Reaction rate and isomer-specific product branching ratios of C 2H + C4H8: 1-butene, cis -2-butene, trans -2-butene, and isobutene at 79 K

Article abstract of DOI:10.1021/jp403637t

The reactions of C₂H radicals with C₄H₈ isomers 1-butene, cis-2-butene, trans-2-butene, and isobutene are studied by laser photolysis-vacuum ultraviolet mass spectrometry in a Laval nozzle expansion at 79 K. Bimolecular-reaction rate constants are obtained by measuring the formation rate of the reaction product species as a function of the reactant density under pseudo-first-order conditions. The rate constants are (1.9 ± 0.5) × 10^-10, (1.7 ± 0.5) × 10 ^-10, (2.1 ± 0.7) × 10^-10, and (1.8 ± 0.9) × 10^-10 cm³ s^-1 for the reaction of C₂H with 1-butene, cis-2-butene, trans-2-butene, and isobutene, respectively. Bimolecular rate constants for 1-butene and isobutene compare well to values measured previously at 103 K using C₂H chemiluminescence. Photoionization spectra of the reaction products are measured and fitted to ionization spectra of the contributing isomers. In conjunction with absolute-ionization cross sections, these fits provide isomer-resolved product branching fractions. The reaction between C₂H and 1-butene yields (65 ± 10)% C₄H₄ in the form of vinylacetylene and (35 ± 10)% C₅H₆ in the form of 4-penten-1-yne. The cis-2-butene and trans-2-butene reactions yield solely 3-penten-1-yne, and no discrimination is made between cis- and trans-3-penten-1-yne. Last, the isobutene reaction yields (26 ± 15)% 3-penten-1-yne, (35 ± 15)% 2-methyl-1-buten-3-yne, and (39 ± 15)% 4-methyl-3-penten-1-yne. The branching fractions reported for the C₂H and butene reactions indicate that these reactions preferentially proceed via CH₃ or C₂H₃ elimination rather than H-atom elimination. Within the experimental uncertainties, no evidence is found for the formation of cyclic species.

Full text of DOI:10.1021/jp403637t

Article
pubs.acs.org/JPCA
Reaction Rate and Isomer-Specific Product Branching Ratios of C2H +
C4H8: 1‑Butene, cis-2-Butene, trans-2-Butene, and Isobutene at 79 K
Jordy Bouwman,† Martin Fournier,‡ Ian R. Sims,‡ Stephen R. Leone,†,§ and Kevin R. Wilson*,§
†Departments of Chemistry and Physics, University of California, Berkeley, California 94720, United States
‡Institut de Physique de Rennes, UMR 6251 du CNRS, Universite
Cedex, France
́ ́ ́
de Rennes 1, 263 Avenue du General Leclerc, 35042 Rennes
§Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States
ABSTRACT: The reactions of C2H radicals with C4H8 isomers 1-butene, cis-2-
butene, trans-2-butene, and isobutene are studied by laser photolysis-vacuum
ultraviolet mass spectrometry in a Laval nozzle expansion at 79 K. Bimolecular-
reaction rate constants are obtained by measuring the formation rate of the
reaction product species as a function of the reactant density under pseudo-first-
order conditions. The rate constants are (1.9 0.5) × 10−10, (1.7 0.5) × 10−10
,
(2.1 0.7) × 10−10, and (1.8 0.9) × 10−10 cm3 s−1 for the reaction of C2H with
1-butene, cis-2-butene, trans-2-butene, and isobutene, respectively. Bimolecular rate constants for 1-butene and isobutene
compare well to values measured previously at 103 K using C2H chemiluminescence. Photoionization spectra of the reaction
products are measured and fitted to ionization spectra of the contributing isomers. In conjunction with absolute-ionization cross
sections, these fits provide isomer-resolved product branching fractions. The reaction between C2H and 1-butene yields (65
10)% C4H4 in the form of vinylacetylene and (35 10)% C5H6 in the form of 4-penten-1-yne. The cis-2-butene and trans-2-
butene reactions yield solely 3-penten-1-yne, and no discrimination is made between cis- and trans-3-penten-1-yne. Last, the
isobutene reaction yields (26 15)% 3-penten-1-yne, (35 15)% 2-methyl-1-buten-3-yne, and (39 15)% 4-methyl-3-penten-
1-yne. The branching fractions reported for the C2H and butene reactions indicate that these reactions preferentially proceed via
CH3 or C2H3 elimination rather than H-atom elimination. Within the experimental uncertainties, no evidence is found for the
formation of cyclic species.
CH + C4H10 → C4H8 + CH3
1CH2 + C3H6 + M → C4H8 + M
1. INTRODUCTION
(R1a)
(R1b)
Reactions of the highly reactive ethynyl (C2H) radical are
important in a large variety of environments, such as
combustion engines, the interstellar medium, and planetary
atmospheres. In the cold, dense atmosphere of Saturn’s largest
moon, Titan, C2H radicals are efficiently formed through the
photolysis of acetylene.1 Comprehensive chemical models,2−5
quantum-mechanical calculations,6,7 and laboratory experi-
ments8−14 indicate that the reactions of C2H radicals with
unsaturated hydrocarbons play an important role in the
formation of larger hydrocarbons, such as polyynes, cyanopo-
lyynes, and polycyclic aromatic hydrocarbons (PAHs), in
Titan’s atmosphere. These species are thought to make up a
large fraction of the characteristic yellow haze that shrouds the
moon.14,15 C2H radicals are also present in the interstellar
medium,16,17 where they are considered to play a crucial role in
the formation of PAHs18 that are thought to be present along
the line of sight of many objects.19,20 C2H radicals are also
considered to be an important species in the formation of
aromatics resulting from the combustion of hydrocarbons.21,22
Alkenes, such as butene isomers, are important as fuels23,24
and as intermediates in the oxidation of hydrocarbons.25,26
They are also considered to play a role in the photochemistry of
Titan’s atmosphere. Lavvas et al.3,4 specifically included the
production of C4H8 in their photochemical model via two
formation routes:27
In that model, Lavvas et al.3,4 considered the destruction of
C4H8 to occur by its reactions with N atoms, C atoms, and a set
of hydrocarbon radicals that includes C2H. They used a room-
temperature reaction rate of 2.6 × 10−10 cm3 s−1 for the C2H +
butene reaction, and, because experimental data were lacking,
they did not distinguish between the reactivity of the different
C4H8 isomers. Moreover, because product branching ratios
were also not available, no specific reaction products were
included in the model.3,4
Synchrotron-photoionization mass spectrometry offers a
sensitive technique for the mass-selective and isomer-specific
disentanglement of the contents of gas mixtures.28,29 This
technique is commonly used in conjunction with a reactor flow
tube at the Advanced Light Source (ALS) at Lawrence Berkeley
National Laboratory (LBNL) to study reactions of radicals with
unsaturated hydrocarbons that are of interest in combustion
chemistry.30−33 Photoionization spectra of reaction products
are recorded by scanning the energy of the vacuum-ultraviolet
(VUV) ionizing radiation from the synchrotron while
Received: April 12, 2013
Revised: May 17, 2013
© XXXX American Chemical Society
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measuring the temporal formation of the products. Isomer-
specific product branching ratios are derived by fitting the
photoionization spectra with calculated or measured ionization
spectra of possible reaction products. No isomer-specific
product branching measurements have been reported for
C2H + butene reactions.
Over the past decades, Laval nozzle expansions have been
employed by a number of groups to study a variety of reactions
at low temperatures.34−39 Reaction rates involving the ethynyl
radical are generally quantified under pseudo-first-order
conditions using chemiluminescence to track the decay of the
radical density.40−45 Using this technique, the bimolecular rate
constants for C2H + 1-butene43,45 and C2H + isobutene45 were
quantified and reported for discrete temperatures ranging from
103 to 296 K. The measured bimolecular-reaction rate
constants for both reactions were found to be fast, suggesting
that these reactions proceed via the barrierless formation of a
complex followed by a unimolecular reaction to form the final
products. To the best of our knowledge, no bimolecular-
reaction rate-constant measurements for C2H + cis-2-butene
and C2H + trans-2-butene have been reported.
2.1. Laval Expansion. A Laval nozzle designed to yield a
mach 4 expansion is mounted in a vacuum chamber pumped by
a 1200 m3/h Roots blower. The Laval nozzle is mounted on a
reservoir block that is filled with a gas mixture by two solenoid
valves. Gas pulses of 6 ms in duration are expanded through the
nozzle into the vacuum chamber at a rate of 30 Hz. During
Laval nozzle operation, the vacuum chamber is kept at 128
mTorr by means of a feedback-loop-controlled butterfly valve
that is mounted on the blower. The temperature of the
resulting collimated expansion is determined as described
previously48 and is 79 2 K.
The gas mixture containing the bath gas (N2), reactant gas
(C4H8), and radical precursor (C2H2) is prepared in a gas
manifold by allowing the gases to flow through individually
calibrated mass-flow controllers. A cylinder is mounted between
the gas mixing manifold and the Laval nozzle to ensure a
thorough mixing of the gases and to reduce pressure
fluctuations in the gas supply. Ultra-high-purity nitrogen is
used as the bath gas. The C2H radical precursor acetylene
(C2H2, Airgas, stabilized by acetone) is passed through a
Matheson-activated charcoal filter to remove residual acetone.
The reactant molecules 1-butene (≥99%, Sigma-Aldrich), cis-2-
butene (≥99%, Sigma-Aldrich), trans-2-butene (≥99%, Sigma-
Aldrich), and isobutene (≥99%, Matheson) are used without
further purification. C5H6 isomer 2-methyl-1-buten-3-yne (99%,
Sigma-Aldrich) is used as received and is diluted in nitrogen to
measure its absolute photoionization spectrum.
The reactions between C2H and C4H8 isomers 1-butene, cis-
2-butene, and isobutene have been studied theoretically. Woon
and Park46 found that these reactions proceed via the
barrierless addition of C2H radical to the sp2 carbons of the
C4H8 isomer, followed by isomerization of the energetic adduct.
Their energetics are
An unfocused ArF excimer laser operating at 193 nm is
pulsed coaxially through the collimated expansion to generate
C2H radicals from the C2H2 radical precursor. C2H radicals
may be electronically and/or vibrationally excited, but the
excited states are rapidly quenched in the expansion by
collisions with the N2 bath gas.49 The number density of C2H2
is kept constant at 1.4 × 1014 cm−3, and the number density of
C2H radicals produced by the photolysis of acetylene is
calculated from the absorption cross section50 (2 × 10−19 cm−2)
assuming a unity quantum yield51 for the formation of C2H
radicals. A typical laser pulse fluence of 7 mJ cm−2 results in a
number density of C2H radicals of 2 × 1011 cm−3. This value is
small when compared to the number density of reactant
molecules ([RH], ranging from ∼4.5 × 1013 to ∼2.0 × 1014
cm−3), ensuring that pseudo-first-order conditions ([C2H] ≪
[RH]) are fulfilled.
2.2. Product Detection. The collimated expansion is
sampled through a 450 μm pinhole in a parabolically shaped
airfoil, and the sampled part of the expansion enters the
detection chamber, which is maintained at ∼10−7 Torr by two
turbopumps during the operation of the pulsed Laval
expansion. The sampled gas is ionized by quasi-continuous
VUV light from the ALS, after which the resulting ions are
detected in a quadrupole mass spectrometer (QMS).
Undispersed synchrotron light with a photon energy of 11.25
eV and a full-width-at-half-maximum of 320 meV is used for the
bimolecular rate constant measurements. The ion counts
arising from 20 000 laser pulses are time-binned in a
multichannel scaler to obtain a kinetic trace at a single [RH]
density.
The exothermicities relative to the reactants for R2a−R2e are in
kcal/mol. The range of exothermicities accounts for the
possibility of forming a set of different isomers within a single
product channel, and a dash (−) denotes that the exit channel
is inaccessible. In addition to the formation of the stable species
listed above, Woon and Park46 also mentioned the formation of
five- and six-membered C6H9 rings that would be formed
without losing a fragment. These species are very energetic and
would require significant stabilization to be formed.
Here, we report the bimolecular-reaction rate constants and
isomer-specific product branching ratios for the reactions of
C2H radicals with C4H8 isomers 1-butene, cis-2-butene, trans-2-
butene, and isobutene measured in a Laval nozzle expansion at
79 K. The bimolecular-reaction rate constants are quantified by
measuring the formation rate of the product species as a
function of the reactant density. Photoionization spectra of the
reaction products are measured, and isomer-specific branching
ratios are derived from fitting the model to these spectra. The
experimental results are compared to calculations, and the
implications for combustion chemistry and the photochemistry
of Titan’s atmosphere are highlighted.
2. EXPERIMENTAL SECTION
Mass spectra of reaction products are recorded with 0.2 amu
resolution. These species are sampled and subsequently ionized
with 11.25 eV light that is dispersed by a 3 m monochromator
with a slit width of 1 mm, which yields an energy resolution of
∼75 meV. For these measurements, ions are extracted and
detected in the QMS, and ion counts are time-binned for 500
The measurements are performed at the chemical dynamics
beamline of the ALS at LBNL in an apparatus that combines a
pulsed Laval nozzle reactor with time-resolved synchrotron-
photoionization mass spectrometry. The experimental appara-
tus has been described in detail in a previous publication;47
therefore only a brief description is given here.
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laser pulses for each mass-channel setting. This results in a 2D
data set that contains ion counts as a function of both time and
mass (Figure 1A). The mass-dependent sensitivity of the QMS
ionization energies are computed using the CBSQB3 basis
set,54,55 which is found to yield reliable values. Ground-state
geometries of the neutral and cation are optimized using the
CBSB7 basis set,39 and Franck−Condon simulations are
performed to simulate the shape of the ionization spectrum.
Ionization cross sections are computed using a model by
Bobeldijk et al.56 Within this model, the ionization cross section
of a molecule at a photon energy of 11.8 eV can be estimated
through the summation of the cross sections (σX−Y) of the
number (n) of bonds X−Y within the molecule:
σtot
=
σX−YnX−Y
∑
(1)
This model gives ionization cross sections that are generally
accurate to within ∼20%.
3. RESULTS
The reactions between C2H radicals and 1-butene, cis-2-butene,
trans-2-butene, and isobutene are studied in the low-temper-
ature Laval nozzle expansion apparatus by photoionization
mass spectrometry. Mass spectra of the reaction products are
recorded, and biomolecular-reaction rate constants are
measured for all four reactants by measuring the temporal
formation of the main product species for a set of reactant
densities under pseudo-first-order conditions. Subsequently,
photoionization spectra of the reaction products are measured
to obtain isomer-specific branching ratios. The results are
described in detail in the following sections.
3.1. Mass Spectra. The formation of reaction products and
photolysis products is recorded as a function of mass and time.
Species in the expansion are ionized with monochromatized
light at 11.25 eV prior to being extracted into the QMS. Mass
information is obtained by stepping through the mass channels
of the QMS with 0.2 amu resolution. Temporal information is
obtained by time-binning ion counts for 500 photolysis laser
pulses at each QMS mass channel. Shown in Figure 1A are the
resulting ion counts that are detected (grayscale) as a function
of the mass-to-charge ratio (m/z in amu on the x axis) and time
(μs on the y axis) by the 193 nm photolysis of a mixture of 1.3
× 1014 cm−3 1-butene and 1.2 × 1014 cm−3 C2H2 in 1.4 × 1016
cm−3 N2. The horizontal line at t = 0 μs is an artifact caused by
the photolysis laser and indicates the start time of the reaction.
From Figure 1A it can be seen that, as is the case for time-of-
flight mass spectrometers, the arrival time of the ions after the
photolysis laser pulse is proportional to the square root of the
ion mass. The arrival time of the ions that are instantaneously
formed from the photolysis of 1-butene and extracted into the
QMS at a fixed potential (−10 V) is shown as a function of m/z
in Figure 1B. Also shown in this figure is the fit to the arrival
time based on
Figure 1. (A) Image of the ion counts formed by ionization at a
synchrotron energy of 11.25 eV (grayscale) as a function of time (y
axis) and m/z ratio (x axis) for the reaction between C2H radicals and
1-butene. (B) Arrival time of ions plotted against the mass-to-charge
ratio together with a fit to the data.
detector is taken into account when determining the absolute
branching ratio for reactions with multiple product channels, as
described in detail in a previous publication.52
Photoionization spectra are measured by scanning the
photon energy from ∼8.5 to ∼11.5 eV while detecting product
ions. VUV light from the ALS is dispersed by the 3 m
monochromator, and a part of the dispersed light is transmitted
through a 600 μm slit, resulting in an energy resolution of ∼36
meV. Ionized product species are detected and typically time-
binned for 2000 laser pulses at each photon energy. The ion
counts are integrated over the duration of the cold expansion
and normalized to the photon flux of the ALS measured with a
NIST-calibrated photodiode. Error bars in the photoionization
spectra display the shot-noise level, which is equal to the square
root of the ion counts.
2.3. Computational Method. Measured photoionization
spectra of the reaction products are modeled using measured
absolute ionization spectra from the literature when available.
Photoionization spectra are computed using Gaussian0953
when the spectra are unavailable from the literature and
when the species cannot be purchased commercially. The
tarr = a + b(m/z)1/2
(2)
Ion counts are integrated for the first ∼150 μs after this arrival
time (tarr) to obtain a mass spectrum that reflects the correct
branching of the products formed in the cold expansion.
Furthermore, integrated ion counts before the photolysis laser
is pulsed are subtracted from the ion counts after the laser is
pulsed to reflect only those species that are formed after the
photolysis laser is pulsed. The resulting mass spectrum exhibits
positive signals for species that are formed from photolysis and
photochemical reactions and negative signals for species that
are depleted. Although the synchrotron photon energy used for
this measurement (11.25 eV) is below the ionization threshold
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Figure 2. Mass spectra of the C2H + 1-butene reaction measured at an ionization energy of 11.25 eV. (A) Mass spectrum of 1-butene in N2
photolyzed at 193 nm. (B) Mass spectrum of acetylene and 1-butene in N2 photolyzed at 193 nm. (C) Reaction products formed from the reaction
between C2H and 1-butene (spectrum B minus spectrum A).
of acetylene (I.E. = 11.4 eV), a negative signal that is caused by
the photolysis of acetylene by the laser is observed at m/z = 26
(C2H2). The detection of the acetylene depletion at 11.25 eV is
explained by the low photon-energy resolution used for this
measurement because there is still some photon flux above the
ionization threshold of acetylene. Depletion is also expected at
the reactant’s mass, but saturation of the QMS detector causes
the ion counts around this mass channel to be nonlinear with
density. For this reason, data are omitted around the mass of
the reactant species (from m/z = 55 to 57).
Table 1. Absorption Cross (σ) Sections at 193 nm,
Ionization Energies (IE), and Fractional Absorption
(Expressed as a Percentage of the Number Density) for the
Radical Precursor and Reactant Species Under Typical
Experimental Conditions
Shown in Figure 2A is a mass spectrum of the products
formed from the 193 nm photolysis of ∼1% 1-butene in
nitrogen (total density 1.4 × 1016 cm−3). From this figure, it can
be seen that strong features in mass channels m/z = 15 (CH3),
27 (C2H3), 29 (C2H5), 39 (C3H3), 41 (C3H5), and 53 (C4H6)
appear upon the photolysis of 1-butene. The formation of these
1-butene photofragments, with the exception of C3H3, is in
agreement with a previous study by Niedzielski et al.57
In Figure 2B, it can be seen that new products are detected at
m/z = 50, 52, and 66 when ∼1% of the C2H radical precursor
(C2H2) is added to the expansion. A mass spectrum showing
only the reaction products is obtained by subtracting the trace
in Figure 2A from the trace in Figure 2B and is displayed in
Figure 2C after multiplication by a factor of 5. The product
detected at m/z = 50 is identified as diacetylene (C4H2) formed
from the reaction between C2H and C2H2, as described in
previous publications.47,52 Bimolecular rate measurements
confirm that the products at m/z = 52 and 66 originate from
the reaction under investigation and are described in detail in
the next section.
absorption path length. Under the experimental conditions
typically used for the mass spectra in Figure 2 (∼1% reactant,
∼1% C2H2 in 1.4 × 1016 cm−3 N2, and ∼7 mJ/cm2 laser
fluence), 1.2% of the 1-butene in the flow absorbs a photon
compared to only 0.4% of the C2H2, yielding larger number
densities for the 1-butene photolysis fragments than C2H
radicals. An overview of the fractions of the reactant and radical
precursor molecules that absorb a 193 nm photon is given in
Table 1.
Analyses similar to that described above for 1-butene are
performed for the other reactant species to isolate reaction
products from photofragments. The mass spectra of these
reaction products are displayed in Figure 3. From this figure, it
can be seen that products at m/z = 50 and 66 are formed from
the reaction between C2H and cis-2-butene and trans-2-butene.
The reaction between C2H and isobutene yields products at
m/z = 50 and 66 and a weak product at m/z = 80. The product
at m/z = 50, which is detected for all four reactions, is assigned
to the C4H2 isomer diacetylene, which is a well-known product
of the C2H + C2H2 reaction.32,37
The product signals detected at m/z = 50, 52, and 66 (Figure
2B) are small compared to the signals arising from the
photolysis of the reactant species. This difference can be largely
explained by the difference in the absorption cross section
between 1-butene and C2H2 at 193 nm (Table 1). The fraction
of the molecules that absorb a photolysis laser photon, γ, can be
expressed as
Φλ
hcNl
γ =
(1 − exp(−Nσl))
(3)
where Φ is the laser fluence, λ is the photolysis laser
wavelength, h is Planck’s constant, c is the speed of light, N
is the number density of the reactant or radical precursor, σ is
the absorption cross section at the laser wavelength, and l is the
Contributions of products to the observed mass channels
formed by side reactions also need to be considered. Radicals
that are formed from the photolysis of the reactant can
potentially react with the reactant itself to form products that
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Figure 4A. The fit routine described in Bouwman et al.52 is used
to fit eq 4 to the measured data and is discussed briefly here.
Figure 3. Mass spectra of the reaction products of C2H reacting with
isobutene, cis-2-butene, and trans-2-butene.
interfere with the C2H reaction products under investigation.
No products are detected in the absence of the radical
precursor (see Figure 2A for 1-butene), and such side reactions
can thus be excluded. Also, side reactions of the radicals formed
from the photolysis of the reactant can potentially react with
acetylene to contribute to the detected product channels:
⧧
C2H3 + C2H2 → C4H5 → C4H4 + H
(R3a)
⧧
C3H5 + C2H2 → C5H7 → C4H4 + CH3
(R3b)
⧧
C3H5 + C2H2 → C5H7 → C5H6 + H
(R3c)
Figure 4. (A) Formation of a reaction product as a function of time at
m/z = 66 (C5H6) measured at 11.25 eV for the C2H + cis-2-butene
reaction at a reactant density of 2.1 × 1014 cm−3 plotted together with
the best Gaussian convoluted fit to the data. (B) Gaussian with unit
area that resembles the system-response function. (C) Fit function
based on eq 4.
These reactions, however, have slow bimolecular rate constants
(kR3a = 7.5 × 10−16 cm3 s−1 at 300 K,34 kR3b = kR3c = 4.5 × 10−19
cm3 s−1 at 300 K35). A chemical kinetics model is employed to
investigate the contributions of these reactions. Under the
experimental conditions used here, these side reactions were
found to have negligible contributions to the product channels
of all the reactions investigated.
First, a value for Qm is determined by calculating the average of
the ion counts for 130 < t < 150 μs. Subsequently, a fit based on
eq 4 and an initial trial value for k1st is computed (Figure 4C).
This trace is convoluted with a 16 μs-wide Gaussian shaped
system-response function (Figure 4B) that is quantified from
the instantaneous formation of radicals from the photolysis of
1-butene. The resulting fit function corrected for the system
response is shown in Figure 4A. The difference between the
resulting Gaussian convoluted fit and the measured data is
quantified by calculating the sum of the residuals. This method
is repeated for a large set of trial values of k1st. Finally, the value
of k1st that yields the lowest sum of the residuals is selected and
is displayed in Figure 4A. The analysis is repeated for the
product formation at all densities, and the resulting values of
k1st are displayed with an estimated error of 3000 s−1 as a
function of reactant density in Figure 5. The reactant density is
corrected for losses due to the photolysis of the reactant.
The analysis described above is performed for the strongest
product channels of the reactions, and the resulting values of
k1st are plotted as a function of calibrated reactant densities in
Figure 5. Also shown in this figure are linear fits to the data.
The intercepts are different for each reactant, and this is most
striking for the isobutene data. The intercepts reflect additional
loss processes of C2H radicals (eq 4). Possible loss processes
are diffusion of radicals out of the expansion, reactions of C2H
with background gases (e.g., oxygen), and reactions of C2H
with radicals in the expansion that are formed from photolysis
of the reactant. Hence, the large intercept for isobutene could
3.2. Reaction Rate Determinations. Reaction rate
constants for the reactions between C2H and C4H8 isomers
are measured by recording the formation rates of product
species as a function of reactant density. The experiments are
performed under pseudo-first-order conditions (i.e., the density
of the radical species [C2H] is much lower than that of the
reactant [RH]). Under these conditions, the formation of
product species [Pm] as a function of time can be expressed as
described in detail in a previous publication:52
[P ] = Q (1 − exp{−(k[RH] +
k [M ])t})
n n
∑
m t
m
(4)
n
km[RH][C2H]0
k[RH] + ∑n kn[Mn]
with Q m
=
(5)
The bimolecular rate constant k is retrieved by measuring the
first-order rate coefficient k1st = k[RH] + ∑nkn[Mn] for a set of
reactant densities [RH]. The slope directly yields the
bimolecular rate constant, and the intercept reflects the sum
over the additional loss processes of the radical, for example,
reactions with C2H2 and other radicals in the expansion. The
prefactor Qm contains information on the product branching
ratios, which will be described in more detail in the next
section.
A typical time trace for the formation of products from the
reaction between C2H and cis-2-butene detected at m/z = 66
after being photoionized at 11.25 eV is displayed in black in
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the isomers of the product species considered in this work
together with their ionization energies is given in Table 2.
Table 2. Product Isomers of the C4H4, C5H6, and C6H8
Species Considered in This Work Together with Their
Structures, Ionization Energies, and Methods of
Determining the Ionization Energies
Figure 5. Measured values of k1st as a function of the reactant density
◊
○
for the reaction between C2H and 1-butene ( ), cis-2-butene ( ),
□
trans-2-butene (Δ), and isobutene ( ) displayed together with linear
fits to the data.
be a result of the much larger absorption cross section of
isobutene that causes 23% of this isomer to be photolyzed.
Bimolecular-reaction rate constants are determined from the
slopes of the linear fits to the data shown in Figure 5. The
resulting bimolecular-reaction rate constants are (1.9 0.5) ×
10−10, (1.7 0.5) × 10−10, (2.1 0.7) × 10−10, and (1.8 0.9)
× 10−10 cm3 s−1 for the reactions of C2H with 1-butene, cis-2-
butene, trans-2-butene, and isobutene, respectively. The errors
in the reaction rate constants are composed of 3σ in the linear
fit together with a contribution of 10% to account for
systematic errors. Furthermore, the uncertainty accounts for
the fact that the derived rate constants are rather sensitive to
the assumed starting point (t0) of the reaction, which is
included in the model fit. The uncertainty in the bimolecular
rate constant for isobutene is slightly larger because an
additional uncertainty caused by the abundant isobutene
photolysis fragments is introduced. The isobutene data seem
to deviate from a linear dependence toward the higher reactant
densities. This could be indicative of clustering in the
expansion, but there is no further evidence for the formation
of such clusters.
The bimolecular-reaction rate constants reported here can be
compared to those measured previously for C2H + 1-
butene43,45 and C2H + isobutene.45 These rate constants
were measured for discrete temperatures ranging from 103 to
296 K and at 103 K are (2.6 0.6) × 10−10 and (1.4 0.3) ×
10−10 cm3 s−1 for the 1-butene and isobutene reactions,
respectively. The bimolecular rate constants reported here are
in agreement with those measured previously and are
consistent with the products being formed from the studied
reactions rather than from side reactions.
3.3.1. C2H + 1-Butene. Figure 6 shows the photoionization
spectrum of the product at m/z = 52 that is formed from the
Figure 6. Photoionization spectrum of the C4H4 (m/z = 52) product
species formed from the reaction between C2H and 1-butene plotted
together with a fit based on the vinylacetylene photoionization
spectrum.
reaction between C2H and 1-butene. The sharp onset at ∼9.6
eV, the overall shape, and the characteristic resonances are
consistent with a single predominant isomer, vinylacetylene. A
fit is made using the measured absolute spectrum of
vinylacetylene from Cool et al.58 and is also shown in Figure
6. An upper limit of 5% is derived for contributions of other
C4H4 isomers by assuming a detection limit of twice the
standard deviation in the baseline noise. A photoionization
spectrum of the C5H6 species formed from the reaction
between C2H and 1-butene and detected at m/z = 66 is shown
in Figure 7. The ionization threshold of the C5H6 product
species detected from the C2H + 1-butene reaction is around
9.8 eV. This ionization threshold is close to the calculated
ionization threshold of 4-penten-1-yne (IE = 9.9 eV) and far
from the thresholds of other possible C5H6 isomers52 (IE = 9.1,
9.2, and 9.3 eV, Table 2). The simulated ionization spectrum of
C5H6 isomer 4-penten-1-yne is shown in Figure 7 and does not
fully capture the measured spectrum. The simulated spectrum
3.3. Isomer-Specific Product Branching Ratios. Photo-
ionization spectra of the reaction products are recorded to
identify the isomers that are formed from the title reactions and
to quantify their respective branching fractions. Product ion
counts are measured as a function of the synchrotron photon
energy and normalized to the photon flux, which is measured
with a NIST-calibrated photodiode. The resulting photo-
ionization spectra are modeled using contributions of absolute
photoionization spectra of isomers that are expected to
contribute to the overall spectrum. Computed spectra are
used when measured spectra are unavailable. An overview of
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The photoionization spectrum of this mass channel is shown in
Figure 9. Also displayed in Figure 9 is the simulated
photoionization spectrum of cis-3-penten-1-yne from Bouwman
et al.52 The calculated ionization onset of cis-3-penten-1-yne
resembles the ionization threshold that is measured exper-
imentally. The vibrational structure at low energies is nicely
captured by the calculated spectrum of cis-3-penten-1-yne, and
a reasonable fit is obtained.
There is a slight mismatch between the measurement and the
calculation for energies exceeding 9.6 eV, but no clear
secondary threshold at ∼9.9 eV is observed. Thus, no
contribution of 2-methyl-1-buten-3-yne to the spectrum is
apparent, and an upper limit of 5% is derived for this species.
The slow rise could be caused by an artifact called charge-
transfer ionization that is caused by inadequate pressure
reduction in the mass spectrometer region.52 In this scenario,
the charge of the ionized reactant, which is present in much
larger abundance, is transferred to the reaction product. As a
result, the shape of the ionization curve will exhibit some
character of the reactant species, which in the case of cis-2-
butene exhibits a slow rise.59
Because the ionization thresholds of cis-3-penten-1-yne and
trans-3-penten-1-yne is separated by less than 100 meV,52,60 an
unambiguous identification of the exact geometrical isomers is
difficult, if not impossible. Thus, although the fit to the data
shown in Figure 9 exhibits very good agreement, the reaction
products are identified as either cis-3-penten-1-yne, trans-3-
penten-1-yne, or a superposition of these geometrical isomers.
Upper limits of 2 and 4% are derived for the formation of C6H8
from H loss and the formation of C4H4 from C2H3 loss,
respectively.
The detection of a product with m/z = 66 from the reaction
between C2H and cis-2-butene can be understood from the
reaction scheme shown in Figure 8. C2H radicals can add to
either of the carbons in the CC bond of cis-2-butene, and by
symmetry this forms a single-adduct species (Adduct 3 in
Figure 8). This adduct can eject an H atom or a CH3 radical.
Hydrogen shifts within adduct 3 could occur, but there is no
experimental proof for such isomerization. The detection of
geometrical isomers of 3-penten-1-yne as the only product
species is in agreement with calculations by Woon and Park.46
3.3.3. C2H + trans-2-Butene. The photoionization spectrum
of the C5H6 product that is formed from the C2H + trans-2-
butene reaction is measured and shown in Figure 10. Also
shown in this figure is a fit to the data based on the simulated
ionization spectrum of trans-3-penten-1-yne from Bouwman et
al.52 The ionization threshold and vibrational structure are well
represented, and an excellent fit is obtained for energies below
∼9.8 eV. At higher photon energies, there is a slight difference
between the measured spectrum and the calculated spectrum,
but no clear ionization threshold is observed. An upper limit of
5% is derived for contributions of 2-methyl-1-buten-3-yne.
Although the ionization onset is captured very nicely by our
simulated value, it is difficult to unambiguously assign the
product to the geometrical isomer trans-3-penten-1-yne, as
noted earlier for the cis-2-butene reaction product. Hence, the
detected species is identified as cis- or trans-3-penten-1-yne or a
superposition of the two.
Figure 7. Photoionization spectrum of C5H6 product species formed
from the reaction between C2H and 1-butene plotted together with a
simulated ionization spectrum of 4-penten-1-yne.
exhibits a sharp onset, whereas the measured spectrum rises
more slowly. The mismatch of the shape can be attributed to
large structural differences between the optimized geometries
of the 4-penten-1-yne neutral and cation ground state. The
resulting poor Franck−Condon overlap can translate into a
slow rise in the measured photoionization spectrum that is not
well captured by the Franck−Condon simulation.
Product species with the general formula C6H8 (m/z = 80)
are accessible on C6H9 potential-energy surface (R2a). The
mass spectrum displayed in Figure 2C, however, shows no
evidence for the formation of this species. From eq 1, an
ionization cross section of 51 MB is computed for C6H8. With
this ionization cross section and a detection limit of 2σ of the
baseline noise, we derived an upper limit of 6% for the
contribution of these species.
A product branching fraction is derived for the formation of
the product at m/z = 52 (vinylacetylene) and m/z = 66 (4-
penten-1-yne) for C2H + 1-butene. The absolute energy-
dependent ionization cross section of vinylacetylene is available
from the literature.58 The absolute ionization cross section of 4-
penten-1-yne (45 MB) was computed in a previous study.52
After correcting for the mass-dependent sensitivity of the
QMS,52 the reaction between C2H and 1-butene is found to
yield (65 10)% vinylacetylene and (35 10)% 4-penten-1-
yne. The errors in the branching fractions are indicated at the
1σ level and mainly result from the uncertainty in the
computed ionization cross section of 4-penten-1-yne.
A possible reaction mechanism for the C2H + 1-butene
reaction is displayed in Figure 8. C2H radical can add to either
of the sp2 carbons of 1-butene, giving rise to two distinct
adducts. Possible pathways from adduct 1 (Figure 8) are H or
CH3 loss to form 3-hexen-1-yne or 4-penten-1-yne, respec-
tively. Adduct 2 can lose an H atom or C2H3 radical to form 3-
ethyl-but-3-en-1-yne or vinylacetylene, respectively. The most
prominent product found experimentally is vinylacetylene with
a branching of 65 10%. This is in excellent agreement with
the computed branching fraction value of ∼58%.46 The
quantity of 4-penten-1-yne amounts to 35 10% and is also
in excellent agreement with the computed value of ∼42%.46 An
upper limit of 6% is put on the detection of signal at m/z = 80.
Within this channel, the cyclic C6H8 molecule methylcyclo-
pentadiene is accessible by a double H shift followed by a ring
closure, but this route is very unlikely.
The reactions of C2H with cis- and trans-2-butene are very
similar, and a possible reaction mechanism for trans-2-butene is
also shown in Figure 8. The C2H radical adds to the π system
of trans-2-butene to form adduct 4. This adduct subsequently
ejects the largest possible fragment (CH3). Upper limits of 2
3.3.2. C2H + cis-2-Butene. The reaction between cis-2-
butene and C2H yields a reaction product at m/z = 66 (C5H6).
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Figure 8. Possible reaction mechanisms for the reaction between C2H and 1-butene, cis-2-butene, trans-2-butene, and isobutene (u.l. denotes an
upper limit).
and 4% are derived for the formation of C6H8 from H loss and
the formation of C4H4 from C2H3 loss, respectively. These
findings are expected on the basis of the cis-2-butene
measurements and calculations reported above.
3.3.4. C2H + Isobutene. The reaction between C2H and
isobutene exhibits two product channels (m/z = 66 and 80,
Figure 3), and ionization spectra have been acquired for both
reaction products. The photoionization spectrum of the C5H6
species at m/z = 66 is displayed in Figure 11. The signal-to-
noise ratio for this measurement is significantly lower than that
for the other reactions. This is caused by the large absorption
cross section of the reactant, which results in the photolysis of
one-quarter of the reactant species before they react. Moreover,
photolysis of the reactant forms deposits on the MgF2 window
through which the photolysis laser enters the Laval flow. Over
the course of an experiment, this reduces the laser power in the
expansion and hence reduces the number density of C2H
radicals that are formed. Further investigations have shown that
these soot deposits indeed cause the laser power in the system
to decay exponentially over time for the isobutene experiments.
C5H6 isomer 2-methyl-1-buten-3-yne is a likely contributor
to the m/z = 66 ionization spectrum in Figure 11. The
ionization spectrum of the commercially available 2-methyl-1-
buten-3-yne is measured in the Laval nozzle expansion relative
to that of isobutene. Subsequently, the spectra are scaled to the
absolute ionization spectrum of isobutene measured by Wang
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Figure 12. Absolute photoionization spectrum of 2-methyl-1-buten-3-
Figure 9. Photoionization spectrum of the C5H6 reaction product
formed from the C2H + cis-2-butene reaction plotted together with a
fit based on a calculated cis-3-penten-1-yne ionization spectrum.
yne measured in the Laval nozzle expansion (1 MB = 10−18 cm2).
9.1 eV. The simulated spectra of C5H6 isomers cis-3-penten-1-
yne and trans-3-penten-1-yne exhibit a sharp onset around this
photon energy and these species are also expected to contribute
to the measured ionization spectrum. The simulated spectrum
and cross section of trans-3-penten-1-yne taken from Bouwman
et al.52 are used for the analysis. No further attempt is made to
distinguish the two geometrical isomers, and it is concluded
that one or both of the geometrical isomers contribute to this
channel.
Contributions of 2-methyl-1-buten-3-yne and cis- and trans-3-
penten-1-yne to the m/z = 66 spectrum are computed from fits
to the measured data. The fact that the radical density, and
inherently the product signal, decreases because of deposits on
the MgF2 laser entrance window needs to be taken into account
in the fit procedure. To do so, the computed spectrum of trans-
3-penten-1-yne and the acquired spectrum of the 2-methyl-1-
buten-3-yne that are used for the fit are multiplied by an
exponential function that accounts for the measured decay of
the laser power. The resulting fit to the measured ionization
spectrum and the individual contributions of the two scaled
C5H6 isomers are shown in Figure 11. The fit to the data nicely
agrees with the resonances in the measured spectrum that arise
from 2-methyl-1-buten-3-yne. Contributions of (57 15)% for
Figure 10. Photoionization spectrum of the C5H6 product species
formed from the reaction between C2H and trans-2-butene plotted
together with a fit to the data based on a calculated ionization
spectrum of trans-3-penten-1-yne.
2-methyl-1-buten-3-yne and (43
15)% for geometrical
isomers cis- and trans-3-penten-1-yne are derived for the m/z
= 66 mass channel of the C2H + isobutene reaction. The errors
in the branching fractions are displayed at the 1σ level and are
dominated by the computed ionization cross section of cis- and
trans-3-penten-1-yne.
A photoionization spectrum is recorded for the C6H8 (m/z =
80) product formed from the reaction between C2H and
isobutene and is displayed in Figure 13. Also displayed in
Figure 13 is the simulated spectrum of C6H8 isomer 4-methyl-
3-penten-1-yne that is likely formed from the reaction. There is
good agreement between the measured and calculated
ionization onsets. Furthermore, the shape of the measured
spectrum is captured by the calculated spectrum, and this signal
is assigned to 4-methyl-3-penten-1-yne.
Figure 11. Photoionization spectrum of the C5H6 species formed from
the reaction between C2H and isobutene plotted together with a
simulated spectrum of trans-3-penten-1-yne and a measured spectrum
of 2-methyl-1-buten-3-yne that are corrected for the decay of the
photolysis laser (see text).
The branching ratio between the m/z = 66 and 80 channels
for the reaction between isobutene and C2H is also determined.
As described above, because the window is covered by deposits
very rapidly when running isobutene at 193 nm, the time-trace
measurements are performed for both mass channels in quick
succession in the absence and presence of the C2H precursor at
a single photon-energy value. The integrated ion counts
resulting from these traces are subsequently corrected for the
et al.59 The measured absolute photoionization spectrum of 2-
methyl-1-buten-3-yne is displayed in Figure 12. The decrease in
signal beyond a photon energy of 11.2 eV is presumably caused
by dissociative ionization, as is common for hydrocarbons.
The ionization spectrum of 2-methyl-1-buten-3-yne matches
the resonances in the acquired spectrum (Figure 11), but it
does not explain the rather sharp ionization threshold at around
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Figure 13. Photoionization spectrum of the product formed from the
reaction between C2H and isobutene and detected at m/z = 80.
mass-dependent QMS sensitivity and the ionization cross
sections of the C5H6 and C6H8 species. The ionization cross
section of C6H8 is computed using eq 1 and is 51 MB. The
branching fractions derived from these signals are (61 15)%
for the m/z = 66 product and (39 15)% for the m/z = 80
product. The errors in the branching fractions are displayed at
the 1σ level and are dominated by the uncertainty in the
computed ionization cross sections.
Figure 14. Overview of the product branching fractions for C2H
reacting with alkenes up to C4. Only product fractions larger than 15%
are depicted.
A schematic of a possible reaction network for the reaction
between C2H and isobutene is shown in Figure 8. The C2H
radical adds to either carbon of the π system of the reactant to
form adduct species 5 and 6. Adduct 5 undergoes direct CH3
elimination to form the stable product 2-methyl-1-buten-3-yne.
Adduct 6 can lose an H atom to form the stable product 4-
methyl-3-penten-1-yne. A hydrogen from 6 to form adduct 7
needs to be included to explain the detection of cis- and/or
trans-3-penten-1-yne. H-atom shifts followed by H-atom or
CH3-radical elimination could lead to the formation of ring
structures, but no experimental evidence is currently found for
these reaction pathways and an upper limit of 10% is derived
for the formation these species.
The branching fractions reported here for the C2H +
isobutene reaction can be compared to calculated values
reported in the literature.46 From the measurements presented
here, we find that (26 15)% is in the form of cis- and trans-3-
penten-1-yne, (35 15)% is in the form of 2-methyl-1-buten-3-
yne, and (39 15)% is in the form of 4-methyl-3-penten-1-yne.
These values qualitatively agree with calculated branching
fractions of 24% for 4-methyl-3-penten-1-yne, 64% 2-methyl-1-
buten-3-yne (C5H6), and 12% trans-3-penten-1-yne reported
by Woon and Park.46 The difference between the measure-
ments and calculations are possibly caused by the assumption
that is made in the computations that addition to the carbon
atoms in the π system has equal probability. Steric effects may
become more dominant for larger molecular systems, possibly
causing larger discrepancies between the computations and
experiments.
the previous study, it was found that the C2H + ethene reaction
yields solely vinylacetylene (C4H4) through H elimination. For
this reaction, this is the only accessible elimination pathway that
leads to the formation of a stable species. For the C2H +
propene reaction, it was found that methyl loss from the initial
adduct is the dominant channel and only a small fraction (15%)
of the product is in the form of various C5H6 isomers that are
formed through H-atom elimination. It was suggested that the
prevalence of the methyl elimination channel is caused by
either a steric effect in the formation of the adduct or by a H-
atom shift after the adduct is formed.51,52
The dominant exit channel of the C2H + 1-butene reaction is
found to be the loss of the vinyl radical to form (65 10)%
vinylacetylene. The other observed channel for the C2H + 1-
butene reaction is the formation of 4-penten-1-yne through
methyl loss and accounts for (35
10)%. The preferred
formation of vinylacetylene over 4-penten-1-yne can have a
similar origin as in propene reaction. Addition to the
nonterminal carbon of the π system can be preferred, forming
adduct 1 (Figure 8) in larger abundance. Alternatively, an H
shift can occur within adduct 1, giving rise to the preferred
formation of vinylacetylene. Although direct H elimination
from the adduct species is accessible, it is not detected.
The cis- and trans-2-butene reactions could proceed via H
elimination, but the data show that methyl elimination is the
only observed pathway. The methyl-loss channel is also
dominant for the C2H + isobutene reaction, but for this
reaction, a significant fraction proceeds via H elimination. This
is likely caused by the fact that H elimination is the only
pathway to a stable product that is directly accessible from
adduct 6 in Figure 8. The direct H-loss pathway competes with
an H shift that exhibits a barrier that is lower by only 1 kcal/
mol.46
4. DISCUSSION
The experimental results presented here provide pivotal
information for understanding the chemistry in combustion
environments and the photochemistry in cold regions, such as
Titan’s atmosphere. This work adds to previously published
work on the C2H + C2H4 and C2H + C3H6 reactions.52 An
overview of the product branching fractions from the previous
study and the work presented here is shown in Figure 14. From
The branching fractions reported here can be compared to
theoretical work. From a multiple-well treatment, Woon and
Park46 derived branching ratios for the C2H + C4H8 reactions
over a range of temperatures (50−300 K) and pressures (10−2
mbar to 1 bar). For the reaction between C2H and 1-butene
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under temperature and pressure conditions where no adduct
stabilization occurs (e.g., for 50−100 K at ca. 10 mbar and
below), they calculate that 58% is in the form of vinylacetylene
(C4H4) formed by C2H5 elimination with the remaining 42% 4-
penten-1-yne formed by CH3 elimination. For C2H + cis-2-
butene, they predict that the only product is trans-3-penten-1-
yne formed by CH3 elimination from the adduct species.
Finally, from their calculations they predict that the reaction
between C2H and isobutene forms 24% 4-methyl-3-penten-1-
yne (C6H8), 64% 2-methyl-1-buten-3-yne (C5H6), and 12%
trans-3-penten-1-yne (C5H6). They report significant stabiliza-
tion of the adduct species toward higher pressures (e.g., for 50−
100 K at ca. 1 mbar and above). The measured product
branching ratios reported in this work agree qualitatively with
these calculated values.
For the reactions of alkenes of up to C4 with C2H, there is a
propensity to proceed via the elimination of the largest possible
radical fragment from the initially formed adduct. The loss of a
vinyl or methyl radical is thus preferred over the loss of an H
atom, and this is likely caused by the larger bond strength of the
C−H bond compared to that of the C−C bond. These results
imply that molecular growth via C2H is fairly inefficient, and an
important consequence of these reactions is the production of
CH3 and C2H5 radicals. Reactions of these radicals may thus
play a more prominent role in the overall chemistry on Titan.
The C2H + C2H4, C3H6, and C4H8 reactions are included in
the latest comprehensive chemical model describing Titan’s
atmospheric chemistry.3 The C2H + propene reaction was
assumed to proceed via H elimination, but experimentally it
was found to occur via CH3 elimination instead.52 In current
photochemical models, no specific products were assigned to
the C2H + C4H8 reactions. The data presented here can be
directly used in refining chemical models that aim to
understand the photochemistry that occurs in Titan’s
atmosphere. Calculations predict that the branching ratios do
not change significantly toward higher temperatures (300 K),46
and this is likely caused by the fact that the reactions exhibit no
entrance barrier. Hence, the branching fractions reported here
can be cautiously used in models describing combustion
reactions. At higher pressures, as predicted by calculations,
significant stabilization of the adduct may occur.46
C2H and 1-butene is found to yield (65 10)% vinylacetylene
and (35 10)% 4-penten-1-yne. The cis-2-butene and trans-2-
butene reactions yield purely cis-3-penten-1-yne, trans-3-
penten-1-yne, or a combination of these two geometrical
isomers. Finally, the isobutene reaction yields (26 15)% cis-
and trans-3-penten-1-yne, (35 15)% 2-methyl-1-buten-3-yne,
and (39 15)% 4-methyl-3-penten-1-yne.
The bimolecular-reaction rate constants and product
branching ratios reported here provide important input for
chemical models that aim to understand hydrocarbon
chemistries, such as combustion chemistry and the chemistry
in the atmosphere of Saturn’s largest moon, Titan. In Titan’s
atmosphere, reactions between unsaturated hydrocarbons and
hydrocarbon radicals are thought to lead eventually to the
formation of the yellow haze that shrouds the moon. This haze
is composed of large, possibly cyclic and/or aromatic
hydrocarbons. Similarly, in combustion environments the
reactions of C2H radicals are considered to play an important
role in the formation of aromatic species. In the work presented
here, no evidence is found for the formation of cyclic molecules
from butenes reacting with C2H, and an upper limit of 10% is
put on this channel. The feedback of CH3 and C2H5 radicals
indicates that subsequent reactions of these radicals may play a
more prominent role in the overall chemistry in these
environments.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: krwilson@lbl.gov. Tel: +1-510-495-2474.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Advanced Light Source and Chemical Sciences Division
(K.R.W. and S.R.L.) are supported by the Director, Office of
Science, Office of Basic Energy Sciences of the U.S.
Department of Energy under contract no. DE-AC02-
05CH11231 at the Lawrence Berkeley National Laboratory.
K.R.W. and S.R.L. are supported in part by NASA grant no.
NNH13AV43I. Support for J.B. was obtained from the
National Science Foundation Engineering Research Center
for Extreme Ultraviolet Science and Technology. Construction
of this Laval instrument was made possible by a National
Aeronautics and Space Administration Planetary Major Equip-
ment grant. I.R.S. thanks the CNRS for the award of sabbatical
funding during the period of this research and the French
5. CONCLUSIONS
Low-temperature bimolecular-reaction rate constants are
measured for the C2H + 1-butene, cis-2-butene, trans-2-butene,
and isobutene reactions. The rates are quantified at 79 K in a
Laval nozzle expansion by measuring the formation of product
species as a function of time for a set of reactant densities under
pseudo-first-order conditions. The rate constants amount to
Programme National de Planet
́
ologie for financial support. M.F.
ieur et de
thanks the French Ministere de l’Enseignement Super
̀
́
(1.9 0.5) × 10−10, (1.7 0.5) × 10−10, (2.1 0.7) × 10−10
,
la Recherche for a doctoral grant. We thank the France-
and (1.8 0.9) × 10−10 cm3 s−1 for the reactions between C2H
and 1-butene, cis-2-butene, trans-2-butene, and isobutene,
respectively. The rate constants of 1-butene and isobutene
compare well to those measured previously, and the rate
constants for cis- and trans-2-butene are reported for the first
time.
Berkeley Fund for financial support.
REFERENCES
■
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(4) Lavvas, P. P.; Coustenis, A.; Vardavas, I. M. Coupling
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