-60.6
2.67 ( 0.12
2.62 ( 0.16
2.48 ( 0.14
2.69 ( 0.05
2.59 ( 0.08
2.66 ( 0.14
2.87 ( 0.09
2.99 ( 0.15
3.35 ( 0.11
57.6-175.9
51.1-165.8
58.0-163.4
38.9-153.6
50.4-169.4
42.3-121.2
60.2-186.7
83.9-190.9
57.3-206.0
12
9
14
14
11
9
20
19
15
1.30
1.27
1.30
1.28
1.29
1.80
1.29
1.28
10.2-61.1
9.55-56.5
9.69-51.1
11.2-59.8
10.2-42.8
12.4-58.2
21.1-58.9
8.72-57.7
R ) CH
3
CHCl, (CH
3
2
CHCl + Cl f products)
-
3
-1
k(CH
3
CHCl + Cl
2
) ) ((3.02 ( 0.14) × 10 12)(T/300 K)-1.89(0.19 cm molecule s-1
191
201
221
241
267
298
298
298
336
363
1.38
1.41
1.43
1.39
1.39
0.54
1.74
1.66
1.28
1.27
0.45-1.61
0.62-4.12
0.57-3.12
1.10-5.29
0.99-8.20
1.05-4.81
1.12-5.30
1.14-4.39
0.95-5.76
1.64-8.08
81.8 ( 2.15
61.0 ( 3.45
59.9 ( 4.24
40.1 ( 3.06
32.4 ( 2.09
31.4 ( 2.23
26.3 ( 2.35
29.3 ( 3.33
25.7 ( 2.90
24.0 ( 2.19
76.7-176.2
77.1-283.8
75.3-222.9
80.2-226.4
82.5-294.9
48.9-172.6
59.6-149.1
68.6-179.1
39.4-171.6
48.2-210.3
46
29
35
22
21
14
11
28
26
11
R ) CH
3
CCl
2
, (CH
3
CCl
2
2
+ Cl f products)
Current data:
k ) ((1.23 ( 0.02) × 10 13)(T/300 K)-0.26(0.10 cm molecule s-1
-
3
-1
1
5
Current and previously measured data combined:
-
15) exp((971 ( 106) K/T)(T/300 K)3.07(0.23 cm molecule s-1
3
-1
240
243
266
298
298
336
336
363
1.6 7k ) ((4.72 ( 1.66) × 10
19.1-89.0
17.2-65.9
22.1-75.0
21.2-65.8
19.0-106
20.3-75.4
19.5-58.6
21.5-111
1.29 ( 0.05
1.38 ( 0.06
1.20 ( 0.07
1.24 ( 0.09
1.18 ( 0.05
1.21 ( 0.14
1.17 ( 0.06
1.21 ( 0.12
47.5-136.4
40.7-109.5
50.8-102.7
43.9-96.9
37.1-140.8
38.7-99.8
37.3-75.6
30.1-129.0
22
18
17
18
18
14
7
1.66
1.68
1.27
1.65
1.28
1.26
1.26
5
a
Range of precursor concentrations used: (0.77-3.0) × 10 molecules cm-3 for CH
13
ClBr, (1.87-6.95) × 10 molecules cm-3 for
12
2
1
3
-3
3 3
CCl
. b Statistical uncertainties shown are 1σ. Estimated overall uncertainty in the
CH
3
CHCl
2
and (0.07-1.42) × 10 molecules cm for CH
measured bimolecular rate coefficients is about (15%.
lamp for ionization for comparison. No differences in the
measured pseudo-first-order kinetics were observed.
K) rate coefficient, and n is the coefficient describing temper-
ature dependence. The rate coefficients measured for the
Precursors bromochloromethane (CH
dichloroethane (CH CHCl , Aldrich, 97%), 1,1,1-trichloroethane
CH CCl , Aldrich, 99%), and reagent chlorine gas (Cl , Messer-
Griesheim, 99.8%) were degassed prior to use by several
freeze-pump-thaw cycles. The carrier gas (He, Messer-
Griesheim, 99.9996%) was used as supplied.
2
ClBr, Fluka, 98%), 1,1-
CH Cl + Cl reaction and for the CH CCl reaction with Cl
2
2
3
2
2
combined with previous data,1 show a discernible curvature
in their double-logarithmic plots and a temperature-dependent
pre-exponential factor had to be used for a better fit. Three
5,16
3
2
(
3
3
2
ˆ
parameter fits of the modified Arrhenius equation k ) A
m
ˆ
exp(B/T)(T/300 K) , where T is temperature in K, and A, B,
and m are fitting parameters, are given separately for the
current data and also for the whole data including previously
measured values by Seetula et al.1 These obtained expres-
sions for the rate coefficients can be found in Table 1. The
preferred fits, i.e., the fits with widest experimental range,
have been included in Figure 2.
Results and Discussion
5,16
The results of the performed measurements and the corre-
sponding experimental conditions are presented in Table 1. The
determined bimolecular R + Cl reaction rate coefficients
2
together with the results from previous studies are shown as a
function of temperature on a double-logarithmic plot in Figure
The only product observed for the reactions studied was
2
+
. For the current results of the CH
Cl reactions, the rate coefficients’ dependence on temperature
2
3
CHCl + Cl
2
and CH
3
CCl
2
CH
2
Cl
2
(m/z ) 88) in CH
2
Cl + Cl
2
1
reaction in agreement with
5,16
the previous work by Seetula et al.
The measured signal is
n
can be expressed by the equation: k ) k300K(T/300 K) , where
T is temperature in K, k300K is the room temperature (T ) 300
shown in the inset of Figure 1 together with the corresponding
CH Cl radical decay profile. The exponential fits to the signals
2

4
808 J. Phys. Chem. A, Vol. 114, No. 14, 2010
Rissanen et al.
it at low pressures (2-11 Torr) with a similar method as in the
present work and obtained k298K(CH
0
3
CHCl + Cl ) ) (4.37 (
2
.70) × 10 cm molecule s-1 in reasonably good agreement
-12
3
-1
with the current value k300K(CH
3
CHCl + Cl ) ) (3.02 ( 0.45)
2
-12
3
-1 -1
18,19
×
10 cm molecule s . Kaiser et al.
measured reaction
2
indirectly in a relative rate determination using 351 nm light
in order to initiate the reactions in a reaction
cell (T ) 298 K and 700 Torr N ), using absorption spectroscopy
for analysis method and CH CHCl + O as a reference reaction.
They obtained 0.42 as a ratio of the CH CHCl + Cl and
CH CHCl + O reaction rate coefficients. Dobis and Benson
studied the CH CHCl + Cl reaction with a VLPR technique
in the millitorr pressure range and obtained k298K(CH CHCl +
in a gross
to photolyze Cl
2
2
3
2
3
2
2
0
3
2
3
2
3
-
13
3
-1 -1
Cl
disagreement with the current result.
The CH CCl + Cl reaction has been studied by Seetula et
2
) ) (1.7 ( 1.0) × 10
cm molecule
s
3
2
2
al.15 both experimentally to measure the kinetics of the reaction
and also using an ab initio calculation to locate the reaction
transition states and energies. The rate coefficients measured
by Seetula et al.15 (T ) 414-873 K, p ) 3.1-7.4 Torr) are
presented with hollow triangles in Figure 2, together with the
current results (filled triangles). The agreement between the two
Figure 2. Double-logarithmic plot of the determined R + Cl
bimolecular reaction rate coefficients shown as a function of temper-
ature. Included are the previously measured values for the CH Cl
hollow squares) and CH CCl (hollow triangles) radical reactions with
Cl measured by Seetula et al.
et al. (hollow circle) and Benson and Dobis (hollow star) on
CH CHCl + Cl reaction. Current results are indicated with solid
symbols. The temperature dependences of the CH Cl + Cl and
CH CCl + Cl reactions are fitted according to expression k ) A exp-
B/T)(T/300 K) . For the CH
the temperature dependence is fitted by equation k ) k300K(T/300 K) .
2
2
(
3
2
set of results is good. The expression: k(CH
3
3
CCl
2
+ Cl ) )
2
1
5,16
2
and the measurements of Knyazev
-13
-0.26(0.10
-1 -1
(
(1.23 ( 0.02) × 10 )(T/300 K)
cm molecule
s
1
7
20
fits well to the data measured in this work. When previous
values15 are taken into account, a two-parameter fit cannot give
an adequate description of the temperature dependence and a
3
2
2
2
ˆ
3
2
2
m
(
3
CHCl + Cl
2
reaction rate coefficients
three-parameter fit is used instead: kcombined(CH
CCl
+ Cl ) )
3
2
2
n
-15
3.07(0.23
(
(4.72 ( 1.66) × 10 ) exp(971 ( 106 K/T)(T/300 K)
3 -1 -1
cm molecule s .
3
The rate coefficients of the CH CHCl radical reaction with
Cl
2
exhibit a negative temperature dependence, which is notably
correspond to the pseudo-first-order decay (k
d
′(CH
2
Cl) ) 113
-1
-1
different from the other two title reactions. The CH CHCl radical
3
(
7 s ) and formation (k
f
′(CH Cl ) ) 107 ( 6 s ) rate
2 2
is also significantly more reactive toward Cl
radicals studied in this work; at room temperature, it is about
0 times more reactive than CH Cl and about 20 times more
than CH CCl . It should, however, be pointed out that the rate
coefficients of CH Cl + Cl and CH CCl + Cl reactions also
possess negative temperature dependences below a certain
temperature. For the CH CCl radical the changing temperature
is about T ) 340 K and for the CH Cl radical about T ) 240
K as seen in Figure 2. An intriguing question is whether the
higher reactivity of CH CHCl radicals, than the other radicals
2
than the other
coefficients and their similar values within the statistical
uncertainties of the fits are taken as an indication that the signals
are formed in the same reaction.
1
2
3
2
The CH
2
2
Cl + Cl reaction has been studied before by
1
3
u
s
i
n
g
t
r
a
n
s
i
t
i
o
n
s
t
a
t
e
t
h
e
o
r
y
a
n
d
B
e
l
l
e
t
a
l
.
1
4
2
2
3
2
2
J
o
h
n
s
t
o
n
e
t
a
l
.
using the BEBO method. These early theoretical studies give
rate coefficients that differ orders of magnitude from the current
3
2
-
15
3
-1 -1
13
14
2
results (k300K ) 3.5 × 10
cm molecule
s
with TST
-
17
cm molecule s-1 with BEBO,
3
-1
and k300K ) 5.9 × 10
3
respectively) and will not be considered here afterward. Direct,
more recent studies have been performed by Seetula et al.15,16
studied in this work, is connected to its strong negative
temperature dependence. Interesting information would be the
value of k(CH
Unfortunately, we are not currently able to reach those
temperatures.
(
T ) 295-873 K, p ) 1.8-5.3 Torr) with essentially similar
3
CHCl + Cl ) at temperatures 600-800 K.
2
LP-PIMS setup as was used in the present study. The agreement
between these values and the bimolecular rate coefficients
obtained in this work is excellent as can be seen in Figure 2.
15,16
The temperature dependence of the CH
3 2
CHCl + Cl reaction
The rate coefficients measured by Seetula et al.
with hollow squares, and the results from the current work are
indicated with filled squares. Considering only the current
are presented
rate coefficients shows a slight deviation from linearity on a
logarithmic scale. Nevertheless, the aforementioned equation
(k ) k300K(T/300 K) ) can still be used to properly describe the
temperature dependence.
n
results, the rate coefficients of the CH
expressed as a function of temperature with equation k(CH
2
Cl + Cl
2
reaction can be
Cl
) ) ((2.11 ( 1.29) × 10 ) exp(773 ( 183 K/T)(T/300
2
-
14
+
Cl
2
It seems obvious that CH CCl radical has lower reactivity
3
2
3
.26(0.67
3
-1 -1
K)
cm molecule s . Taking the previous results into
Cl + Cl ) )
2 3
toward Cl than CH CHCl. The same can be expected for
CH Cl, although the reasons for the behavior in reactivity can
be different. In the CH CCl case, the radical site is hardest to
attack because of the sterical hindrance of the two relatively
large Cl atoms and a methyl group attached to it; this could
account, the expression becomes: kcombined(CH
2
2
2
-
14
2.52(0.13
(
(5.18 ( 1.06) × 10 ) exp(525 ( 63 K/T)(T/300 K)
3
2
3
-1 -1
15
cm molecule s . Seetula et al. also studied the reaction
transition states by ab initio calculations and found that as the
electron density of the radical center decreases the transition
make the reactivity lower compared to CH Cl. Because of the
2
state of the R + Cl
The room temperature rate coefficient of the CH
2
reaction becomes tighter.
electron-withdrawing inductive effect of the halogen atoms in
3
CHCl +
the CH
than in CH
compared to CH
3
CCl
Cl and CH
CHCl and CH
2
radical, the radical site is more electron deficient
CHCl. This can cause the lower reactivity
Cl radicals. In the CH CHCl
1
7
Cl
2
reaction has been measured by Knyazev et al., by Kaiser
2
3
et al.1 and by Dobis and Benson. Knyazev et al. measured
8,19
20
17
3
2
3

Kinetics of the R + Cl2 Reactions
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4809
radical, the electron density of the radical site should be higher
than that in the CH
to serve as an electron donor. Therefore the reactivity of
CH CHCl should be even higher than the reactivity of CH Cl,
2
Cl radical due to the methyl group’s ability
3
2
and the 10-fold difference in the room temperature reaction rate
coefficient is noteworthy to be pointed out.
The reactivity differences among radical-molecule reactions
toward a common reactant have been related to readily obtain-
able radical properties by making correlations of the observed
rate coefficients against a chosen molecular property, to gain
understanding about the underlying reasons for different reactiv-
ity. Linear correlations have been sought by making free energy
22
plots for the reactions under study. The logarithm of the room
temperature rate coefficient is usually taken as a measure of
the free energy of activation in similar types of reactions and is
plotted against different radical properties. One example of such
a procedure is to plot the logarithm of the room temperature
rate coefficient (log(k300K)) of the radical-molecule reaction
against the electron affinity of the molecular reagent (EA(reac-
tant)) subtracted from the ionization potential of the radical
(
IP(R)); [log(k300K) vs (IP(R) s EA(reactant))]. Paltenghi et al.23
observed that this expression provides a good simultaneous
correlation for oxygen and ozone reactions with alkyl radicals,
instead of just correlating the logarithm of reaction rate
coefficients with the ionization potential of the alkyl radicals
(
IP(R)) in one set of reactions. Another such example is to plot
the log(k300K) for the radical-molecule reaction against ∆Elec-
tronegatiVity of the reactive radical, which is given in an
arbitrary scale based on the simple sum of Pauling electronega-
tivities2 of the substituent atoms/groups. ∆ElectronegatiVity
of the radical is used for estimating the electron-withdrawing
inductive effects of the substituents in the radical center. This
4,25
2
6
scale was first introduced by Thomas and transported to gas
16,27
kinetics by Gutman and co-workers
who found it to be useful
in explaining the observed differences in the rate coefficients
of the methyl and halogenated methyl radical reactions with
HI. The similar linear relationship was observed to hold for
larger alkyl radicals when a group electronegativity value of
2
7
1
.82 was assigned to the methyl group as a substituent. It was
Figure 3. (a) ∆ElectronegatiVity of the radical (R) plotted against the
room temperature (RT ) 298 ( 3 K) rate coefficient in current and
also shown to correlate linearly with the rate coefficients of the
16
28
R + Cl
Two of these linear correlations were observed to hold for
reactions and they are
shown in parts a and b of Figure 3. The room temperature rate
coefficients are plotted against radicals’ ∆ElectronegatiVity(R)
Figure 3a) and electron affinity EA(R) (Figure 3b) on semi-
logarithmic plots. A good linear correlation spanning almost 6
orders of magnitude can be recognized in the plots. In Figure
a the ∆ElectronegatiVities(R) of the fluorinated radicals have
been calculated using an effective electronegativity value 2.8
for isolated fluorine atom (instead of Paulings’ value 3.98) to
2
and R + Br
2
reactions.
selected R + Cl
2
reactions. Electronegativities were taken from
2
4,25
Pauling,
and an electronegativity value of 1.82 was assigned to the
methyl group in accordance with ref 27. Fluorinated radicals have been
included to the correlation by assigning an effective electronegativity
value of 2.8 to a fluorine atom as a substituent, as was briefly noted in
the text and discussed in ref 16. The rate coefficients for the selected
the current and selected similar R + Cl
2
3
0
31
32
R + Cl
2
reactions were taken from CH
3
,
CF
3
,
CCl
3
,
CHBrCl and
I, CF , n-C , and
3 7 4 9
and i-C H and t-C H . The line in the picture
(
1
5
16
,33
CHCl
n-C
2
,
CH
2
Br and CH
2
2
Cl and CFCl
2
C
2
H
5
3 7
H
35,36
37
4
H
9
,34 s-C
4
H
9
,
is an unweighted, linear least-squares fit to the data. The current results
are indicated by star symbols. (b) Electron affinity of the radical (R)
plotted against the room temperature (RT ) 298 ( 3 K) rate coefficient
3
2
in current and selected R + Cl reactions. Electron affinities were taken
from ref 29 except for CH
substituted ethyl radicals, the EA of the radical was unavailable and
was estimated with the help of ref 40, e.g., for CH CCl : EA(CH CCl
EA(CH CH ) + (EA(CHCl ) - EA(CH )). The rate coefficients for
the selected R + Cl reactions were taken from: CH
2
I3 and for CHBrCl. In the case of
8
39
have them fit into the same correlation as has been discussed
previously.16 For the methyl group, an electronegativity value
3
2
3
2
)
1
.82 was used, as has been briefly discussed above.
As seen in Figure 3a, the rates of R + Cl reactions are
)
3
2
2
3
2
30
31
32
2
3
,
CF
3 3
, CCl ,
affected by the electron densities in the radical centers; the rate
of the reaction decreases together with decreasing electron
density. For example a Cl atom, or halogen atoms in general,
when substituted in the radical center withdraw electrons from
the center and decrease the electron density. Chlorine molecule
15
16
33
CHBrCl and CHCl
2
,
CH
2
Br and CH
and i-C
2
I, CF
and t-C
2
Cl and CFCl
2
,
2 5
C H
3
4
35,36
37
and n-C
3
H
7
,
s-C
4
H
9
,
3
H
7
4
H
9
.
The line in the
picture is an unweighted, linear least-squares fit to the data. The current
results are indicated by star symbols.
2
9
has a large electron affinity (EA(Cl
2
) ≈ 2.40 eV ) and
that the more loosely the electron is bound to the radical (lower
the ionization energy of the radical) the faster the reaction rate
should be, i.e., rate coefficients of these reactions should
correlate with IE(R). However, this correlation was not observed.
consequently when electron density is higher in the radical
center, as it is for alkyl substituted radicals, the rates of R +
Cl reactions are enhanced (Figure 3a). This could also suggest
2

4
810 J. Phys. Chem. A, Vol. 114, No. 14, 2010
Rissanen et al.
The rates increase with the electron density in the radical site,
but not with the looseness of the weakest bound electron (i.e.,
lower IE(R)).
Kinetics and Photochemical Data for Use in Stratospheric Modelling:
Evaluation Number 15; Publication 06-2; National Aeronautics and Space
Administration, Jet Propulsion Laboratory, California Institute of Technol-
ogy: Pasadena, CA, 2006.
The room temperature rate coefficients of the R + Cl
2
(3) Chiltz, G.; Goldfinger, P.; Huybrechts, G.; Martens, G.; Verbeke,
G. Chem. ReV. 1963, 63 (4), 355.
reactions also correlate fairly well with the electron affinity of
the radical (R) (Figure 3b). The increasing electron affinity
(4) Poutsma, M. In Methods in Free-Radical Chemistry; Huyser, E. S.,
Ed.; Marcel Dekker: New York, 1969; Vol. 1, pp 79-193.
5) Benson, S. W. U.S. Patent No. 4,199,533, 1980.
(6) Senkan, M. S. Chem. Eng. Prog. 1987, 83 (12), 58.
7) Cormier, S. A.; Lomnicki, S.; Backes, W.; Dellinger, B. EnViron.
Health Perspect. 2006, 114 (6), 810.
2
reduces the radical reactivity toward Cl . This may be under-
stood in the following way. The higher the electron affinity of
the radical, the more the radical holds the electron and does
(
(
not share it with Cl
the formation of the bond between the radical center and Cl
2
, which also has high EA. Consequently
in
(
(
8) Violi, A.; D’Anna, A.; D’Alessio, A. Chemosphere 2001, 42, 463.
9) Scientific Assessment of Ozone Depletion: 2006, Global Ozone
2
the reaction can be hindered by the higher the electron affinity
of the radical.
By inspection of the results presented in parts a and b of
Figure 3, some general conclusions can be drawn. (i) Replacing
hydrogen atoms with halogen atoms in an alkyl radical decreases
Research and Monitoring ProjectsReport No. 50; WMO (World Meteo-
rological Organization): Geneva, Switzerland, 2007; 572 pp.
(
10) (a) http://periodic.lanl.gov/elements/17.html, 28. 09. 2009. (b) http://
en.wikipedia.org/wiki/Chlorine, 28. 09. 2009.
11) http://www.health.state.ny.us/environmental/emergency/chemical_
terrorism/chlorine_tech.htm; 30.09.2009.
12) Wayne, R. P. Chemistry of atmospheres, 3rd ed.; Oxford University
Press, Inc.: New York, 2000.
(
(
the R + Cl
carbon chain at R-position to the radical center increases the R
Cl rate coefficients. (iii) Branching at the R-position seems
2
reaction rate coefficients. (ii) Lengthening the
(
(
13) Johnston, H. S.; Goldfinger, P. J. Chem. Phys. 1962, 37, 700.
14) Bell, T. N.; Perkins, K. A.; Perkins, P. G. J. Phys. Chem. 1977,
+
2
to have a stronger rate enhancing influence than just the carbon
chain length but more studies with the isomeric radicals are
needed to further clarify this issue.
81, 2610.
(
15) Seetula, J. A. J. Chem. Soc., Faraday Trans. 1998, 94, 3561.
(16) Seetula, J. A.; Gutman, D.; Lightfoot, P. D.; Rayes, M. T.; Senkan,
S. M. J. Phys. Chem. 1991, 95, 10688.
(17) Knyazev, V. D.; Bencsura, A.; Dubinsky, I. A.; Gutman, D.; Melius,
C. F.; Senkan, S. M. J. Phys. Chem. 1995, 99, 230.
1
5
As has been discussed above, Seetula et al. studied the
CH Cl + Cl and CH CCl + Cl reaction transition states by
ab initio calculations, together with other alkyl and halogenated
alkyl radical reactions with Cl and found that as the electron
density of the radical center decreases, the transition state of
the R + Cl reaction becomes tighter. Compared to the other
radicals in this study, the CH CHCl radical has higher electron
2
2
3
2
2
(
18) Kaiser, E. W.; Rimai, L.; Schwab, E.; Lim, E. C. J. Phys. Chem.
992, 96, 303.
19) Kaiser, E. W.; Wallington, T. J. J. Phys. Chem. 1995, 99, 8669.
1
2
(
(20) Dobis, O.; Benson, S. W. J. Phys. Chem. A 2000, 104, 5503.
(21) Eskola, A. J.; Timonen, R. S. Phys. Chem. Chem. Phys. 2003, 5,
2
2
557.
22) Marston, G.; Monks, P. S.; Wayne, R. P. In General Aspects of
3
(
density in the radical center due to the methyl group substitution
and it would be interesting to have an ab initio calculation of
the potential energy surface for the reaction path with the
transition state.
the Chemistry of Radicals; Alfassi, Z. B.; Ed.; John Wiley & Sons Ltd.:
New York, 1999; pp 429-471.
(23) Paltenghi, R.; Ogryzlo, E. A.; Bayes, K. D. J. Phys. Chem. 1984,
8, 2595.
8
(24) Pauling, L. Nature of the Chemical Bond, 3rd ed,; Cornell University
Press: Ithaca, NY, 1960; pp 88-95.
Conclusion
(25) Allred, A. L. J. Inorg. Nucl. Chem. 1961, 17, 215.
(
(
26) Thomas, T. D. J. Am. Chem. Soc. 1970, 92, 4184.
27) Seetula, J. A.; Gutman, D. J. Phys. Chem. 1991, 95, 3626.
Three chlorinated alkyl radical reactions with Cl
studied in direct time-resolved measurements. The measured
rate coefficients for two of the reactions studied (CH Cl + Cl
and CH CCl + Cl ) show an interesting behavior as temperature
2
have been
(28) Timonen, R. S.; Seetula, J. A.; Niiranen, J.; Gutman, D. J. Phys.
Chem. 1991, 95, 4009.
2
2
(
29) Linstrom, P. J.; Mallard, W. G. NIST Chemistry Webbook, NIST
3
2
2
Standard Reference Database Number 69, June 2005,; National Institute
of Standards and Technology: Gaithersburg, MD (http://webbook.nist.gov).
(30) Eskola, A. J.; Timonen, R. S.; Marshall, P.; Chesnokov, E. N.;
Krasnoperov, L. N. J. Phys. Chem. A 2008, 112, 7391.
is varied: a negative temperature dependence below a specific
limiting temperature and a positive dependence on temperature
above it. The CH
these temperatures and has steep negative temperature depen-
dence over the whole experimental range covered. Linear
correlations of the determined rate coefficients against radical
properties have been sought in order to gain understanding about
3
2
CHCl + Cl reaction behaves differently at
(
31) Kaiser, E. W.; Wallington, T. J.; Hurley, M. D. Int. J. Chem. Kinet.
995, 27, 205.
32) DeMare, G. R.; Huybrechts, G. Trans. Faraday Soc. 1968, 64, 1311.
1
(
(33) Timonen, R. S.; Russell, J. J.; Gutman, D. Int. J. Chem. Kinet.
986, 18, 1193.
1
2
(
34) Eskola, A. J.; Lozovsky, V. A.; Timonen, R. S. Int. J. Chem. Kin.
007, 39, 614.
35) Tyndall, G. S.; Orlando, J. J.; Wallington, T. J.; Dill, M.; Kaiser,
E. W. Int. J. Chem. Kinet. 1997, 29, 43.
the factors affecting radical reactivity in R + Cl
CH Cl , from the CH Cl + Cl reaction, was the only observed
product in the studied reactions.
2
reactions.
2
2
2
2
(
(
36) Lenhardt, T. M.; McDade, C. E.; Bayes, K. D. J. Chem. Phys. 1980,
7
2, 304.
Acknowledgment. R.S.T. and M.P.R. acknowledge the
support from the CoE of the Academy of Finland.
(
(
37) Timonen, R. S.; Gutman, D. J. Phys. Chem. 1986, 90, 2987.
38) Born, M.; Ingemann, S.; Nibbering, N. M. M. J. Am. Chem. Soc.
1
994, 116, 7210.
References and Notes
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Ion Processes 2000, 194, 103.
40) Eskola, A. J. PhD Thesis, University of Helsinki, Helsinki, 2007
http://urn.fi/URN:ISBN:978-952-10-3786-3).
(
(
1) Tsang, W. Combust. Sci. Technol. 1990, 74, 99.
(
2) Sander, S. P.; Friedl, R. R.; Ravishankara, A. R.; Golden, D. M.;
(
Kolb, C. E.; Kurylo, M. J.; Molina, M. J.; Moortgat, G. K.; Keller-Rudek,
H.; Finlayson-Pitts, B. J.; Wine, P. H.; Huie, R. E.; Orkin, V. L. Chemical
JP909419V