H-atom yields from the photolysis of acetylene and from the reaction of C2H with H2, C2H2, and C 2H4

Article abstract of DOI:10.1021/jp908285t

The photolysis of acetylene at 193 nm has been investigated as a source of the ethynyl radical, C₂H, for product branching ratio studies, particularly the formation of H atom product as the photolysis, producing a 1:1 ratio of C2H and H, provides an internal calibration. Previous literature had suggested that C2H and H may only be a minor component of acetylene photolysis at 193 nm. Acetylene was photolyzed at low laser energy densities (<7 mJ cm^-2), with H atoms being observed as a function of time by VUV laser induced fluorescence. When C₂H was reacted with C₂H ₂, a reaction that is known to produce H atoms with unit yield, the ratio of photolytic H atom production to chemical production was 0.96 (0.03. The rate coefficient for the reaction of C₂H with C₂H ₂ could accurately be retrieved from the time evolution of the H atom signal. The results suggest that acetylene photolysis at low laser energies is a good source of C₂H for product branching studies, and the technique has been applied to the reactions of C₂H with ethene and propene. For the reaction with ethene between 23 and 81 Torr, the yield of H is 0.94 (0.06, suggesting that an addition eliminationmechanism dominates with the formation of vinylacetylene and H atoms. For the reaction of C₂H with propene, no H atom product was observed, putting a lower limit of <5% for H atom production. Possible explanations for the low H atom yield are discussed. The implications of these results in combustion and planetary atmospheres are briefly considered. Copyright

Full text of DOI:10.1021/jp908285t

J. Phys. Chem. A 2010, 114, 4735–4741
4735
H-Atom Yields from the Photolysis of Acetylene and from the Reaction of C H with H ,
2
2
†
C H , and C H
2
2
2
4
Tam a´ s Kov a´ cs, Mark A. Blitz, and Paul W. Seakins*
School of Chemistry, UniVersity of Leeds, Leeds, LS2 9JT, United Kingdom
ReceiVed: August 27, 2009; ReVised Manuscript ReceiVed: January 8, 2010
The photolysis of acetylene at 193 nm has been investigated as a source of the ethynyl radical, C
2
H, for
product branching ratio studies, particularly the formation of H atom product as the photolysis, producing a
:1 ratio of C H and H, provides an internal calibration. Previous literature had suggested that C H and H
may only be a minor component of acetylene photolysis at 193 nm. Acetylene was photolyzed at low laser
1
2
2
-
2
energy densities (<7 mJ cm ), with H atoms being observed as a function of time by VUV laser induced
fluorescence. When C H was reacted with C , a reaction that is known to produce H atoms with unit yield,
the ratio of photolytic H atom production to chemical production was 0.96 ( 0.03. The rate coefficient
for the reaction of C H with C could accurately be retrieved from the time evolution of the H atom signal.
The results suggest that acetylene photolysis at low laser energies is a good source of C H for product branching
studies, and the technique has been applied to the reactions of C H with ethene and propene. For the reaction
with ethene between 23 and 81 Torr, the yield of H is 0.94 ( 0.06, suggesting that an addition elimination
mechanism dominates with the formation of vinylacetylene and H atoms. For the reaction of C H with propene,
2
2 2
H
2
2 2
H
2
2
2
no H atom product was observed, putting a lower limit of <5% for H atom production. Possible explanations
for the low H atom yield are discussed. The implications of these results in combustion and planetary
atmospheres are briefly considered.
Introduction
The ethynyl radical, C
production of soot during combustion and in the production
with the different products having a potentially significant effect
on subsequent chemistry
2
H, plays an important role in the
1
2,3
-1
of polyenes in the reducing atmospheres of the outer planets
C H + O f HCO + CO ∆ H ) -620 kJ mol
2
2
r
3
and in the interstellar medium. The radical can undergo two
main types of reaction; a direct abstraction typified by the
(R4a)
-
1
reaction with H
2
or saturated alkanes
f H + 2CO ∆ H ) -547 kJ mol
r
(R4b)
C H + H f C H + H
(R1)
(R2)
2
2
2
2
-1
f CH + CO2 ∆ H ) -363 kJ mol
r
(R4c)
C H + CH f C H + CH
3
2
4
2
2
-
1
f C O + OH ∆ H ) -154 kJ mol
2
r
(
R4d)
Alternatively, for reactions with unsaturated hydrocarbons or
radical species such as O , the reaction proceeds via the
formation of a complex that can either break up to yield products
or be stabilized by collisions with a bath gas. The high heat of
2
-1
f HCCO + O ∆ H ) -73 kJ mol
r
(
R4e)
formation of the ethynyl radical (∆
f
H C
2
H ) 567 ( 1 kJ
-1 4,5
mol
) means that in many cases a variety of product channels
For the reaction of C H with ethene, there are two thermo-
dynamically accessible channels
2
are thermodynamically accessible, including the production of
electronically excited products.
2
The reaction of C H with acetylene is typical of the complex-
forming mechanism; in this case, only one product is thermo-
dynamically accessible from the dissociation of the complex
-
1
C H + C H f C H + C H ∆ H ) -90.4 kJ mol
2
2
4
2
2
2
3
r
(
R5a)
-
1
f C H + H ∆ H ) -95.4 kJ mol
4
4
r
C H + C H f C H* f C H + H
(R3)
2
2
2
4
3
4
2
(R5b)
However, for the reaction of C H with O , an important reaction
in combustion, a large number of product channels are accessible
2 2
Reaction R5a could potentially take place via either a direct
abstraction or an addition elimination mechanism.
The kinetics of C
2
H have received considerable attention,
†
Part of the special section “30th Free Radical Symposium”.
To whom correspondence should be addressed. Tel: +44 (0)113
6-8
particularly in the studies led by Leone and coworkers, Curl,
*
,10
11,12
3
436568. Fax: +44 (0)113 3436565. E-mail: p.w.seakins@leeds.ac.uk.
Glass, and coworkers,9 and Peeters and coworkers.
Sims
1
0.1021/jp908285t  2010 American Chemical Society
Published on Web 01/27/2010

4
736 J. Phys. Chem. A, Vol. 114, No. 14, 2010
Kov a´ cs et al.
and coworkers have made measurements at very low temper-
fluorescence detection axis mutually perpendicular to each other.
The pressure within the cell was controlled by a needle valve
located between the exit of the cell and the vacuum pump. The
pressure in the cell was measured by a baratron-type pressure
gauge, and all reactions were carried out at room temperature
(295 ( 2 K).
1
3
atures using a continuous-flow Laval system, and Leone and
coworkers have more recently measured C H kinetics around
H has predominantly
been generated by the excimer laser photolysis of C or
CF H at 193 nm and monitored by IR absorption. Additional
studies have been performed by Laufer and Fahr, who have
2
1
4
1
00 K using a pulsed Laval system. C
2
2
H
2
3 2
C
2
C H radicals were generated by the photolysis of acetylene
authored a review on the reactions of C
2
H and other unsaturated
using a GAM EX5 excimer laser operating at 193 nm. H atoms
were monitored by VUV laser-induced fluorescence at the
Lyman-R transition at 121.56 nm. The technique has been used
radicals.5
In contrast with the kinetics, much less is known about the
products of C H radical reactions. Quantitative product studies
1
9
2
before in this laboratory to monitor H atom yields from CH,
CN,2 and CH
0,21
22
reactions. A 308 nm excimer laser (Lambda
of radical reactions are challenging experiments because many
of the spectroscopic techniques used to monitor radical con-
centrations do not provide absolute concentrations. The calcula-
tion of radical concentrations from photolysis fluence measure-
ments is subject to many uncertainties. An alternative approach
is to use a calibration reaction, a reaction of the radical that
produces the target product with a well-defined, ideally 100%,
product yield. Issues around quantitative product determinations
2
Physik LPX 100) pumped dye laser (Lambda Physik FL2002)
-1
was used for probing. Radiation (364.68 nm, ∼5-7 mJ pulse )
was produced from a DMQ dye and was frequency tripled in
krypton-argon mixture (Kr/Ar 1:2.5). Light (364.68 nm) was
focused by a quartz lens (focal length ) 15 cm) into a glass
cell attached directly to the cell containing ∼1000 Torr of Kr/
Ar; the exact mixture varied depending on the laser energy.
Lyman-R radiation was coupled to the cell perpendicularly to
1
5
have recently been reviewed.
For C H studies, reactions R1 and R3 provide a suitable
calibration reaction for H atoms because H + C and H +
, respectively, are the only possible products under our
experimental conditions. C can be produced from reaction
2
2
the photolysis beam through a MgF window.
2
H
2
Fluorescence was detected perpendicularly to both laser
beams by a solar blind photomultiplier tube (PMT, Perkin-Elmer
CPM-18). The power of the VUV radiation was monitored by
a second solar blind PMT (Thorn EMI Gencom, RFI/B-214 FV
model) with exactly the same path length in the cell. Some of
the probe light was reflected using quartz flat at 45° through a
VUV interference filter and onto the second PMT. This resulting
signal was used directly as the absorption signal and also to
normalize the VUV LIF signal taking into account any temporal
power fluctuations of the probe laser and attenuation by
absorption.
4 2
C H
4
H
3
11
R1 at high pressures; however, previous calculations have
shown that stabilization should not occur under our conditions,
a prediction verified in the current study. Uncertainties exist
over the yield of C
the most convenient source of C
2
H from the 193 nm photolysis of acetylene,
H. Early studies suggested that
2
2
triplet acetylene or vinylidene (H CC) was the dominant
product;16 secondary reactions of such species could distort the
kinetics of H atom formation. The photolysis of acetylene is
particularly appealing for H atom studies because C
are formed in identical amounts from the primary photodissocia-
tion
2
H and H
By varying the time delay between the photolysis and probe
beams, a time profile of the H atom concentration was recorded.
The time between photolysis and probing was varied by a digital
delay generator type DG 535 (Stanford Research Systems). The
PMT outputs were converted by an AD converter and recorded
and stored on a PC. In typical, the time profiles consisted of
C H + hν(193 nm) f C H + H
(R6)
2
2
2
2
00 points, where each time point was the average of 10-20
potentially providing an internal calibration17 of C
shots and the time interval between the points was 0.1 µs.
Photolysis laser powers were measured by a Gentec energy type
power meter.
The power dependence of the acetylene photodissociation was
investigated in detail. By varying the applied voltage to the
excimer or by focusing or defocusing the photolysis laser beam
2
H and H.
However, alternative multiphoton dissociations that yield H
atoms or secondary chemistry will negate this advantage.
18
More recently Lauter et al. have demonstrated that reaction
R6 is the dominant channel for acetylene dissociation at 193
nm with a quantum yield of 0.94 ( 0.12, suggesting that, in
contrast with the earlier studies, acetylene photolysis should be
(
using either metal mesh filters or cylindrical lens), the light
15
18
energy densities could be varied from 5 × 10 to 3 × 10
a clean source of C
the suitability of acetylene photolysis as a source of C
2
H. The objective of this article is to confirm
H for
-
2
photon cm . To investigate the yield of H atoms from both
the photolysis of acetylene and the reaction of C H with calibrant
and test molecules, the photolysis of N O at 193 nm in the
presence of hydrogen was used as a well-characterized H-atom
source. Photolysis of N O at 193 nm leads exclusively to the
2
H-atom product studies and to see whether the potential
advantages of an internal calibration can be realized. We
subsequently apply this technique to determine the H-atom yield
2
2
from the reaction of C
2
H with ethene (R5a,R5b) and propene
2
1
production of N
2
and excited oxygen atoms, O( D)
C H + C H f products
(R7)
2
3
6
1
N O + hν(193 nm) f N + O( D)
(R8)
2
2
Experimental Section
1
7
-3
Reactions were carried out in a slow flow excimer laser flash
photolysis apparatus with H-atom detection by VUV laser-
Sufficient hydrogen (typically 3.3 × 10 molecule cm ) is
added to ensure that all oxygen atoms produced react rapidly
induced fluorescence. The C
2
H precursor, acetylene, reagent (if
with H
The H atom concentration can be calculated from the well-
determined N
10 cm ), unit quantum yield, and the measured laser photon
density.
2
to generate an essentially instantaneous H atom signal.
appropriate), and bath gas were metered using calibrated mass
flow controllers and premixed before being flowed into the
stainless steel reaction vessel. The reaction vessel was a stainless
steel six-way cross with the photolysis, probe laser, and
2
O absorption cross section at 193 nm (8.95 ×
-20
2 23

H Atom Yields from the Photolysis of Acetylene
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4737
Figure 2. Bimolecular plot (k′
3 2 2
vs [C H ]) for reaction R3 at 298 K
and 23 Torr. The bimolecular rate coefficient obtained as the gradient
Figure 1. Typical experimental H atom profiles following the
-
10
3
-1 -1
1
4
is (1.16 ( 0.06) × 10
cm molecule
s , where the errors are
photolysis of acetylene (pressure 42 Torr Ar, [C
molecule cm , laser fluence ) 7 mJ cm ). The solid line is a fit to
reaction R1.
2
H
2
] ) 1.6 × 10
-
3
-2
statistical only at the 1 sigma level.
TABLE 1: Summary of 298 K Rate Coefficients for the
Reaction C H + C
2
2 2
H
Argon (BOC Pureshield) and helium (BOC VP grade,
010 × k298/cm
3
1
9
9.9999%) were admitted directly from cylinders without
-1 -1
author
this work
date molecule
s
comments
purification, whereas acetylene (Aldrich, 99.9+%, HPLC grade),
ethylene (Aldrich, 99.9+%, HPLC grade), and propylene
2009
2000
1.2 ( 0.2
1.3 ( 0.1
monitoring H atom
production
monitoring
(
Aldrich, 99.9+%, HPLC grade) were stored diluted in helium
Ceursters et al.11
in glass bulbs following freeze-pump-thaw purifications.
chemiluminescence via
reaction of C
2
H + O
2
Farhat et al.10
1993
1993
1992
1.6 ( 0.3
1.2
monitoring C H by
Results and Discussion
2
IR absorption
a. Reaction with Acetylene at Low Photolysis Powers.
Figure 1 shows an experimental profile of the H atom signal
following the photolysis of an acetylene and argon mixture at
low excimer fluences (7 mJ cm ). A prompt rise in H atom
signal is seen to be consistent with the photolysis process
Pederson et al.6
monitoring C
IR absorption
4 2
monitoring C H by
2
H by
Koshi et al.24
1.5 ( 0.3
-
2
mass spectrometry or
H atoms by VUV
absorption
monitoring H atom
production by VUV
resonance absorption
Shin and Michael43 1991
Stephens et al.9
1.4
1.5
C H + hν(193 nm) f C H + H
(R6)
2
2
2
1987
monitoring C
2
H by
followed by a slower exponential rise in the signal associated
with the reaction of C H with acetylene. Under these conditions,
IR absorption
2
1
1
-3
where [C
2
H
2
] . [C
2
H] ([C
2
H]
0
) 1.4 × 10 molecules cm
the expected 1:1 relationship is observed on the bais of C
H being the sole products of acetylene photolysis at low powers
2
and C H + H being the sole products of reaction R3 at these
2
H +
and ∼1/1000th [C
2
H
2
]), the production of H atoms should be
controlled by pseudo-first-order kinetics. Traces such as Figure
were fit with eq E1, which accounts for both prompt photolytic
4
1
pressures (20-300 Torr argon).
production and the chemical production of H atoms and
subsequent slow removal due to diffusion and reaction with
acetylene.
2
Further evidence of the sole production of C H and H at low
laser fluences comes from the good agreement obtained when
we compare our observed H atom signal from acetylene
photolysis with that from N
, for which the cross section and yield of H (via O D + H
are known. The calculated quantum yield of H at 193 nm with
2
O photolysis in the presence of
1
SH,Rk3′
H
2
2
)
-k 3′ t
)
(e
- e-kdt) + SH,Pe-kdt
(E1)
If,H
kd - k′
3
15
-2
a laser photon fluence of 8.2 × 10 photons cm is 1.02 (
.05.
The rise in the concentration of H atoms from reaction R3
0
where If,H is the observed H atom fluorescence, SH,R is the signal
associated with H atoms produced in reaction R3, k′ is the
pseudo-first-order rate coefficient for the removal of C H (k′
[C ] + c), k is the rate coefficient for H atom loss
mainly via diffusion), and SH,P is the signal associated with
3
can clearly be seen, and by carrying out measurements over a
range of acetylene concentrations, it is possible to extract the
bimolecular rate coefficient for reaction R3. Figure 2 shows an
example of a bimolecular plot (pseudo-first-order rate coefficient
2
3
)
(
k
3
2
H
2
d
prompt H production from acetylene photolysis.
2 2
for H production vs [C H ]), and the resultant bimolecular rate
The ratio of SH,R/SH,P was 0.96 ( 0.03 for 48 experiments
coefficient (the gradient of plot) is compared with literature
values in Table 1.
-
2
carried out at low photolysis laser fluences (<7 mJ cm ) and
14
-10
3
-1 -1
with acetylene concentrations from (1.5 to 2.3) × 10 molecule
Our determination, (1.2 ( 0.2) × 10 cm molecule s ,
-3
cm and total pressures (23-98 Torr). Under these conditions,
where the error includes both statistical uncertainties and

4
738 J. Phys. Chem. A, Vol. 114, No. 14, 2010
Kov a´ cs et al.
1
27
with rate coefficients for C
2
+ C
2
H
2
of (4.1 ( 0.4) or (3.5 (
28
-10
3
-1 -1
3
1
.0) × 10 cm molecule
s
and for C
2
2 2
+ C H of (1.1
The reaction proceeds
-
10
3
-1 -1 27,28
(
0.1) × 10 cm molecule s .
via the formation of the diacetylene complex, and the expected
29
products of diacetylene dissociation are C H + H. However,
4
these molecular beam studies29 also show that the lifetime of
the diacetylene, which is in a deep well with barriers to
-
1
dissociation of 577 and 544 kJ mol to reagents and products,
respectively, is long compared with the rotational period, even
-
1
at a collision energy of 29 kJ mol . Under our room-
temperature thermal conditions, the complex lifetime is expected
to be much longer, and hence it is likely that the diacetylene
complex is collisionally stabilized under our experimental
conditions, explaining the reduction of reactive H-atom signal
on the formation of C
Adding oxygen to the system provides a sensitive diagnostic
for the presence of C . In the presence of oxygen, a very strong
2
.
2
Figure 3. Variation of the ratio of prompt to reactive H with photon
fluence.
VUV emission could be observed at photolysis laser powers
where there was significant deviation from the 1:1 photolytic/
reactive H atom ratio. Photolysis laser powers were reduced
until the chemiluminescence disappeared. The origin of the
chemiluminescence will be discussed in a future paper on the
estimates of systematic errors (flow controller and pressure
calibrations), is in excellent agreement with literature values of
C
2
H/O
2
system.
k3,298, which have been measured using a wide variety of
18
Lauter et al. found that C
2
H photolysis becomes important
only at fluences above 100 mJ cm . They typically used 2-45
mJ cm photolysis fluence, and single photon absorption was
techniques including monitoring both reagents and products.
This excellent agreement suggests that there are no unexpected
-2
-2
2
4
sources of H in our system. Koshi et al. measured identical
reported under these conditions. Our results suggest that there
values for k3,298 by monitoring H atom production by VUV
may be ∼10% C
2
H photolysis at the upper end of their linear
photolysis region, which may have been within their error limits
absorption and C
this precluded significant involvement of vinylidene, which
would produce C + H on reaction with acetylene, distorting
the kinetics of C versus H production. Interestingly Koshi
4 2
H by mass spectrometry. They argued that
for detection. Low powers of 121.6 nm probe radiation
H
4 2
2
-2
(
<25 µJ cm ) were enough to generate additional H atom signal,
4
H
2
2
presumably from C H photolysis, followed by H detection in
the same laser pulse. The low concentrations of C H and low
2
et al. do show a figure for H-atom production; the figure shows
equal concentrations of photolytic and reactive H, but Koshi et
al. did not exploit this for product studies.
The invariance of the H atom yield with pressure (20-300
4 3
Torr argon) shows that stabilization of the C H radical is not
significant up to 300 Torr. This is to be expected from the
potential energy surface (PES) of reaction R3. Ceusters et al.11
have calculated the PES using B3LYP or CCSD(T) calculations
18
probe energies negate this process under our experimental
conditions, but some effects from the high intensities of the
3
64.8 nm light in combination with the VUV radiation were
observed. These could be avoided by adjusting the lens focusing
the light into the tripling cell such that the intensity in the
reaction cell was reduced.
c. H Atom Yields from the Reaction of C
2
H and H
2
. The
at the 6-311++G(d,p) level. The formation of C
4
H
3
(-239 kJ
reaction of C
2
H with hydrogen
-1
mol relative to entrance channel) is barrierless, and the loose
-1
barrier for production of H + C
entrance channel. Alternatively, the initial C
isomerize via a 1,2 H-atom shift with a barrier of ∼173 kJ mol
to produce the more stable HCt C-C·dCH radical (-285 kJ
4
H
2
is ∼73 kJ mol below the
4
H
3
* radical can
C H + H f C H + H
(R1)
2
2
2
2
-1
2
-
1
mol ) with respect to entrance channel. The only calculated
fate of this radical is also H atom elimination with a barrier of
should show the same behavior as reaction R3, that is, a 1:1
relationship between the H produced from C H photolysis and
2
2
-1
∼
187 kJ mol .
the H produced from reaction R1, and Figure 4a shows that
this indeed is the case. Low concentrations of acetylene
b. Acetylene Photolysis at High Laser Fluences. As the
1
3
-3
excimer laser pulse energy density is increased above 10 mJ
(typically ∼1 × 10 molecule cm ) and high concentrations
-2
cm , the ratio of prompt to reactive H increases above unity.
The prompt photolysis signal increases with laser pulse energy
in a nonlinear fashion indicating that multiphoton processes are
involved. The reactive H atom signal decreases in comparison
with the prompt signal, and the relationships of the two signals
are shown in Figure 3. The symmetry of the plot suggests that
the source of the additional photolytic H comes at the expense
of hydrogen are used to ensure that a majority of the C H reacts
2
with hydrogen. In Figure 4a, the rise time now corresponds to
the pseudo-first-order rate coefficient, k′ , for reaction of C H
1
2
and H , and Figure 4b shows the resultant bimolecular plot.
2
The measured bimolecular rate coefficient is compared with
previous literature values in Table 2.
As expected from a simple abstraction reaction, there was
no dependence of the H-atom yield with total pressure (20-100
2
of the reactive H, that is, that it is the photolysis of C
+ H that is the source of the photolytic H.
The coproduct of C H photolysis, C , can be formed in either
the ground singlet state X Σ
2
H to give
C
2
Torr Ar). The prompt and reactive H atom signal from the C H/
2
2
H
2
system showed a very similar dependence with laser power
as the earlier C H/C system. The increase in prompt H can
again be explained by C H photolysis, and the lack of reactive
H atom signal is due to the very slow rate of reaction of either
1
+
3
g
, the first excited triplet state a Π
u
,
2
2 2
H
2
5
or possibly higher states. Previous studies indicate that
predominantly ground-state C (6.6:1) is formed from 193 nm
photolysis of acetylene. However both forms of C are reactive
2
2
26
-16
3
-1 -130
2
state of C
2
with H
2
(<3 × 10 cm molecule
s
).

H Atom Yields from the Photolysis of Acetylene
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4739
Figure 5. Variation of H atom yield from the reaction of C
as a function of argon bath gas pressure.
2
H and
2 4
C H
Figure 6. Schematic of the potential energy surface for C
All energies are in kilojoules per mole.
2
2 4
H + C H .
C
2
H
4
(reactions R5a and R5b). The average H atom yield is
.94 ( 0.06 (errors are 1σ) over a total pressure of 23-81 Torr.
Figure 5 also shows that there is no systematic variation of yield
with pressure; therefore, stabilization of the C * intermediate
does not appear to be occurring at these pressures. The only
other thermodynamically accessible products are C + C
which could be formed either from a direct abstraction or
following a rearrangement of the energized C intermediate;
0
Figure 4. (a) H atom profile from reaction R1 (pressure 24 Torr He,
1
4
-3
17
[
C
2
H
2
] ) 7.9 × 10 molecule cm , [H
2
] ) 3.0 × 10 molecules
4
H
5
-
3
cm ). (b) Bimolecular plot for reaction R1 at 298 K and 24 Torr
-
13
3
helium. The bimolecular rate coefficient is (5.15 ( 0.42) × 10 cm
-
1
-1
H
2 2
2 3
H ,
molecule
s , where the errors are statistical at the 1 sigma level.
TABLE 2: Summary of 298 K Rate Coefficients for the
Reaction C H + H
H
5
4
2
2
however, for the reasons outlined below, we would not expect
reaction R5a to contribute significantly.
013 × k298/cm
3
1
-
1
-1
8
author
this work
Peeters et al.12
date molecule
s
comments
Opansky and Leone have correlated activation energies of
2
C H abstraction reactions with bond dissociation energies. Direct
2009
2002
5.2 ( 0.8
5.6 ( 0.6
monitoring H
atom production
abstraction by C H from C would be expected to have a
2
2 4
H
positive activation energy; indeed, high-level calculations by
Temelso et al.31 do calculate barrier heights of 4-8 kJ mol
for reaction R5a at various levels of theory. However, the overall
C
2
H monitored via
chemiluminescence
from C H + O
H monitored
via IR absorption
H monitored
via IR absorption
decrease in C
-1
2
2
Opansky and Leone8 1996
4.6 ( 0.7
4.7
C
2
8,13
reaction exhibits a negative temperature dependence and a
similar room temperature rate coefficient as reaction R3; the
overall kinetics appear to be more compatible with an addition
elimination mechanism.
Farhat et al.10
1993
1992
C
2
Koshi et al.24
4.6
4 2
H
product measured
with increasing H
Following addition, two different 1,2-H-atom migrations
could occur with the resultant isomers fragmenting to form either
vinylacetylene + H (reaction R5b) or acetylene + vinyl (reaction
R5a). The enthalpy of formation of the initial vinyl radical has
2
Stephens et al.9
1987
4.8
C
H monitored via
2
IR absorption
0
-1 32
been calculated as ∆ H (0 K) 389 kJ mol , and the enthalpies
4 5
of the other two C H isomers have been calculated by both
f
d. H-Atom Yield from the Reaction of C
C H . The studies of the H atom yield from ethene and propene
3 6
2
H + C
2
H
4
and
Hansen et al.32 and Wheeler et al. Figure 6 shows a schematic
of the possible PES adapted from the above references. Some
barrier heights are unknown, but by comparison with analogous
systems, we do expect these to be below the energy of the
entrance channel.
33
were slightly complicated by the production of H atoms from
substrate photolysis. Background traces in the absence of
acetylene were recorded to take into account this source of H.
2
Figure 5 shows the H atom yield from the reaction of C H with

4
740 J. Phys. Chem. A, Vol. 114, No. 14, 2010
Kov a´ cs et al.
Although the formation of vinyl and acetylene is likely to be
by C
2
H from isobutane predict a negative activation energy
possible via isomerizations with barriers below the entrance
channel, the observation of near-unity H-atom yield suggests
that these isomerizations, which would occur via relatively tight
transition states, cannot compete with the looser transition states
for H-atom elimination either from the initially formed adduct
for this abstraction system with a relatively weak C-H
-
1 36
tertiary bond (DH298 ) 403 kJ mol
), and earlier measure-
ments by Hoobler et al.37 do indeed measure a slight negative
temperature dependence for reaction R7 and a fast rate
-
11
3
-1
-1
coefficient of 9.6 × 10
cm molecule
s
at room
HCt CCH
2
CH
2
· or from the more stable HCt CCH·CH
3
temperature. The C-H bond leading to allyl is significantly
-
1 38
radical formed via a 1,2-H-atom migration (given the tight
transition state for the 1,2 migration, the contribution from this
channel is, however, likely to be limited). The isoelectronic
weaker (DH298 ) 369 kJ mol
), and given the correlation
between activation energies of abstraction reactions and bond
8
dissociation energies observed by Opansky and Leone, one
reaction of CN with C
2
H
4
also produces essentially unity H-atom
2
would expect H-atom abstraction by C H to form allyl and
acetylene to be barrierless and possibly competitive with
addition/elimination.
e. Implications. This work has demonstrated that the H-atom
2
yield of C H reactions can be studied using the photolysis of
yield.2
0,34
No H-atom signal was observed from the reaction of C
2
H +
propene at 50 Torr total pressure. Uncertainties in the analysis
put an upper limit of 5% on H-atom production. Various reasons
can be postulated for the lack of H-atom production from
reaction R7: (1) Under our experimental conditions, collisional
acetylene at low laser energy densities at 193 nm as an internal
calibration. This reaction can now be used as the basis for the
stabilization of this system (which is larger than C
is essentially complete. (2) 1,2-H migration from the initial
adduct, forming the more stable (CH )CH -C·H-Ct CH
radical, is more rapid than H-atom elimination. Methyl radicals
will be preferentially produced from the fragmentation of
2
H + C
2
H
4
)
study of H-atom yields from other C
2
H reactions, and the
H-atom yields from O
paper.
2
and NO will be the subject of a future
3
2
Internal calibration systems have the major advantage that
the H-atom signal does not need to be corrected for the
different detection efficiency in the presence of either the
calibrant or test reagents. This is particularly important if
(
3 2
CH )CH -C · H-Ct CH. (3) Direct abstraction to form the
resonantly stabilized allyl radical occurs.
Whereas it is not possible to be definitive with the current
experimental measurements, some pointers to the products can
be gained from comparison with the isoelectronic CN + propene
reaction (R8a-R8d), which has recently been investigated
2 2
H is used as the calibration reaction because H can quench
2
0
Lyman-R fluorescence. However, internal calibrations do
introduce a significant alternative source of error in that the
signal directly following photolysis, at zero time, needs to
be accurately determined. For normal calibration reactions,
this is a straightforward process for exponential growth,
which starts at zero signal at zero time. However, for internal
calibrations, this approach is not possible, introducing
additional random errors and limiting the concentration of
substrate that can be introduced.
2
1,35
experimentally and through theoretical calculations.
CN + C H f H + C H N
(R8a)
(R8b)
(R8c)
(R8d)
3
6
4
5
f CH + C H N
3
3
3
f C H N
4
6
The reaction of C
importance in both combustion chemistry and in the reducing
atmospheres of the outer planets and their satellites. C species
can be formed either by stabilization at high pressures of the
H * intermediate formed in reactions R5aand R5b or from
2 2 4
H with C H (R5a and R5b) is of potential
f HCN + C H
3
5
4 5
H
For this reaction (and the larger CN + iso-butene reaction), a
C
4
5
21
pressure-dependent H-atom yield was observed over the range
4 5
the addition of H atoms to vinylacetylene. The reactions of C H
of 2-200 Torr (i.e., both reactions R8a and R8c were occurring),
suggesting that whereas the initial complex formed between C H
2
and propene might be collisionally stabilized, complete stabi-
lization of the complex would not be expected under our
experimental conditions.
isomers with acetylene have been implicated in the first steps
of soot formation.3 In planetary atmospheres, the formation
of vinylacetylene is one example of the growth of larger
functionalized compounds. Such molecules are important in the
synthesis of yet larger compounds and hazes, but also, because
they photolyze at longer wavelengths than simple alkenes or
9,40
Extrapolation of the H-atom yield from reaction R8a to zero
2
1
41
pressure predicted an H-atom yield of ∼50%. This experi-
mental yield was consistent with preferential, but not exclusive,
formation of the secondary radical following CN addition and
competition between 1,2-H atom migration, with subsequent
methyl elimination, and H atom dissociation from the secondary
radical. Broadly similar results were obtained in the more recent
alkynes, they can be a source of radicals lower in the
atmosphere.
A recent model for haze formation in Titan’s atmosphere42
includes only the C H + H channel from reactions R5a and
4
4
R5b, and this work confirms that this will be the only important
reaction under such conditions. The same model has H +
unidentified coproducts for reaction of C H with C H (reaction
35
study of Trevitt et al. using photoionization mass spectrometry
to observe cyanoethene (+CH ) or C N (+H) species. In the
2
3
6
3
4
H
5
R7). In this case, the current studies suggest that this is incorrect
and that for alkenes with allylic hydrogens, the efficiency of
low-pressure study of Trevitt et al., H atom elimination was
the minor channel (25 ( 15%), but error bars of the zero-
pressure H-atom yields overlap. It is possible that the analogous
C
2
H reactions in promoting molecular growth may need to be
adjusted.
C
2
H system behaves differently with a more facile 1,2-H-atom
migration. Given the similarities in behavior between the C
and CN + ethene systems, this seems unlikely, but either high-
level calculations or experimental observation of the CH yield
2
H
Acknowledgment. This work was funded with support from
the EPSRC under grant GR/T28560/01.
3
are needed to confirm the role of isomerizations following
addition.
References and Notes
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2
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