Low-temperature rate coefficients of C2H with CH4 and CD4 from 154 to 359 K

Article abstract of DOI:10.1021/jp9532677

Rate coefficients for the reaction C₂H + CH₄ → C₂H₂ + CH₃ and C₂H + CD₄ → C₂HD + CD₃ are measured over the temperature range 154-359 K using transient infrared laser absorption spectroscopy. Ethyl radicals are produced by pulsed the laser photolysis of C₂H₂ in a variable temperature flow cell, and a tunable color center laser probes the transient removal of C₂H (X²Σ⁺ (0,0,0)) in absorption. The rate coefficients for the reactions of C₂H with CH₄ and CD₄ both show a positive temperature dependence over the range 154-359 K, which can be expressed as k_CH(4) = (1.2±0.1) × 10^-11 exp[(-149±12)/T] and k_CD(4) = (8.7±1.8) × 10^-12 exp[(-650±61)/T] cm³ molecule^-1 s^-1, respectively. The reaction of C₂H + CH₄ exhibits a significant kinetic isotope effect at 300 K of k_CH(4)/k_CD(4) = 2.5±0.2. Temperature dependent rate constants for C₂H + C₂H₂ were also remeasured over an increased temperature range from 143 to 359 K and found to show a slight negative temperature dependence, which can be expressed as k_C(2)H(2) = 8.6 × 10^-16 T^1.8 exp[(474±90)/T] cm³ molecule^-1 s^-1.

Full text of DOI:10.1021/jp9532677

4888
J. Phys. Chem. 1996, 100, 4888-4892
Low-Temperature Rate Coefficients of C2H with CH4 and CD4 from 154 to 359 K
Brian J. Opansky and Stephen R. Leone*,†
JILA, National Institute of Standards and Technology and UniVersity of Colorado, Department of Chemistry
and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0440
ReceiVed: NoVember 6, 1995; In Final Form: January 2, 1996X
Rate coefficients for the reaction C2H + CH4 f C2H2 + CH3 and C2H + CD4 f C2HD + CD3 are measured
over the temperature range 154-359 K using transient infrared laser absorption spectroscopy. Ethynyl radicals
are produced by pulsed laser photolysis of C2H2 in a variable temperature flow cell, and a tunable color
center laser probes the transient removal of C2H (X2Σ+ (0,0,0)) in absorption. The rate coefficients for the
reactions of C2H with CH4 and CD4 both show a positive temperature dependence over the range 154-359
K, which can be expressed as kCH ) (1.2 ( 0.1) × 10-11 exp[(-491 ( 12)/T] and kCD ) (8.7 ( 1.8) ×
4
4
10-12 exp[(-650 ( 61)/T] cm3 molecule-1 s-1, respectively. The reaction of C2H + CH4 exhibits a significant
kinetic isotope effect at 300 K of kCH /kCD ) 2.5 ( 0.2. Temperature dependent rate constants for C2H +
4
4
C2H2 were also remeasured over an increased temperature range from 143 to 359 K and found to show a
slight negative temperature dependence, which can be expressed as kC H ) 8.6 × 10-16 T1.8 exp[(474 (
2
2
90)/T] cm3 molecule-1 s-1.
Introduction
This work is part of an ongoing project to measure, for the
first time, low-temperature rate constants of the C2H radical
with various hydrocarbons present in Titan’s atmosphere. In
planetary atmospheres, such as Titan’s, the ethynyl radical can
catalyze the dissociation of CH4 to form methyl radicals, CH3.1,2
C2H2 + hν f C2H + H
(1)
(2)
(3)
C2H + CH4 f C2H2 + CH3
net
CH4 f CH3 + H
The recombination of two methyl radicals produces ethane,
C2H6, which can be transported downward to the moon’s
surface. Hence, the reaction C2H + CH4 is of central
importance in photochemical models of Titan to understand why
ethane is so abundant in Titan’s atmosphere.1,2
Figure 1. Schematic of the experimental setup.
these results provide an experimental basis for theoretical studies
on primary isotope effects.
This is the first low-temperature study of the reactions
C2H + CH4(CD4) f C2H2(C2HD) + CH3(CD3) (4)
Experimental Technique
Briefly, the kinetics of C2H + CH4 and CD4 are studied using
transient infrared laser absorption spectroscopy. A schematic
of the experimental setup is shown in Figure 1. The low-
temperature kinetics measurements are described in detail in
an earlier paper.6 The only significant change was to add
Brewster windows to the flow cell to incorporate a multipass
arrangement for the probe laser beam to increase the absorption
path length.
Ethynyl radicals are produced in the meter long variable
temperature flow cell by a pulsed excimer laser at 193 nm. The
excimer laser was operated at 55 mJ/pulse at a repetition rate
of 10 Hz. Acetylene has an absorption cross section of 1.35 ×
10-19 cm2 at 193 nm7 and a quantum yield of approximately
0.26 for C2H production.8 After accounting for UV laser loss
on the windows and absorption in air, the C2H concentration
was calculated to be no greater than 4.6 × 1010 cm-3 for the
highest acetylene pressures (the acetylene number density was
over the temperature range 154-359 K. A complete study of
the reaction C2H + C2H2 from 170 to 350 K, another critical
reaction in Titan’s atmosphere, was already reported by this
laboratory;3 here, these measurements have also been extended
to 143-359 K. Low-temperature rate constants of reaction 4
will help decide which reaction schemes for ethane production
are consistent with data from Voyager IRIS (infrared interfer-
ometer spectrometer) results.4,5 In addition, NASA plans to
launch a mission called Cassini, which is intended to study
Saturn and its moon Titan some time in 1996. The rate
coefficients measured in this experiment will be used to model
Titan’s atmosphere to compare calculated gas densities with
measured concentrations from the Cassini mission. In addition,
† Staff Member, Quantum Physics Division, National Institute of
Standards and Technology.
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-4888$12.00/0 © 1996 American Chemical Society

Low-Temperature Rate Coefficients of C2H with CH4 and CD4
J. Phys. Chem., Vol. 100, No. 12, 1996 4889
home-built scanning Michelson interferometer wavemeter10 is
used to monitor the color center’s wavelength.
The probe beam, after three to five multipasses, is directed
onto a 50-MHz 77 K Ge:Au detector which has a 20-mm2
sensitive area. The transient signals are amplified and then
coadded using a 100-MHz digital oscilloscope. Typical single-
shot traces have a signal-to-noise ratio of 20-30. For a typical
run, transient signals from 1000 excimer pulses are averaged.
The amplitude of the transient C2H signal is found to be linear
with color center probe power.
In all experiments, acetylene, helium, and methane or
methane-d4 were flowed through a mixing cell before entering
the reaction cell. Helium is used to thermally equilibrate the
mixture with the cell walls. In earlier experiments, sulfur
hexafluoride, SF6, was used to vibrationally and electronically
quench the C2H;3,6 this was omitted from this study because it
was found to have no effect on the relatively slow ground-state
removal rates, as discussed below. All gases are obtained
commercially with the following purities: He, 99.99%; C2H2,
99.6%; CH4, 99.99%; CD4, 99.6%. The acetone in the C2H2 is
removed by passing the gas through an activated charcoal filter.
Partial pressures of each gas are determined by calibrated mass
flow meters and the measured total pressure inside the cell. With
the use of isopentane as a cooling solvent, temperatures as low
as 143 K can be reached. For measurements taken above 300
K, heated water was used as the solvent.
Analysis of Kinetic Data
In this study we observe reactions from the ground state of
C2H (X2Σ+ (0,0,0)) directly. For accurate measurements it is
necessary that the electronic and vibrationally excited states of
C2H be fully quenched. Work done by Glass et al. under similar
conditions reports that relaxation of the C2H (X(0,0,1)) state
occurs in approximately 1 µs.9 Therefore, if sufficient time has
elapsed, complete vibrational and electronic relaxation should
have occurred before any ground-state measurements are made.
The data is fit only beginning with times a factor of 3 longer
than the rise time to ensure complete relaxation of upper
vibrational states of C2H. In previous studies SF6 was used as
a vibrational quencher.3,6 Temperature dependent measurements
with and without SF6 were taken, and the measured rate
constants were found to be equal. Therefore, SF6 was not used
for this study.
Figure 2. (a) Decay of C2H with C2H2. The solid line is the fitted
single-exponential decay. (b) Residuals to the fit. The oscillations at
lower frequencies are due to the noise from the color center laser.
in the range (1.6-6.4) × 1014 cm-3 in the CH4 (CD4)
experiments). For these experiments a 1000-1500-fold excess
of CH4 (CD4) with respect to C2H2 is always present. Contribu-
tions from secondary or radical-radical reactions can be
neglected since the time between collisions for C2H and CH4
(CD4) is 1000 times shorter than the time between collisions of
two C2H radicals. The transverse flow arrangement in the cell
allows high laser repetition rates with minimal photolysis of
the same gas volume. With typical linear flow rates of 1.9 ×
1020 molecules s-1, about 90% of which is helium, the photolysis
volume is replenished every 10 laser pulses. Kinetic experi-
ments are performed at various methane (methane-d) densities
((0.2-2.5) × 1017 cm-3). The total helium density is in the
range (0.4-2.4) × 1018 cm-3, and the methane density is held
at 0.7 × 1017 cm-3 to test for a pressure dependence. In the
experiments involving just the reaction C2H + C2H2, the
acetylene number density was in the range (0.2-6.0) × 1015
cm-3, and the helium number density was in the range (0.32-
3.2) × 1018 cm-3. For the highest acetylene pressures, the C2H
Prior to measuring rate coefficients for the reaction C2H +
CH4 (CD4), an accurate temperature dependence investigation
of the rate constant for
C2H + C2H2 f C4H2 + H
(5)
had to be completed. Earlier measurements could only be made
down to 170 K, and here it was desired to extend the results to
at least 143 K.3 The experiments were done under pseudo-
first-order conditions where [C2H2] . [C2H] by a factor of 500-
1500. The rate equation for reaction 5 integrates to
concentration is estimated to be no greater than 6.7 × 1011 cm-3
.
[C2H]t ) [C2H]0 exp(-kobst)
(6)
Therefore, a 300-9000-fold excess of C2H2 with respect to C2H
is always present. Contributions from radical-radical reactions
can be neglected since the time between collisions for two C2H
radicals is 1000 times longer at the highest C2H density than
the time between collisions for C2H and C2H2.
where
kobs ) kC H [C2H2]
(7)
2
2
The observed rate coefficients, kobs, are calculated by fitting
the observed decay traces to a single-exponential decay plus a
constant, eq 8, to fit the zero level of the base line
A high-resolution color center laser tuned to the Q11(9) line
at 3593.68 cm-1 of the A 2Π-X 2Σ transition probes the
transient concentration of ethynyl radical in absorption.9
A
scanning Fabry-Perot spectrum analyzer is used to ensure that
the color center is running on one longitudinal mode, and a
y ) A exp(-kobst) + constant
(8)

4890 J. Phys. Chem., Vol. 100, No. 12, 1996
Opansky and Leone
TABLE 1: Summary of Rate Constants for C2H + C2H2
from 143 to 359 K
temp
(K)
kC H
temp
(K)
kC H
2 2
2
2
(cm3 molecule-1 s-1
)
(cm3 molecule-1 s-1
)
359a
350b
296a
295b
273b
263b
256b
245b
242b
239b
233b
232b
225b
223b
223b
213b
(1.3 ( 0.1) × 10-10
(1.2 ( 0.2) × 10-10
(1.4 ( 0.1) × 10-10
(1.3 ( 0.2) × 10-10
(1.2 ( 0.2) × 10-10
(1.2 ( 0.2) × 10-10
(1.3 ( 0.2) × 10-10
(1.3 ( 0.2) × 10-10
(1.2 ( 0.2) × 10-10
(1.3 ( 0.2) × 10-10
(1.2 ( 0.2) × 10-10
(1.3 ( 0.2) × 10-10
(1.2 ( 0.2) × 10-10
(1.3 ( 0.2) × 10-10
(1.2 ( 0.2) × 10-10
(1.4 ( 0.2) × 10-10
211b
207b
201b
199b
197b
193b
193b
191a
188b
185b
170b
163a
155a
153b
143a
(1.3 ( 0.2) × 10-10
(1.3 ( 0.2) × 10-10
(1.3 ( 0.2) × 10-10
(1.3 ( 0.2) × 10-10
(1.4 ( 0.1) × 10-10
(1.3 ( 0.2) × 10-10
(1.3 ( 0.2) × 10-10
(1.5 ( 0.1) × 10-10
(1.2 ( 0.2) × 10-10
(1.4 ( 0.2) × 10-10
(1.4 ( 0.2) × 10-10
(1.8 ( 0.2) × 10-10
(1.6 ( 0.2) × 10-10
(1.8 ( 0.2) × 10-10
(1.9 ( 0.2) × 10-10
a Measurements in this work. b Measurements made in a previous
study.3
Figure 3. Plot of observed C2H removal coefficient kobs versus C2H2
concentration. These data were taken at 155 K, and kC H2 for this plot
2
is (1.6 ( 0.2) × 10-10 cm3 molecule-1 s-1
.
where A is the pre-exponential factor, t is the time, and kobs is
the observed rate coefficient; see Figure 2. The fits for kobs are
then plotted against their respective [C2H2] concentrations, and
the gradient is kC H ; see Figure 3. The uncertainty in kC H is
2
2
2 2
calculated on the basis of the uncertainties in fitting kobs
,
measuring [C2H2], and measuring the temperature. This leads
to relative errors in kC H of 10-13%, depending on the
2
2
temperature.
Table 1 is a summary of the rate constants measured for C2H
+ C2H2 down to 143 K. The data from ref 3 and the new data
can be fit approximately to the Arrhenius equation. However,
the data is best fit by the equation kC H ) 8.6 × 10-16 T1.8
2
2
exp[(474 ( 90)/T] cm3 molecule-1 s-1; see Figure 4.
All experiments involving methane were performed under
pseudo-first-order conditions in which [CH4], [CD4], and [C2H2]
. [C2H]. The rate of change of [C2H] can be expressed as
Figure 4. Arrhenius plot of the experimental data for C2H + C2H2;
(b) this work; (O) data taken from ref 2. The fit to all the data is
d[C2H]/dt ) -[C2H](kCX [CX4] + kC H [C2H2]) (9)
4
2 2
given by 8.6 × 10-16 T1.8 exp[(474 ( 90)/T] cm3 molecule-1 s-1
.
where CX4 ) CH4 or CD4. After integration
[C2H]t ) [C2H]0 exp(-kobst)
Table 2 summarizes the kinetic measurements in these
(6′)
experiments for kCH and kCD from 154 to 359 K. The data
4
4
indicate that the rate coefficients kCH and kCD have a positive
4
4
temperature dependence from 154 to 359 K, which can be
expressed as kCH ) (1.2 ( 0.1) × 10-11 exp[(-491 ( 12)/T]
kobs ) k [CX4] + k [C2H2]
(10)
(11)
CX4
C2H2
4
and kCD ) (8.7 ( 1.8) × 10-12 exp[(-650 ( 61)/T] cm3
4
molecule-1 s-1, respectively; see Figure 6. The reaction of C2H
+ CD4 exhibits a significant kinetic isotope effect at 300 K of
kCH /kCD ) 2.5 ( 0.2.
kobs - kC H [C2H2] ) kCX [CX4] ) k′CX
4
2
2
4
4
4
The observed decay rates, kobs, are obtained by fitting the
observed traces to eq 8. They are then corrected for the
contribution of the C2H2 precursor reacting with C2H using both
previous measurements and new measurements down to 143
K, as noted in eq 11. Since C2H reacts 100 times faster with
C2H2 then it does with CX4, the contribution to kobs from C2H2
Discussion
Previous measurements of the reaction C2H + CH4 were
performed at room temperature and are summarized in Table
3. Rendlund et al. monitored the CH (A-X) product chemilu-
minescence from the C2H + O2 reaction with and without CH4.11
+ C2H is kept below 40%. The values of k′CX are plotted
against their respective CX4 concentrations, and a linear least
4
They measured a room temperature value of kCH ) (4.8 ( 1.0)
4
squares fit is used to determine kCX ; see Figure 5. The
× 10-12 cm3 molecule-1 s-1. Okabe photolyzed C2H2 at 147
4
uncertainty in kCX is calculated by combining the accumulated
nm and determined a ratio of kCH /kC H ) 0.032 ( 0.0018.
4
4
2 2
uncertainties in the corrected decay fits, which include errors
in kC H , with the uncertainties in temperature and the measure-
Using the present value of kC H at room temperature of (1.3 (
2
2
0.2) × 10-10 cm3 molecule-1 s-1, one obtains a value for kCH
2
2
4
ment of [CX4]. The error in kCX is typically 10-20%.
of 4.2 × 10-12 cm3 molecule-1 s-1 12
.
Laufer photolyzed
4

Low-Temperature Rate Coefficients of C2H with CH4 and CD4
J. Phys. Chem., Vol. 100, No. 12, 1996 4891
TABLE 3: Summary of Rate Constants for C2H + CH4 at
Room Temperature
ref
kCH4 (cm3 molecule-1 s-1
)
temp (K)
this work
Renlund11
Laufer13
Okabe12
Lander14
(2.3 ( 0.1) × 10-12
(4.8 ( 1.0) × 10-12
(1.2 ( 0.2) × 10-12
4.2 ( 10-12a
300
298
297
298
298
(3.0 ( 0.3) × 10-12
a Recalculated using the measured ratio and the present-day C2H +
C2H2 rate constant (1.3 ( 0.2) × 10-10 cm3 molecule-1 s-1
.
10-12 and kCH ) (1.2 ( 0.1) × 10-11 exp[(-491 ( 12)/T]
4
cm3 molecule-1 s-1, respectively. From the Arrhenius fit, the
energy of activation for C2H + CH4 is equal to 4.3 ( 0.1 kJ
mol-1. Rate coefficient measurements for C2H + CD4 at 300
K and from 227 to 359 K result in a kCD ) (0.9 ( 0.1) ×
4
10-12 and kCD ) (8.7 ( 1.8) × 10-12 exp[(-650 ( 61)T] cm3
4
molecule-1 s-1, respectively. The energy of activation for C2H
+ CD4 from 227 to 359 K is equal to 5.4 ( 0.5 kJ mol-1. No
pressure dependence was found for kCH at 300 K over the
4
experimental range (0.4-2.4) × 1018 cm-3 variation in helium
Figure 5. Plot of k′CX versus [CX4] at 359 K: (O) C2H + CH4; (b)
C2H + CD4. The rate 4constants kCH and kCD4 are equal to (3.1 ( 0.4)
4
buffer gas (15-75 Torr) and for a methane density of 0.7 ×
× 10-12 and (1.5 ( 0.2) × 10-12 cm3 molecule-1 s-1, respectively.
1017 cm-3. Over this number density kCH ) (2.4 ( 0.2) ×
4
10-12 cm3 molecules-1 s-1. Both of these reactions, C2H +
CH4 (CD4), show a positive temperature dependence, consistent
with reactions involving a hydrogen abstraction as the rate-
determining step and a positive activation energy.
Our results are most consistent with those of Lander et al.
Both experiments monitor the direct disappearance of C2H. The
difference in kCH at 300 K between this work and the work by
4
Lander et al. can be attributed to experimental error in
determining number density and error in fitting the observed
signal. In the experiment by Renlund et al. the chemilumines-
2
cence of CH (A ∆) produced by the reaction C2H + O2 was
used to monitor [C2H]. They were able to measure kCH by
4
keeping the concentration of O2 constant while varying the
concentration of CH4. Renlund et al. measured a value for kCH4
at 300 K that is a factor of 2 greater than the rate constant
reported in this work. They reported in a later publication that
the reason for the larger kCH was due to vibrationally and or
4
2
2
electronically excited C2H producing the CH (A ∆-X Π)
emission.15 Okabe measured kCH /kC H to be 0.032 ( 0.0018.
4
2 2
This leads to a value of kCH that is ∼2 times faster at room
4
Figure 6. Arrhenius plot of C2H + CX4: (O) C2H + CH4; (b) C2H
temperature than the kCH reported in this work when using (1.3
4
+ CD4. The Arrhenius fits for kCH and kCD4 are given by (1.2 ( 0.1)
4
( 0.2) × 10-10 cm3 molecule-1 s-1 for kC H
. However,
× 10-11 exp[(-491 ( 12)/T] and (8.7 ( 1.8) × 10-12 exp[(-650 (
2
2
Okabe’s result involved measuring φ0C H /φC H , which is the
61)/T] cm3 molecule-1 s-1, respectively.
4
2
4 2
quantum yield ratio of diacetylene without (φ0C H ) and with
4
2
TABLE 2: Summary of Rate Constants for C2H + CH4
(CD4) from 154 to 359 K
(φC H ) CH4 in the mixture and not a direct measurement of the
4
2
reaction of C2H with CH4.
temp
(K)
kCH
temp
(K)
kCD
4
4
The work presented here on C2H + C2H2 further supports
the reasoning that C4H3q is formed via an addition mechanism
with no barrier to formation. The short-lived intermediate then
immediately dissociates to C4H2 + H. A previous study on
C2H + C2H2 by this lab reported that within experimental error
there was no temperature dependence over 170-350 K.3 More
recent results by Van Look et al. also showed no evidence for
a temperature dependence from 295 to 450 K.16 They measured
rate coefficients of the reaction C2H + O2 by monitoring CH-
(A 2∆ f X 2Π) chemiluminescence. The rate coefficients for
C2H with C2H2 were taken from the y-intercepts of their k′
versus [O2] plots. An Arrhenius fit to their data is reported as
kC H ) (1.3 ( 0.2) × 10-10 exp[(0 ( 10)/T] cm3 molecule-1
(cm3 molecule-1 s-1
)
(cm3 molecule-1 s-1
)
359
300
226
190
159
157
154
(3.1 ( 0.4) × 10-12
(2.3 ( 0.1) × 10-12
(1.2 ( 0.1) × 10-12
(8.7 ( 0.2) × 10-13
(5.2 ( 0.6) × 10-13
(4.7 ( 1.0) × 10-13
(5.5 ( 1.0) × 10-13
359
300
227
190
(1.5 ( 0.2) × 10-12
(0.9 ( 0.1) × 10-12
(5.2 ( 0.1) × 10-13
(2.8 ( 0.6) × 10-13
CF3C2H in the presence of CH4, and the product [C2H2] was
monitored by absorption at 152 nm over time.13 Laufer obtained
a kCH ) (1.2 ( 0.2) × 10-12 cm3 molecule-1 s-1. Finally,
4
Lander et al. used a diode laser to measure the transient
depletion of C2H (X 2Σ+(0,0,0)) in absorption.14 They measured
2
2
s-1. Despite previous measurements for the rate coefficients
of C2H + C2H2, which show no definite temperature dependence
above room temperature,3,9,16,17 new data presented in this study
a kCH ) (3.0 ( 0.3) × 10-12 cm3 molecule-1 s-1 at 300 K.
4
The present rate coefficient measurements for C2H + CH4
at 300 K and from 154 to 359 K are kCH ) (2.3 ( 0.1) ×
4

4892 J. Phys. Chem., Vol. 100, No. 12, 1996
Opansky and Leone
does reveal a small negative temperature dependence in the rate
Acknowledgment. We gratefully acknowledge the National
Aeronautics and Space Administration for support of this
research and the Department of Energy for additional support.
coefficients for C2H + C2H2.
The measured kinetic isotope effect at 300 K is kCH /kCD
)
4
4
2.5 ( 0.2. Unfortunately, no transition-state calculations have
been done on this reaction to be able to compare theory with
experiment. Judging by the magnitude of the isotope effect and
the reaction mechanism involved, the reaction C2H + CH4 (CD4)
exhibits a large primary isotope effect in which the main
contribution is the difference in zero-point energies between
the initial states and the transition states.18-20 Hopefully this
research will encourage theoretical studies of C2H + CH4 (CD4)
for comparison.
In addition to the kinetic isotope effect, there is the possibility
for tunneling to occur in this reaction between the H (D) atom
of methane and the carbon atom of the C2H radical. Unfortu-
nately, no curvature is detected in the Arrhenius plot over 154-
359 K. This does not eliminate the possibility of tunneling
taking place. If the experiment could be done over a larger
temperature range, the possibility of detecting curvature in the
Arrhenius plot would be more favorable.
Results based on the effect of reaction 2 on photochemical
models of Titan are forthcoming. There is little question that
this reaction plays a key role in determining the ethane
concentration on Titan, but recent experiments by Mordaunt et
al.21 have uncovered a new CH4 photolysis channel that will
affect current photochemical models. Mordaunt et al. have
shown that the direct photodissociation of CH4 to CH3 + H is
the main source of methyl radicals in Titan’s atmosphere. Both
the discovery by Mordaunt et al. and the data in this work should
help to clarify the production of ethane in Titan’s atmosphere.
Updated versions2 of the Yung et al1 model and more low-
temperature rate coefficients of pertinent reactions are needed
to determine whether a complete analysis will match forthcom-
ing spacecraft observations.
References and Notes
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(2) Toublanc, D.; Parisot, J. P.; Brillet, J.; Gautier, D.; Raulin, F.;
McKay, C. P. Icarus 1995, 2, 113.
(3) Pedersen, J. O. P.; Opansky, B. J.; Leone, S. R. J. Phys. Chem.
1993, 97, 6822.
(4) Coustenis, A.; Bezard, B.; Gautier, D. Icarus 1989, 80, 54.
(5) Coustenis, A.; Bezard, B.; Vis. Astron. 1991, 34, 11.
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