Rate constants and the H atom branching ratio of the reactions of the methylidyne CH(X2Π) radical with C2H2, C2H4, C3H4 (methylacetylene and allene), C3H6 (

Article abstract of DOI:10.1039/b812810c

The reactions of the CH radical with several unsaturated hydrocarbons C₂H₂ (acetylene), C₂H₄ (ethylene), C₃H₄ (methyl-acetylene and allene), C₃H ₆ (propene) and C

Full text of DOI:10.1039/b812810c

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Rate constants and the H atom branching ratio of the reactions
2
of the methylidyne CH(X P) radical with C H , C H , C H
2
2
2 4
3 4
(
methylacetylene and allene), C H (propene) and C H (trans-butene)
3 6 4 8
Jean-Christophe Loison*ab and Astrid Bergeatab
Received 25th July 2008, Accepted 16th September 2008
First published as an Advance Article on the web 30th October 2008
DOI: 10.1039/b812810c
The reactions of the CH radical with several unsaturated hydrocarbons C
H
2 2
2 4
(acetylene), C H
(
3 4 3 6 4 8
ethylene), C H (methyl-acetylene and allene), C H (propene) and C H (trans-butene) were
2
studied at room temperature, in a low-pressure fast-flow reactor. CH(X P, v = 0) radicals were
obtained from the reaction of CHBr with potassium atoms. The overall rate constants at 300 K
3
À10
À10
are CH + C H : (3.6 Æ 0.6) Â 10 , CH + C H : (3.1 Æ 0.6) Â 10 , CH + C H
2
2
2
4
3
4
À10
À10
(
(
methyl-acetylene): (3.4 Æ 0.6) Â 10 , CH + C
H
3 4
(allene): (3.6 Æ 0.6) Â 10 , CH + C
3 6
H
À10
À10
3
cm
propene): (4.2 Æ 0.8) Â 10
and CH + C
H
4 8
(trans-butene): (4.0 Æ 0.80) Â 10
À1 À1
molecule
s
(errors are cited at the level of Æ1s). Absolute atomic hydrogen production was
determined by vacuum ultra-violet (VUV) resonance fluorescence, H production from the CH + CH
reaction being used as a reference. Observed H branching ratios for these CH reactions were:
C H : 0.90 Æ 0.08, C H : 0.94 Æ 0.08, C H (methyl-acetylene): 0.98 Æ 0.08, C H (allene):
4
2
2
2
4
3
4
3
4
0
.97 Æ 0.08, C H (propene): 0.78 Æ 0.10, C H (trans-butene): 0.69 Æ 0.12 (errors are cited at the
3
6
4
8
level of Æ1s). A compilation of the available kinetic data on these reactions has been made in order to
propose rate coefficients for each possible channel of the different reactions for astrochemical models.
Introduction
CH + unsaturated hydrocarbon reactions. Identification of the
final products and determination of branching ratios are parti-
cularly important for obtaining reliable modeling of complex
chemistry such as interstellar chemistry and combustion. If the
global rate constant of CH + hydrocarbons is quite well
The methylidyne radical, CH, is extremely reactive and plays a
major role in various chemical environments ranging from
1
hydrocarbon combustion, dense interstellar clouds (ISCs)
2
3
4
5
and in the atmospheres of Titan, Neptune and Triton. The
reactions of the CH radical provide a way to synthesize long
hydrocarbons and complex organic molecules in dense inter-
studied, there are only two previous experimental H branching
13,14
ratio determinations for the CH + C H reaction
and one
2
2
14
for the CH + C H reaction. There are also theoretical studies
2
4
2
stellar clouds (ISCs) and planetary atmospheres. The kinetics
15–19
2
20,21
4
for the CH + C
2
H
and CH + C
2
H
reactions and two
of CH reactions with unsaturated hydrocarbons have been
previously studied for temperatures ranging from 300 K to
coupled ab initio/RRKM studies on the evolution of the excited
22,23
allyl radical (C
3
H
5
)
which is supposed to be the main
reaction.
6
–9
10
6
1
50 K, down to 23 K for CH + C H , CH + C H and
2 2 2 4
intermediate of the CH + C
H
2 4
1
-C H and down to 77 K for CH + C H , C H and C H .
1
4
8
2
2
3
4
3
6
We have performed kinetics experiments using a selective
source of CH radicals in a low-pressure fast-flow reactor at
room temperature. The overall rate constants were obtained
studying the decay of the CH radical by laser induced fluores-
cence or by OH chemiluminescence after addition of small
The experimental temperature dependence of the global rate
constants of the CH + unsaturated hydrocarbon reactions
suggest that these reactions proceed without any barrier.
Although there is still debate about the exact mechanism of
the CH + unsaturated hydrocarbons reactions, the inter-
mediate results from the insertion of the CH radical into a
C–H or single C–C bond, or addition to the double or triple
C–C bond, leading to a triplet radical which then quickly
evolves. The absence of any pressure dependence for the rate
constant shows that stabilization is not competitive with dis-
sociation of the intermediates. As the CH radical has a high
2
amount of O (CH + O - OH(A S) + CO), the hydrocarbon
2
2
being introduced in excess; corrections for radical loss by
24–27
diffusion have been validated in previous studies.
Absolute
product branching ratios of the CH + hydrocarbons reactions
were estimated for the channels yielding H atoms by comparison
26
with the CH + CH - C H + H (100%) ratio, the H atoms
4 2 4
were probed by resonance fluorescence in the VUV. Com-
parisons with previous experimental and theoretical studies are
2
À1 12
enthalpy of formation (D H (CH(X P)) = 596.40 kJ mol ),
f
298
there are many thermodynamically accessible channels for the
2 2 2 4
made for the CH + C H and CH + C H reactions.
a
Universite´ de Bordeaux, UMR 5255, ISM, Groupe Astrochimie,
3
51 cours de la Libe´ration, F-33405 Talence Cedex, France.
Experimental measurement
E-mail: jc.loison@ism.u-bordeaux1.fr; Fax: +33 (0)5 40 00 66 45
CNRS, UMR 5255, ISM, Groupe Astrochimie, 351 cours de la
Libe´ration, F-33405 Talence Cedex, France
b
The experimental setup has been described in detail
2
4–28
previously
and only a brief summary is given here. The
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s
setup consists of a fast-flow reactor, i.e., a 36-mm Teflon
Results
inner tube with four optical ports for detection. The CH
radicals are produced in an ‘‘injector’’ which slides in the
reactor. At the end of the injector, the CH radicals are
mixed with the hydrocarbon flow. Then, the distance (d)
between the end of the injector and the observation windows
is directly proportional to the reaction time under plug
flow conditions. The distance can be varied over the range
Overall rate constant
The pseudo-first-order decays of the CH radical fluorescence
signal were measured at different concentrations of hydro-
carbons introduced in large excess. To eliminate mixing effects
at short reaction times, only the last stages of the decay (after 3 cm
from the injector exit) have been taken to determine the
pseudo-first-order rate constants. The measured rate constants
were then corrected for radial and axial diffusion using
0
–100 mm with 0.5 mm precision, allowing kinetic studies
À1
between 0 and 3.3 ms (the flow velocity is around 30 m s ).
The pressure, is measured by a capacitance manometer
2
9
26
Keyser’s formula, as done previously with good results.
(
Barocel 0–10 Torr), and the flow rates are adjusted by thermal
A typical measurement of the second-order rate constant is
displayed in Fig. 1, for the CH + C H reaction, where axial
mass flow controllers.
2
2
The CH radicals were produced from the CHBr + 3 K -
3
and radial corrected pseudo-first-order rate constants are
plotted versus the hydrocarbon concentration. The main
source of errors in our measurements was the important radial
and axial diffusion correction. Moreover, the high wall
removal rate constant, due to wall deposit of potassium,
associated with these diffusional processes, leads to the limit
conditions of the plug-flow approximation and the errors
quoted take into account these uncertainties. The second-
order rate constants of the CH reaction with the various
hydrocarbons concerned by this study are summarized in
Table 1 and are compared with previous measurements. The
present results are in relatively good agreement with previous
measurements, considering the variety of experimental setups.
There are fairly substantial scatters in the measurements of the
CH + 3 KBr overall reactions which can be separated into
three elementary bromine abstractions. As all of the
K + CHBr - KH + CBr (x Z 0) reactions are endoergic,
x
x
this source can only produce CH radicals in their ground
vibrational state which has also been checked experimentally.
As a large excess of potassium is introduced in the injector
compared to the CHBr
3
concentration, the precursors
(
CHBr , CHBr , and CHBr) concentrations in the fast flow
3
2
2
5
reactor are very small (as check in a previous study by fitting
the LIF time-dependent signals of CHBr and CH radicals with
kinetic modelisation of the three bromine atoms stripping) and
will not interfere in our study. The K atoms are not reactive
with non-halogenated hydrocarbon molecules. The typical
conditions in the reactor are the following: P = 2 Torr, [K] o
2 2 2 4
rate coefficients for the CH + C H and CH + C H
1
2
À3
3
.0 Â 10 molecule cm , [CHBr
3
, CHBr
2
12
, and CHBr] o
reactions. However, no systematic deviation can be identified
in the various studies, even for the lowest values of Butler
6
et al. In fact, the measurements of the other reactions they
1
0
À3
À3
,
1
.0 Â 10 molecule cm and [CH] E 1.0 Â 10 molecule cm
the carrier gas being He with a stated purity of 99.995%. CHBr
99%) was used without any further purification. The different
3
(
have studied are in better agreement with those of others
groups. We thus propose to adopt, at 298 K, the average
value (fit by weighted least squares analysis) of (3.4 Æ 0.4) and
hydrocarbons were used directly from the cylinder with a stated
purity 499.995%.
À10
3
À1 À1
The CH radicals were probed by laser-induced fluores-
cence (LIF) using a ND:YAG laser (Quantel YG 581C)
pumped dye laser (around 100 mJ by pulse) and exciting the
(
3.0 Æ 0.3) 10
cm molecule s for the CH + C H and
2
2
C H rate coefficients, respectively.
2
4
2
2
2
2
CH (A D ’ X P) near 431 nm or by OH (A S - X P)
chemiluminescence detection with an interference filter around
Product branching ratios
Hydrogen atom production from the CH + hydrocarbon
reactions was measured relative to the H atom production
3
05 nm, with electronically excited OH being produced by
introducing a very small concentration of O for kinetics
experiments (kinetic contribution of the CH + O reac-
2
2
tion was always inferior to 5% of the CH + hydrocarbons
contribution).
Hydrogen atoms were detected by resonance fluorescence
1
2
o
1 2
using the 2p
a
P - 1s S transition at 121.6 nm (L ).
Atom excitation was achieved with the microwave discharge
2
4–28
lamp previously described.
We also used the microwave
discharge lamp in an absorption setup to check the absorp-
tion of H and hydrocarbons in the reactor. Typically the
maximum H atom concentration attenuate the incident radia-
a
tion by a few percent at the L (H atoms concentration range
1
1
À3
from 2–8 Â 10 atom cm ) and the light attenuation
by hydrocarbons in this wavelength range is inferior to
0
.1%. Thus, the conditions of the presently reported experi-
ments ensure the linear dependence of the atomic fluores-
cence signal versus the lamp emission intensity and the H
atom concentration, and also the negligible influence of
hydrocarbon absorption.
Fig. 1 Pseudo-first-order rate constant of the CH + C H reaction
2
2
2 2
versus the C H concentration.
6
56 | Phys. Chem. Chem. Phys., 2009, 11, 655–664
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À10
À3
À1 À1
Table 1 Overall rate constants at room temperature in 10
cm molecule s
À10
298 K/10
À3
À1 À1
Reactions
k
cm molecule s
References
8
CH + C
CH + C
CH + C
2
2
3
H
H
H
2
4
4
3.2 (Æ0.2)
Thiesemann et al.
10
4
4
2
3
.0
Canosa et al.
7
Berman et al.
.2 (Æ0.4)
.2 (Æ0.4)
.6 (Æ0.6)
6
Butler et al.
This work
9
2.85 (Æ0.02)
Thiesemann et al.
10
3
3
2
3
.2
Canosa et al.
7
Berman et al.
.65 (Æ0.3)
.1 (Æ0.8)
.1 (Æ0.6)
6
Butler et al.
This work
6
(H
3
C–CRCH)
4.6 (Æ1.5)
Butler et al.
This work
3
4
.4 (Æ0.6)
.03 (Æ0.13) (170 K)
1
1
1
1
1
1
Daugey et al.
This work
Daugey et al.
This work
Daugey et al.
This work
Canosa et al.
CH + C
3
3
H
4
6
(H
2
3
CQCQCH
2
)
3.6 (Æ0.6)
.01 (Æ0.16) (170 K)
4.2 (Æ0.8)
4
CH + C
H
(H
C–CHQCH
2
)
3
.86 (Æ0.20) (170 K)
CH + C
CH + C
4
H
H
8
(trans-Butene)
(1-Butene)
4.0 (Æ0.8)
10
4
8
3.70 (Æ0.25)
from the CH + CH reaction by resonance fluorescence in the
4
production. We used the Wu and Kern estimation for
3
3
VUV. As the H atom branching ratio is known to be equal to
CH + C H
3
reactions and arbitrary H atom productions
4
2
for the CH + CH4 reaction in our conditions, these
6
1
for the others reactions. As the C H + hydrocarbon reaction
2
relative determinations of the branching ratio for the
CH + hydrocarbon reactions can be converted to absolute values.
In order to measure the relative H atom production, the
produces a maximum of 10% of the total H atoms present in
the experiment, the uncertainties of these branching ratios are
included in the total uncertainty of the H product branching
ratios of the CH + hydrocarbon reactions. Typical traces of H
atom concentrations, deduced from the fluorescence signals,
versus the distance (i.e. the reaction time) are shown in Fig. 2
fluorescence signal is recorded successively for the CH + CH
and the CH + hydrocarbon reactions. The CH and hydro-
4
4
carbon concentrations were adjusted in order to have equi-
valent global first-order rate constants, the CH production
being constant during a period of more than one hour. This
operation was repeated several times, alternately for different
CH4 and hydrocarbon concentrations, under different pre-
ssures and different CHBr3 concentrations. In our experi-
mental conditions we have to be aware that secondary
reactions occur. (i) We checked that the molecules and radicals
produced by the CH + hydrocarbon reactions do not react
with the reactant hydrocarbon (the radical products are
4 2 2
for the CH + CH and CH + C H reactions. In the
experimental conditions of Fig. 2, the contribution from
CH + CH - C H + H in the presence of CH or C H is
2
4
2
2
close to 10% and the contribution to the H atom concentration
from C H + C H - C H + H is also close to 10%.
2
2
2
4
The two total H atom concentration traces during CH + CH
and CH + C are very similar. As the C H + CH reaction
gives C + CH and no H atom, the CH + C
reaction has a smaller H branching ratio than the CH + CH
4
2
H
2
2
4
2
H
2
3
2
H
2
4
mainly H or CH
reaction with unsaturated hydrocarbons). (ii) There are two
other sources of H atoms: one is the CH + CH - C H + H
3
, both of them presenting a barrier towards
reaction. The total uncertainty in the H atom branching ratio
is estimated to be 8% in this case. As the H atom production
2
4
from the CH + CH reaction is the same when CH and
reaction and the second source is the C H + hydrocarbon
2
hydrocarbons are added with equivalent global first-order rate
constants, the uncertainty comes mainly from the amount of
C H radical formed and the subsequent C H + hydrocarbons
reactions. These two sources have to be taken in account as
they could produce H atoms. To determine the H atom produc-
tion of CH + hydrocarbon reactions, we performed simulations
2
2
reactions.
of H production, including the CH + CH - C
reaction, the C H + hydrocarbon reaction, the mixing
effect and wall reactions for each experimental condition:
CH alone, CH + CH and CH + hydrocarbon. The kinetic
parameters used in the simulation are listed in Table 2. The H
atoms branching ratio for the hydrocarbons
reactions are not known and are estimated in the following
2
H + H
We have performed simulations for all the experimental H
branching ratios for the CH + hydrocarbon reactions. The
results are presented in Table 3.
2
4
C
2
H
+
Discussion
3
0
Overall rate constant
way. Ab initio calculations for the C H + C H shown that
2
2
2
H production is equal to 100% for this reaction. The C H + C H
4
The high values of the rate constants for these reactions
suggest that there are no barriers on the potential-energy
surfaces for these reactions. This absence of a barrier is
confirmed by low temperature kinetics studies performed with
the CRESU technique, down to 23 K for the CH + C H ,
2
2
3
1
leads to the HC H CH
radical which should mainly
evolve toward HC HCH and so leading to 100% of
3
2
2
3
3
HC
3 2 3 4
HCH + H (see discussion on CH + C H reactions).
For the others reactions, H atom abstraction can become
3
2
2
2–35
10
important
as well as others exit channel such as CH
3
CH + C H and CH + 1-butene reactions and down to 77 K
2 6
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3
Table 2 Reaction mechanism used in simulations of atomic H production (k298 K in cm molecule
À1 À1
s )
a
Reactions
k
298 K
References
3.0 Â 10À10
36
Dean et al.
Bergeat et al.
CH + CH - C
2
H + H
28
À10
b
CH + C
2
H - C
- C
3
2
H + H
+ H
3.0 Â 10
À10
37
CH + CH
4
H
4
0.9 Â 10
Blitz et al.
2
6
Fleurat-Lessard et al.
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
À10
CH + C
CH + C
CH + C
CH + C
CH + CH
CH + CH
CH + CH
CH + CH
2
2
2
2
H
H
H
H
2
2
4
4
- C
- C
- C
- C
3
3
3
2
H
H + H
2
+ H
3.4 Â 10
À10
2
0.2 Â 10
À10
H
H
3
+ H
+ CH
3.0 Â 10
À12
2
3
6.0 Â 10
À10
3
3
2
2
CCH - C
CCH - C
CCH
CCH
4
H
H
3
2
+ H
+ H
3.3 Â 10
À12
4
2
7.0 Â 10
À10
2
- C
- C
4
H
H
3
+ H
+ H
3.5 Â 10
À11
2
4
2
2
1.0 Â 10
À10
CH + C
CH + C
CH + C
CH + C
3
3
4
4
H
6
H
6
H
8
H
8
4
- C
- C
- C
- C
- C
H
4 6
H
4 4
H
4 8
H
4 6
H
2 2
+ H
+ CH
+ H
+ CH
+ CH
3.3 Â 10
À10
3
0.9 Â 10
À10
2.6 Â 10
À10
3
1.1 Â 10
À12
38
C
C
C
C
C
C
C
C
C
C
C
C
C
C
2
2
2
2
2
2
2
2
2
2
2
2
2
2
H + CH
3
3.0 Â 10
Opansky et al.
Ceursters et al.
À10
30
H + C
H + C
H + C
H + CH
H + CH
H + CH
H + CH
H + CH
H + CH
H + C
H + C
H + C
H + C
2
2
2
H
H
H
2
4
4
- C
- products
- C + H
4
H
2
+ H
1.3 Â 10
À10
39
1.2 Â 10
Opansky et al.
Estimation
À10
4
H
4
1.2 Â 10
À10
32
3
3
3
2
2
2
CCH - products
CCH - C
CCH - C
1.9 Â 10
Hoobler et al.
Wu and Kern
Hoobler et al.
Hoobler et al.
À11
33
2
H
H
2
+ CH
+ H
2
CCH
1.7 Â 10
À10
32,33
5
4
1.7 Â 10
À10
32
CCH
CCH
CCH
2
2
2
- products
- C
- C
1.7 Â 10
À11
33
2
H
H
2
+ CH
+ H
2
CCH
1.7 Â 10
Wu and Kern
Hoobler et al.
À10
32,33
5
4
1.5 Â 10
À10
40
3
3
4
4
H
H
H
H
6
6
8
8
- products
- C + H
- products
2.4 Â 10
Vaktin et al.
Estimation
À10
5
H
6
1.4 Â 10
À10
41,42
2.1 Â 10
Vakhtin et al.
Estimation
À10
- C
6
H
8
+ H
0.8 Â 10
a
3
In cm molecule
À1 À1
b
s
.
Assumed to be equal to CH + CH.
Table 3 Branching ratio of atomic hydrogen production
Reactions
H atom yields
References
CH + C
CH + C
2
H
2
4
0.90 Æ 0.08
This work
Boullart et al.
9
13
4
0
1
0
.85 + À15
.05 Æ 0.09
.98 (RRKM)
1
McKee et al.
Nguyen et al.
1
9
2
H
0.94 Æ 0.08
This work
McKee et al.
1
4
1
0
.09 Æ 0.14
2
2,23
.98 Æ 0.01 (RRKM) Davis et al.
CH + C
3
H
4
(methylacetylene) 0.98 Æ 0.08
This work
This work
This work
This work
CH + C
CH + C
CH + C
3
3
4
H
4
H
6
H
8
(allene)
0.97 Æ 0.08
0.78 Æ 0.10
0.69 Æ 0.12
(trans-butene)
1
1
for CH + methylacetylene, CH + allene and CH + propene.
15–19
Ab initio calculations on the CH + C H ,
20,21
CH + C H
2 4
2
2
2
7
and CH + C H reactions show that the first step without a
2
6
potential energy barrier is (i) the insertion of the CH radical
into a C–H bond or eventually a single C–C bond, or
(
ii) the addition to the double or triple C–C bond, resulting
in a chemically-activated radical. As for all of the reactions
studied here, there are several open exothermic exit channels,
thus stabilization of any adduct cannot compete with dissocia-
tion and the transient radical rapidly decomposes. Therefore
the rate-limiting process is the CH insertion and/or addition.
The same argument can be applied to the CH radical reacting
with larger alkenes or alkynes. The similarity in the
overall rate constants for the reactions studied here, where
Fig. 2 H atom concentrations, for the CH + CH
reactions. The open squares are the experimentally measured H atom
concentrations during the CH + CH reaction and the continuous line
is the simulation, the filled circles are the total experimental measured
H atom concentrations during the CH + C reaction and the
dashed line is the simulation, the filled stars are the H atom concen-
4 2 2
and CH + C H
4
2
H
2
trations from the CH + CH reaction in the absence of CH
and the dashed-point line is the simulation.
4 2 2
or C H
6
58 | Phys. Chem. Chem. Phys., 2009, 11, 655–664
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À1
the overall rate constants at room temperature are equal to
À10
is only 60 kJ mol below the entrance channel, a change of
À1
À3
À1 À1
3
–4 Â 10 cm molecule
s
, could be qualitatively explained
À4 kJ mol will lead to a higher H
2
production in better
by the dominance of the dispersion interaction, as well as the quite
large contribution from the dipole-induced dipole term, as shown
agreement with the two last experimental results. Further-
more, the dissociation onsets for propargyl determined by
44
43
by Georgievskii and Klippenstein.
McCunn et al. are significantly lower than the ab initio
predictions, indicating that the ab initio predictions could be
too high, so the RRKM predictions could underestimate the
Products
CH + C
H
2
channel. Considering all the experimental data and theo-
2
H
2
. Detailed ab initio calculations from Vereecken
retical calculations, there is no doubt that the C H + H
2
3
1
6
19
et al. and Nguyen et al. indicate that the first step of this
reaction is the CH insertion into one C–H bond, leading to
propargyl, or the CH addition to the triple bond, leading
channel is the major one, the overlap of the experimental data
leading to the value of 95%. Considering the various
experimental data and RRKM calculations, we propose the
following branching ratios:
to cyclic C
3 2
H . As there are several open exothermic exit
channels, stabilization of any adduct cannot compete with
1
7,19
dissociation.
A simplified representation of the minimum
CH + C H - HCCCH + H 85 Æ 6%
2
2
energy paths (MEP) from their calculations is presented in
Fig. 3.
- c-C H + H 10 Æ 6%
3
2
Our experimental H production for this reaction has been
determined to be equal to 90 Æ 8%. There are two previous
- HCCC + H 5 Æ 3%
2
1
experimental measurements: one from Boullart et al. at
3
+
9
6
00 K equal to 85
+15
% for the C H + H channel and
CH + C
initial step of the CH + C
theoretical calculations on the evolution of the subsequent
2 4
H . There are some theoretical calculations on the
À15
% for the C H + H channel, and one another from
3
2
9,20,21
1
5
H
2 4
reaction,
and detailed
À9
3
2
1
4
McKee et al. equal to 105 Æ 9% for the C H + H channel.
3
2
2
2,23
There is also one study of the unimolecular dissociation of the
C
3
H
5
isomers,
evolution related to the unimolecular dis-
4
4
propargyl radical by McCunn et al. with identification of H
and H products, without a branching ratio determination but
with dissociation onset for the various channels. The study
sociation of the allyl radical. The most recent and reliable
,21
9
calculations
have predicted that the insertion of CH in one
2
C–H bond leading to allyl radical, and the addition of CH
to the double CQC bond, leading to cyclo-propyl, do not
possess any potential energy barrier in the entrance valley.
Detailed quantum chemical calculations at various levels by
4
5
of the photodissociation of propargyl radicals at 248 nm
leading to a propargyl radical with a similar energy to that
obtained from the CH + C - C reaction) gives 97.6%
for the C + H (HCCCH + H) channel and 2.4% for the
C H + H channel. Combined ab initio and RRKM calcula-
(
2
H
2
3 3
H
9
H
2
Thiesemann et al. strongly suggest that the initial step of the
3
reaction is dominated by the addition. However, because of
the much lower energy of the allyl radical compared to the
cyclo-propyl radical, associated to high isomerization rates,
this allyl radical will be the dominant isomer and the relative
product yields will not be sensitive to the initial branching.
A simplified representation of the calculated MEP is presented
in Fig. 4.
3
2
tions predict the yield of H atoms to be 97% according to
1
Vereecken et al. and 98% according to Nguyen et al.; in
7
19
1
good agreement with the results of McKee et al. and
4
4
5
Goncher et al. and within the error bars of the data of
1
3
Boullart et al. and our result. Despite the greater exothermi-
city of the C H + H channel, the low H yield comes from the
tight transition state for H elimination. Ab initio calcula-
3
2
2
Our experimental H production for this reaction has been
determined to be equal to 94 Æ 8%, in relatively good
agreement (within error bars) with the previous experimental
2
tions on such transition state energies have a precision of 4 to
À1
1
0 kJ mol , and as the energy location of this transition state
1
4
measurement from McKee et al. of 109 Æ 14%. There
are also several photodissociation studies of the allyl radical
at 248 nm (leading to an excited allyl radical with a similar
2 2
Fig. 3 Simplified representation of the lowest MEP of the CH + C H
À1
16
reaction (energies in kJ mol from Vereecken et al. and Nguyen
2 4
Fig. 4 Simplified representation of the lowest MEP of the CH + C H
19
À1
22
reaction (energies in kJ mol from Davis et al. and Stranges et al. ).
23
et al. ).
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4
6,47
ꢁ
energy to the CH + C
2
H
4
- C
3
H
5
reaction): two of them
converted to CH
comparison with the C–C
are several other stable C H isomers, mainly i-C H
5
3
– CH–CRCH with a 1,2-H migration by
were only able to detect H (or D) atom (associated to allene
3 3
H
- H C–CRCH case. There
2
production and not to cyclo-propene), and two other studies in
4
998 and 2008 detect H as the main channel (84 and 95%
4
5
4
8
23
ꢁ ꢁ
(H C –CHQCCH ) and n-C H (H CQCH–CHQCH ), with
2 2 4 5 2
1
respectively) but also 16% and 5% respectively, for the
CH + C H channel. Furthermore, two RRKM calculations
the possibility of interconversion through isomerization by 1,2 H
51
transfer and with another open exit channel (C H + C H :
2
3
2
2,23
2
3
2
2
2
À1
studies
on the excited allyl dissociation branching ratio
À248 kJ mol relative to the reactants energy). However,
isomerization between the i-C or n-C isomers and the
ꢁ ꢁ
3 3 2
CH – CH–CRCH or CH –CRC– CH isomers seems to be
give similar conclusions. They predict 98% of H atom produc-
H
4 5
4 5
H
tion (mainly allene + H) and 2% of CH + C . Among
3
2
H
2
2
those calculations, Davis et al. have estimated RRKM
2
51
impossible: it has already dissociated before isomerization can
occur. So the reaction can lead to
error limits by varying the transition state energy between
À1
Æ4 kJ mol , and they get a variation of Æ1% for the H atom
ꢁ
À1
CH + C H - CH –CH –CRCH D H = À380 kJ mol
3
4
2
2
r
production, this small effect being due to the large amount of
À1
energy above the transition state (typically B220 kJ mol ).
ꢁ
À1
-
-
CH –CRC– CH D H = À486 kJ mol
3
2
r
Considering the various experimental data and the reliability
of the RRKM calculations, there is no doubt that the
H CQCQCH + H atom channel is the major one, with a
ꢁ
H = À474 kJ molÀ1
CH
3
– CH–CRCH D
r
2
2
small contribution from the CH
3
+ C
H
2 2
channel. We
ꢁ
ꢁ
The CH –CH
2
2
–CRCH and CH
3
– CH–CRCH can directly
propose the following branching ratio:
evolve to an H atom loss, which leads to H
À1
2
CQHC–CRCH + H
without or with a very
(
D
r
H
=
À280 kJ mol
)
CH + C H - H CQCQCH + H 98 Æ 2%
2
4
2
2
small barrier, or to H2 elimination, which leads to
À1
ꢁ
CHQCH–CRCH + H (D H = À279 kJ mol ) and
2
r
-
CH
3
+ C
2
H
2
2 Æ 1%
ꢁ
À1
H CQ C–CRCH + H (D H = À239 kJ mol ). The other
2
2
r
ꢁ
intermediate, CH –CRC– CH , can only convert to
3
2
CH
clearly shows that H atom production is the main exit channel
0.98 Æ 0.08). There is no theoretical work on this reaction.
Based on our knowledge of the CH reactivity and the theoretical
+
C
H
3 4
(methylacetylene). Our measurement
À1
CH
2
QCQCQCH
2
+ H (D
r
H = À249 kJ mol /CH + C
3 4
H )
ꢁ
as 1,3 H transfer leading to CH
3
– CH–CRCH seems
loss where the hydrogen
atoms come from the same carbon atom or 1,2-H loss where the
hydrogen atoms come from two neighboring carbon atoms,
(
51
2 2
impossible. The H loss, either 1,1-H
31,49–54
2
calculations on the C H system,
4
we propose a mecha-
5
nism for this reaction and a schematic representation of our
proposed MEP is presented in Fig. 5. From experimental and
always have
a
substantial activation barrier versus
À1
9,16,21,26,55
theoretical works,
the exit channel, for example 40 kJ mol
À1
calculated for
ꢁ
we know that the CH radical can
ꢁ
H + C –CRCH and 200 kJ mol for H + HC QCH .
2
2
2
insert into a sigma C–H bond and eventually into a sigma C–C
bond, or add to double and triple C–C bonds. So the first step of
this reaction can be either CH insertion into one C–H bond
H atom loss from the radical does not possess any barrier in the
1
7,19
exit channel. In the case of CH + C H ,
the RRKM
formation could play a role only
channel is much lower than the other channels, which is
not the case here. As for the CH + C reaction, the H
branching ratio will only play a minor role, and our observation
2%) may be even too high as H elimination leads to the very
stable H CQCH–CRCH molecule. Considering our experi-
2
2
ꢁ
ꢁ
calculations show that the H
if the H
2
leading to CH –CH
2
2
–CRCH or to CH
3
–CRC– CH
2
, inser-
ꢁ
2
tion into the sigma C–C bond leading to CH
3
– CH–CRCH, or
2 2
H
2
4 5
addition to the triple bond, leading to cyclo-C H (methyl
cyclo-propenyl). However this addition leads to a much less
stable isomer: if methyl cyclo-propenyl is formed, it is likely to be
(
2
mental data and the exothermicity of the reaction, we can
consider that the H atom production is equal to 100%.
3 4
CH + C H (allene). As with the other reactions, our
measurement of 0.97 Æ 0.08 clearly shows that H atom
production is the main exit channel. There is no theoretical
work on this system although it is a different part of the
potential energy surface of the CH + CH –CCH reaction.
3
A simplified representation of the MEP, taken from Miller
5
1
et al., for this reaction is presented in Fig. 6. There are only
two different first steps for this reaction: CH insertion into one
ꢁ
C–H bond leading to H
2
C –CHQCQCH
2
(i-C
4
5
H ) or addi-
tion to one of the double bonds of the allene leading to a quite
unstable cyclo-C , in fact a less stable cyclic isomer than for
CH + C , CH + C and CH + methylacetylene. If this
4
H
5
2
H
2
2 4
H
addition occurs, this cyclic isomer is likely to be rapidly
ꢁ
converted to H C –CHQCQCH (i-C H ). Thus we have
Fig.
5
Simplified representation of the lowest MEP of the
À1
2
2
4
5
CH + methylacetylene reaction (energies in kJ mol from Parker
only to consider the i-C H adduct formation. The i-C H
4 5 4 5
51
evolution has been studied in detail by Miller et al. The
31
and Cooksy ).
6
60 | Phys. Chem. Chem. Phys., 2009, 11, 655–664
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À1
51
Fig. 6 Simplified representation of the lowest MEP of the CH + allene reaction (energies in kJ mol from Miller et al. ).
ꢁ
H
2
C –CHQCQCH
2
adduct mainly evolves through C–H
study. There is no theoretical work for this reaction but there
is a detailed theoretical study of C isomers and their iso-
bond dissociation to H
À1
2
CQCQCQCH
2
+ H (D
r
H =
4 7
H
56
À258 kJ mol /CH + allene) with a very minor part to
merizations or dissociation transition states. A schematic
representation of their theoretical calculations relevant for the
CH + propene reaction is presented in Fig. 7. The first step of
ꢁ
n-C H (H CQCH–CHQCH ). This in turn yields mainly
4
5
2
H CQCQCQCH2 + H and a very minor production of
2
C H + C H . Considering the relative stability of i-C H and
2
this reaction can be either (i) CH insertion into one C–H bond
ꢁ
3
2
2
4
5
n-C H
4 5
isomers associated to the tight transition state for the
+ C exit channel can only be a very
minor one. As in the case of the CH + C
CH + C reactions, the stability gain for the H
leading to CH –CH
2
2
–CHQCH
2
or to the most stable isomers
ꢁ
ꢁ
isomerization, the C
H
2 3
H
2 2
CH –C( CH
3
2
)Q CH
2
and CH
3
–CHQCH– CH
2
, (ii) CH inser-
ꢁ
2
H
2
and the
elimina-
tion into the sigma C–C bond leading to CH
3
– CH–CHQCH
2
ꢁ
2
H
4
2
2 CH
3
–CHQCH– CH
bond, leading to cyclo-C H
4 7
2
, or (iii) CH addition to the double
(2-methyl cyclo-propyl). This
addition leads to a much less stable isomer which will convert
ꢁ
tion channel is too low (HCRC– CQCH
2
+ H
2
D
r
H =
À1
À278 kJ mol /CH + allene) to overcome the tightness of the
ꢁ
ꢁ
ꢁ
transition state, and the H
2
elimination exit channel can only be
to CH
3
– CH–CHQCH
2
2 CH
3
–CHQCH– CH
2
and not to
a very minor one. These conclusions are in good agree-
ment with our experimental results, and we can consider thet
CH + allene leads only to H formation, likely associated with
the H CQCQCQCH formation.
CH –C( CH )Q CH as isomerization happens through ring
3
2
2
5
opening without H transfer. So the reaction can lead to:
7
ꢁ
À1
À1
CH + C H - CH –CH –CHQCH D H = À395 kJ mol
3
6
2
2
2
r
2
2
ꢁ
-
-
CH –C( CH )Q CH D H = À461 kJ mol
3
2
2
r
CH + C
H
3 6
(propene). Our measurement clearly shows that
H atom production is an important channel (78 Æ 10%) but
ꢁ
H = À463 kJ molÀ1
CH
3
– CH–CHQCH
2
D
r
much less abundant than for the previous reactions in this
Among these three isomers, two of them will lead mainly to
ꢁ
H+H CQCH–CHQCH formation and the CH –C( CH )Q CH
2
2
3
2
2
isomer only to CH + H CQCQCQCH formation (we neglect
3
2
2
H2 formation in this case). As the isomerization between
ꢁ
ꢁ
CH
CH
3
– CH–CHQCH
2
(or
CH –CH
2
2
–CHQCH
2
)
and
ꢁ
3
–C( CH
2
)Q CH
2
isomers needs a rearrangement of the C
56
skeleton through a tight cyclic structure of three carbon atoms,
it is non-competitive with C–H or C–CH bond dissociation
3
56
which proceeds through a small barrier. So for this reaction,
the relative product branching ratios are very sensitive to
the initial CH attack. Our experimental H production for this
reaction is measured to be equal to 78 Æ 8% and clearly shows
ꢁ
that the CH –C( CH )Q CH isomer is formed in the CH attack
3
2
2
as the two other isomers which cannot isomerize to
ꢁ
CH –C( CH )Q CH lead unambiguously to H atom production.
3
2
2
Fig. 7 Simplified representation of the lowest MEP of the CH + C
À1
H
3 6
CH + C H (trans-butene). This case is close to the CH +
4 8
56
reaction (energies in kJ mol from Miller et al. ).
propene reaction, where H atom production is still an
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important channel (69 Æ 12%) but far from being the only
one. There is no theoretical work for this reaction to the best
of our knowledge. The first step of this reaction can be
either (i) CH insertion into one C–H bond leading
production. For the CH + allene reaction there is less
2 2
ambiguity: we consider also only H CQCQCQCH + H
formation. For the CH + propene reaction, considering our
experimental data and the fact that there are only two avail-
able exit channels, we propose the following branching ratio:
78 Æ 10% for H CQCH–CHQCH + H and 22 Æ 10% for
ꢁ
to CH –CH –CHQCH–CH or to the most stable
2
2
3
ꢁ
CH –C( CH )QCH–CH , (ii) CH insertion into the sigma
3
2
3
2
2
ꢁ
C–C bond leading to CH
3
– CH–CHQCH–CH
iii) CH addition to the double bond, leading to cyclo-C
dimethyl-1,2-cyclo-propyl). As for CH + C , this addition
3
,
or
H
2
CQCQCQCH
2
+ CH
3
. The three last branching ratios
(
(
H
5 9
should be used with care as they are highly speculative.
2
H
4
For the temperature dependence we use the modified
b
leads to a much less stable isomer and is likely to be converted
ꢁ
Arrhenius form k = A (T/300) exp(Àg/T). For the reac-
to CH
3
– CH–CHQCH–CH
3
(as isomerization happens through
tion of CH with unsaturated hydrocarbons, the kinetic beha-
vior is similar to that observed for the reactions of CH with
57
ring opening without H transfer ). So the reaction leads to:
2 2 2 4
C H and C H , with high values for the rate constants and
ꢁ
CH + C H - CH –CH –CHQCH–CH
4
8
2
2
3
non pronounced maxima near 50–100 K. The origin of
these maxima is still unknown, but with the exception of the
CH + C H reaction, the variation of the rate constant
À1
D H = À457 kJ mol
r
2
4
ꢁ
-
-
CH –C( CH )Q CH–CH
3
2
3
between 23 and 600 K is very small and within the experi-
mental errors. We propose the use of a constant value for all of
À1
D H = À535 kJ mol
r
2 4
these reactions with the exception of the CH + C H reac-
ꢁ
CH – CH–CHQCH–CH
tion for which we use a temperature-dependent rate coeffi-
cient expression fitted to the experimental values (A =
3
3
À1
D H = À540 kJ mol
r
À10
3
cm molecule
À1 À1
3
.5 Â 10
s
, b = À0.33 and g = 14 K)
Among these three isomers two of them will lead only to
as shown in Fig. 8. For this fit we add extra points at 10 and
À10
À1
3 À1 À1
cm molecule s at
H + H
butene) and the last one, CH
produce CH + H
trans-butene) (H formation can be neglected in this case).
2
CQCH–CHQCH–CH
3
(À322 kJ mol /CH + trans-
15 K to force a value of 3.0 Â 10
ꢁ
3
–C( CH
2
)Q CH–CH
(À304 kJ mol /CH +
3
will only
10 K to get a reasonable value in the 10–20 K range. It can
be seen that even for the CH + C H reaction, the constant
À1
3
2
CQCQCH–CH
3
2
4
À10
3
cm molecule s in the 23–800 K
À1 À1
2
value of k = 3.3 Â 10
À10
However 1,4-H and 1,5-H migrations should be quick in this
range is very close (within 50%) to the 3.5 Â 10
À0.33
case, even if they involve one double C–C bond. This should
ꢁ
(
T/300 K)
exp(À14 K/ T) expression.
allow the isomerization to CH –CH – CH–CHQCH and
3
2
2
The extrapolation of the H branching ratio measurements at
300 K to low temperature is somewhat hazardous. However,
leads to the most stable CH + H CQCH–CHQCH
2
3
2
À1
1
6
(
À360 kJ mol /CH + trans-butene) exit channel. The relative
2 2
we can use the ab initio calculations on the CH + C H and
9
product yields are thus eventually sensitive to the initial
branching and coupled ab initio/RRKM calculations are
needed to estimate the influence of the isomerization. Our
experimental H production for this reaction is measured equal
to 69 Æ 7% and clearly shows that other channels are open,
2 4
CH + C H systems. When isomerization between the
various intermediates is quick, the lowest intermediate radical
3
likely mainly CH formation.
Astrochemical importance
As CH, C , C , allene, methylacetylene and very recently
2
8
H
2
2 4
H
5
propene have been detected in cold interstellar clouds, it is
necessary to provide detailled rate constant at 298 K and
formulae expressing how these detailled rate coefficients vary
with temperature for astrochemical models. For the branching
ratio at 298 K we propose speculative values (‘‘educated
guess’’) based on our H atoms production and theoritical data
from litterature presented in the discussion on the products.
For the reactions of CH with C H and C H we used the
2
2
2
4
1
7,19,22,23
RRKM estimations.
For the CH + methylacetylene
reaction, we neglect CH insertion into a CH bond of the
methyl group, as it leads to a less stable isomer, and CH
insertion into the sigma C–C bond, because of the hindrance.
Fig. 8 Temperature dependence of the rate coefficient for the
2 4
CH + C H reaction. Open circles are the measurements from Canosa
That means that we consider only H
formation and not H CQCH–CRCH + H. Moreover,
contrary to the CH + C case, the presence of a low energy
2
CQCQCQCH
2
+ H
10
et al., filled circles are from Thiesemann et al., open squares are
9
2
7
from Berman et al. and the present measurements as well as the
H
2
6
ones of Butler et al. at 296 K are shown as filled stars. The conti-
2
À10
À0.33
exit channel, associated with a quick isomerization of the first
methyl cyclo-propenyl adduct, should avoid the methyl-cyclo-
propyl formation. So we consider only H CQCQCQCH + H
nuous line is the fit with the expression k = 3.5 Â 10
(T/300)
3
À1 À1
exp(À14/T) cm molecule
s
, and the dashed line is the average
À1 À1
À10
3
cm molecule
value k = 3.3 Â 10
s .
2
2
6
62 | Phys. Chem. Chem. Phys., 2009, 11, 655–664
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will be the dominant isomer, and the relative product yields
are not sensitive to the initial branching and so to the
temperature. Furthermore, as the exit barrier or exit channel
is well below the energy of the reactants and as RRKM
calculations give reliable results indicating good energy dis-
tribution in the adducts, the energy variation of the reactants
in the 300–10 K range is negligible versus the intermediate
energy formation. The only critical case is when the cyclic
radical from CH addition to double or triple CC bonds
converts to an isomer with a different carbon skeleton
than that from CH insertion. In this case, as isomerization
involving carbon skeleton reorganization could not be com-
petitive with C–H or C–C bond dissociation, the relative
product yields can be sensitive to the initial branching between
addition and insertion if these processes have not the same
addition of the CH radical to the double or triple bond will
be required to verify our proposed branching ratios. However,
as astrochemical, combustion and planetary atmospheric
models use rate constants and branching ratios, we think it
is important to propose branching ratios associated with a
critical discussion. For this purpose, we are currently engaged
in the creation of a new database, KIDA: kinetic database for
astrochemistry (http://kida.obs.u-bordeaux1.fr/), which is an
international project to create an interactive database for
reaction rate coefficient useful in the chemical modeling of
the interstellar chemistry and in planetary atmospheres with
author-submitted discussion on the quality of the data.
Acknowledgements
temperature dependence as suggested by Thiesemann et al. for
9
the CH + C H4 reaction, where insertion could be a
We would like to thank Kevin M. Hickson for proof reading.
The support of this research by the ‘‘Programme National de
Physique et Chimie du Milieu Interstellaire’’ is gratefully
acknowledged.
2
temperature-dependent process. However, this is only the
case for CH + methylacetylene where the main exit channel
is
CH
branching ratio and for CH + propene where addition leads
only to H CQCH–CHQCH + H and insertion leads to
3
CQCH–CHQCH + H and H CQCQCQCH + CH .
H
+
C
4
H
4
with formation of the two isomers
(
2
QCH–CRCH and H CQCQCQCH ) with unknown
2
2
References
2
2
1 W. A. Sanders and M. C. Lin, in Chemical Kinetics of Small
Organic Radicals, ed. Z. B. Alfassi, CRC, Boca Raton, FL, 1986.
H
2
2
2
2
2
E. Herbst, H.-H. Lee, D. A. Howe and T. J. Millar, Mon. Not. R.
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