Crossed-beam reaction of carbon atoms with hydrocarbon molecules. IV. Chemical dynamics of methylpropargyl radical formation, C4H5, from reaction of C(3Pj) with propylene, C3H6 (X1A′)

Article abstract of DOI:10.1063/1.473543

The reaction between ground state carbon atoms and propylene, C₃H₆, was studied at average collision energies of 23.3 and 45.0 kJ mol^-1 using the crossed molecular beam technique. Product angular distributions and time-of-flight spectra of C₄H₅ at m/e=53 were recorded. Forward-convolution fitting of the data yields a maximum energy release as well as angular distributions consistent with the formation of methylpropargyl radicals. Reaction dynamics inferred from the experimental results suggest that the reaction proceeds on the lowest ³A surface via an initial addition of the carbon atom to the π-orbital to form a triplet methylcyclopropylidene collision complex followed by ring opening to triplet 1,2-butadiene. Within 0.3-0.6 ps, 1,2-butadiene decomposes through carbon-hydrogen bond rupture to atomic hydrogen and methylpropargyl radicals. The explicit identification of C₄H₅ under single collision conditions represents a further example of a carbon-hydrogen exchange in reactions of ground state carbon with unsaturated hydrocarbons. This versatile machine represents an alternative pathway to build up unsaturated hydrocarbon chains in combustion processes, chemical vapor deposition, and in the interstellar medium.

Full text of DOI:10.1063/1.473543

Crossed-beam reaction of carbon atoms with hydrocarbon molecules. IV. Chemical
dynamics of methylpropargyl radical formation, C4H5, from reaction of C(3Pj) with
propylene, C3H6 (X1A)
R. I. Kaiser, D. Stranges, H. M. Bevsek, Y. T. Lee, and A. G. Suits
Citation: The Journal of Chemical Physics 106, 4945 (1997); doi: 10.1063/1.473543
View online: http://dx.doi.org/10.1063/1.473543
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/106/12?ver=pdfcov
Published by the AIP Publishing
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Crossed-beam reaction of carbon atoms with hydrocarbon molecules. IV.
Chemical dynamics of methylpropargyl radical formation, C H ,
4
5
3
1
from reaction of C( P ) with propylene, C H (X AЈ)
j
3
6
R. I. Kaiser, D. Stranges,a) H. M. Bevsek, Y. T. Lee,b) and A. G. Suits
Department of Chemistry, University of California, Berkeley, California 94720 and Chemical Sciences
Division, Berkeley National Laboratory, Berkeley, California 94720
͑
Received 3 October 1996; accepted 19 December 1996͒
The reaction between ground state carbon atoms and propylene, C H , was studied at average
3
6
Ϫ1
collision energies of 23.3 and 45.0 kJ mol using the crossed molecular beam technique. Product
angular distributions and time-of-flight spectra of C H at m/eϭ53 were recorded. Forward-
4
5
convolution fitting of the data yields a maximum energy release as well as angular distributions
consistent with the formation of methylpropargyl radicals. Reaction dynamics inferred from the
3
experimental results suggest that the reaction proceeds on the lowest A surface via an initial
addition of the carbon atom to the ␲-orbital to form a triplet methylcyclopropylidene collision
complex followed by ring opening to triplet 1,2-butadiene. Within 0.3–0.6 ps, 1,2-butadiene
decomposes through carbon–hydrogen bond rupture to atomic hydrogen and methylpropargyl
radicals. The explicit identification of C H under single collision conditions represents a further
4
5
example of a carbon–hydrogen exchange in reactions of ground state carbon with unsaturated
hydrocarbons. This versatile machine represents an alternative pathway to build up unsaturated
hydrocarbon chains in combustion processes, chemical vapor deposition, and in the interstellar
medium. © 1997 American Institute of Physics. ͓S0021-9606͑97͒01812-6͔
I. INTRODUCTION
The spectroscopic identification of CH radicals, even in oxi-
dative hydrocarbon flames, opens an alternative pathway to
Sophisticated combustion models of oxidative hydrocar-
bon flames postulate that synthesis of polycyclic aromatic
hydrocarbons ͑PAHs͒ and formation of soot particles are
strongly related and are initiated by stepwise reaction of
smaller hydrocarbon radicals to cyclohexadienyl ͓reactions
5
C H isomers, since atomic hydrogen reacts in these media
4
5
with CH to atomic carbon and molecular hydrogen ͓reaction
3
͑
8͔͒ followed by a likely addition of C( P ) to the carbon–
j
carbon double bond of propylene to form an internally ex-
cited ͑*͒ triplet C H isomer ͓reaction ͑9͔͒. This collision
1͒–͑2͔͒ or benzene ͓reaction ͑3͔͒:1,2
4
6
͑
complex can decompose via hydrogen emission to C H :
4
5
i/n-C H ϩC H →1-C H ,
͑1͒
͑2͒
͑3͒
4
5
2
2
6
7
CH͑X ⌸1/2͒ϩH͑2S ͒→C͑ P ͒ϩH ͑X ⌺ ͒,
2
3
1
ϩ
g
͑8͒
͑9͒
i/n-C H ϩC H →c-C H ,
1/2
j
2
4
5
2
2
6
7
i/n-C H →C H ϩH.
6
7
6
6
3
C͑ P ͒ϩC H →C H*→C H ϩH.
j
3
6
4
6
4
5
However, the explicit mechanism to form distinct C H iso-
4
5
3
mers is still elusive. Wang and Frenklach as well as Millar
2
Besides their latent combustion relevance, propylene and
C H isomers are expected to contribute to interstellar chem-
istry. Although neither C H nor C H have been identified
and Melius assume reaction of vinyl radicals with acetylene
via a long-lived, rovibrationally excited ͑*͒ C H adduct
4
5
4
5
which fragments either to C H or is stabilized in a third
3
6
4
5
4
4
in the interstellar medium ͑ISM͒ explicitly, the extraterres-
trial propylene existence seems reasonable since unsubsti-
tuted C H was detected in the circumstellar envelope of the
body collision ͑M͒:
C H ϩC H →C H* ,
͑4͒
͑5͒
͑6͒
2
3
2
2
4
5
2
4
evolved carbon star IRCϩ10216 and the two lowest alkynes
acetylene ͑C H ͒ and methylacetylene ͑CH CCH͒ are ubiq-
C H*→C H ϩH,
4
5
4
4
2
2
3
6
C H*ϩM→C H ϩM.
uitous in the ISM. Beyond its interstellar relevance, Voy-
4
5
4
5
ager data depict propylene in Titan’s and in the upper Nep-
Weissman et al. pointed out reaction ͑4͒ should lead exclu-
7–10
tunian atmosphere.
Recent MeV and keV ion induced
4
sively to n-C H , but isomerization might transform iso to
11
4
5
collision cascade simulations in hydrocarbon ices as well
as planetary atmospheres show a production rate of, e.g.,
2
normal isomers via reaction ͑7͒, Fig. 1:
12
i-C H ϩH→n-C H ϩH.
͑7͒
70 suprathermal knock-on carbon atoms per impinging 12
keV carbon atom originating from the solar radiation field.
Since the knock-on atoms are born with kinetic energies up
to 1 keV, they cannot form stable chemical bonds and sur-
vive reducing planetary atmospheres to kinetic energies less
4
5
4
5
a͒Present address: Dipartimento Chimica, Universita La Sapienza, Piazzale
A. Moro 5, 00185 Rome, Italy.
Present address: Academia Sinica, Nankang, Taipei, 11529, Taiwan.
b͒
J. Chem. Phys. 106 (12), 22 March 1997
0021-9606/97/106(12)/4945/9/$10.00
© 1997 American Institute of Physics
4945
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4946
Kaiser et al.: Reaction of carbon atoms with hydrocarbon molecules. IV
TABLE I. Experimental beam conditions and 1␴ errors: most probable ve-
locity v , speed ratio S, most probable relative collision energy with the
0
propylene molecules, Ecoll , center-of-mass angle, ␪CM , and composition of
the carbon beam?: no information available.
Ϫ1
Ecoll , kJ molϪ1
Beam
v0 , m s
S
␪CM
C :C :C
1 2 3
3
C( P )/Ne 2090Ϯ50 5.2Ϯ0.3
23.3Ϯ1.0
45.0Ϯ4.0
•••
52.7Ϯ0.9 1:0.5:1.5
j
3
C( P )/He 3000Ϯ75 2.6Ϯ0.2
42.5Ϯ1.5
?
j
C H
783Ϯ12 8.3Ϯ0.2
•••
•••
3
6
action
͑
͑11͔͒,27
and
tricarbon-hydrides
͓reactions
12a/b͔͒:28–30
3
1
2
2
C͑ P ͒ϩC H ͑X A ͒→l-C H ͑X B ͒ϩH͑ S1/2͒,
͑10͒
j
2
4
g
3
3
2
3
1
2
2
C͑ P ͒ϩCH CCH͑X A ͒→nϪC H ͑X AЈ͒ϩH͑ S1/2͒,
j
3
1
4
3
͑
11͒
3
1
ϩ
g
2
2
C͑ P ͒ϩC H ͑X ⌺ ͒→c-C H͑X B ͒ϩH͑ S1/2͒ ͑12a͒
j
2
2
3
2
2
2
→
l-C H͑X ⌸ ͒ϩH͑ S1/2͒, ͑12b͒
3 j
and initially formed triplet collision complexes. Here, we
3
investigate the detailed chemical dynamics of C( P ) reac-
j
tive encounters with propylene at nominal collision energies
Ϫ1
Ϫ1
of 23.3 kJ mol and 45.0 kJ mol to tetracarbon hydrides
C H (xϭ0–5) of potential interstellar and combustion
4
x
FIG. 1. Structures of distinct C H isomers. Enthalpies of formations are
given in the text.
chemistry relevance.
4
5
II. EXPERIMENTAL SETUP
than ca. 10 eV. At the endpoints of their trajectories, atomic
carbon can react with unsaturated hydrocarbons such as pro-
pylene via reaction ͑9͒.
Reactive scattering experiments are performed in a uni-
versal crossed molecular beam apparatus.31 Briefly, a pulsed
supersonic carbon beam was generated via laser ablation of
graphite at 266 nm from a quadrupled Nd:YAG laser.32 The
present design of our carbon source differs slightly from Ref.
Despite the astrochemical and combustion potential, the
characterization of the doublet C H potential energy surface
4
5
͑PES͒ has been neglected. Low and high pressure pyrolysis
3
2. Both micro switches that triggered the polarity switch of
experiments indicate that 1- or 3-methylpropargyl ͑1͒/͑2͒
the stepper motor were replaced by a resistance based posi-
tion indicator interfaced to the rotating graphite rod. This
improved version prevents shorting of the micro switch cir-
cuit as graphite condenses during the operation. The 30 Hz,
35–40 mJ laser output is focused onto a rotating carbon rod,
and ablated carbon atoms are seeded into neon ͑99.999%,
Bay Area Gas͒ or helium ͑99.999%, Bay Area Gas͒ released
by a Proch-Trickl pulsed valve. A four slot chopper wheel is
mounted after the ablation zone and selects a 9.0 ␮s segment
of the seeded carbon beam. Table I compiles the experimen-
tal beam conditions. The pulsed carbon beam and a continu-
ous propylene ͑99.995%, Matheson͒ beam with 558Ϯ11
Torr backing pressure pass through skimmers and cross at
90° in the interaction region of the scattering chamber at
0
Ϫ1
hold the global minimum ͓⌬ H (1)ϭ295Ϯ10 kJ mol
;
f
0
⌬fH (2)ϭ294Ϯ3 kJ molϪ1͔,13–16 Fig. 1. Two less stable
0
0
radicals, 1,3-butadienyl-2 and 1,3-butadienyl-1 follow with
0
Ϫ1
0
⌬fH (3)ϭ344Ϯ10 kJ mol
and ⌬ H (4)ϭ357Ϯ10
0
f 0
Ϫ1
3,16,17
kJ mol , respectively.
Additionally, the enthalpy of
formation of the bicyclo͓1.1.0͔-butyl, ͑5͒, as well as the
butynyl-1 radical, ͑6͒, were determined to be 427Ϯ33 and
Ϫ1 18
485Ϯ17 kJ mol
.
Due to their high reactivity, strained
C H isomers ͑7͒–͑10͒ could only be identified in solid ma-
4
5
trices at 77 K via ESR spectroscopy after irradiating distinct
60
hydrocarbon precursors with Co ␥-rays, but thermodynami-
cal data are missing.1
9–24
Finally, a methylallyl isomer which
even resisted matrix trapping was stabilized by metal atoms
25
Ϫ1
as a ␮ -diosmiumate complex ͑11͒.
relative collision energies of 23.3 and 45.0 kJ mol . Reac-
3
The work reported here is part of an ongoing project to
tively scattered products were detected in the plane of the
beams using a rotatable detector with a Brink-type electron-
elucidate the potential energy surfaces and chemical reaction
3
33
dynamics of carbon atoms in their C( P ) electronic ground
impact ionizer, quadrupole mass filter, and a Daly ion
j
state with unsaturated hydrocarbons under single collision
conditions. Papers I–III disclosed precise information on the
formation of propargyl ͓reaction ͑10͔͒,26 ␣-ethinylvinyl ͓re-
detector34 at different laboratory angles between 5.0° and
60.0° with respect to the carbon beam. Velocity distributions
of the products were recorded using the time-of-flight ͑TOF͒
J. Chem. Phys., Vol. 106, No. 12, 22 March 1997
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Kaiser et al.: Reaction of carbon atoms with hydrocarbon molecules. IV
4947
FIG. 2. Lower: Newton diagram for the reaction C( P )ϩC H (X 1AЈ) at a
3
j
3
6
Ϫ1
collision energy of 23.3 kJ mol . The circle stands for the maximum
center-of-mass recoil velocity assuming no internal excitation. Upper: Labo-
ratory angular distribution of product channel at m/eϭ53. Circles and 1␴
error bars indicate experimental data, the solid lines the calculated distribu-
tions for the upper and lower carbon beam velocity. CM designates the
center-of-mass angle. The solid lines point to distinct laboratory angles
whose TOFs are shown in Fig. 4.
FIG. 3. Lower: Newton diagram for the reaction C( P )ϩC H (X 1AЈ) at a
3
j
3
6
Ϫ1
collision energy of 45.0 kJ mol . The circle stands for the maximum
center-of-mass recoil velocity assuming no internal excitation. Upper: Labo-
ratory angular distribution of product channel at m/eϭ53. Circles and 1␴
error bars indicate experimental data, the solid lines the calculated distribu-
tions for the upper and lower carbon beam velocity. Dotted lines show the
1
3
contribution from C( D ), dashed lines from C( P ). CM designates the
2
j
center-of-mass angle. The solid lines point to distinct laboratory angles
whose TOFs are shown in Fig. 5.
technique. Reference angles were chosen at 55° and 40° to
calibrate fluctuating carbon beam intensities and mass dial
settings at the quadrupole controller.
For the physical interpretation of the scattering data it is
necessary to transform the laboratory data into the center-of-
mass ͑CM͒ reference frame. A forward-convolution routine
C H parent in the ionizer, and exothermic channels 7–9 are
4
5
absent within detection limits ͑Table II͒. Additionally, no
radiative association to C H (m/eϭ54) or higher masses
4
6
is used to fit the TOF spectra and the product angular distri-
were observed. Endothermic channels 10 and 11 could not
bution in the laboratory frame ͑LAB distribution͒.3
5,36
This
Ϫ1
be opened at relative collision energies up to 45.0 kJ mol
procedure initially guesses the angular flux distribution T(␪)
employed in our experiments.
and the translational energy flux distribution P(E ) in the
T
center-of-mass system ͑CM͒. TOF spectra and the LAB dis-
B. LAB distribution and TOF spectra
tribution are calculated from T(␪) and P(E ) and refined
T
iteratively until a reasonable fit is achieved. The ultimate
outcome is the generation of a velocity flux contour map
showing the intensity as a function of angle and velocity in
the CM frame. This map serves as an image of the reaction
and contains all information of the scattering process.
Figures 2 and 3 show the most probable Newton dia-
3
1
grams of the reaction C( P )ϩC H (X AЈ) together with
j
3
6
the laboratory angular distributions ͑LAB͒ of the C H prod-
4
5
Ϫ1
uct at collision energies of 23.3 and 45.0 kJ mol , respec-
tively. At higher energy, the carbon beam contains contribu-
1
tions from electronically excited C( D ) state, whose
2
III. RESULTS
reaction dynamics with acetylene, ethylene, methylacetylene,
and propylene are subject of a forthcoming article.37 As the
collision energy Ecoll rises, the maximum of the LAB distri-
bution shifts from the center-of-mass angle ␪CMϭ52.7Ϯ0.9°
A. Reactive scattering signal
Reactive scattering signal was only detected at m/eϭ53,
i.e., C H , cf. Figs. 2–5. TOF spectra of lower m/e values
Ϫ1
͑Ecollϭ23.3 kJ mol ͒ to a slightly forward position of about
4
5
Ϫ1
3
between 52 and 48 were monitored and reveal identical pat-
terns. Therefore, this signal originates in cracking of the
40.0° versus ␪CMϭ42.5Ϯ1.0° ͓Ecollϭ45.0 kJ mol ; C( P )
j
contribution only͔. These data suggest a reduced lifetime of
J. Chem. Phys., Vol. 106, No. 12, 22 March 1997
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4948
Kaiser et al.: Reaction of carbon atoms with hydrocarbon molecules. IV
FIG. 4. Time-of-flight data at m/eϭ53 for laboratory angles 35.0, 40.0,
FIG. 5. Time-of-flight data at m/eϭ53 for laboratory angles 25.0, 30.0,
35.0, 40.0, 45.0, and 50.0° at a collision energy of 45.0 kJ mol . Open
Ϫ1
Ϫ1
5
0.0, 52.5, 55.0, and 60.0° at a collision energy of 23.3 kJ mol . Open
circles represent experimental data, the solid line the fit. TOF spectra have
been normalized to the relative intensity at each angle.
circles represent experimental data, the solid line the fit. Dotted lines show
1
3
the contribution from C( D ), dashed lines from C( P ). TOF spectra have
2
j
been normalized to the relative intensity at each angle.
the decomposing C H complex with rising collision energy.
4
6
mum translational energy is consistent with the formation of
methylpropargyl isomers. Further, the most probable transla-
tional energy yields an order-of-magnitude of the barrier
Further, both LAB distributions are very broad and spread
about 50° in the scattering plane. This finding proposes a
large energy release into translational degrees of freedom of
the C H and H products as well as a center-of-mass trans-
height in the exit channel. Both P(E )s show a broad plateau
T
4
5
lational energy distribution peaking away from zero. A com-
parison of the scattering range at lower energy with the limit
circle of the methylpropargyl isomers correlates with the sig-
nal cut off at 25° and suggests a significant contribution of
these isomers to the reactive scattering signal.
3
1
TABLE II. Thermochemistry of the reaction C( P )ϩC H (X AЈ) to C H
j
3
6
4
x
products. Channels were calculated for the most stable isomer, i.e., C H
4
4
͑
butatriene; channel 7͒, C H ͑␣-ethinylvinyl; channel 8͒, C H ͑diacetylene;
4
3
4
2
channel 9͒, C H ͑tetracarbonhydride; channel 10͒, and C ͑tetracarbon,
4
4
channel 11͒.
Free reaction enthalpy
C. Center-of-mass translational energy distributions,
P(ET)
at 0 K, ⌬ H͑0 K͒,
R
Ϫ1
#
Exit channel
kJ mol
2
2
1
2
HCCCHCH (X AЉ)ϩH ( S1/2)
Ϫ220Ϯ10
Ϫ221Ϯ3
Ϫ171Ϯ10
Ϫ158Ϯ10
Ϫ88Ϯ33
Ϫ30Ϯ17
Ϫ442Ϯ2
Ϫ5Ϯ35
The translational energy distributions P(E ) and the an-
gular distributions T(␪) in the center-of-mass frame are pre-
sented in Figs. 6 and 7. Best fits of TOF spectra and LAB
3
T
2
2
CH CCCH (X B )ϩH ( S1/2)
3
2
1
2
2
3
4
5
6
7
8
9
0
1
H CHCCCH (X AЈ)ϩH ( S1/2)
2 2
2
2
H CHCHCHC (X AЈ)ϩH ( S1/2)
c-C H (X AЈ)ϩH ( S1/2)
4 5
C H CC (X AЈ)ϩH ( S )
2 5 1/2
C H (X A )ϩH (X ⌺ )
C H (X AЈ)ϩH (X ⌺ )ϩH( S1/2)
2
distributions were achieved with P(E )s extending to
2
2
T
Ϫ1
Ϫ1
2
2
Emaxϭ200–225 kJ mol and 260–280 kJ mol , respec-
tively. These energy cutoffs can be utilized to identify the
product isomer if their energetics are well separated. At
lower collision energy, Emax suggests formation of methyl-
propargyl and/or 1,3-butadienyl radicals within the error lim-
1
1
ϩ
g
4
4
2
g
2
1
ϩ
2
4
3
2
g
1
ϩ
1
ϩ
C H (X ⌺ )ϩ2H (X ⌺ )
Ϫ279Ϯ12
ϩ243Ϯ5
ϩ232Ϯ5
4
2
2
g
2
g
ϩ
1
ϩ
2
1
1
C H (X ⌺ )ϩ2H (X ⌺ )ϩH( S1/2)
4 2 g
C (X 3⌺ )ϩ3H (X ⌺ )
Ϫ
g
1
ϩ
g
4
2
Ϫ1
its cf. Fig. 1 and Table III. At Ecollϭ45.0 kJ mol , the maxi-
J. Chem. Phys., Vol. 106, No. 12, 22 March 1997
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Kaiser et al.: Reaction of carbon atoms with hydrocarbon molecules. IV
4949
FIG. 6. Lower: Center-of-mass angular flux distribution for the reaction
FIG. 7. Lower: Center-of-mass angular flux distribution for the reaction
3
1
Ϫ1
3
1
Ϫ1
C( P )ϩC H (X AЈ) at a collision energy of 23.3 kJ mol . Upper:
C( P )ϩC H (X AЈ) at a collision energy of 45.0 kJ mol . Upper:
j
3
6
j 3 6
Center-of-mass translational energy flux distribution for the reaction
Center-of-mass translational energy flux distribution for the reaction
3
1
Ϫ1
3
1
Ϫ1
C( P )ϩC H (X AЈ) at a collision energy of 23.3 kJ mol . Dashed and
C( P )ϩC H (X AЈ) at a collision energy of 45.0 kJ mol . Dashed and
j
3
6
j 3 6
solid lines limit the range of acceptable fits within 1␴ error bars.
solid lines limit the range of acceptable fits within 1␴ error bars.
between 15 and 60 kJ molϪ1, indicating a tight exit transition
state as well as a significant electron density change from the
C H complex to the products. A potential energy barrier in
carbon atom and the leaving hydrogen atom must be located
forward peaking at higher collision energy requires that the
on opposite sites of the rotation axis of the fragmenting com-
plex.
4
6
the exit channel is further indicated by the large fraction of
total available energy released into translational motion of
the products, i.e., 31Ϯ4% and 34Ϯ3% at 23.3 and 45.0
The lack of polarization of the angular distribution at
Ϫ1
Ecollϭ23.3 kJ mol is the result of a poor correlation be-
Ϫ1
kJ mol , respectively.
tween the initial and final angular momentum vectors, L and
LЈ, respectively.2
6,38,39
Calculating the maximum impact pa-
rameter bmax and the maximum orbital angular momentum
Lmax within the orbiting limit and approximating the
Lennard-Jones coefficient C6 according to Hirschfelder
et al.40 with the ionization potentials of C( Pj) and propy-
lene of 11.76 eV, and 10.36 eV, respectively, and polariz-
D. Center-of-mass angular distributions, T(␪)
3
As the collision energy rises, the shape of both T(␪)s
changes significantly. At lower collision energy, the T(␪) is
isotropic and symmetric around ␲/2 suggesting that the
decomposing C H complex has a lifetime longer than its
3
Ϫ30
Ϫ1
3
abilities of C( Pj) and propylene, 1.76*10
m
and
Ϫ30
3 41
6.24*10
m , bmax yields bmax͑23.3 kJ mol ͒ϭ3.7 Å,
4
6
Ϫ1
Ϫ1
rotational period or that the exit transition state is
bmax͑45.0 kJ mol ͒ϭ3.3 Å, Lmax͑23.3 kJ mol ͒ϭ122ប and
2
6,38,39
Ϫ1
symmetric.
With rising collision energy, the center-of-
Lmax͑45.0 kJ mol ͒ϭ151ប. If we compare this order-of-
mass angular distribution peaks forward with respect to the
carbon beam and shows an intensity ratio at the poles of
I(0°)/I(180°)ϭ6Ϯ3. This trend is reflected in the center-
magnitude calculation with the final orbital angular momen-
tum LЈ as derived from acceptable exit impact parameters,
of-mass flux contour maps I(␪)ϳT(␪) P(E ) as well, cf.
*
T
TABLE III. Lifetime t in ps of triplet cis and trans 1,2-butadiene calculated
Figs. 8 and 9. Our results strongly indicate a reduced lifetime
of the C H intermediate and suggest an osculating complex:
Ϫ1
for rotations about the A, B, and C axis at collision energy of 45.0 kJ mol
.
4
6
2
Errors are 28%. Units of principal moments of inertia are given in amu Å .
a complex formation takes place, but the well depth along
the reaction coordinate is too shallow to allow multiple rota-
tions, and the complex decomposes with a random lifetime
distribution before one full rotation elapses. In Sec. IV, we
identify this complex and estimate its lifetime. Further, the
Isomer
IA
IB
IC
t (A)
t (B)
t (C)
cis
trans
14.5949
6.7936
79.3413
99.8642
93.9362
106.6578
0.05
0.02
0.3
0.4
0.35
0.55
J. Chem. Phys., Vol. 106, No. 12, 22 March 1997
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4950
Kaiser et al.: Reaction of carbon atoms with hydrocarbon molecules. IV
FIG. 8. Contour flux map distribution for the reaction C( P )ϩC H (X 1AЈ) at a collision energy of 23.3 kJ molϪ1
3
.
j
3
6
cf. Sec. V and Fig. 10, we find LϽ0.2LЈ. Therefore, most of
the initial orbital angular momentum channels into rotational
excitation of the C H products to yield a flat angular distri-
of the carbon atom with the ␲-electron density, and no in-
sertion into the olefinic C–H bond was verified.26 Finally,
the hydrogen atoms screen the aliphatic C–C bond in propy-
lene from insertion: even hot atom tracer experiments with
4
5
bution at lower collision energy. This result is the direct
11
3
44
consequence of large impact parameters leading to C H6
C( P ) show no insertion into C–C single bonds.
4
j
complex formation and the inability of the hydrogen atom to
carry away a significant amount of orbital angular momen-
tum.
B. Addition pathway
The reaction dynamics of the title reaction are controlled
by addition of the carbon atom to the olefinic carbon–carbon
bond of the propylene molecule under C symmetry to form
V. DISCUSSION
1
In this section, we investigate energetically feasible re-
action pathways on the triplet C H PES via insertion of the
methylcyclopropylidene, cf. Fig. 10. This pathway does not
require solely an on-axis approach of the C( Pj) p-orbital
3
4
6
electrophile carbon atoms into the C–H– and C–C bonds of
propylene as well as addition to the ␲-molecular orbital. The
experimental CM angular and translational energy distribu-
tions are then compared to what is expected based on these
postulated channels. Since no C H intermediate fulfills re-
toward the ␲-molecular orbital, but permits trajectories in
which p-orbitals are skewed with respect to the propylene
plane. This opens larger impact parameters than typical C3H6
bond dimensions compared to the large b dominated opacity
3
function as discussed in Sec. IV D. Initially, C( Pj) attacks
4
6
42
quirements for intersystem crossing, the discussion is re-
either the ␣-carbon atom, i.e., the neighboring carbon atom
to the CH3 group, or the ␤-C atom holding the CH2 unit prior
to ring closure. If we apply the framework of regioselectivity
of electrophilic attacks to substituted olefines,45 we discrimi-
nate between these two options. This concept predicts the
electrophilic attack is directed preferentially to the carbon
3
stricted to the triplet surface. We establish that C( P ) inter-
j
acts with the ␲-electron density to form methylcyclo-
propylidene followed by ring opening to triplet 1,2-
butadiene. This complex decomposes to a methylpropargyl
radical and atomic hydrogen.
center retaining the maximum spin density of the first excited
A. Insertion pathway
3
␲
␲* state. Since ab initio calculations show that the ␤-C
As outlined in the previous section, large impact param-
eters up to bϭ3.7 Å contribute predominantly to the reactive
atom holds a spin density of 1.232 versus 1.216 at the ␣-C,
3
C( P ) should attack preferentially the ␤-position. In addi-
j
scattering signal. If we compare this dimension with impact
tion, the methyl group reduces the cone of acceptance on the
␣-C atom as well and directs the approach even more to the
␤-position. The pathway to methylcyclopropylidene corre-
3
parameters to C( P ) insertion into the olefinic C–H– and
j
C–C as well as aliphatic C–H bonds of propylene, we esti-
mate an upper limit of only 1.4, 0.9, and 1.5 Å. Therefore,
any insertion can be very likely ruled out. This conclusion
lates with previous 11C( P ) bulk experiments, in which cy-
3
j
44
clopropylidene intermediates were trapped as spiranes.
correlates strongly with related systems studied recently in
The fate of the methylcyclopropylidene is governed ei-
ther by hydrogen migration to methylcyclopropane isomers
or conrotatory ring opening to cis/trans 1,2-butadiene. Any
H rearrangement, however, can be excluded: the methylcy-
clopropane isomers would undergo a subsequent C–H bond
3
our group. Here, no C( P ) insertion into an aliphatic C–H
j
bonds of CH ͑Ref. 43͒ and CH group in methylacetylene
4
3
2
7
was found.
Further, the chemical dynamics of the
3
C( P )/C H system are solely determined by the interaction
j
2
4
FIG. 9. Contour flux map distribution for the reaction C( P )ϩC H (X 1AЈ) at a collision energy of 45.0 kJ molϪ1
3
.
j
3
6
J. Chem. Phys., Vol. 106, No. 12, 22 March 1997
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Kaiser et al.: Reaction of carbon atoms with hydrocarbon molecules. IV
4951
FIG. 10. Schematic representation of the lowest energy pathways on the triplet C H PES and structures of potentially involved collision complexes. ?: no
4
6
information available ͑Refs. 13–25,41͒.
rupture to a carbontricycle. Even the energetically most fa-
vorable isomer, methylcyclopropenyl, is at least ca. 100
teracting H atoms ͑pathway II͒, the transition state in I is
anticipated to be energetically less favorable. Hence, trans-
1,2-butadiene should be formed preferentially. Since no ab
initio frequencies of the transition states are available, we
cannot quantify the relative fraction of the clockwise versus
counterclockwise direction.
Ϫ1
kJ mol less stable than methylpropargyl. Therefore, the re-
Ϫ1
action exothermicity of only 130 kJ mol cannot account
for the high energy cutoffs of both translational energy dis-
tributions. Therefore, the conrotatory ring opening to 1,2-
butadiene remains the only open path. This ring opening can
proceed via a clockwise–clockwise ͑I͒ as well as counter-
1,2-butadiene either undergoes hydrogen rearrangement
to 1,3-butadiene, dimethyacetylene, or ethyl-acetylene or
fragments via C–H bond rupture to methylpropargyl. Since
these H migrations are symmetry allowed, reaction to 1,3-
butadiene is expected to involve a barrier much less than at
clockwise–counterclockwise ͑II͒ rotation of the CH and
2
CH͑CH ͒ units, cf. Fig. 11. Pathway I leads solely to cis
3
1,2-butadiene, whereas II forms only the trans isomer. Since
Ϫ1
an enhanced repulsive potential between the CH group and
least 140 kJ mol from 1,2-butadiene to acetylene deriva-
3
the hydrogen atom is expected in pathway I versus two in-
tives. However, both acetylene derivatives can likely be ex-
FIG. 11. Schematic representation of methycyclopropylidene ring opening to triplet cis/trans 1,2-butadiene. Top: clockwise–clockwise; bottom:
counterclockwise–counterclockwise.
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4952
Kaiser et al.: Reaction of carbon atoms with hydrocarbon molecules. IV
TABLE IV. Fractional energy release into the translational
Erot͘, and vibrational, ͗Evib͘ degrees of freedom calculated for Kϭ0 at both
collision energies.
ET
͗ ͘
, rotational,
cluded from further discussion because the reaction proceeds
via the lowest energy pathway. In addition, the rigorous
identification of methylpropargyl at collision energy of 45.0
͗
Ϫ1
Ecollϭ23.3 kJ molϪ1
Ecollϭ45.0 kJ molϪ1
kJ mol explicitly rules out a ͓1,3͔-H-shift to 1,3-butadiene,
since the last would decompose to 1,3-butadienyl-1 or 1,3-
butadienyl-2. Even at our lowest applied collision energy,
the formation of 1,3-butadienyl-2 seems rather unlikely.
͗
͗
ET
Erot
͗Evib
͘
͘
͘
31Ϯ4
15Ϯ2
57Ϯ2
34Ϯ3
7Ϯ1
57Ϯ2
First, only the lowest limit of our high energy P(E ) cutoff
T
overlaps with the maximum error as expected from thermo-
dynamical data. Further, an increase in collision energy by
only 20 kJ mol hardly explains the formation of only 1,3-
Ϫ1
Since reaction with collision times less then 0.1 ps follow
direct dynamics,46 T(␪) should be strongly forward peaked
butadienyl-2 at lower, but methylpropargyl radicals at higher
collision energy. Therefore, we conclude the reaction most
likely proceeds via 1,2-butadiene and a carbon-hydrogen
bond cleavage to methylpropargylene. Since no symmetry
element is conserved from the initial addition to the final
Ϫ1
at the collision energy of 45.0 kJ mol . Therefore, rotations
around the A axis of cis/trans 1,2-butadiene can be clearly
ruled out and only B- and/or C-like rotations account for the
reactive scattering signal. Here, the almost in-plane C-like
rotations give rise to extremely low K states of 1,2-
butadiene. This in-plane rotation could be associated with
preferentially low K values populated in the methylpropargyl
product ͑Sec. V D͒.
3
fragmentation step, the reaction can proceed on the A sur-
face.
If we compare these dynamics with those of the reaction
3
26
C( P )ϩC H to propargyl and atomic hydrogen, we find
j
2
4
that the additional modes of the CH group in the triplet
3
D. Exit transition state and energy partitioning
1,2-butadiene intermediate enhance the lifetime of the de-
composing complex as compared to triplet allene. This yields
a symmetric center-of-mass angular distribution at 23.3
kJ mol collision energy as the result of a decomposing
Since both translational energy distributions P(E )s
peak at 15–60 kJ mol , the C–H bond rupture in triplet
,2-butadiene does not represent an ideal RRKM system
T
Ϫ1
Ϫ1
1
1,2-butadiene complex holding a lifetime equal or exceeding
with a loose transition state. Our data rather indicate a sig-
nificant geometry change from the fragmenting complex to
the methylpropargyl radicals as evident from carbon–carbon
bond lengths reduced by 0.1–0.2 Å. Further, the bond angle
of the allene-subunit in 1,2-butadiene opens up by 50°–55°
to a linear C–C–C–chain. The order of magnitude of the exit
barrier is consistent when compared to the reaction
its rotational period. As the collision energy increases, the
angular distribution changes to a more forward peaked one
as a consequence of the reduced 1,2-butadiene lifetime. The
alternative interpretation of a symmetric exit transition state
contributing to a symmetric angular distribution can be ruled
out, since the fragmenting 1,2-butadiene belongs to the C1
point group, and no H atoms can be interconverted to depart
with equal probability into the center-of-mass angles ␪ and
3
26
C( P )ϩC H →l-C H ϩH. Here, the allene intermediate
j
2
4
3
3
fragments to propargyl radicals and atomic hydrogen, and
the barrier ranges between 28 and 43 kJ mol
Ϫ1
␲-␪.
.
In addition, we analyze the partition of total available
energy into product rotation, vibration, and translation. This
enables us to compare the collision energy dependent frac-
tional energy release in the vibrational degrees of freedom.
When these data are compared to the C/C H system, the
C. Rotation axis and lifetime of the 1,2-butadiene
complex
In the following paragraph, we investigate the rotational
motion of the cis/trans 1,2-butadiene complexes to discrimi-
nate between 1- and 3-methylpropargyl. Since the angular
distribution is forward peaked at higher collision energy, this
requires the attacking carbon atom and the leaving hydrogen
to be located on opposite sites of the rotation axis. Therefore,
trans 1,2-butadiene complexes excited to B- and C-like ro-
tations decompose solely through C–H1 rupture to
2
4
role of the propylene CH group to boost the lifetime of the
3
fragmenting complex is elucidated. Since both methylprop-
argyl radicals represent prolate asymmetric tops with asym-
metry parameters ␬ϭϪ0.99 and Ϫ0.81, 3- and 1-methylpro-
pargyl, respectively, their energy levels are approximated as
those of an ideal, rigid symmetric top. Since our crossed
beam experiments cannot resolve the rotational structure, no
information on the K distribution can be supplied. Hence, we
estimate the maximum vibrational energy release choosing
Kϭ0. Our calculation reveals an almost constant fraction of
57Ϯ2% channeling into the vibration, cf. Table IV. The re-
3-methylpropargylene. On the other hand, the cis isomer can
fragment via C–H /H ͑A-rotations͒ as well as C–H rupture
2
3
2
͑C-rotations; C-axis perpendicular to the molecular plane͒ to
1
-methylpropargyl, but via C–H cleavage ͑B-rotations͒ to
1
3
26
3-methylpropargyl.
action C( P )ϩC H →l-C H ϩH however, shows a de-
j
2
4
3
3
We reduce the feasible rotational axis further, if we es-
creasing fraction of vibrational energy release from 50% to
43% in the triplet allene intermediate as the collision energy
rises. These patterns clearly indicate an enhanced partition of
the total available energy into vibration as the vibrational
degrees of freedom rise from 15 ͑decomposing allene com-
plex, C H ͒ to 24 ͑fragmenting 1,2-butadiene, C H ͒.
timate the lifetime of 1,2-butadiene in terms of the osculating
complex model. Based on the intensity ratio of 6Ϯ3 at the
poles of the angular distribution, the lifetime is calculated to
be approximately one third of a rotation period, i.e., 0.02–
0.05 ps ͑A-axis͒, and 0.3–0.55 ps ͑B/C-axes͒, cf. Table III.
3
4
4
6
J. Chem. Phys., Vol. 106, No. 12, 22 March 1997
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Kaiser et al.: Reaction of carbon atoms with hydrocarbon molecules. IV
4953
1
VI. CONCLUSIONS
P. R. Westmoreland, A. M. Dean, J. B. Howard, and J. P. Longwell, J.
Phys. Chem. 93, 8171 ͑1989͒.
J. A. Millar and C. F. Melius, Comb. Flame 91, 21 ͑1992͒.
H. Wang and M. Frenklach, J. Phys. Chem. 98, 11 465 ͑1994͒.
M. A. Weissman and S. W. Benson, J. Phys. Chem. 92, 4080 ͑1988͒.
W. C. Hung, M. L. Huang, Y. C. Lee, and Y. P. Lee, J. Chem. Phys. 103,
The reaction between ground state carbon atoms,
2
3
4
5
3
1
C( P ), and propylene, C H (X AЈ), was studied at average
j
3
6
Ϫ1
collision energies of 23.3 and 45.0 kJ mol
using the
crossed molecular beam technique. This reaction proceeds
3
9941 ͑1995͒.
on the A surface via an addition of the carbon atom to the
6
7
E. Herbst, Angew. Chem. 102, 627 ͑1990͒.
Symposium on Titan, ESA, ESA Special Publication 338 ͑ESTEC, Noord-
wijk, 1992͒.
J. I. Lunne, Chemical Eng. News 1995, 52.
D. Toublanc, J. P. Parisot, J. Brillet, D. Gautier, F. Raulin, and C. P.
McKay, Icarus 113, 2 ͑1995͒.
␲-orbital to form triplet methylcyclopropylidene, followed
by ring opening to cis/trans-1,2-butadiene. Within 0.3–0.6
ps, this complex decomposes via carbon–hydrogen bond
rupture to atomic hydrogen and methylpropargyl radicals.
8
9
3
Compared to the C( P )/C H system studied recently in our
j
2
4
2
6
10
group, the methyl group enhances the lifetime of the de-
composing intermediate and reduces the cone of acceptance
of the approaching carbon atom to propylene. This effects
the approach geometries significantly and closes the strongly
forward-scattered microchannel to high K-states excited pro-
pargyl isomer, l-C H , as found in Ref. 26. Finally, the CH
K. H. Baines, M. E. Mickelson, L. E. Larson, and D. W. Ferguson, Icarus
114, 328 ͑1995͒.
1
1
1
1
2
3
R. I. Kaiser and K. Roessler, Astrophys. J. 475, 144 ͑1997͒.
A. M. Mansour, H. A. Elkhalk, and K. Roessler, Report Jul-3002 ͑1994͒.
K. Ohta, M. Shiotani, J. Sohma, and A. Hasegawa, M. C. R. Symons,
Chem. Phys. Lett. 136, 465 ͑1987͒.
T. T. Nguyen and K. D. King, J. Phys. Chem. 85, 3130 ͑1981͒.
T. T. Nguyen and K. D. King, Int. J. Chem. Kin. 14, 613 ͑1982͒.
R. D. Kern, H. J. Singh, and C. H. Wu, Int. J. Chem. Kin. 20, 731 ͑1988͒.
R. P. Duran, V. Amorebieta, and A. J. Colussi, J. Phys. Chem. 92, 636
14
3
3
3
3
15
group breaks the symmetry of the reaction surface to A vs
3
3
16
AЉ as found in the C( P )/C H system. The explicit iden-
j
2
4
17
tification of C H under single collision conditions represents
4
5
͑
1988͒.
an additional example of a carbon–hydrogen exchange in
reactions of ground state carbon with unsaturated hydrocar-
bons. This versatile concept represents an alternative path-
way to form hydrocarbon radicals in combustion processes,
chemical vapor deposition, and the interstellar medium.
Further investigations of this reaction should focus on
the hitherto unresolved isomer assignment, i.e., 1- versus
1
1
8
9
P. K. Chou and S. R. Kass, J. Am. Chem. Soc. 113, 697 ͑1991͒.
P. J. Krusic, J. P. Jesson, and J. K. Kochi, J. Am. Chem. Soc. 91, 4566
͑
1969͒.
20
C. Roberts and J. C. Watson, J. Chem. Soc. Perkin. Trans. 879 ͑1983͒.
F. W. King and H. Schlegel, J. Magn. Res. 22, 389 ͑1976͒.
T. T. Nguyen and K. D. King, Int. J. Chem. Kin. 14, 613 ͑1982͒.
T. T. Nguyen and K. D. King, J. Phys. Chem. 85, 3130 ͑1981͒.
S. Olivena and A. Scole, J. Am. Chem. Soc. 113, 87 ͑1991͒.
H. Chen, B. F. G. Johnson, J. Lewis, D. Braga, and F. Grepioni, J. Orag-
nometallic Chem. 398, 159 ͑1990͒.
2
2
2
2
2
1
2
3
4
5
3-methylpropargyl. If the adiabatic ionization potentials dif-
fer by more than 0.5 eV, this system resembles an ideal
candidate to be investigated via VUV photon induced ioniza-
tion on the chemical dynamics beamline at the advanced
light source ͑ALS͒. Further, photolysis of the reaction prod-
ucts at 193 nm in the interaction region yields HϩC H or
26
R. I. Kaiser, Y. T. Lee, and A. G. Suits, J. Chem. Phys. 105, 8705
͑
1996͒.
2
2
7
8
R. I. Kaiser, D. Stranges, Y. T. Lee, and A. G. Suits, J. Chem. Phys. 105,
8721 ͑1996͒.
R. I. Kaiser, Y. T. Lee, and A. G. Suits, J. Chem. Phys. 103, 10 395
4
4
C HϩC H ͑1-methylpropargyl͒, but CH ϩC H and
2
2
4
3
3
2
͑
1995͒.
C H ϩCH in the case of 3-methylpropargyl. Finally, par-
3
3
2
29
30
R. I. Kaiser, C. O. Ochsenfeld, M. Head-Gordon, Y. T. Lee, and A. G.
Suits, Science 274, 1508 ͑1996͒.
C. Ochsenfeld, R. I. Kaiser, Y. T. Lee, A. G. Suits, and M. Head-Gordom,
J. Chem. Phys. ͑to be published͒.
Y. T. Lee, J. D. McDonald, P. R. LeBreton, and D. R. Herschbach, Rev.
Sci. Instrum. 40, 1402 ͑1969͒.
R. I. Kaiser and A. G. Suits, Rev. Sci. Instrum. 66, 5405 ͑1995͒.
G. O. Brink, Rev. Sci. Instrum. 37, 857 ͑1966͒.
N. R. Daly, Rev. Sci. Instrum. 31, 264 ͑1960͒.
M. S. Weis, Ph.D. thesis, University of California, Berkeley ͑1986͒.
M. Vernon, LBL-Report 12 422 ͑1981͒.
R. I. Kaiser, D. Stranges, Y. T. Lee, and A. G. Suits ͑unpublished͒.
W. B. Miller, S. A. Safron, and D. R. Herschbach, Discuss Faraday. Soc.
tially deuterated propylene represents an excellent option if
the 1,2-3-deutero-butadiene complex fragments via C–D
bond rupture, we will detect 1-methylpropargyl at m/eϭ53;
H atom loss, however, gives rise to m/eϭ54 pattern. The
mission continues.
31
3
3
2
3
Note added in proof. Prior to acceptance of this manu-
script, we performed a crossed beam reaction of C( P ) with
3
j
34
35
36
37
the second C H isomer, cyclopropylene. However, no reac-
3
6
tive scattering signal could be probed, and the reaction cross
section is at least 50 times lower as compared to the title
reaction.
38
44 108, 291 ͑1967͒.
3
4
9
0
W. B. Miller, Ph.D. thesis, Harvard University, Cambridge ͑1969͒.
J. O. Hirschfelder, C. F. Curtiss, and R. B. Bird, Molecular Theory of
Gases and Liquids ͑Wiley, New York, 1954͒.
Handbook of Chemistry and Physics ͑CRC Press, Boca Raton, 1995͒.
N. J. Turro, Modern Molecular Photochemistry ͑University Science
Books, Milla Valley, 1991͒.
R. I. Kaiser, Y. T. Lee, and A. G. Suits ͑unpublished͒.
See discussion in Ref. 26.
S. S. Shaik and E. Canadell, J. Am. Chem. Soc. 112, 1446 ͑1990͒.
R. D. Levine, and R. B. Bernstein, Molecular Reaction Dynamics and
Chemical Reactivity ͑Oxford University Press, Oxford, 1987͒.
ACKNOWLEDGMENTS
R.I.K. is indebted the Deutsche Forschungsgemeinschaft
for a post-doctoral fellowship. We thank the electronics
shop, Department of Chemistry, UC Berkeley, for designing
the new resistance triggered polarity switchbox of our carbon
source. This work was supported by the Director, Office of
Energy Research, Office of Basic Energy Sciences, Chemical
Sciences Division of the U.S. Department of Energy under
Contract No. De-AC03-76SF00098.
4
4
1
2
4
4
3
4
45
46
J. Chem. Phys., Vol. 106, No. 12, 22 March 1997
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