Comparing the dynamical effects of symmetric and antisymmetric stretch excitation of methane in the Cl+CH4 reaction

Article abstract of DOI:10.1063/1.1647533

The effects on the gas-phase reaction of atomic chlorine with methane by two nearly isoenergetic C-H stretching motions were examined. It was observed that the rovibrational distributions and state-selected differential cross sections of the HCl and CH₃ products from the two vibrationally excited reactions are nearly indistinguishable. The reactions of symmetric-stretched excited or antisymmetric-stretch excited methane with atomic chlorine follow closely related product pathways. The results show that the reactive event involves only a single C-H oscillator and that vibrational excitation of a C-H stretch increases reactivity by opening the cone of acceptance to allow peripheral rections.

Full text of DOI:10.1063/1.1647533

Comparing the dynamical effects of symmetric and antisymmetric stretch excitation of
methane in the Cl + CH 4 reaction
Hans A. Bechtel, Jon P. Camden, Davida J. Ankeny Brown, and Richard N. Zare
Citation: The Journal of Chemical Physics 120, 5096 (2004); doi: 10.1063/1.1647533
View online: http://dx.doi.org/10.1063/1.1647533
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 120, NUMBER 11
15 MARCH 2004
Comparing the dynamical effects of symmetric and antisymmetric stretch
excitation of methane in the Cl¿CH4 reaction
Hans A. Bechtel, Jon P. Camden, Davida J. Ankeny Brown, and Richard N. Zarea)
Department of Chemistry, Stanford University, Stanford, California 94305-5080
͑Received 2 December 2003; accepted 18 December 2003͒
The effects of two nearly isoenergetic C–H stretching motions on the gas-phase reaction of atomic
chlorine with methane are examined. First, a 1:4:9 mixture of Cl2 , CH4 , and He is coexpanded into
a vacuum chamber. Then, either the antisymmetric stretch (␯3ϭ3019 cmϪ1) of CH4 is prepared by
direct infrared absorption or the infrared-inactive symmetric stretch (␯1ϭ2917 cmϪ1) of CH4 is
prepared by stimulated Raman pumping. Photolysis of Cl2 at 355 nm generates fast Cl atoms that
initiate the reaction with a collision energy of 1290Ϯ175 cmϪ1 ͑0.16Ϯ0.02 eV͒. Finally, the nascent
HCl or CH3 products are detected state-specifically via resonance enhanced multiphoton ionization
and separated by mass in a time-of-flight spectrometer. We find that the rovibrational distributions
and state-selected differential cross sections of the HCl and CH3 products from the two vibrationally
excited reactions are nearly indistinguishable. Although Yoon et al. ͓J. Chem. Phys. 119, 9568
͑2003͔͒ report that the reactivities of these two different types of vibrational excitation are quite
different, the present results indicate that the reactions of symmetric-stretch excited or
antisymmetric-stretch excited methane with atomic chlorine follow closely related product
pathways. Approximately 37% of the reaction products are formed in HCl( ϭ1,J) states with little
v
rotational excitation. At low J states these products are sharply forward scattered, but become almost
equally forward and backward scattered at higher J states. The remaining reaction products are
formed in HCl( ϭ0,J) and have more rotational excitation. The HCl( ϭ0,J) products are
v
v
predominantly back and side scattered. Measurements of the CH3 products indicate production of a
non-negligible amount of umbrella bend excited methyl radicals primarily in coincidence with the
HCl( ϭ0,J) products. The data are consistent with a model in which the impact parameter governs
v
the scattering dynamics. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1647533͔
I. INTRODUCTION
used quasiclassical trajectory calculations to determine that
the symmetric stretch of a linear triatomic molecule is more
efficient at promoting reaction than a comparable amount of
excitation in the antisymmetric stretch. Since then, a variety
of theoretical methods have been used to examine the rela-
tive reactivity of the symmetric and antisymmetric stretches
in a number of polyatomic reaction systems.21–31 Palma and
Clary29 performed four-dimensional quantum scattering cal-
culations on the O(3P)ϩCH4→OHϩCH3 system and found
the symmetric stretch of CH4 to be more reactive than the
antisymmetric stretch. Fair et al.32 found similar results with
wave packet calculations on the ClϩH2O→HClϩOH reac-
tion: the symmetric stretch of H2O is more reactive than the
antisymmetric stretch. They attributed the increased reactiv-
ity to the adiabatic flow of vibrational energy into local-
mode OH excitations pointing either toward ͑proximal͒ or
away ͑distal͒ from the approaching Cl atom for the symmet-
ric and antisymmetric stretches of H2O, respectively.
Despite numerous theoretical studies, few experimental
investigations have compared the effects of the symmetric
and antisymmetric stretches on chemical reactions. Yoon
et al.33 examined the relative reactivity of the stretch–bend
combination vibrations of CH4 in the ClϩCH4→HCl
ϩCH3 reaction using infrared excitation and action spectros-
copy. They found the symmetric stretch–bend combination
(␯1ϩ␯4) more reactive than the antisymmetric stretch–bend
Experimental and theoretical work has shown that re-
agent vibrational excitation can have dramatic effects on
chemical reactions. One of the most obvious effects of vibra-
tional excitation is an increase in reactivity. Indeed, Polanyi1
found that vibrational excitation is more efficient than trans-
lational energy in promoting endoergic atom plus diatom re-
actions with late reaction barriers. For systems involving
polyatomic reagents, the extra degrees of freedom associated
with polyatomics can complicate this simple picture and lead
to a large number of different vibrational motions. Several
groups2–16 have demonstrated that excitation of certain vi-
brational motions can localize energy in specific parts of the
polyatomic reagent and lead to dramatic bond- and
mode-selectivity.17–19 The first examples of such behavior
involved reactions of fast H atoms with various isotopes of
water.2,3 This method of vibrational control has recently been
extended to larger systems, such as the reaction of methane
with atomic chlorine13,14,16 or nickel surfaces.15
Although most of these bond- and mode-selective stud-
ies have focused on the effects of rather different vibrational
motions, the effects of seemingly similar motions on chemi-
cal reactivity is also of particular interest. In 1979, Schatz20
a͒
Author to whom all correspondence should be addressed; electronic mail:
zare@stanford.edu
0021-9606/2004/120(11)/5096/8/$22.00
5096
© 2004 American Institute of Physics
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J. Chem. Phys., Vol. 120, No. 11, 15 March 2004
Excitation in the ClϩCH4 reaction
combination (␯3ϩ␯4) by a factor of 1.9Ϯ0.5. Direct com-
parisons of the symmetric and antisymmetric stretch, how-
ever, could not be made because the symmetric stretch (␯1)
of CH4 is infrared inactive and the effects of the bending
mode (␯4) on the reaction are not known. Recently, Yoon
et al.34 exploited the reduced symmetry of CH3D, which
makes both the symmetric and antisymmetric stretches infra-
red active. They found that the symmetric stretch is seven
times more reactive than the antisymmetric stretch in the
ClϩCH3D→HClϩCH2D reaction.
The large difference in reactivity between these two
seemingly similar C–H stretching motions is surprising, es-
pecially considering that both C–H stretches are expected to
map effectively onto the reaction coordinate. Indeed, we
might expect the antisymmetric stretch to be more reactive
than the symmetric stretch because some of the C–H bonds
in the antisymmetric stretch extend more than the bonds in
the symmetric stretch.35 Clearly, the dynamics of vibra-
tionally excited reactions are more complicated than this
simple picture, which appears to be invalidated by the pre-
dicted and measured increased reactivity of the symmetric
stretch over the antisymmetric stretch. The large difference
in reactivity raises the possibility that these two vibrationally
excited reactions proceed via different mechanisms. The
present study contradicts this supposition in part. Instead, we
suggest that the reactions of these two differently prepared
vibrationally excited reagents must follow a similar pathway
leading to product formation. Thus, it is possible that the
initial preparation of the reagents affects the reactivity, but
not the dynamics of the reaction.
Here, we use the photoloc technique to examine the ef-
fects of the symmetric (␯1) and antisymmetric (␯3) stretches
on the dynamics of the ClϩCH4→HClϩCH3 reaction. Sim-
pson et al.36 previously used the same technique to examine
the ClϩCH4(␯3) reaction with an unprecedented level of
detail, obtaining rotational distributions, state-selected differ-
ential cross sections ͑DCSs͒, and information on the effects
of rotational and vibrational alignment on chemical reactiv-
ity. These measured quantities have recently been used to
differentiate the effects of two nearly isoenergetic vibrations
on the ClϩCH2D2 reaction16 and are known be sensitive
probes of chemical dynamics. We use stimulated Raman
pumping ͑SRP͒ to excite the fundamental of the symmetric
stretch (␯1) and infrared excitation to excite the fundamental
of the triply degenerate antisymmetric stretch (␯3). To re-
duce any systematic errors, both reactions were performed
under identical conditions. We find the repeated measure-
ments of the ClϩCH4(␯3) reaction to be in excellent agree-
ment with previous measurements.36
FIG. 1. Energy level diagram for the reaction of atomic chlorine with vi-
brationally excited methane. The symmetric stretch (␯1ϭ2917 cmϪ1) is
prepared by stimulated Raman pumping ͑SRP͒ and the antisymmetric
stretch (␯3ϭ3019 cmϪ1) is prepared by direct infrared absorption ͑IR͒.
Photolysis of Cl2 at 355 nm provides 1290 cmϪ1 of collision energy with an
energy spread determined from the formulas of van der Zande et al. ͑Ref.
54͒ at 15 K.
provides 1290Ϯ175 cmϪ1 of translational energy in the cen-
ter of mass frame. Excitation of the symmetric stretch (␯1)
provides 2917 cmϪ1 of vibrational energy, whereas excita-
tion of the antisymmetric stretch (␯3) provides 3019 cmϪ1 of
vibrational energy.
The methods and experimental apparatus have been de-
scribed in detail previously,36,39 therefore, only the primary
features are presented here. A 1:4:9 mixture of molecular
chlorine ͑Matheson, research grade, 99.999%͒, methane
͑Matheson, 99.999%͒, and helium ͑Liquid Carbonic,
99.995%͒ is supersonically expanded into the extraction re-
gion of a linear time-of-flight ͑TOF͒ spectrometer having a
Wiley-McLaren configuration.40 The vibrational state of CH4
is prepared by SRP (␯1) or direct IR excitation (␯3). The
reaction is initiated by the photolysis of Cl2 with linearly
polarized 355 nm light, which produces monoenergetic Cl
atoms primarily in the ground state (2P3/2) with an anisot-
ropy parameter ␤ϭϪ1.41 After a 20–80 ns time delay, the
HCl or CH3 products are state selectively ionized by 2ϩ1
resonance-enhanced multiphoton ionization ͑REMPI͒, sepa-
rated by mass, and detected by microchannel plates. The re-
active signal from vibrationally excited methane is separated
from backgrounds by modulating the SRP or IR light and
subtracting the resultant signals on a shot-by-shot basis.
The SRP radiation required to excite the symmetric
stretch (␯1) of CH4 is generated by a Nd3ϩ:YAG laser ͑Con-
tinuum PL8020͒ and a pulsed dye laser ͑Quanta-Ray, PDL-
3͒. Although the Nd3ϩ:YAG second harmonic ͑532 nm͒ is
more commonly used as the pump source for SRP, we found
that the 532 nm light excited the Cl2 precursor, resulting in
large ion backgrounds that interfere with the reaction signal.
To avoid these backgrounds, we use the output of the dye
II. ENERGETICS AND EXPERIMENTAL PROCEDURES
Figure 1 displays the relevant energetics of the Cl
ϩCH4→HClϩCH3 reaction. The reaction is slightly
endothermic,37 ⌬Hϭ600 cmϪ1 ͑1.7 kcal/mol͒, and has an
activation barrier in the 800–1300 cmϪ1 ͑2.4–3.6 kcal/mol͒
or 1300–1900 cmϪ1 ͑3.6–5.5 kcal/mol͒ range, based on
experimental38 or ab initio calculations,28 respectively. The
combination of translational and vibrational energy is used to
overcome the reaction barrier. Photolysis of Cl2 at 355 nm
laser ͑Exciton LDS 821͒ to generate ϳ812 nm light (␭
)
pump
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5098
J. Chem. Phys., Vol. 120, No. 11, 15 March 2004
Bechtel et al.
and the fundamental from the Nd3ϩ:YAG laser to generate
1064 nm light (␭Stokes). The 1064 nm light is combined col-
linearly with the 812 nm light using an 800 nm high reflec-
tor. In order to find the frequency condition between the
pump and Stokes lasers, a photodiode is used to detect the
coherent anti-Stokes Raman ͑CARS͒ signal at ϳ657 nm
which is generated in a separate cell containing ϳ20 Torr of
CH4 . The CARS spectrum consists of only Q-branch transi-
tions because the ␯ vibration is the totally symmetric C–H
1
stretch. The resolution of our dye laser ͑ϳ0.1 cmϪ1͒ proved
to be insufficient to fully resolve individual Q-branch mem-
bers. Typical laser energies of the 812 and 1064 nm light are
ϳ25 and 200 mJ, respectively. The pump and Stokes beams
are focused into the chamber with an fϭ45 cm CaF2 lens.
The IR radiation required to excite the antisymmetric
stretch (␯3) of CH4 is generated in a two-step process in-
volving difference-frequency mixing and optical parametric
amplification. First, mid-IR light at ␭ϭ3.3 ␮m is generated
via difference-frequency mixing by combining the 1064 nm
fundamental of an Nd3ϩ:YAG laser ͑Continuum PL9020͒
with the ϳ804 nm output of a dye laser ͑Continuum
ND6000, Exciton LDS 821͒ in a lithium niobate (LiNbO3)
crystal. The mid-IR radiation is then parametrically amplified
in a second LiNBO3 crystal pumped by another 1064 nm
beam to produce approximately 10 mJ of the requisite light.
The frequency condition of the IR light is found by using a
photoacoustic cell containing ϳ10 Torr of CH4 . Once the
reaction signal is found, care is taken to attenuate and defo-
cus the IR light to avoid two-photon absorption to the first
overtone of the antisymmetric stretch (2␯3), which produces
FIG. 2. Rotational distributions of the ͑a͒ HCl( ϭ1,J) and ͑b͒ HCl(
ϭ0,J) products from the reaction of atomic chlorine with vibrationally ex-
v
v
cited methane. The HCl( ,J) populations from the ClϩCH (␯ ) reaction
v
4
1
are represented by open squares and solid lines, and the HCl( ,J) popula-
v
tions from the ClϩCH4(␯3) reaction are represented by closed circles and
dotted lines. The HCl( ϭ0,J) and HCl( ϭ1,J) populations are scaled rela-
v
v
tive to one another. The error bars represent 95% confidence intervals of
replicate measurements.
stretch-excited methyl radical. The ␯ vibrational state is pre-
3
pared on the Q-branch bandhead or on single R-branch lines.
tropic TOF profiles are analyzed to estimate the amount of
internal energy deposited into the co-product by a method
described in previous publications.39,45
With the exception of a slight increase in forward scattered
behavior for HCl( ϭ1) products,36 the data show no strong
v
dependence on the CH4 rotational state.
The photolysis light is generated from the third harmonic
of a Nd3ϩ:YAG laser ͑Continuum PL9020͒, and the probe
light for REMPI is generated by frequency doubling the fun-
damental of a dye laser output ͑Quanta Ray DCR-2A,
Lambda Physik FL 2002, Exciton LD489 or DCM/LDS698͒
III. RESULTS
A. HCl product state distributions
For the ClϩCH4(␯1) and ClϩCH4(␯3) reactions Figs.
2͑a͒ and 2͑b͒ present the integral cross sections for HCl(
v
in
a BBO crystal. The HCl products are detected
ϭ1,J) and HCl( ϭ0,J), respectively. The HCl( ϭ0,J)
v
v
3
via the f ⌬2 –X 1⌺ϩ(0,0), F 1⌬2 –X 1⌺ϩ(0,0), F 1⌬2
–X 1⌺ϩ(1,1), and the E 1⌺ϩ –X 1⌺ϩ(0,1) bands.42,43 The
methyl radical products are detected by the 3pz 2A2Љ–X 2B1Љ
band.44 Approximately 2 mJ of ϳ240 nm light is used to
probe the HCl products, and less than 1.5 mJ of ϳ330 nm
light is used to probe the CH3 products. The probe light is
focused into the chamber using a fϭ50 cm fused-silica lens.
A photoelastic modulator ͑PEM-80, Hinds International
Inc.͒ flips the direction of the photolysis laser polarization
between parallel and perpendicular to the TOF axis on an
every-other-shot basis in order to obtain the isotropic Iiso
ϭIʈϩ2IЌ and anisotropic Ianisoϭ2(IʈϪIЌ) components of
the core-extracted TOF profiles. The isotropic TOF profile
removes any dependence on the photolysis spatial anisotropy
and thus provides a direct measurement of the speed distri-
bution. These profiles are analyzed and converted into DCSs
by a method similar to that of Simpson et al.36 The aniso-
populations are obtained by detecting the m/zϭ36 (H35Clϩ)
ion signal while scanning the probe laser over the Q, R, and
S branches of the F 1⌬2 –X 1⌺ϩ(0,0) 2ϩ1 REMPI band and
the Q branch of the f ⌬2 –X 1⌺ϩ(0,0) 2ϩ1 REMPI band.
3
Signal intensities are converted into relative populations us-
ing empirical correction factors determined by leaking room
temperature HCl into the vacuum chamber. The HCl(
v
ϭ1,J) populations are obtained by detecting the m/zϭ1
(Hϩ) ion signal from dissociative ionization of HCl while
scanning the probe laser over the Q branch of the
E 1⌺ϩ –X 1⌺ϩ(0,1) 2ϩ1 REMPI band. The correction fac-
tors of Simpson et al.36 are used to convert the HCl( ϭ1)
v
signal intensity to population. The HCl( ϭ0,J) and HCl(
v
v
ϭ1,J) rotational distributions are scaled with respect to each
other by recording the intensity of the R(1) member of the
F 1⌬2 –X 1⌺ϩ(1,1) 2ϩ1 REMPI band in the same scan as
the R(5) member of the F 1⌬2 –X 1⌺ϩ(0,0) 2ϩ1 REMPI
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5099
J. Chem. Phys., Vol. 120, No. 11, 15 March 2004
Excitation in the ClϩCH4 reaction
FIG. 3. Co-added 2ϩ1 REMPI spectra of the CH3 products from the ClϩCH4(␯1) reaction, black line, and the ClϩCH4(␯3) reaction, gray line. The Q
branch of the 000 band is presented off scale in order to see the rotational structure of the O, P, R, and S branches. The integrated intensity ratio of the 000, 211,
and 222 bands for the ClϩCH4(␯1) reaction is 0.80Ϯ0.03:0.18Ϯ0.02:0.02Ϯ0.01. The integrated intensity ratio of the three bands for the ClϩCH4(␯3)
reaction is 0.76Ϯ0.02:0.21Ϯ0.02:0.03Ϯ0.01. The errors represent 95% confidence intervals of replicate measurements.
band. The vibrational correction factor for the relative sensi-
tivity of the diagonal bands of the F 1⌬2 –X 1⌺ϩ band is
obtained by tuning the IR laser to various HCl rovibrational
lines and scanning the probe laser over the depleted
F 1⌬2 –X 1⌺ϩ(0,0) lines and the enhanced F 1⌬2
–X 1⌺ϩ(1,1) lines. Every effort was made to ensure that the
probe laser power and focusing conditions were identical for
Franck–Condon factors are unknown and substantial predis-
sociation occurs in the upper electronic state. Although at-
tempts have been made to quantify these values,46,47 we
choose instead to make only qualitative comparisons of the
CH3 product REMPI spectra from the ClϩCH4(␯1) and Cl
ϩCH4(␯3) reactions.
Figure 3 clearly shows that the CH3 product-state distri-
bution of the ClϩCH4(␯1) reaction is similar to the CH3
product-state distribution of the ClϩCH4(␯3) reaction. The
rotational structure of the two reactions in the 000 band region
appears indistinguishable, indicating that the ground state
methyl radical products have essentially identical rotational
distributions. Both rotational distributions have no observ-
able population in states higher than Nϭ7. Moreover, both
spectra show that the O and S branches are enhanced over
the P and R branches. There also appears to be an enhance-
ment of the even lines over the odd lines for the O and S
branches. The enhancement of the O and S branches and the
alternating intensities suggest that the methyl radical prod-
ucts are formed preferentially in low K states,46 indicating
that the methyl radical is rotating about a C2 axis that passes
through a C–H bond rather than rotating about the C3 axis
that passes through the C atom perpendicular to the plane of
the molecule.
the measurement of the HCl( ,J) integral cross sections
v
from the ClϩCH4(␯1) and the ClϩCH4(␯3) reactions. The
uncertainties represent 95% confidence intervals of replicate
measurements and include the error in the determination of
the correction factors.
As shown in Fig. 2, the HCl integral state distributions
for the ClϩCH4(␯1) and the ClϩCH4(␯3) reactions are
nearly identical. For both reactions, 37Ϯ7% of the reaction
products are formed in HCl( ϭ1,J) states. The average en-
v
ergy in rotation for the HCl( ϭ1) products is 41Ϯ3 and
v
53Ϯ3 cmϪ1, and the average energy in rotation for the
HCl( ϭ0) products is 301Ϯ95 and 292Ϯ93 cmϪ1 for the
v
ClϩCH4(␯1) and the ClϩCH4(␯3) reactions, respectively.
Accounting for the ϳ100 cmϪ1 difference in the vibrational
frequencies of the two C–H stretches, the average energy in
rotation for the HCl( ϭ1) products is only ϳ6% of the
v
available energy for both the ClϩCH4(␯1) and the Cl
ϩCH (␯ ) reactions. Although the HCl( ϭ0) rotational
v
4
3
distributions are much warmer than the HCl( ϭ1) rotational
v
Both reactions produce non-negligible amounts of um-
brella bend excited methyl radical. The integrated intensity
ratio of the 000, 211, and 222 bands for the ClϩCH4(␯1) reac-
tion is 0.80Ϯ0.03:0.18Ϯ0.02:0.02Ϯ0.01, whereas the inte-
grated intensity ratio of the three bands for the Cl
ϩCH4(␯3) reaction is 0.76Ϯ0.02:0.21Ϯ0.02:0.03Ϯ0.01.
The uncertainties given represent 95% confidence intervals
of replicate measurements. Yoon et al.34 observed a similar
ratio of ground state to umbrella bend excited CHD2 prod-
distributions, the average energy in rotation is still only a
small fraction of the available energy: ϳ8% for both reac-
tions.
B. CH3 product state distributions
Figure 3 displays the 2ϩ1 REMPI spectra of the CH3
products from the ClϩCH4(␯1) and ClϩCH4(␯3) reactions
obtained with linearly polarized light. The Q branch of the 000
band is presented off-scale in order to display the rotational
structure of the O, P, R, and S branches and the progression
of bands resulting from umbrella bend excitation (211 and 222
bands͒. Obtaining rovibrational state distributions from the
3pz 2A2Љ–X 2BЉ1 REMPI scheme is nontrivial because the
ucts in the reaction of ClϩCHD3 (␯ or ␯4). Although it is
1
possible that the ClϩCH4(␯3) reaction generates more um-
brella bend excited methyl radical products than the Cl
ϩCH4(␯1) reaction, the difference is not discernible within
our uncertainty.
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5100
J. Chem. Phys., Vol. 120, No. 11, 15 March 2004
Bechtel et al.
The isotropic TOF profile is a measurement of the ve-
locity distribution of the products. Under perfect core-
extraction conditions, there is a one-to-one relationship be-
tween the TOF shift and the velocity of the product ions in
the lab frame. Thus, large positive TOF shifts correspond to
fast products moving initially toward the detector, and large
negative TOF shifts correspond to fast products moving ini-
tially away from the detector. The distribution of product
lab-frame speeds can be extracted by fitting the isotropic
TOF profile with a set of basis functions generated by Monte
Carlo simulation, as described in a previous publication.39
These speed distributions can be converted into DCSs with
knowledge of the internal energy of the co-product state. We
obtain this information by fitting the anisotropic TOF profile,
which is a measurement of the product speed-dependent spa-
tial anisotropy and provides a means of determining the av-
erage internal energy deposited in the co-product.39
Unfortunately, the kinematics of the HCl( ϭ1,J) prod-
v
ucts constrain the measurable spatial anisotropy such that it
is of little aid in determining the energy deposited in the
methyl fragment. Because most of the HCl( ϭ1,J) product
v
intensity occurs outside the allowed speed range for products
that are generated in coincidence with umbrella bend excited
methyl radical, we assume that the methyl radical consumes
no energy and all the excess is present in translation. More-
over, the spatial anisotropy of the CH3(␯2ϭ1) product
shows that minimal energy is deposited into the HCl co-
products, indicating that the formation of HCl( ϭ1,J) prod-
ucts in coincidence with CH3(␯2ϭ1) products is a minor
channel. We assume that the CH3(␯2ϭ1) products are
v
FIG. 4. Isotropic IisoϭIʈϩ2IЌ and anisotropic Ianisoϭ2(IʈϪIЌ) components
of the core-extracted TOF profiles of the ͑a͒ HCl( ϭ1,Jϭ1) products, ͑b͒
v
the HCl( ϭ1,Jϭ2,3) products, ͑c͒ the HCl( ϭ0,Jϭ5) products, and ͑d͒
v
v
the CH3(␯2ϭ1) products from the reaction of atomic chlorine with vibra-
tionally excited methane. The profiles in the left-hand column are from the
ClϩCH4(␯1) reaction, and the profiles in the right-hand column are from
the ClϩCH4(␯3) reaction. The open circles are the measured isotropic TOF
profiles, the open squares are the measured anisotropic TOF profiles, and the
solid lines are the results of the fit.
formed entirely with HCl( ϭ0,J) products and that the
v
HCl( ϭ0,J) co-products have the same rotational distribu-
v
tion as shown earlier. The spatial anisotropy of the HCl(
v
ϭ0,Jϭ5) products also indicates that minimal energy is de-
posited into the methyl radical. We are unable to differentiate
between HCl( ϭ0,Jϭ5) products formed in coincidence
v
with methyl radical in the ground state or umbrella bend
excited, but the DCS associated with each co-product state is
the same within our error bars.
Figure 5 shows the resulting DCSs of the different prod-
uct states from both the ClϩCH4(␯1) and the Cl
ϩCH4(␯3) reactions. As suggested by the TOF profiles, the
scattering distributions for each product state are nearly iden-
C. State-to-state scattering distributions
Figure 4 shows the isotropic IisoϭIʈϩ2IЌ and aniso-
tropic Ianisoϭ2(IʈϪIЌ) core-extracted TOF profiles of the
HCl( ϭ1,Jϭ1), HCl( ϭ1,Jϭ2,3), HCl( ϭ0,Jϭ5), and
v
v
v
CH ( 2ϭ1) products from the ClϩCH4(␯1) and Cl
v
3
ϩCH (␯ ) reactions. The HCl( ϭ1,Jϭ1) TOF profiles are
v
4
3
tical for both reactions. The HCl( ϭ1,Jϭ1) DCSs are both
v
obtained on the R(1) line of the F 1⌬2 –X 1⌺ϩ(1,1) band;
sharply peaked in the forward scattered region. The HCl(
v
the HCl( ϭ1,Jϭ2,3) TOF profiles are obtained on the over-
v
ϭ1,Jϭ2,3) DCSs are also peaked in the forward scattered
region, but have more intensity in the backward scattered
lapped Q(2) and Q(3) lines of the F 1⌬2 –X 1⌺ϩ(1,1) tran-
sition; the HCl( ϭ0,Jϭ5) TOF profiles are obtained on the
v
region than the HCl( ϭ1,Jϭ1) DCSs. The HCl( ϭ0,J
v
v
3
Q(5) line of the f ⌬2 –X 1⌺ϩ(1,1) band; and the CH3(␯
2
ϭ5) products are predominantly backward and side scat-
tered, whereas the CH3(␯2ϭ1) products are predominantly
forward and side scattered.
ϭ1) TOF profiles are obtained on the Q branch of the 211
band of the 3pz 2A2Љ–X 2B1Љ band. Accurate CH3(␯ϭ0)
TOFs could not be obtained because the transfer of popula-
tion to the C–H stretching states in CH4 necessarily depletes
the signal arising from the reaction of Cl with ground-state
IV. DISCUSSION
CH4 . The ground-state reaction does not produce CH3(␯
As shown in Figs. 2–5, the rovibrational distributions
and scattering distributions of the products from the Cl
ϩCH4(␯1) and ClϩCH4(␯3) reactions are nearly indistin-
guishable. In fact, the only difference discernable within our
2
ϭ1), however, and thus we could obtain CH3(␯2ϭ1) TOF
profiles. The TOF profiles have a clear dependence on prod-
uct state; but for each product state, the TOF profiles from
the ClϩCH4(␯1) reaction are nearly indistinguishable from
the TOF profiles of the ClϩCH4(␯3) reaction.
signal to noise and resolution is a slightly warmer HCl(
v
ϭ1) rotational distribution for the ClϩCH4(␯3) reaction.
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J. Chem. Phys., Vol. 120, No. 11, 15 March 2004
Excitation in the ClϩCH4 reaction
FIG. 6. Schematic model of the observed scattering behavior from the Cl
ϩCH (␯ ) reaction and the ClϩCH (␯ ) reaction. The HCl( ϭ0,J) prod-
v
4
1
4
3
ucts result from ͑a͒ glancing collisions that cause side-scattering and rota-
tional excitation. The HCl( ϭ1,J) products are formed via two competing
v
mechanisms: ͑b͒ stripping and ͑c͒ rebound. The stripping mechanism leads
to forward-scattered HCl( ϭ1,J) products with little rotational excitation,
v
and the rebound mechanism leads to backward-scattered HCl( ϭ1,J) prod-
v
ucts that have more rotational excitation.
ping’’ mechanism imparts little torque on the HCl( ϭ1)
v
products, and consequently leaves them rotationally cold.
The other mechanism arises from collisions at low-impact
parameter causing the HCl( ϭ1) products to ‘‘rebound’’ in
v
the backscattered direction. The impulse release associated
with the redirection of the Cl initial velocity should cause
these backscattered products to be more rotationally excited.
Indeed, this behavior is exactly what we observe. The low
J HCl( ϭ1) products are sharply forward scattered,
v
FIG. 5. State-to-state differential cross sections for the ͑a͒ HCl( ϭ1,J
v
whereas the higher J HCl( ϭ1) products are more equally
v
ϭ1) products, ͑b͒ the HCl( ϭ1,Jϭ2,3) products, ͑c͒ the HCl( ϭ0,J
v
v
forward and back scattered. The steric measurements of Sim-
pson et al.36 further support the proposed ‘‘stripping’’
mechanism by showing that the forward scattered behavior
ϭ5) products, and ͑d͒ the CH3(␯2ϭ1) products from the reaction of atomic
chlorine with vibrationally excited methane. The DCSs of the products from
the ClϩCH4(␯1) reaction are represented by open squares and solid lines,
and the DCSs of the products from the ClϩCH4(␯3) reaction are repre-
sented by closed circles and dotted lines. The error bars represent 95%
confidence intervals of replicate measurements.
of the HCl( ϭ1,Jϭ1) products results from T-shaped tran-
v
sition state geometries.
The observed HCl( ϭ0,J) products are rotationally ex-
v
cited and predominantly back and side scattered. The CH3
REMPI spectrum and the measured spatial anisotropy show
that little internal energy is deposited into the methyl frag-
ment, with only the umbrella excited bending mode signifi-
cantly excited. Because little energy is consumed by the in-
Because the HCl( ϭ1,J) products are close to the energetic
limit of the reaction, they are more likely to be sensitive to
small differences in the reaction energetics. Consequently,
v
the difference in the HCl( ϭ1) rotational distribution sim-
v
ply may arise from the extra ϳ100 cmϪ1 in the antisymmet-
ric stretch (␯3). The remaining rovibrational distributions
and scattering distributions, however, are identical within our
signal-to-noise ratio, from which we conclude that the reac-
tive mechanisms of the two reactions are the same.
ternal modes of the products, the HCl( ϭ0,J) products must
v
experience an impulsive kick to rid the reactants of excess
energy. This impulsive kick is expected to excite the HCl
products rotationally and is likely to occur in the direction of
the C–H bond in the transition state. Unless the transition
state rotates significantly, the C–H bond will be on the hemi-
sphere pointing toward the Cl-atom approach. Thus, the im-
Simpson et al.36 proposed
a model for the Cl
ϩCH4(␯3) reaction in which the impact parameter deter-
mines where the products are scattered and how the energy is
partitioned between vibrational, rotational, and translational
energy. Based on the results shown above, we believe the
same model explains the rovibrational and scattering distri-
butions of the ClϩCH4(␯1) reaction. Figure 6 illustrates the
different mechanisms present in the reaction of atomic chlo-
pulse release will cause the HCl( ϭ0) products to be back-
v
ward and side scattered and cause the CH3 radical to rotate
preferentially about its C–H bond, which is what we observe
experimentally.
The CH3 product is believed to behave primarily as a
spectator during the reaction because of the low degree of
methyl radical excitation. Other studies involving reactions
of atomic chlorine with overtone excited methane and isoto-
pomers also show that the methyl radical does not participate
in the reaction.13,16,45 Based on the measured spatial anisot-
ropy and energetic constraints, the umbrella bend excited
methyl radical products are believed to be formed predomi-
rine with vibrationally excited methane. Because the HCl(
v
ϭ1,J) DCS changes as the rotational number is increased,
we believe there are two competing mechanisms that form
HCl( ϭ1,J) products. The dominant mechanism corre-
v
sponds to the reaction of Cl with a peripheral H atom, result-
ing in forward scattered HCl( ϭ1) products. This ‘‘strip-
v
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5102
J. Chem. Phys., Vol. 120, No. 11, 15 March 2004
Bechtel et al.
reaction coordinate, leading to an increased reactivity of the
symmetric stretch over the antisymmetric stretch. Although
the vibrationally adiabatic model successfully predicts the
increased reactivity of the symmetric stretch over the anti-
symmetric stretch, there are limitations to its predictive abili-
ties. First, the model suggests that excitation of the antisym-
metric stretch should not enhance the reactivity at all, in
contrast to experimental measurements that estimate the vi-
brational enhancement factor over the ground state reaction
to be ϳ30 for the ClϩCH4(␯3) reaction.52 Yoon et al. attrib-
uted the residual reactivity of the antisymmetric stretch to
collision-induced mode-mixing, which was beyond the scope
of their model. Our results are consistent with the hypothesis.
Second, the model suggests that the symmetric stretch should
couple energy more efficiently into HCl product vibration,
nantly in coincidence with HCl( ϭ0) products, not HCl(
ϭ1) products. Furthermore, it is apparent from the DCS of
the CH3(␯2ϭ1) products that these products are formed by a
v
v
similar mechanism as the HCl( ϭ0,J) products as the
v
CH3(␯2ϭ1) scattering distributions are near mirror images
of the HCl( ϭ0,J) scattering distributions. We postulate
v
that the CH3(␯2ϭ1) products are generated from Cl colli-
sions at low to medium impact parameter and the source of
the bend excitation is the transformation of the methyl radi-
cal from a pyramidal geometry to a planar geometry in the
transition state region.
The above-presented impact parameter model is rooted
on the idea that vibrational excitation opens the cone of ac-
ceptance by localizing energy along the reaction coordinate
and reducing the line-of-centers energy,48 thereby allowing
peripheral reactions.49–51 The nearly indistinguishable rovi-
brational distributions and scattering distributions of the Cl
ϩCH4(␯1) and ClϩCH4(␯3) reactions indicate that the re-
active mechanisms of these two reactions are similar and
suggest that the reactive event involves only a single C–H
oscillator. Indeed, rovibrational and scattering distributions
from the ClϩCHD3(␯1) reaction,36 in which there is only
one vibrationally excited C–H oscillator, are remarkably
similar to the rovibrational distributions and scattering distri-
butions of the ClϩCH4(␯1) and ClϩCH4(␯3) reactions.
Thus, a symmetric C–H stretch appears to behave just as an
antisymmetric C–H stretch in controlling product internal-
state distribution and angular distribution. The relative
phases of the other C–H oscillators in CH4 seem to be in-
consequential.
This simple picture is in apparent contradiction with
theoretical21–31 and experimental results33,34 in which the
symmetric stretch is found to be more reactive than the an-
tisymmetric stretch. Our measurements do not necessarily
invalidate these previous studies, however, because it is pos-
sible for the symmetric stretch to be more reactive than the
antisymmetric stretch and for the ClϩCH4(␯1) and Cl
ϩCH4(␯3) reactions to still have identical product rovibra-
tional and scattering distributions. Moreover, our measure-
ments can actually be used to support the conclusions de-
rived from the experiments of Yoon et al.34 Because they
performed action spectroscopy on only the 000 band of the
CHD2 product, one of the uncertainties in their experiment
was whether or not the state distributions of the symmetric
and antisymmetric stretch excited reactions were the same.
Our measurements indicate that their assumption of identical
state distributions is most likely valid. Thus, we believe that
the initial preparation of the reagents alters the reactivity, but
the dynamics leading to product formation follow a common
pathway, which could be promoted by vibrational mixing
during the collision event.
producing a larger HCl( ϭ1):HCl( ϭ0) ratio for the Cl
v
v
ϩCH4(␯1) reaction. The model also predicts that the CH3
products from the ClϩCH4(␯3) reaction should be vibra-
tionally excited because the vibrational energy of the anti-
symmetric stretch is localized into the ‘‘distal’’ or nonreac-
tive C–H bonds. In contrast, we observe the same HCl(
v
ϭ1):HCl( ϭ0) ratio for both the ClϩCH (␯ ) and the Cl
v
4
1
ϩCH4(␯3) reactions, and we do not observe vibrationally
excited CH3 from the ClϩCH4(␯3) reaction. Truhlar and
co-workers28,53 have cautioned previously that the assump-
tion of vibrational adiabaticity may not hold along the entire
reaction path. Thus, the initial reactant vibrational motions
may not correlate well to the product vibrational motions in
the asymptotic region. Another limitation of the vibrational
adiabatic model proposed by Yoon et al.34 as well as other
theoretical models,29 is the restriction on the Cl atom to have
zero impact parameter and to make a collinear approach to
the C–H bond. Based on our DCS measurements, we believe
that vibrational excitation opens the cone of acceptance to a
large range of impact parameters. Moreover, we believe that
greater than 30% of the products, namely the forward scat-
tered HCl( ϭ1,J) products, result from collisions at high
v
impact parameter and a T-shaped geometry in the transition
state region. The effects of these different reactive geom-
etries are not incorporated in the current theoretical models.
Clearly, higher dimensionality models are necessary to fully
understand the differences between the effects of the sym-
metric and antisymmetric stretches on the ClϩCH4 reaction.
Although the source of the increased reactivity of the sym-
metric stretch as compared to the antisymmetric stretch may
not be fully established, we find that it does not cause any
differences in product state or scattering distributions.
V. CONCLUSION
We have measured rovibrational and state-selected scat-
tering distributions for the HCl and CH3 products from the
ClϩCH4(␯1) and ClϩCH4(␯3) reactions. Detailed compari-
sons of these quantities show that there is no difference
within our resolution and measurement uncertainty between
the mechanisms of the ClϩCH4(␯1) and ClϩCH4(␯3) reac-
tions, despite theoretical and experimental results that show
the symmetric stretch (␯1) to be more reactive than the an-
tisymmetric stretch (␯3). The results presented here suggest
that the reactive event involves only a single C–H oscillator
Yoon et al.34 proposed a vibrationally adiabatic model
for the ClϩCH4 reaction similar to the models of Fair
et al.32 for the ClϩH2O reaction and Halonen et al.30 for the
reaction of CH4 on nickel surfaces: the approach of the Cl
atom causes the vibrational energy of the CH4 symmetric
and antisymmetric stretches to become localized in the
proximal and distal C–H bonds, respectively. As a conse-
quence, the symmetric stretch has more energy along the
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5103
J. Chem. Phys., Vol. 120, No. 11, 15 March 2004
Excitation in the ClϩCH4 reaction
and that vibrational excitation of a C–H stretch increases
reactivity by opening the cone of acceptance to allow periph-
eral reactions. We have presented a model in which the im-
pact parameter governs the state distributions and scattering
angle of the products.
Our results represent a counterexample of the mode se-
lectivity observed previously with other vibrationally excited
direct reactions. The lack of difference between the dynami-
cal effects of these nearly isoenergetic vibrations on the Cl
ϩCH4 reaction suggest that a symmetric C–H stretch be-
haves as an antisymmetric C–H stretch in determining prod-
uct formation. The apparent discrepancies between our re-
sults and others dictate that further experimental and
theoretical investigations are necessary to fully understand
the role that vibrations play in polyatomic reactions.
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