Bimolecular rate constant and product branching ratio measurements for the reaction of C2H with ethene and propene at 79 K

Article abstract of DOI:10.1021/jp301015b

The reactions of the ethynyl radical (C₂H) with ethene (C ₂H₄) and propene (C₃H₆) are studied under low temperature conditions (79 K) in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by 193 nm photolysis of acetylene (C ₂H₂) and the reactions are studied in nitrogen as a carrier gas. Reaction products are sampled and subsequently photoionized by the tunable vacuum ultraviolet radiation of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The product ions are detected mass selectively and time-resolved by a quadrupole mass spectrometer. Bimolecular rate coefficients are determined under pseudo-first-order conditions, yielding values in good agreement with previous measurements. Photoionization spectra are measured by scanning the ALS photon energy while detecting the ionized reaction products. Analysis of the photoionization spectra yields-for the first time-low temperature isomer resolved product branching ratios. The reaction between C₂H and ethene is found to proceed by H-loss and yields 100% vinylacetylene. The reaction between C₂H and propene results in (85 ± 10)% C₄H₄ (m/z = 52) via CH₃-loss and (15 ± 10)% C₅H₆ (m/z = 66) by H-loss. The C ₄H₄ channel is found to consist of 100% vinylacetylene. For the C₅H₆ channel, analysis of the photoionization spectrum reveals that (62 ± 16)% is in the form of 4-penten-1-yne, (27 ± 8)% is in the form of cis-and trans-3-penten-1-yne and (11 ± 10)% is in the form of 2-methyl-1-buten-3-yne.

Full text of DOI:10.1021/jp301015b

Article
pubs.acs.org/JPCA
Bimolecular Rate Constant and Product Branching Ratio Measurements
for the Reaction of C2H with Ethene and Propene at 79 K
Jordy Bouwman,† Fabien Goulay,†,§ Stephen R. Leone,†,‡ and Kevin R. Wilson*,‡
†Departments of Chemistry and Physics, University of California Berkeley, Berkeley, California 94720, United States
‡Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States
ABSTRACT: The reactions of the ethynyl radical (C2H) with
ethene (C2H4) and propene (C3H6) are studied under low
temperature conditions (79 K) in a pulsed Laval nozzle apparatus.
Ethynyl radicals are formed by 193 nm photolysis of acetylene
(C2H2) and the reactions are studied in nitrogen as a carrier gas.
Reaction products are sampled and subsequently photoionized by
the tunable vacuum ultraviolet radiation of the Advanced Light
Source (ALS) at Lawrence Berkeley National Laboratory. The
product ions are detected mass selectively and time-resolved by a
quadrupole mass spectrometer. Bimolecular rate coefficients are
determined under pseudo-first-order conditions, yielding values in good agreement with previous measurements. Photoionization
spectra are measured by scanning the ALS photon energy while detecting the ionized reaction products. Analysis of the photoionization
spectra yieldsfor the first timelow temperature isomer resolved product branching ratios. The reaction between C2H and ethene
is found to proceed by H-loss and yields 100% vinylacetylene. The reaction between C2H and propene results in (85 10)% C4H4
(m/z = 52) via CH3-loss and (15 10)% C5H6 (m/z = 66) by H-loss. The C4H4 channel is found to consist of 100% vinylacetylene.
For the C5H6 channel, analysis of the photoionization spectrum reveals that (62 16)% is in the form of 4-penten-1-yne, (27 8)% is
in the form of cis- and trans-3-penten-1-yne and (11 10)% is in the form of 2-methyl-1-buten-3-yne.
Over the past few decades, Laval nozzle expansions have been
employed by a number of groups to measure low temperature
bimolecular reaction rate constants of radical-neutral reac-
tions.23−28 Typically, these systems employ laser induced
fluorescence or chemiluminescence to measure the decay rate of
a radical species as a function of reactant density. In this manner,
the bimolecular reaction rate constants for ethene29−31 and
propene30,31 reacting with C2H have been measured experimen-
tally at Titan-relevant temperatures and are found to be near the
collision limit. Based on the lack of pressure dependence, the
slightly negative temperature dependence, and thermodynamic
considerations, it was argued that these reactions proceed via an
addition−elimination mechanism.29−31 Currently, the distribution
of products from these reactions under Titan-relevant conditions
cannot be retrieved via the optically based methods used to
quantify these low temperature rate coefficients.
INTRODUCTION
■
The reactive ethynyl (C2H) radical plays a crucial role in the
complex chemistries of planetary atmospheres, such as that of
Saturn’s largest moon, Titan.1−4 Titan’s atmosphere is cold
(T = 70−180 K) and dense,5 and it consists of mostly nitrogen
(>98%) with trace amounts of hydrocarbons.6 C2H radicals are
formed through the UV photolysis of acetylene (C2H2) by solar
radiation.7,8 Subsequent reactions of C2H with small unsaturated
hydrocarbon species result in molecular growth,9−11 leading to
larger polyynes, aromatic molecules, and aerosols that are thought
to make up the haze that shrouds the moon.12−14 The C2H radical
is also ubiquitous in the interstellar medium15,16 and is considered
to be a central species in the formation of the polycyclic aromatic
hydrocarbons (PAHs) in outflows of carbon rich stars.17,18
Trace amounts of ethene (C2H4) have been detected in Titan’s
stratosphere through mid-IR observations by the Voyager I
mission and later by the Infrared Space Observatory (ISO).19,20
Propene (C3H6) has been identified in Titan’s ionosphere by
Cassini’s Ion Neutral Mass Spectrometer (INMS)21 and is also
expected to be present in Titan’s lower atmosphere, where
photoinduced processes dominate the neutral chemistry.2−4 Both
ethene and propene are predicted to play a key role in the
evolution of the chemical constituents of Titan’s atmosphere.3,4,22
Thus, accurate reaction rate constants and isomer-specific product
distributions of ethene and propene reacting with C2H under
Titan-relevant conditions are needed for accurate modeling of
Titan’s atmospheric chemistry.
Theory predicts that two exit channels are thermodynami-
cally accessible for the reaction between ethene and ethynyl32
‡
C H + C H → C H
4 5
(R1)
2
4
2
‡
C H → C H + H (−2.3 to − 26.5 kcal/mol)
4
5
4 4
(R1a)
Received: January 31, 2012
Revised: March 18, 2012
Published: March 19, 2012
© 2012 American Chemical Society
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Woon and Park34 attempted to predictunder Titan-relevant
conditionsthe product branching ratios of species formed from
reactions between C2H and a set of alkenes, including ethene and
propene. By means of large basis set DFT calculations in con-
junction with a multiple-well treatment, they found that vinyl-
acetylene is the dominant reaction product from C2H reacting
with both ethene and propene. Furthermore, they predict that
the C5H6 species formed from the H-loss channel from C2H
reacting with propene can consist of a set of different isomers.34
Here we present, for the first time, low-temperature isomer
specific product branching ratios measured by mass spectrom-
etry for the C2H radical reacting with ethene and propene.
Reaction products are identified and quantified by modeling the
measured photoionization spectra using absolute photoioniza-
tion spectra from the literature and DFT computed photo-
ionization spectra of possible isomers. Additionally, bimolecular
rate coefficients have been measured for both reactions by
analyzing the time-resolved formation of the product species.
The reaction rate coefficients are compared with previously
measured values. The implications of the branching ratios for
Titan’s hydrocarbon chemistry are highlighted.
‡
C H → C H + C H (−22.2 kcal/mol)
(R1b)
4
5
2 3
2 2
The range of exothermicities associated with the formation of C4H4
denotes the formation of the three C4H4 isomers vinylacetylene,
butatriene and methylenecyclopropene. The C2H3 + C2H2 product
channel R1b is accessible through an addition−elimination process
on the C4H5 potential energy surface. This is found to be highly
improbable, since multiple steps are required.32 Alternatively, this
product channel can be reached directly by hydrogen abstraction.
This pathway is also unlikely to contribute at Titan-relevant
temperatures, since there is a barrier of 1.4 kcal/mol (∼470 K)
associated with it.33 The most probable pathway is the formation
of the C4H4 isomer vinylacetylene R1a, which is exothermic
by 26.5 kcal/mol.32,34
Zhang et al.35 performed a crossed molecular beam study on
the reaction between C2H (X2Σ+) and C2H4 at high collision
energy (E = 4.92 kcal mol−1). They found that vinylacetylene is
the sole product formed from this reaction. Kovac
́
s et al.8
studied the reaction between C2H and C2H4 in a slow flow
excimer laser flash photolysis setup at room temperature and at
pressures between 23 and 81 Torr. They quantified the H-atom
yield by a Vacuum Ultraviolet (VUV) Ly-α laser induced
fluorescence scheme and found that the yield of H-atoms from
this reaction is close to unity, implying that the C4H4 isomer
vinylacetylene is the sole reaction product. No low temperature
product detection data are available for this reaction.
EXPERIMENTAL SECTION
■
Measurements are performed in a pulsed Laval nozzle
apparatus coupled to tunable VUV synchrotron radiation
from the Advanced Light Source (ALS) at Lawrence Berkeley
National Laboratory. The experimental setup has been
described in detail in a previous publication36 and will be
described only briefly here.
Theory predicts that from the reaction between C2H and
propene an energetic C5H7 adduct species is formed without a
barrier. The adduct subsequently isomerizes by H-atom, methyl
(CH3) or C3H5 elimination to form stable reaction products:34
Laval expansion. A Laval nozzle, designed to yield a Mach
4 expansion, is mounted inside a vacuum chamber on a
reservoir block that is filled with gas by two pulsed solenoid
valves. The vacuum chamber is pumped down by a roots
blower and the pressure in the vacuum chamber during Laval
operation is maintained at 145 mTorr by a feedforward-loop
controlled butterfly valve mounted on the intake of the roots
blower. A flow of nitrogen slip gas is maintained to reduce
pressure fluctuations during Laval operation. The system runs
at a repetition rate of 10 Hz with a gas pulse duration of 5 ms.
Acetylene (C2H2, Airgas, stabilized by acetone) is used as the
C2H precursor gas and is passed through a charcoal cartridge
filter to remove the acetone. Nitrogen boil off from a liquid N2
dewar is used as the carrier gas. The reactant gases ethene (C2H4,
Airgas, 99%) and propene (C3H6, Sigma Aldrich, ≥99%) are used
as commercially available. The acetylene, nitrogen, and reactant
gas are supplied to the Laval nozzle through individual calibrated
mass flow controllers. A cylinder is mounted between the mass
flow controllers and the nozzle assembly to ensure good mixing of
the radical precursor, reactant and bath gas.
A calibrated pressure transducer is mounted on the Laval
nozzle reservoir block to monitor the stagnation pressure in the
reservoir during the expansion. The Laval nozzle assembly is
mounted on a stepper-motor-controlled movable linear trans-
lator and the position of the nozzle can be controlled by
means of a LabView program. A second pressure transducer is
mounted in the vacuum chamber and can be manually inserted
into the flow perpendicular to the expanding gas. The resulting
setup allows for accurate determination of both the uniformity
of the expansion as well as the temperature of the expanding
gas as described in detail by Sims et al.37 The temperature is
found to be 79 K with variations smaller than 2 K.
‡
C H + C H → C H
5 7
(R2)
3
6
2
‡
C H → C H + CH (−40.6 kcal/mol)
5 7
(R2a)
4
4
3
‡
C H → C H + H (−14.8 to − 28.0 kcal/mol)
5 7 5 6
(R2b)
‡
C H → C H + C H (−23.4 or − 28.7 kcal/mol)
5 7 3 5 2 2
(R2c)
The ranges of exothermicites for the formation of C5H6 depicts
the formation of the isomers trans-penten-1-yne, cis-3-penten-1-
yne, 2-methyl-1-buten-3-yne, 4-penten-1-yne, and cyclopropy-
lacetylene. The two energies associated with R2c denote the
formation of two isomers of the C3H5 radical. Direct hydrogen
abstraction by C2H from propene is expected to proceed via a
barrier33 and this reaction path is thus unlikely to contribute at
low temperatures. The formation of C2H2 + C3H5 via an
addition−elimination pathway R2c, however, is feasible.
The reaction between C2H and propene has been studied
experimentally by monitoring the H-atoms originating from the
reaction at room temperature.8 From their experiments, Kovac
́
s
et al.8 detected no H-atoms and derived an upper limit of 5% for
the hydrogen loss channel R2a. They point out three possible ex-
planations for the nondetection of hydrogen atoms: (i) Collisional
stabilization of the adduct, (ii) 1,2-hydrogen migration within the
adduct, thereby forming the (CH3)CH2C·HCCH species, is more
rapid than H-elimination from the adduct. Subsequent methyl loss
from the (CH3)CH2−C·H−CCH radical produces C4H4, and (iii)
Direct H-abstraction to form the resonantly stabilized allyl radical.
To date, no product detection or product branching measure-
ments at any temperature are available to test their hypothesis.
Ethynyl radicals (C2H) are formed from the acetylene
precursor by coaxially pulsing an unfocused ArF excimer laser
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(193 nm) through the collimated expansion. This method is
known to yield vibrationally and electronically excited C2H
radicals, but these are rapidly quenched in the expansion.38 The
absorption cross section39 of C2H2 at 193 nm is 2 × 10−19 cm2
and the quantum yield of C2H radicals is unity8 resulting in a
number density of C2H radicals of ∼8 × 1010 cm−3. Number
densities of the reactant species, [R], range from 1 × 1013 cm−3
to 25 × 1013 cm−3, so that the kinetic measurements are
performed under pseudofirst-order conditions ([C2H] ≪ [R]).
Product Detection. Part of the collimated expansion is
sampled through a 450 μm pinhole in a parabolically shaped
airfoil mounted downward of the expansion. During an
experiment, the region after the pinhole sampling is kept at
high vacuum (∼10−7 Torr) by two 2000 L/s turbomolecular
pumps. The molecular beam formed after the pinhole is ionized
with the quasi-continuous tunable radiation from the ALS. The
resulting ions are extracted and mass selectively detected using
a Quadrupole Mass Spectrometer (QMS). The ion counts
measured by the QMS are recorded as a function of time using
a multichannel scaler. For the kinetic measurements, ion counts
vs time are recorded for 5,000 laser pulses to obtain a kinetic
trace at a single reactant density setting. The time resolution of
the kinetic measurements is determined by the QMS response
function. The QMS response function is quantified by
measuring the instantaneous formation of vinyl (C2H3) and
methyl (CH3) radicals upon 193 nm excimer laser photolysis of
propene. The response of the QMS is found to be well
represented by a Gaussian profile with a Full-Width-at-Half-
Maximum (fwhm) of 15 μs.
channeltron efficiency. Assuming that the mass discrimination
can be described by the polynomial:42
2
α = A + Bm + Cm
i
(2)
i
i
with mi the mass of species i, eq 1 can be rewritten as
det
S
(E)
cor
i
2
S
(E) =
= Λ(A + Bm + Cm )
i i
i
σ (E)N
(3)
i
i
where Sicor(E) is the signal corrected for cross sections σi(E)
and isotopic fractions Ni.
In Figure 1, the values of Sicor(E) measured at an ionization
energy of 15 eV and normalized to the value at m/z = 16 are
displayed against mass of methane, krypton and xenon. The
figure shows that the sensitivity curve is fairly flat with slightly
higher sensitivity toward the lower masses, which is common
for quadrupole mass spectrometers.42 The polynomial fit to the
corrected signal is also displayed in Figure 1. This curve is later
The ionizing radiation from the ALS is dispersed in a 3 m
monochromator and a portion of the dispersed light enters the
ionization region through a 600 μm slit, resulting in an energy
resolution of approximately 25 meV. For the measurements of
photoionization spectra described here, the energy of the
ionizing radiation is scanned from ∼8 to 11.3 eV, while
detecting the ion counts in a single mass channel. Ion counts
are typically time binned for 600 laser pulses for each
synchrotron photon energy. Subsequently, the ion counts are
corrected for the photon energy dependent flux of the ALS,
which is measured with a NIST calibrated VUV photodiode.
The mass dependent sensitivity of the quadrupole mass
spectrometer detector is measured, which is essential for
determining the absolute branching ratio for reactions with
multiple product channels. This is done by measuring the
response of the QMS detector to a calibration gas mixture
containing accurate concentrations of species for which the
Figure 1. Ionization cross section and isotopic-fractions-corrected
photoionization signal of methane, krypton and xenon at 15 eV
photon energy, normalized to the signal at m/z = 16.
used to correct for the QMS sensitivity for determining
absolute branching ratios between product channels.
Computational Method. Measured photoionization spec-
tra are modeled with photoionization spectra from the
literature to identify products and to obtain branching ratios.
The overall shape and onset of a photoionization spectrum
of likely reaction products, for which no photoionization data
are available from the literature, are simulated by calculating
oscillator strengths within the Franck−Condon approximation.
To this end, electronic structures are calculated for the ground
state and ionized species with the Gaussian 03 package.43 The
CBS-QB3 method of Petersson and co-workers44,45 is used to
obtain reliable energies, optimized bond distances, force
constants and frequencies. Transition probabilities from the
neutral ground state to the ionized ground state are computed,
including full Duschinsky rotation for all symmetric modes,
with the PESCAL package.46,47 The calculated spectra are
convoluted with the ALS resolution of 25 meV and
subsequently integrated, resulting in photoionization spectra.
Absolute absorption cross sections are estimated using the
method developed by Bobeldijk et al.48
ionization cross sections are well established (CH4, 1%, σ15eV
=
25.4 MB,40 with 1 MB = 10−18 cm2, Kr, 0.5%, σ15eV = 45.0
MB,41 and Xe, 1%, σ15eV = 61.1 MB41). The detected ion counts
Sidet(E) for species i at photon energy E can be written as
det
S
(E) = Λσ (E)N α
i i
(1)
i
i
where Λ is a mass-independent instrument response function,
which includes the geometry of the ionization region, photon
flux and signal averaging. σi(E) is the absolute ionization cross
section of species i as a function of energy, and Ni is the
concentration of the species in the gas expansion corrected
for the fractional natural abundance of i. αi is the mass
discrimination factor of the QMS, which includes the
RESULTS AND DISCUSSION
■
Low Temperature Product Detection. Figure 2A shows
an image of the ion counts detected by the QMS as a function
of mass and time obtained for the reaction between C2H and
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propene. The image is composed of time traces that are
recorded by stepping through the mass channels of the QMS
with a resolution of 0.1 amu. The product species are ionized at
a synchrotron photon energy of 11.25 eV prior to being
detected. Each individual time trace is recorded by time binning
the ion counts for 200 laser pulses. The horizontal line of high
ion counts at t ≈ −40 μs is caused by the photolysis laser and
shows when the reaction is initiated.
obtained by subtracting the integrated ion counts before the
photolysis laser is pulsed from the integrated ion counts in the
kinetic window t = 0 and 150 μs. The resulting mass spectrum
reflects time-dependent products that are formed from chemical
reactions in the Laval expansion. Ionized reaction products are
detected at m/z = 50, 52, and 66.
The product detected at m/z = 50 is diacetylene (C4H2)
formed from the side reaction between C2H radical and its
precursor (C2H + C2H2 → C4H2 + H) and has been a subject
of a previous publication.36 Time dependent products detected
at m/z = 52 (C4H4) and m/z = 66 (C5H6) are produced by the
reaction of C2H with propene (R2a and R2b, respectively). The
time independent signal at m/z = 58 is likely caused by acetone
from the C2H2 cylinder that has not been fully removed by
the charcoal filter.
Figure 2B shows a vertical slice of the image integrated from
m/z = 51.5 to 52.5. The resulting trace depicts the formation of
For the reaction between C2H and ethene, products are
detected in mass channel m/z = 50 (C4H2) and m/z = 52
(C4H4). Similar to the reaction between C2H and propene, the
mass detected at m/z = 50 is attributed to the formation of
diacetylene. The product detected at m/z = 52 originates from
the reaction between C2H and ethene R1a.
Rate Coefficient Determinations. The bimolecular rate
coefficient determinations reported here are made by
monitoring the time-dependent formation of the reaction
products, rather than the decay of the radical species. The
measurements are performed under pseudo-first-order con-
ditions, i.e. the concentration of the radical species is much
lower than that of the reactant species ([C2H] ≪ [R]). The
C2H radical in the expansion is converted by reactions with the
reactant molecule, R, at rate constant k to form reaction
products
k
C H + R → products
(R3)
2
The reaction between the radical and reactant can lead to
the formation of multiple distinct stable products Pm, with
accompanying coproducts Ym such as H-atoms or CH3 radicals
k
m
C H + R ⎯⎯→ P + Y
m
(R4)
2
m
with:
k
= k, and m = 1, 2, 3, ...
∑
m
(4)
m
The formation of product species Pm is now given by
d[P ]
dt
m
= k [C H] [R]
m
2
t
(5)
The branching ratio, BR, of product species, Pm, is defined as
[P ]
k
k
m
k
m
m
BR =
=
=
∑ [P ]
∑ k
m
m
m
(6)
m
Figure 2. Formation of products as a function of time and mass for the
reaction between propene and C2H. (A) The number of ion counts
(grayscale) as a function of time (in μs on the vertical axis) and mass
(in amu on the horizontal axis) recorded at a synchrotron ionization
energy of 11.25 eV. (B) A time trace obtained by slicing the image
vertically from m/z = 51.5 to 52.5. (C) A mass spectrum of the time-
dependent reaction products.
Besides reactions with the reactant species, the radical
undergoes side reactions with for example the radical precursor
molecule (C2H2) or trace amounts of oxygen in the vacuum
chamber. In general, one can write that side reactions with
molecules Mn, excluding the reactant molecule R, at reaction
rate kn form the products Xn and accompanying fragments Wn
the product species at m/z = 52 as a function of time at a
pressure of 145 mTorr and a temperature of 79 K. Figure 2C is
k
n
C H + M → X + W
n
(R5)
2
n
n
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with:
n = 1, 2, 3..., and M ≠ R
(7)
n
The decay of the C2H radical concentration as a function of
time can be expressed by
d[C H]
2
= −[C H](k[R] +
k [M ])
n n
∑
2
dt
(8)
n
with the solution to this differential equation
[C H] = [C H] exp[−(k[R] +
k [M ])t]
n n
∑
2
t
2
0
(9)
n
After substituting eq 9 in eq 5 one can write for the formation
of product Pm
[P ] = Q (1 − exp[−(k[R] +
k [M ])t])
n n
∑
m t
m
(10)
n
with
k [R][C H]
0
m
2
Q
=
m
(k[R] + ∑ k [M ])
n
n
(11)
n
The formation of product species Pm proceeds with the first-
order reaction rate coefficient
k
= k[R] +
k [M ]
n n
∑
1st
Figure 3. Kinetic measurements for the reaction between C2H and
C2H4. (A) Ion counts in mass channel m/z = 52 vs time recorded at a
synchrotron photon energy of 10.2 eV. The red line depicts the best fit
to the data. (B) Values of k1st as a function of reactant density
displayed together with a linear fit to the data.
(12)
n
The first-order rate coefficients, k1st, are measured for a set of
reactant densities [R]. The bimolecular rate coefficient for the
reaction between the radical and the reactant, k, is the slope of
the values of k1st versus the reactant density, [R]. The intercept
of the fit on the y axis reflects the sum of all other loss processes
that are independent of the reactant concentration.
C2H + Ethene. Figure 3A depicts the time dependent
formation of the product species at m/z = 52 for a selected
reactant density. The product species are ionized at a
synchrotron photon energy of 10.2 eV, prior to being detected
in the QMS.
Products formed from side reactions may contribute to
the measured time traces of the product species and their
contributions to the measured mass channels need to be
quantified. Ethene photolysis at 193 nm yields vinyl radicals
(C2H3) and H atoms with a quantum yield of 0.1649
not interfere with the products formed from the reaction under
investigation. Contributions to mass channel m/z = 52 of
products formed from side reactions of residual acetone from
the acetylene cylinder, or acetone photodissociation products,
can also be ruled out.
A fit routine is used for obtaining the bimolecular rate
coefficient and is described here. First, the routine determines
the pre factor Qm (eq 10) by calculating the average ion counts
ranging from 100−125 μs. Subsequently, a fit based on eq 10
is generated with an initial guess for the first-order rate
coefficient, k1*st. Next, the model fit is convoluted with a
Gaussian profile with unit area that accounts for the response
function of the QMS detector. The sum of the residuals of the
convoluted fit and the measured time trace is determined. The
routine is repeated for a set of values for k1*st and the value that
results in the lowest sum of residuals is selected as the best fit.
In Figure 3A, the best fit is plotted together with the measured
time trace at a C2H4 density of 1.68 × 1014 cm−3. The fit
routine is subsequently repeated for the time traces of the other
density settings.
193 nm
C H ⎯⎯⎯⎯⎯⎯⎯⎯→ C H + H Φ = 0.16
(R6)
2
4
2 3
Vinyl radicals produced by C2H4 photolysis can subsequently
react with C2H2 in the expansion to form C4H5 which, after
losing a hydrogen atom, also yields products in mass channel
m/z = 52
‡
C H + C H → C H → C H + H
(R7)
2 3
2 2
4 5
4 4
Figure 3B displays the first-order rate coefficients with
conservative error bars of 3000 s−1 as a function of reaction
density together with a linear fit to the data. The error in values
of k1st originates mainly from the uncertainty in the starting
point (t0) of the reaction, which is obscured by the QMS response.
The absorption cross section of ethene at 193 nm and 140 K
is small50 (σC H = 1 × 10−20 cm2) compared to the C2H2 cross
2
4
section (σC H = 2 × 10−19 cm2) and the resulting number
2
4
density of vinyl radicals is low compared to the number density
of C2H radicals in the flow. Furthermore, the reaction of vinyl
radicals with C2H2 exhibits a barrier to formation of the initial
adduct of 4.8 kcal/mol and reaction rates of vinyl radicals are
typically very slow, even at high temperatures (2.6 × 10−14 cm3 s−1
at T = 630 K).8,51 Thus, side reactions of vinyl radicals will
A bimolecular rate coefficient of (1.3
0.1) × 10−10 cm3s−1
for the reaction between C2H and C2H4 is obtained from
the weighted linear fit in Figure 3B. The error in the
bimolecular rate coefficient depicts the 1σ confidence level.
This rate constant is in good agreement with previously
measured values. Chastaing et al.30 found that the reaction
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between C2H and C2H4 at 112 K using argon as a bath gas
proceeds at a rate of 1.59 0.004 × 10−10 cm3 s−1. Vakhtin
et al.31 found that the bimolecular rate for C2H reacting with
C2H4 is 1.4 0.4 × 10−10 cm3 s−1 at 103 K and using N2 as a
bath gas.
C2H + Propene. Products formed from the reaction between
propene and C2H are detected at m/z = 52 and 66 (Figure 2).
A typical time trace of ion counts detected at m/z = 52 for a
single reactant density setting is displayed in Figure 4A. Similar
mass channels (m/z = 52 and 66) can be ruled out. The time
traces can thus be used to obtain the rate of the reaction between
C2H and propene.
Figure 4B displays the first-order rate coefficients, k1st, as a
function of the reactant density, [C3H6]. The overall change in
reactant density caused by laser photolysis is small (<0.5%) and
hence not considered in further analysis. Also indicated in
Figure 4B is a weighted linear fit to the data. The bimolecular
rate coefficient for the reaction between C2H and propene is
found to be (2.1 0.2) × 10−10 cm3 s−1, with the error in the
bimolecular rate coefficient displaying the 1σ confidence level.
This rate is in good agreement with previously measured
rates. Chastaing et al.30 found that, at 112 K using argon as a
bath gas, the reaction between C2H and C3H6 proceeds at a
rate of 2.35 0.09 × 10−10 cm3 s−1. Vakhtin et al.31 found that
the reaction between C2H and C3H6 proceeds at a rate of 2.4
0.6 × 10−10 cm3 s−1 at 103 K and using N2 as a bath gas.
Product Branching Ratios. In a separate set of experi-
ments, photoionization spectra were recorded by scanning the
photon energy and measuring ion counts at the mass channel
under investigation. The resulting photoionization spectra are
analyzed to obtain information on the isomers formed from the
low temperature chemical reaction.
At a synchrotron photon energy, E, the number of detected
ions in a mass channel, IE, is proportional to the total ionization
cross section (σE), which is given by
I ∝ σ =
χ σ
E,m
m
∑
E
E
(13)
m
where σE,m is the cross section of isomer m at photon energy E
and χm is the molar fraction of isomer m. The sum over the
molar fractions is unity
χ
= 1
∑
m
(14)
m
Equation 13 allows for branching measurements under the
condition that all isomers contributing to a mass channel are
ionized.
C2H + Ethene. The reaction between C2H and C2H4 results
in a sole reaction product at m/z = 52. A photoionization
spectrum of this mass channel, normalized to the number of ion
counts at a photon energy of 10.35 eV is shown in Figure 5.
Figure 4. Kinetic measurements for the reaction between C2H and
C3H6. (A) Time-binned reaction product ion counts in mass channel
m/z = 52 recorded at a synchrotron photon energy of 10.2 eV. The
red line depicts the best fit to the data. (B) A plot of the values of k1st
as a function of reactant density displayed together with a linear fit to
the data.
to the reaction between C2H and ethene, side reactions may
contribute to the observed time traces and need to be
quantified.
Propene has an absorption cross section of 7.6 × 10−19 cm2
at the wavelength of the ArF excimer laser. The quantum yield
of C2H3 radicals from photolysis of propene is 0.3652
193 nm
C H ⎯⎯⎯⎯⎯⎯⎯⎯→ C H + CH Φ = 0.36
(R8)
3
6
2 3
3
The resulting number density of vinyl radicals in the flow is
five times larger than that of the C2H radicals. Reactions of
C2H3 with C2H2 R7 yields C4H4 and can possibly interfere with
the reaction under investigation. The bimolecular rate constant
of the C2H3 radical reacting with C2H2 is very slow51 and a
kinetic model shows that the products formed from the side
reaction do not contribute significantly to the product observed
at m/z = 52. Contributions of side reactions involving residual
acetone or acetone photoproducts to the either of the expected
Figure 5. Photoionization spectrum for mass channel m/z = 52
plotted together with a model fit and the individual contributions of
vinylacetylene and ethene for the C2H + ethene reaction.
The error bars in Figure 5 comprise the shot noise in the
number of detected ion counts (√N).
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There are two ionization thresholds visible, located at ∼9.6
and ∼10.5 eV. An excellent fit to the measured photoionization
spectrum (Figure 5) can be made with contributions of the
absolute photoionization spectra of vinylacetylene and ethene
taken from measurements by Cool et al.53 The model fit and
the individual contributions of ethene and vinylacetylene are
also displayed in the Figure 5.
The detection of a large contribution of ethene (m/z = 28) at
the mass channel of the product species (m/z = 52) is
surprising and could point to poor mass filtering by the QMS.
In this scenario, signals obtained before the photolysis laser is
pulsed could be used to subtract the contribution of the
reactant molecule, which has a number density much larger
than the product species ([R] > 1000[Pm]). However, the
integrated ethene signal measured before the laser is pulsed
accounts for less than 10% of the total ethene contribution to
the photoionization spectrum. Thus, the large majority of the
ion counts in mass channel m/z = 52 that scales with the
ethene photoionization spectrum is likely caused by charge-
transfer ionization between the neutral reaction product and
the ionized reactant molecule, rather than poor mass filtering.
Figure 6. Photoionization spectrum of mass channel m/z = 52 formed
from the reaction between C2H and C3H6 and normalized at a photon
energy of 10.35 eV plotted together with a fit to the data and the
individual contributions of propene and vinylacetylene to the fit.
of vinylacetylene and propene. The fit and the individual
contributions of vinylacetylene and propene are also displayed
in Figure 6. Similar to the reaction between C2H and ethene,
vinylacetylene is the sole C4H4 isomer detected at m/z = 52
and upper limits of 4% are derived for contributions of other
C4H4 isomers. The contribution of the ions that scale with the
reactant ionization curve is much smaller than for the C2H
reacting with ethene. This could be indicative of a smaller
charge transfer cross section for C3H6+ and C4H4.
+
+
C H + C H → C H + C H
(R9)
2
4
4
4
2
4
4
4
If charge transfer causes the reaction product signal to scale
with the reactant ionization cross section, the apparent
contribution of the reactant to the photoionization spectrum
will depend on the charge transfer cross section of the product
species and the reactant. When a kinetic trace is measured
above the ionization threshold of the reactant, such as that
displayed in Figure 4, there is a contribution of ions formed
from charge transfer to the recorded trace. This is not expected
to disturb the rate constant determination, since it only adds a
contribution to the ion counts with the same time dependence.
Modeling efforts are needed to elucidate and possibly reduce
the contribution of charge transfer to the observed signal.
Analysis of the photoionization spectrum (Figure 5) points
to vinylacetylene as the sole reaction product from C2H
reacting with C2H4. An upper limit of 2% is derived for
contributions of the two other C4H4 isomersbutatriene and
methylenecyclopropenethat are thermodynamically accessi-
ble R1a. This is in agreement with earlier findings by Zhang
et al.35, who detected vinylacetylene as the sole product from
high energy collisions between C2H and ethene. Furthermore,
Figure 7A shows a photoionization spectrum measured for
mass channel m/z = 66. The spectrum is corrected for
Kovac
́
s et al. found a hydrogen atom yield of 0.94
0.06,8
which implies that C4H4 is the only reaction product.
Our findings are also in agreement with high level quantum
mechanical calculations by Woon and Park34 and Krishtal
et al.32 Krishtal et al. report that the formation of vinylacetylene
is the dominant channel in this reaction. The formation of
vinylacetylene, which is exothermic by 26.6 kcal/mol, can most
readily occur via direct H-elimination of the initial adduct.
Several other two step and three step processes also lead to the
formation of vinylacetylene. The thermodynamically accessible
formation of C2H2 + C2H3, which has an exothermicity of 22.2
kcal/mol is only feasible via three or more step processes,
which involve 1,3 H-migration or isomerizations with high
barriers. The formation of C2H2 + C2H3 in therefore very
unlikely. Woon and Park34 report similar results.
Figure 7. (A) Photoionization spectrum at m/z = 66 formed from the
reaction between C2H and C3H6 plotted together with a model fit to
the data. (B) Simulated photoionization spectra of the C5H6 isomers
considered in this work plotted together with a measured propene
photoionization spectrum. Isomers labels: (1) trans-3-penten-1-yne
(light blue), (2) cis-3-penten-1-yne (dark blue), (3) 2-methyl-1-buten-
3-yne (orange dash-dotted), and (4) 4-penten-1-yne (purple dashed).
(5) Also displayed in the figure is the photoionization spectrum of the
reactant molecule propene (green dotted). Note: All ionization spectra
in B are normalized at 11.8 eV.
C2H + Propene. Figure 6 shows a photoionization spectrum
at m/z = 52 normalized to the signal at a photon energy of
10.35 eV and displayed together with error bars that indicate
the shot noise in the ion counts. A fit to the photoionization
spectrum is made with the absolute photoionization spectrum
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background counts and plotted together with error bars that
display the shot noise in the ion count measurement (√N). An
ionization onset is visible at a photon energy of 9.1 eV. The
signal at m/z = 66 reflects the formation of C5H6 isomers from
the reaction. Contribution of many C5H6 isomers to the
photoionization spectrum can be ruled out, since the ionization
onset of these species is lower than the onset observed in
Figure 7. The remaining C5H6 isomers that may possibly
contribute to the measured signal are 2-methyl-1-buten-3-yne,
trans-3-penten-1-yne, cis-3-penten-1-yne, and 4-penten-1-yne
and are listed in Table 1 together with their ionization energies
taken from the literature.54
formed from the reaction of C2H and propene. The geometrical
isomers trans- and cis-3-penten-1-yne fit the ionization onset at
9.1 eV and trace the shape of the photoionization spectrum at
energies below 9.3 eV. Trans- and cis-3-penten-1-yne account
for (27 8)% of the C5H6 product. The spectral resolution of
our data is insufficient to discriminate between the two
geometrical isomers. A small contribution of 2-methyl-1-buten-
3-yne results from the fit, but there is a large error associated
with it (11%
10%). No distinct ionization threshold is
observed at 9.9 eV, the onset of 4-penten-1-yne. The quality of
the fit, however, improves significantly when this isomer is
included and the contribution of 4-penten-1-yne is found to be
(62 16)%. All errors reported here indicate the 1σ confidence
level of the fit. Absolute errors in the branching ratios within the
C5H6 channel may be larger, since these branching ratios are based
on simulated ionization spectra and cross sections.
The mass spectrum in the bottom frame of Figure 2 reflects
the branching ratio between the two product channels of the
reaction between C2H and propene (eq 6). As pointed out,
there is a contribution of propene to each of the mass channels
that needs to be subtracted before a branching ratio can be
determined. Additionally, the data are corrected for the
instrument sensitivity according to eq 3 in order to obtain an
accurate branching ratio between the two mass channels. The
reaction between C2H and propene is found to yield (85 10)%
in the m/z = 52 (C4H4) channel and (15 10)% in the m/z =
66 (C5H6) channel. The error bar is rather large, since the signal
at C5H6 is corrected for calculated ionization cross sections,
which have large uncertainties. Additionally, the error in the
contribution of propene to this channel is large. An overview of
the branching ratios of the reaction between propene and C2H is
given in Table 2.
The results presented here can be compared with a recent
theoretical study by Woon and Park.34 A detailed C5H7
potential energy surface based on data taken from their paper
is displayed in Figure 8. The two sites on propene that are
available for C2H addition are the two sp2 carbon atoms. In
their analysis, Woon and Park assume that both addition
channels are equally probable; that is, they do not consider
steric effects. The lowest pathway to form products from the
initial adduct are the direct CH3 elimination from intermediate
1 in Figure 8, or a 1,2-hydrogen shift in reaction intermediate 2
to form intermediate 3 with subsequent CH3 elimination. The
next lowest pathway they report is the formation of 3-penten-1-
yne. This finding is in good agreement with the results
presented here, although they do not distinguish between the
geometrical isomers of 3-penten-1-yne. Somewhat less
favorable is the formation of 4-penten-1-yne, which is observed
in the experiments reported here with an abundance larger than
the two geometrical isomers of 3-penten-1-yne combined. This
could be due to the complication of the analysis caused by the
overlapping ionization spectra of 4-penten-1-yne and propene,
which is reflected in the large error bars. The formation of
C2H2 and C3H5 is slightly less favorable. This channel has not
been observed, because both mass channels are obscured by the
strong signals of the radical precursor and reactant molecule,
respectively. The formation of cyclopropylacetylene has the
largest barrier and is not observed in the experiments reported
here. The formation of 2-methyl-1-buten-3-yne from adduct 1
has to compete with the more favorable CH3 elimination and is
found to have a small contribution (11% 10%) to the measured
photoionization spectrum reported here. From their simu-
lations, no stabilization of the adducts or intermediates is
Table 1. Names, Molecular Structures, Literature Values of
Ionization Energies (from Bieri et al.54), and Calculated
Adiabatic Ionization Energies (CBS-QB3 Method, This
Work) of the C5H6 Isomers Considered Here
Photoionization spectra for the isomers listed in Table 1 are
not available from the literature, so quantum mechanical
calculations are employed to simulate the ionization spectra.
The calculated ionization energies are listed in Table 1 and are
in very good agreement with literature values, where available.
The simulated photoionization spectra normalized at an
ionization energy of E = 11.8 eV, are plotted in Figure 7B.
Also displayed in Figure 7B is the photoionization spectrum of
propene measured in the Laval system normalized to the ion
yield at 11.8 eV.
In absence of absolute ionization cross sections, and in an
attempt to quantify branching ratios from the measured data,
a model by Bobeldijk et al.48 is employed to estimate the ioniza-
tion cross sections of the C5H6 isomers at a photon energy of
11.8 eV. Within the framework of this model, the cross section of
the isomer is computed as the sum of the absorption cross
sections of bonds X−Y in the molecule, according to
σ
=
σ
n
∑
tot
X−Y X−Y
(15)
where σtot is the total cross section of the molecule, σX−Y is the
cross section of bond X−Y, and nX−Y the number of bonds
X−Y in the molecule. All four C5H6 isomers that possibly
contribute to the photoionization spectrum have the same
chemical bonds and their ionization cross sections at 11.8 eV
are evaluated to be 45 × 10−18 cm2.
A model fit is made to the measured photoionization
spectrum and is displayed in Figure 7A. The onset observed at
9.6 eV is fit by a contribution from the reactant molecule,
propene, which accounts for a large fraction (52% 10%) of
the signal observed at m/z = 66. The remaining (48 10)% of
the ionization spectrum is fit with C5H6 isomers that are
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Table 2. Overview of the Low Temperature Product Branching Ratios Measured for the Reaction between C2H and C3H6
a
No discrimination is made between cis- and trans-3-penten-1-yne and the value depicts the sum of the two geometrical isomers.
Figure 8. Schematic of the C5H7 potential energy surface based on data taken from Woon and Park.34 Energies are indicated in kcal/mol relative to
the reactants.
expected under the experimental conditions reported here and
they predict a yield of ∼95% for the formation of vinyl-
acetylene, which is in agreement with this work (85% 10%).
The reaction between propene and C2H has been studied
at room temperature by measuring the H-atom yield by VUV Ly-α
than H-elimination and subsequently methyl radicals will be
preferentially produced from the (CH3)CH2−C·H−CCH radical
(iii) Direct H-abstraction to form the resonantly stabilized allyl
́
radical. Additionally, Kovacs et al. put their measurements in
perspective by comparing their results to measurements on the
laser induced fluorescence.8 Kovac
́
s et al. do not detect H-atoms
isoelectronic CN + propene reaction.55 Trevitt et al.55 reported
and put an upper limit of 5% for the hydrogen elimination
channel. They postulated three reasons for this result: (i) Colli-
sional stabilization of the adduct (ii) 1,2-hydrogen migration form-
ing the more stable (CH3)CH2−C·H−CCH radical is more rapid
that the H-atom loss channel comprises (41
reaction products.55 The similarities between the C2H and CN
reacting with ethene led Kovacs et al. to conclude that it is
10)% of the
́
possible, but unlikely, that 1,2-hydrogen migration is more
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dominant in the C2H + propene system than in the CN +
propene system.
The reaction between C2H and C2H4 is found to yield
exclusively vinylacetylene via H-atom elimination from the
initially formed energetic C4H5 adduct. Upper limits of 2% are
derived for the energetically accessible isomers butatriene and
methylenecyclopropene. This is in agreement with studies
performed at room temperature and at high collision energy.8,35
Considering these experiments and the work presented here,
we conclude that the formed reaction products do not change
with temperature.
The products formed from the reaction between C2H and
C3H6 are studied for the first time. An initial energetic C5H7
adduct forms stable products via two reaction channels; CH3
loss leading to the formation of vinylacetylene, and H atom
elimination leading to the formation of C5H6 isomers. The low
temperature product branching between these two channels is
found to be (85 10)% for the CH3 loss channel and (15 10)%
for the H loss channel. The C4H4 channel is found to consist of
vinylacetylene only, with upper limits of 4% on contributions by
other C4H4 isomers.
The formation of C4H4 is identified as the dominant channel
(85% 10%) for the C2H + propene reaction. This observation
can be explained twofold. First, a steric effect can result in a
preferred addition site for the C2H radical, to form adduct 1,
which preferably loses a methyl group to form C4H4. Second,
as postulated by Kovac
́
s et al.,8 and calculated by Woon and
Park34 the 1,2-hydrogen shift to form adduct 3 can be
competitive with the accessible H-elimination pathways.
Elimination of a methyl group after the 1,2- hydrogen shift
results in the formation of vinylacetylene as the main channel.
The detection of (15
reported here versus the upper limit of 5% derived by Kovac
et al. can be caused by the pressure difference between the two
experiments. The measurements reported by Kovacs et al.
10)% in the C5H6 product channel
́
s
́
are performed at a pressure of 50 Torr, which is significantly
higher than the pressure in the Laval expansion reported here
(145 mTorr). The higher pressure can result in collisional
stabilization of the C5H7 adduct and can lead to a smaller
quantity of detectable H-atoms.
The relative branching for the C5H6 isomers is also
determined based on photoionization spectrum measurements
and simulations. The isomer 4-penten-1-yne is found to be the
main contributor and accounts for (62 16)% of the C5H6
channel. A branching ratio of (27 8)% is derived for the two
geometrical isomers cis- and trans-3-penten-1-yne, while the
isomer 2-methyl-1-buten-3-yne is found to have a minor
Implications for Titan’s Photochemistry. Vinylacetylene
is the dominant product of the reactions of ethene and propene
with C2H. The reaction of vinylacetylene with C2H has recently
been studied in a combined experimental theoretical work.56
On the basis of their calculations, Zhang et al.56 predict that
the reaction occurs via barrierless addition of C2H to vinyl-
acetylene, forming a C6H5 adduct. The barrierless addition
implies that the reaction will be fast, also at Titan-relevant
temperatures. The C6H5 adduct subsequently loses a hydrogen
atom and the most exothermic product channel of this reaction
is the formation of the C6H4 isomer o-benzyne. Zhang et al.56
identified vinyldiacetylene, hexa-3-ene-1,5-diyne, and o-benzyne
as reaction products, but could not determine accurate
branching ratios. Their calculations predict a branching of
∼10% for the formation of o-benzyne at low collision energy,
when the C2H radical adds to either the terminal ethylenic or
middle ethylenic carbon of vinylacetylene. Accurate branch-
ing ratio measurements at low temperatures are needed to
confirm the formation of ortho-benzyne under Titan-relevant
conditions.
The characteristic yellow haze that shrouds Titan consists of
PAHs and large hydrocarbon species that are thought to be formed
by photochemical reactions.3 In combustion environments,
o-benzyne is thought to play a role in the formation of PAHs
and soot.57,58 Similarly, o-benzyne can play a role in the formation
of PAHs in low temperature environments. The reactions of C2H
with the alkenes ethene and propene and the subsequent reactions
of vinylacetylene with C2H may thus impact our understanding of
the formation of haze in Titan’s cold atmosphere.
contribution of (11
10)%. No measurements are available
from the literature to ascertain the temperature dependence of
these branching ratios.
Reactions between the small unsaturated hydrocarbons
ethene and propene, and the ethynyl radical are important in
the chemical evolution of the cold atmosphere of Saturn’s
largest moon, Titan. The results presented here unambiguously
identify vinylacetylene as the dominant reaction product at low
temperatures. The derived branching ratios can be directly used
in models that predict the chemical evolution of the Titan’s
atmosphere.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: krwilson@lbl.gov.
Present Address
§Department of Chemistry, West Virginia University, Morgan-
town, West Virginia 26506, United States.
Notes
The authors declare no competing financial interest.
CONCLUSIONS
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. The support of personnel
(J.B. and F.G.) for this research by the National Aeronautics and
Space Administration (Grant No. NNX09AB60G is gratefully
acknowledged. Support for J.B. was also obtained from the
National Science Foundation Engineering Research Center for
Extreme Ultraviolet Science and Technology. The authors would
Bimolecular rate coefficients have been obtained by measuring
the time-resolved formation of product species from the reaction
between C2H and C2H4, and C3H6 at 79 K. The measured
rate constants (kC H (1.3 0.1) × 10−10 cm3 s−1 and kC H (2.3
2
4
3
6
0.2) × 10−10 cm3 s−1) are in very good agreement with pre-
viously measured values based on the decay rate of the
radical species. Furthermore, the product branching ratios for the
title reactions have been measured for the first time at low
temperature.
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(36) Soorkia, S.; Liu, C.-L.; Savee, J. D.; Ferrell, S. J.; Leone, S. R.;
Wilson, K. R. Rev. Sci. Instrum. 2011, 82, 124102.
(37) Sims, I. R.; Queffelec, J.-L.; Defrance, A.; Rebrion-Rowe, C.;
Travers, D.; Bocherel, P.; Rowe, B. R.; Smith, I. W. M. J. Chem. Phys.
1994, 100, 4229−4241.
like to thank Dr. John D. Savee (Sandia National Laboratory) for
the many useful discussions.
REFERENCES
■
(1) Yung, Y. L.; Allen, M.; Pinto, J. P. Astrophy. J. Suppl. Ser. 1984, 55,
465−506.
(38) Sander, R. K.; Tiee, J. J.; Quick, C. R.; Romero, R. J. J. Chem.
Phys. 1988, 89, 3495−3501.
(2) Wilson, E. H.; Atreya, S. K. J. Geophys. Res. 2004, 109.
(3) Lavvas, P.; Coustenis, A.; Vardavas, I. Planet. Space Sci. 2008, 56,
27−66.
(39) Benilan, Y.; Smith, N.; Jolly, A.; Raulin, F. Planet. Space Sci.
2000, 48, 463−471.
(40) Samson, J. A. R.; Haddad, G. N.; Masuoka, T.; Pareek, P. N.;
Kilcoyne, D. A. L. J. Chem. Phys. 1989, 90, 6925−6932.
(41) Samson, J. A. R.; Stolte, W. C. J. Electron Spectrosc. Relat.
Phenom. 2002, 123, 265−276.
(42) Wood, K. V.; Grange, A. H.; Taylor, J. W. Anal. Chem. 1978, 50,
1652−1654.
(43) Frisch, M. J. et al. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
(44) Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson,
G. A. J. Chem. Phys. 1999, 110, 2822−2827.
(45) Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson,
G. A. J. Chem. Phys. 2000, 112, 6532−6542.
(4) Krasnopolsky, V. A. Icarus 2009, 201, 226−256.
(5) Fulchignoni, M.; et al. Nature 2005, 438, 785−791.
(6) Clark, R. N. et al. J. Geophys. Res. 2010, 115.
(7) Seki, K.; Okabe, H. J. Phys. Chem. 1993, 97, 5284−5290.
́
(8) Kovacs, T.; Blitz, M. A.; Seakins, P. W. J. Phys. Chem. A 2010,
114, 4735−4741.
(9) Stahl, F.; v. R. Schleyer, P.; H. F., S. III; Kaiser, R. I. Planet. Space
Sci. 2002, 50, 685−692.
(10) Goulay, F.; Osborn, D. L.; Taatjes, C. A.; Zou, P.; Meloni, G.;
Leone, S. R. Phys. Chem. Chem. Phys. 2007, 9, 4291−4300.
(11) Soorkia, S.; Trevitt, A. J.; Selby, T. M.; Osborn, D. L.; Taatjes,
C. A.; Wilson, K. R.; Leone, S. R. J. Phys. Chem. A 2010, 114, 3340−
3354 PMID: 20141190.
(46) Ervin, K. M. PESCAL, Fortran program, 2010.
(47) Ervin, K. M.; Ramond, T. M.; Davico, G. E.; Schwartz, R. L.;
Casey, S. M.; Lineberger, W. C. J. Phys. Chem. A 2001, 105, 10822−
10831.
(12) Jones, B. M.; Zhang, F.; Kaiser, R. I.; Jamal, A.; Mebel, A. M.;
Cordiner, M. A.; Charnley, S. B. Proc. Natl. Acad. Sci. U.S.A. 2011, 108,
452−457.
(48) Bobeldijk, M.; van der Zande, W. J.; Kistemaker, P. G. Chem.
(13) Mebel, A. M.; Kislov, V. V.; Kaiser, R. I. J. Am. Chem. Soc. 2008,
130, 13618−13629.
Phys. 1994, 179, 125−130.
(49) Borrell, P.; Cervenka, A.; Turner, J. W. J. Chem. Soc. B 1971,
2293−2298.
(14) Lavvas, P.; Sander, M.; Kraft, M.; Imanaka, H. Astrophys. J. Lett.
2011, 728, 80−+.
(50) Wu, C. Y. R.; Chen, F. Z.; Judge, D. L. J. Geophys. Res. 2004,
109.
(15) Tucker, K. D.; Kutner, M. L.; Thaddeus, P. Astrophys. J. Lett.
1974, 193, L115−L119.
(51) Knyaev, V. D.; Stoliarov, S. I.; Slagle, I. R. Symp. (Int.) Combust.
1996, 26, 513−519.
(16) Beuther, H.; Semenov, D.; Henning, T.; Linz, H. Astrophys. J.
Lett. 2008, 675, L33−L36.
(52) Orkin, V. L.; Huie, R. E.; Kurylo, M. J. J. Phys. Chem. A 1997,
101, 9118−9124.
(17) Allamandola, L. J.; Tielens, A. G. G. M.; Barker, J. R. Astrophys. J.
Lett. 1985, 290, L25−L28.
(53) Cool, T. A.; Wang, J.; Nakajima, K.; Taatjes, C. A.; Mcllroy, A.
Int. J. Mass Spectrom. 2005, 247, 18−27.
(18) Frenklach, M.; Feigelson, E. D. Astroph. J. 1989, 341, 372−384.
́
(19) Coustenis, A.; Bezard, B. Icarus 1995, 115, 126−140.
(54) Bieri, G.; Burger, F.; Heilbronner, E.; Maier, J. P. Helv. Chim.
Acta 1977, 60, 2213−2233.
(20) Coustenis, A.; Salama, A.; Schulz, B.; Ott, S.; Lellouch, E.; h.
Encrenaz, T.; Gautier, D.; Feuchtgruber, H. Icarus 2003, 161, 383−
403.
(55) Trevitt, A. J.; Soorkia, S.; Savee, J. D.; Selby, T. S.; Osborn,
D. L.; Taatjes, C. A.; Leone, S. R. J. Phys. Chem. A 2011, 115, 13467−
13473.
(21) Waite, J. H.; Young, D. T.; Cravens, T. E.; Coates, A. J.; Crary,
F. J.; Magee, B.; Westlake, J. Science 2007, 316, 870−875.
(22) Lavvas, P.; Coustenis, A.; Vardavas, I. Planet. Space Sci. 2008, 56,
67−99.
(56) Zhang, F.; Parker, D.; Kim, Y. S.; Kaiser, R. I.; Mebel, A. M.
Astrophys. J. 2011, 728, 141.
(57) Friedrichs, G.; Goos, E.; Gripp, J.; Nicken, H.; Schoenborn,
J.-B.; Vogel, H.; Temps, F. Z. Phys. Chem. 2009, 223, 387−407.
(58) Comandini, A.; Brezinsky, K. J. Phys. Chem. A 2012, in press.
(23) Rowe, B. R.; Dupeyrat, G.; Marquette, J. B.; Gaucherel, P.
J. Chem. Phys. 1984, 80, 4915−4921.
(24) Dupeyrat, G.; Marquette, J. B.; Rowe, B. R. Phys. Fluids 1985,
28, 1273−1279.
(25) Atkinson, D. B.; Smith, M. A. J. Phys. Chem. 1994, 98, 5797−
5800.
(26) Atkinson, D. B.; Jaramillo, V. I.; Smith, M. A. J. Phys. Chem. A
1997, 101, 3356−3359.
(27) Lee, S.; Hoobler, R. J.; Leone, S. R. Rev. Sci. Instrum. 2000, 71,
1816−1823.
(28) Nizamov, B.; Leone, S. R. J. Phys. Chem. A 2004, 108, 1746−
1752.
(29) Opansky, B. J.; Leone, S. R. J. Phys. Chem. 1996, 100, 19904−
19910.
(30) Chastaing, D.; L. James, P.; R. Sims, I.; W. M. Smith, I. Faraday
Discuss. 1998, 109, 165−181.
(31) Vakhtin, A. B.; Heard, D. E.; Smith, I. W. M.; Leone, S. R. Chem.
Phys. Lett. 2001, 348, 21−26.
(32) Krishtal, S. P.; Mebel, A. M.; Kaiser, R. I. J. Phys. Chem. A 2009,
113, 11112−11128.
(33) Nguyen, H. M. T.; Chandra, A. K.; Carl, S. A.; Nguyen, M. T.
J. Mol. Struct. Theochem 2005, 732, 219−224.
(34) Woon, D. E.; Park, J.-Y. Icarus 2009, 202, 642−655.
(35) Zhang, F.; Kim, Y. S.; Kaiser, R. I.; Krishtal, S. P.; Mebel, A. M.
J. Phys. Chem. A 2009, 113, 11167−11173 PMID: 19522519..
3917
dx.doi.org/10.1021/jp301015b | J. Phys. Chem. A 2012, 116, 3907−3917