Kinetics of reactions of CN with chlorinated methanes

Article abstract of DOI:10.1016/j.cplett.2008.06.014

The kinetics of reactions of CN with the chlorinated methanes CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄ were investigated over the temperature range 298-573 K, using laser induced fluorescence (LIF) spectroscopy. At 298 K, rate constants of 9.0 ± 0.3 × 10^-13, 8.8 ± 0.4 × 10^-13, 9.0 ± 0.5 × 10^-13 and 4.3 ± 0.6 × 10^-13 cm³ molecule^-1 s^-1 were measured, respectively. A small positive temperature dependence was observed, as well as kinetic isotope effects of k_H/k_D ～ 2.14-2.25. These data along with product detection experiments strongly suggest that hydrogen abstraction dominates these reactions.

Full text of DOI:10.1016/j.cplett.2008.06.014

Chemical Physics Letters 460 (2008) 64–67
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Kinetics of reactions of CN with chlorinated methanes
*
Vaishali Samant, John F. Hershberger
Department of Chemistry and Molecular Biology, North Dakota State University, Fargo, ND 58105, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The kinetics of reactions of CN with the chlorinated methanes CH3Cl, CH2Cl2, CHCl3 and CCl4 were inves-
tigated over the temperature range 298–573 K, using laser induced fluorescence (LIF) spectroscopy.
Received 15 May 2008
In final form 5 June 2008
Available online 11 June 2008
At 298 K, rate constants of 9.0 0.3 ꢀ 10ꢁ13
,
8.8 0.4 ꢀ 10ꢁ13
,
9.0 0.5 ꢀ 10ꢁ13 and 4.3 0.6 ꢀ
10ꢁ13 cm3 moleculeꢁ1 sꢁ1 were measured, respectively. A small positive temperature dependence was
observed, as well as kinetic isotope effects of kH/kD ꢂ 2.14–2.25. These data along with product detection
experiments strongly suggest that hydrogen abstraction dominates these reactions.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Thermodynamic data were taken from JANAF tables [12], except
for ClCN [19]. A major goal of this study is to determine the dom-
Halogenated alkanes play an important role in stratospheric
chemistry and combustion chemistry. Many groups have studied
reaction kinetics of halogenated alkanes with different reactive
species for example OH, Cl, etc. [1,2]. Reactions of CN radical are
also important in combustion chemistry as it is an important inter-
mediate in the formation of NOx by oxidation of HCN with OH [3,4].
Reactions of CN with saturated and unsaturated hydrocarbons
have been extensively studied [5–11]. CN radical reacts with satu-
rated hydrocarbons by direct hydrogen abstraction, resulting in
formation of HCN products [9].
In this work we report a study of the kinetics of CN radical reac-
tions with chlorinated methanes. No previous literature exists on
the kinetics of these reactions. Two different product channels
are thermodynamically possible, hydrogen abstraction and chlo-
rine atom abstraction
inant product channel in these reactions.
2. Experimental section
Rate constant measurements were performed using LIF (laser
induced fluorescence) techniques. ICN was used as a photolytic
precursor for CN radicals
ICN þ h
m
ð248 nmÞ ! CN þ I
ð6Þ
Details of the LIF apparatus used for kinetics measurements
have been described in a previous publication [13]. The photolysis
laser light was provided by an excimer laser (Lambda Physik COM-
PEX 200) operating at 248 nm and a repetition rate of 4 Hz. Typical
photolysis pulse energies were 2–6 mJ, resulting in typical initial
[CN]0 concentrations of ꢂ1–2 ꢀ 1013 molecule-cmꢁ3. CN radicals
were detected by LIF, using 387.088 nm probe light (ꢂ0.5 mJ/
pulse), which was produced by frequency doubling 774.176 nm
light from a dye laser (Continuum ND-6000) pumped by the sec-
ond harmonic of a Nd:YAG laser (Continuum Surelite II-10). The
probe laser was operated at a repetition rate of 8 Hz. The photolysis
and probe beams were made collinear using a dichroic mirror and
copropagated down a 88-cm Pyrex reaction cell. Fluorescence was
detected 90° from the laser beams using an R508 photomultiplier
tube. The unamplified PMT signal was recorded by a boxcar inte-
grator (Stanford Instruments model 250) with a 10–60 ns delay
and a 400 ns gate width, and averaged on a computer. A digital de-
lay generator (Stanford DG535) was used to vary the delay be-
tween excimer and dye laser pulses in order to produce a LIF
signal vs. time profile. The CN concentration was assumed to be
proportional to the LIF signal. The static cell was resistively heated
to collect data at elevated temperatures up to 573 K. Gases intro-
duced into the reaction cell were allowed to mix for 5 min to en-
sure complete mixing and thermal equilibration.
CN þ CH4 ! HCN þ CH3
D
H0298 ¼ ꢁ87:34 kJ molꢁ1
ð1Þ
CN þ CH3Cl ! HCN þ CH2Cl
! ClCN þ CH3
D
H0298 ¼ ꢁ107:9 kJ molꢁ1
ð2aÞ
ð2bÞ
D
H0298 ¼ ꢁ71:36 kJ molꢁ1
CN þ CH2Cl2 ! HCN þ CHCl2
D
H0298 ¼ ꢁ124:36 kJ molꢁ1
ð3aÞ
ð3bÞ
! ClCN þ CH2Cl
D
H0298 ¼ ꢁ87:91 kJ molꢁ1
CN þ CHCl3 ! HCN þ CCl3
D
H0298 ¼ ꢁ126:2 kJ molꢁ1
D
ð4aÞ
ð4bÞ
! ClCN þ CHCl2
H0298 ¼ ꢁ108:55 kJ molꢁ1
CN þ CCl4 ! ClCN þ CCl3
D
H0298 ¼ ꢁ126:25 kJ molꢁ1
ð5Þ
* Corresponding author. Fax: +1 701 231 8831.
E-mail address: john.hershberger@ndsu.edu (J.F. Hershberger).
0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.06.014

V. Samant, J.F. Hershberger / Chemical Physics Letters 460 (2008) 64–67
65
In some experiments, FTIR spectra of reactants and products
298 K
373 K
473 K
573 K
150
100
50
were recorded by transferring the contents of the reaction cell into
a shorter 12-cm cell, and collecting spectra using a Thermo Nicolet
NEXUS 470 FTIR spectrometer. Also, some experiments used a high
resolution infrared diode laser spectrometer as described previ-
ously [14].
ICN (Aldrich) was purified by vacuum sublimation to remove
dissolved air. SF6 and CH3Cl (Matheson) were purified by repeated
freeze–pump–thaw cycles at 77 K. CH2Cl2 (Alfa Acer), CHCl3 and
CCl4 (Aldrich) were purified by repeated freeze–pump–thaw cycles
at 77 K to remove dissolved air.
Typical experimental conditions were PICN = 0.05–0.2 Torr,
PChlorinated Methane = 0–0.9 Torr and PSF6 = 0.5–1.0 Torr. The reaction
cell was evacuated and refilled after every kinetic run.
3. Results and discussion
0
0
200
400
600
800
1000
CH3Cl PressuremTorr
3.1. Total rate constant measurements
Fig. 2. Pseudo-first-order decay rates of CN LIF signals as a function of CH3Cl
pressure at different temperatures.
The decay of CN was measured in the absence of chlorinated
methane reagent; a very small decay rate was observed, as shown
in Fig. 1. This decay is attributed to reactions of CN radical with it-
self or with traces of oxygen present in the system. When a reagent
such as CH3Cl was added to the reaction mixture the decay rate of
CN was observed to increase substantially as shown in Fig. 2.
Above about 10 mTorr of CH3Cl, pseudo-first-order conditions ap-
ply, leading to an exponentially decay function:
Similar set of experiments were performed for CH2Cl2, CHCl3
and CCl4. Figs. 3–5 shows a plot of k0 vs. reagent pressures at differ-
ent temperatures for CH2Cl2, CHCl3 and CCl4, respectively. For reac-
tion of CN with CHCl3 and CCl4 at high temperatures some
saturation effect was observed, i.e. at higher pressures
(P400 mTorr) of reactant gas (CHCl3 and CCl4) no change in the
k0 value observed for these reactions. The LIF intensity was ob-
served to decrease with increasing reactant gas pressure. This sug-
gests that dark reactions, possibly involving ICN precursor
molecules, are taking place at higher temperature. This effect
was also observed with CH2Cl2 but only at higher pressures of
P800 mTorr. We carried out some experiments to confirm the
existence of dark reactions (described below). The result of the
dark reaction was to limit reliable data collection to T 6 373 K for
CHCl3 and to room temperature for CCl4.
The rate constant values for all these reactions are summarized
in Table 1. The rate constant values obtained for CN reaction with
partially chlorinated methanes are higher than that obtained with
methane at all temperatures [6]. Fig. 6 shows the Arrhenius plot for
measured rate constants for CN reactions with CH3Cl and CH2Cl2.
The data can be fit to the following expressions for reactions (2)
and (3), respectively (error bars represent one standard deviation):
½CNꢃ ¼ ½CNꢃ0 expðꢁk0tÞ
ð7Þ
ð8Þ
k0 ¼ k½CNꢃ þ kD
where k0 is the observed pseudo-first-order rate constant, k is the
desired bimolecular rate constant for the CN + chlorinated methane
reaction, and kD is the decay rate of CN in the absence of added re-
agent. Several processes contribute to kD, including diffusion of CN
radicals out of the probed volume, self-reaction of CN radicals, and
reaction with trace amounts of O2. Fig. 2 shows a plot of k0 vs. CH3Cl
pressure at different temperature (298, 373, 473 and 573 K). The
slope of this plot gives the bimolecular rate constants (k) for
CN + CH3Cl reaction at different temperatures. These values are
summarized in Table 1.
= PCH Cl = 0 mTorr
2
1
3
k2ðCN þ CH3ClÞ ¼ ð4:9 ꢄ 0:53Þ ꢀ 10ꢁ11 expðꢁ1213 ꢄ 45=TÞ
k3ðCN þ CH2Cl2Þ ¼ ð6:8 ꢄ 1:6Þ ꢀ 10ꢁ11 expðꢁ1309 ꢄ 94=TÞ
= PCH Cl = 800 mTorr
3
The respective activation energy (Ea) values are listed in Table 1.
The Ea value determined for reaction (4) is very approximate, as we
could measure k values at only two temperatures for this reaction.
Because of the presence of dark reactions at elevated temperatures,
we were unable to determine an Ea value for reaction (5). The
observation of positive activation energies of similar magnitude
to that observed in the CN + CH4 reaction is consistent with a direct
H atom abstraction mechanism, channels (2a), (3a), and (4a).
0
-1
-2
3.2. Investigation of dark reactions
0
20
40
60
To monitor the dark reactions taking place in the presence of
CHCl3 and CCl4, FTIR absorption measurements were carried out
for the reaction mixture at room temperature and at 573 K. For
these measurements the reaction mixtures were first filled in the
LIF cell and allowed to mix and thermally equilibrate for 10 min
Time ms
Fig. 1. Laser-induced fluorescence signals of CN as a function of excimer-dye delay
time. Reaction conditions:PICN = 100 mTorr, Pchloromethane = variable, PSF6 = 900 mTorr
at 298 K.

66
V. Samant, J.F. Hershberger / Chemical Physics Letters 460 (2008) 64–67
Table 1
Rate constant and activation energy values for CN reaction with methane and chlorinated methanes at different temperatures
Reagent
Rate constant (cm3 moleculeꢁ1 sꢁ1
)
Ea (kJ/mol)
298 K
373 K
473 K
573 K
a
CH4
7.28 ꢀ 10ꢁ13
1.24 ꢀ 10ꢁ12
2.12 ꢀ 10ꢁ12
3.27 ꢀ 10ꢁ12
8.32
10.08
10.8
ꢂ8.53
CH3Cl
CH2Cl2
CHCl3
CCl4
9.0 0.3 ꢀ 10ꢁ13
8.8 0.4 ꢀ 10ꢁ13
9.0 0.5 ꢀ 10ꢁ13
4.3 0.6 ꢀ 10ꢁ13
1.9 0.2 ꢀ 10ꢁ12
1.9 0.5 ꢀ 10ꢁ12
1.8 0.3 ꢀ 10ꢁ12
3.5 0.5 ꢀ 10ꢁ12
4.0 0.2 ꢀ 10ꢁ12
6.0 0.5 ꢀ 10ꢁ12
7.5 0.5 ꢀ 10ꢁ12
a
Values taken from Ref. [8].
150
100
50
298 K
298 K
373 K
473 K
573 K
10
5
0
0
0
200
400
600
800
1000
0
200
400
600
800
1000
CCl4 Pressure mTorr
CH2Cl2 PressuremTorr
Fig. 5. Pseudo-first-order decay rates of CN LIF signals as a function of CCl4 pressure
at RT.
Fig. 3. Pseudo-first-order decay rates of CN LIF signals as a function of CH2Cl2
pressure at different temperatures.
60
CH3 Cl
298 K
373 K
CH2 Cl2
-26
40
20
0
-28
0.002
0.003
0
200
400
600
800
1000
1/T
CHCl3 Pressure mTorr
Fig. 6. Arrhenius plot of the CN + CH3Cl and CN + CH2Cl2 reactions.
Fig. 4. Pseudo-first-order decay rates of CN LIF signals as a function of CHCl3
pressure at different temperatures.
CN + CHCl3: FTIR spectra of CHCl3 were recorded at 298 K and at
573 K; no significant difference was observed in the spectral char-
acteristics at these two temperatures. These spectra match well
with the literature spectra. Similarly FTIR spectra of CHCl3 in the
presence of ICN were also recorded; this time we observed a differ-
ence in spectral characteristics at different temperatures. The
298 K CHCl3 spectrum remained unchanged in the presence of
ICN. At 573 K, however, the CHCl3 spectrum in the presence of
ICN shows an absorption at 3300 cmꢁ1, which is evidence of HCN
and then transferred to another cell for FTIR measurements. Two
reaction mixtures used for recording FTIR spectra are (i) chlori-
nated methane and (ii) chlorinated methane + ICN. Spectra were
recorded both before and after firing 100 photolysis laser shots.
A similar set of experiments was performed at 573 K. Typical pres-
sures of the gases in reaction mixtures used for these measure-
ments are 300 mTorr of ICN and 4 Torr of chlorinated methanes.
The following general features were observed:

V. Samant, J.F. Hershberger / Chemical Physics Letters 460 (2008) 64–67
67
Table 2
formation. CHCl3 + ICN spectra were also recorded at both temper-
atures after firing photolysis laser shots. In this case the character-
istics absorption for HCN was observed at both the temperatures.
These experimental observations demonstrate that HCN is a prod-
uct (possibly the major product) of reaction (3), and that HCN is
also formed at the high temperature without firing the photolysis
laser, suggesting a dark reaction at high temperature which inter-
feres with the rate constant measurements.
CN + CCl4: FTIR spectra of CCl4 recorded at 298 K and 573 K
matches with the literature CCl4 spectrum at 298 K. CCl4 + ICN
spectra were recorded after firing 100 shots of laser at both tem-
peratures. These spectra have a weak absorption feature at 2000–
2200 cmꢁ1 indicating formation of ClCN. This product was also
formed at higher temperature without any photolysis laser firing,
again suggesting a dark reaction at high temperature.
Rate constant values for CN reaction with deuterated methane and chlorohydrocar-
bons at 298 K
Reagent
Rate constant (cm3 moleculeꢁ1 sꢁ1
)
kH/kD
a
CD4
1.74 ꢀ 10ꢁ13
3.4
CD3Cl
CD2Cl2
CDCl3
4.0 0.2 ꢀ 10ꢁ13
3.9 0.2 ꢀ 10ꢁ13
4.2 0.3 ꢀ 10ꢁ13
2.25
2.25
2.14
a
Values taken from Ref. [6].
secondary isotope effects. Our results, however, are consistent with
a primary isotope effect, nearly of the magnitude of the literature
values for CH4. This provides further evidence that these reactions
proceed primarily via direct hydrogen abstraction, channels (2a),
(3a), and (4a).
CN + CH2Cl2: In the case of CH2Cl2 the recorded FTIR spectra at
298 K and 573 K matches with the reported spectrum at RT. At
higher temperature CH2Cl2 + ICN spectrum recorded without firing
laser shots shows only a trace of HCN, indicating only a slight
amount of dark reaction.
4. Conclusions
We have reported the first measurement of the kinetics of CN
reaction with chlorinated methanes. These reactions are slightly
faster than that of CN + CH4. Several pieces of evidence, including
the observation of positive activation energies, kH/kD ratios consis-
tent with a primary isotope effect, and product detection experi-
ments all indicate that the reaction of CN with partially
chlorinated species CH3Cl, CH2Cl2 and CHCl3 are dominated by di-
rect hydrogen atom abstraction and that CNCl formation is a minor
or insignificant channel, except for the slower CN + CCl4 reaction.
3.3. Detection of ClCN products
High-resolution infrared diode laser absorption spectroscopy
was used for the detection of ClCN. (Unfortunately, diode laser
detection of HCN products was not feasible because of the lack of
available diode lasers with emission near 3300 cmꢁ1). Several
groups have reported the infrared absorption spectrum of ClCN
[15–18]. Theoretical calculations of IR spectra have also been re-
ported [19–21]. No high resolution IR absorption spectrum for
ClCN has been reported, but all previous studies have indicated
that ClCN has an absorption band in the 2000–2200 cmꢁ1 range.
Using a high resolution diode laser, we have monitored the IR
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