Crossed-beam reaction of carbon atoms with hydrocarbon molecules. II. Chemical dynamics of n-C4H3 formation from reaction of C(3Pj) with methylacetylene, CH3CCH (X1A1)

Article abstract of DOI:10.1063/1.472677

The reaction between ground-state carbon atoms, C(³P_j), and methylacetylene, CH₃CCH (X¹A₁), was studied at average collision energies of 20.4 and 33.2 kJ mol^-1 using the crossed molecular beams technique. Product angular distributions and time-of-flight spectra of C₄H₃ at m/e=51 were recorded. Forward-convolution fitting of the data yields weakly polarized center-of-mass angular flux distributions isotropic at lower, but forward scattered with respect to the carbon beam at a higher collision energy. The translational energy flux distributions peak at 30-60 kJ mol^-1 and show an average fractional translational energy release of 22%-30%. The maximum energy release as well as the angular distributions are consistent with the formation of the n-C₄H₃ radical in its electronic ground state. Reaction dynamics inferred from these distributions indicate that the carbon atom attacks the π-orbitals of the methylacetylene molecule via a loose, reactant like transition state located at the centrifugal barrier. The initially formed triplet 1-methylpropendiylidene complex rotates in a plane almost perpendicular to the total angular momentum vector around the BC-axes and undergoes [2,3]-hydrogen migration to triplet 1-methylpropargylene. Within 1-2 ps, the complex decomposes via C-H bond cleavage to n-C₄H₃ and atomic hydrogen. The exit transition state is found to be tight and located at least 30-60 kJ mol^-1 above the products. The explicit identification of the n-C₄H₃ radical under single collision conditions represents a further example of a carbon-hydrogen exchange in reactions of ground state carbon atoms with unsaturated hydrocarbons. This channel opens a versatile pathway to synthesize extremely reactive hydrocarbon radicals relevant to combustion processes as well as interstellar chemistry.

Full text of DOI:10.1063/1.472677

Crossedbeam reaction of carbon atoms with hydrocarbon molecules. II. Chemical
dynamics of nC4H3 formation from reaction of C(3 P j ) with methylacetylene, CH3CCH
(X 1 A 1)
R. I. Kaiser, D. Stranges, Y. T. Lee, and A. G. Suits
Citation: The Journal of Chemical Physics 105, 8721 (1996); doi: 10.1063/1.472677
View online: http://dx.doi.org/10.1063/1.472677
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/105/19?ver=pdfcov
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Crossed-beam reaction of carbon atoms with hydrocarbon molecules. II.
Chemical dynamics of n-C4H3 formation from reaction of C(3Pj) with
methylacetylene, CH3CCH (X1A1)
R. I. Kaiser, D. Stranges,a) Y. T. Lee,b) and A. G. Suitsc)
Department of Chemistry, University of California, Berkeley, California 94720
and Chemical Sciences Division, Berkeley National Laboratory, Berkeley, California 94720
͑Received 28 June 1996; accepted 9 July 1996͒
The reaction between ground-state carbon atoms, C͑3Pj͒, and methylacetylene, CH3CCH (X1A1),
was studied at average collision energies of 20.4 and 33.2 kJ molϪ1 using the crossed molecular
beams technique. Product angular distributions and time-of-flight spectra of C4H3 at m/eϭ51 were
recorded. Forward-convolution fitting of the data yields weakly polarized center-of-mass angular
flux distributions isotropic at lower, but forward scattered with respect to the carbon beam at a
higher collision energy. The translational energy flux distributions peak at 30–60 kJ molϪ1 and
show an average fractional translational energy release of 22%–30%. The maximum energy release
as well as the angular distributions are consistent with the formation of the n-C4H3 radical in its
electronic ground state. Reaction dynamics inferred from these distributions indicate that the carbon
atom attacks the ␲-orbitals of the methylacetylene molecule via a loose, reactant like transition state
located at the centrifugal barrier. The initially formed triplet 1-methylpropendiylidene complex
rotates in a plane almost perpendicular to the total angular momentum vector around the BگC-axes
and undergoes ͓2,3͔-hydrogen migration to triplet 1-methylpropargylene. Within 1–2 ps, the
complex decomposes via C–H bond cleavage to n-C4H3 and atomic hydrogen. The exit transition
state is found to be tight and located at least 30–60 kJ molϪ1 above the products. The explicit
identification of the n-C4H3 radical under single collision conditions represents a further example of
a carbon–hydrogen exchange in reactions of ground state carbon atoms with unsaturated
hydrocarbons. This channel opens a versatile pathway to synthesize extremely reactive hydrocarbon
radicals relevant to combustion processes as well as interstellar chemistry. © 1996 American
Institute of Physics. ͓S0021-9606͑96͒00739-8͔
0.08 kJ molϪ1 ͑dark clouds͒, gas phase reactions under ther-
I. INTRODUCTION
modynamic equilibrium conditions must have little or no
The interstellar medium ͑ISM͒ consists of gas and sub
␮m sized grain particles with averaged number densities of 1
barriers and involve only two body collisions. Ternary en-
counters occur only once in a few 109 years, and can be
H atom cmϪ3 and 10Ϫ11 grains cmϪ3. Its chemical composi-
excluded considering mean interstellar cloud lifetimes of 106
tion is dominated by hydrogen and helium ͑H:HeϷ1:0.1͒,
years.4 The first chemical equilibrium models of interstellar
whereas the biogenic elements oxygen, carbon, and nitrogen
chemistry satisfy these criteria and focus on ion-molecule
contribute Ϸ0.001 relative to atomic hydrogen.1–2 Compris-
reactions, radiative associations, as well as dissociative re-
ing approximately 99% neutrals and 1% ions, interstellar
combination between cations and electrons to advance inter-
radicals, atoms, and molecules are not distributed homoge-
stellar chemistry.5,6 This approach, however, involves reac-
neously, but primarily localized in interstellar clouds as well
as outflow of carbon stars.1–2 Diffuse ͑hot͒ clouds hold num-
tion chains with subsequent collisions, and often cannot
ber densities n up to 100 molecules cmϪ3 and mean transla-
reproduce observed structural isomer ratios as well as num-
ber densities for example of C3H and C3H2.7 The inclusion
tional temperatures T of about 100 K, whereas in dense
͑cold, dark, molecular͒ clouds typical scenarios range be-
tween nϭ102–106 cmϪ3 and Tϭ10–40 K. Molecules in the
outflow of carbon stars contribute only a minor amount, but
temperatures can rise up to 4000 K,3 and a more complex
chemistry is expected as compared to interstellar clouds.
Since the average kinetic energy of the interstellar spe-
cies is confined to typically 0.8 kJ molϪ1 ͑diffuse clouds͒ and
of alternative, one step, exothermic neutral–neutral reactions
into chemical models of the circumstellar envelope surround-
ing the carbon star IRCϩ10216 and the dark cloud TMC-1
occurred only gradually.8–16 However, the ad hoc postulation
of spin conservation and simple thermochemistry clearly
demonstrate the urgency of systematic laboratory examina-
tions probing detailed chemical dynamics and reaction prod-
ucts of neutral–neutral encounters.
a͒
Present address: Dipartimento Chimica, Universita La Sapienza, Piazzale
Very recently, these studies were initiated investigating
exothermic atom-molecule reactions of atomic carbon in its
3Pj electronic ground state with unsaturated hydrocarbons
A. Moro 5, 00185 Rome, Italy.
b͒
Present address: Academia Sinica, Nankang, Taipei, 11529, Taiwan.
c͒
Author to whom correspondence should be addressed.
J. Chem. Phys. 105 (19), 15 November 1996
0021-9606/96/105(19)/8721/13/$10.00
© 1996 American Institute of Physics
8721
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8722
Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
via the crossed beam technique as a potential source of car-
bon hydride and the propargyl radical7,17
Although atomic carbon is only a minor species in oxidative
flames, it is assumed to contribute significantly to combus-
tion chemistry,36 and the carbon–hydrogen exchange chan-
nel ͓Eq. ͑3͔͒ could synthesize several C4H3 isomers as well
as advance diamond synthesis in methylacetylene flames.37
However, despite the potential astrochemical and com-
bustion relevance, the experimental as well as theoretical
characterization of the C4H3 PES is far from being complete,
Fig. 1. n-C4H3 (1a/b) holds the global minimum on the
doublet C4H3 PES ͑⌬fHϭ486Ϯ3 kJmolϪ1͒,28,38–39 followed
by a 40Ϯ1 kJmolϪ1 less stable iso isomer ͑2͒ ͑⌬fHϭ526Ϯ4
kJmolϪ1͒.28,38 The structure of n-C4H3 has not yet been re-
solved. Gay et al. calculated a bent, ␣-ethinylvinyl carbon
skeleton, whereas ESR studies in an argon matrix at 4 K
ϩ
g
3
2
C P ͒ϩC H X1⌺ ͒→1/c ?͒ϪC HϩH S ͒,
͑1͒
͑2͒
͑
͑
2
͑
͑
j
2
3
1/2
C P ͒ϩC H X1A ͒→C H X2B ͒ϩH S ͒.
3
2
͑
͑
4
͑
3
͑
j
2
g
3
2
1/2
The explicit identification of this carbon–hydrogen exchange
channel opens a versatile synthetic pathway to carbon bear-
ing species. Analogous to Eq. ͑1͒, reaction of C͑3Pj͒ with
methylacetylene, CH3CCH, is expected to yield hitherto un-
observed interstellar C4H3 isomer͑s͒
3
C P ͒ϩCH CCH X1A ͒→C H ϩH.
͑3͒
͑
͑
j
3
1
4
3
Its prospective methylacetylene precursor and atomic carbon
have been widely observed in the molecular clouds OMC-118
and TMC-119 with fractional abundances relative to hydro-
2
support a linear structure with B2 electronic ground state of
the butatrienyl radical.40–41 Very recently, high level ab ini-
gen between 4ϫ10Ϫ9 and 6.3ϫ10Ϫ9
.
Likewise,
methylacetylene20–22 as well as C4H3 isomers are included in
a photochemical model of Titan, Jupiter, and Saturn.23–24
The authors postulate C4H3 formation via three body recom-
bination
**
tio calculations at the SCF-CISD͑6-311G ͒ level depict a
quasilinear structure (1a) and an interconversion barrier be-
tween both bent C4H3 conformations (1b) of only 3
kJmolϪ1 42
Since the C4H3 isomers are extremely reactive,
.
isolation is restricted to a low temperature matrix ͑n-C4H3͒
or trapping as an 1,3-␮3-dimetalated cyclic species (M1) as
well as 1,3,3-␮4-trimetalated chain isomer (M2).43–44
In this paper, we investigate the detailed chemical dy-
namics of the atom-neutral reaction of C͑3Pj͒ with
methylacetylene, CH3CCH(X1A1) under single collision
conditions at collision energies of 20.4 and 33.2 kJmolϪ1 as
provided in crossed molecular beam experiments. The in-
sights into the reaction dynamics disclose precise informa-
tion on the hitherto unexplored triplet C4H4 and doublet
C4H3 potential energy surface ͑PES͒ under well-defined col-
lision energies, and potential exit channel͑s͒ to C4H3 iso-
mers.
HϩC4H2ϩM→MϩC4H3,
͑4͒
although cosmic ray produced carbon atoms survive the re-
ducing atmospheres25 and might react via Eq. ͑3͒.
Besides its potential interstellar relevance, a scavenged
C4H3 isomer in acetylene/oxygen flames26 is expected to
play a significant role in formation of the first aromatic ring
in sooting combustion flames. Wang et al. postulated a step-
wise ring growth initiated by Eq. ͑5͒ or ͑6͒ to the phenyl
radical or benzyne27
n-C4H3ϩC2H2ϩM→C6H5ϩM,
n-C4H3ϩC2H2→C6H4ϩH,
͑5͒
͑6͒
whereas Walsh outlined i-C4H3 reacts with only a minor
entrance barrier as well.28
However, no reliable information is available on the
mechanism of C4H3 radical formation. Miller et al. sug-
gested pathways via Eqs. ͑7͒–͑10͒,29 followed by intercon-
version of i/n-C4H3 ͓Eq. ͑11͔͒
II. EXPERIMENTAL SETUP AND DATA ANALYSIS
Owing to the high reactivity of prospective open shell
products, reactions must be performed under single collision
conditions to identify the primary reaction products. These
requirements are achieved here using a universal crossed mo-
lecular beam apparatus described in Ref. 45 in detail. A
pulsed supersonic carbon beam was generated via laser ab-
lation of graphite at 266 nm.46 The 30 Hz, 35–40 mJ output
of a Spectra Physics GCR-270-30 Nd:YAG laser is focused
onto a rotating carbon rod. Ablated carbon atoms are seeded
into neon or helium released by a Proch–Trickl pulsed valve
operating at 60 Hz, 80 ␮s pulses, and 4 atm backing pres-
sure. A four slot chopper wheel mounted 40 mm after the
ablation zone selects a 9.0 ␮s segment of the seeded carbon
beam. Table I compiles the experimental beam conditions.
The pulsed carbon and a continuous methylacetylene beam
at 515Ϯ10 Torr backing pressure pass through skimmers,
and cross at 90° in the interaction region of the scattering
C3H2ϩ3CH2→n-C4H3ϩH,
͑7͒
͑8͒
lϪC3H3ϩCH→i/n-C4H3ϩH,
CH2CHCCHϩOH→H2Oϩi/n-C4H3,
CH2CHCCHϩH→H2ϩi/n-C4H3,
iϪC4H3ϩH↔C4H4↔n-C4H3ϩH.
͑9͒
͑10͒
͑11͒
Alternatively, acetylene dimerization ͓Eq. ͑12͔͒,30–31 recom-
bination with C2H ͓Eq. ͑13͔͒,32 and thermal decomposition
33–35
of vinylacetylene intermediates ͓Eq. ͑14͔͒
might yield
C4H3
2C2H2→Hϩi-C4H3,
͑12͒
͑13͒
͑14͒
C2HϩC2H2→i-C4H3,
CH2CHCCH→i/n-C4H3ϩH.
J. Chem. Phys., Vol. 105, No. 19, 15 November 1996
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Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
8723
FIG. 1. Structures of low lying C4H3 isomers as well as metalated radicals M1 and M2. A linear isomer of Eq. ͑4͒, analogous to the c/1ϪC3H isomer pair,
has not been investigated. Since cϪ and 1ϪC3H differ only by 8Ϯ4 kJmolϪ1, the linear isomer ͑5͒ holds approximately ⌬fHϭ560 kJmolϪ1. Other enthalpies
of formations for the isomers are ͑1͒ 486, ͑2͒ 526, ͑3͒ 541, ͑4͒ 553, ͑6͒ 572, ͑7͒ 604, ͑8͒ 613, ͑9͒ 660, and ͑10͒ 766 kJmolϪ1
.
chamber. Reactively scattered products were detected in the
plane of the beams using a rotable detector consisting of a
Brink-type electron-impact ionizer,47 quadrupole mass filter,
and a Daly ion detector48 at different laboratory angles in
5.0° steps between 5.0° and 60.0° with respect to the carbon
beam. The velocity distribution of the products was recorded
using the time-of-flight ͑TOF͒ technique49 choosing a chan-
nel width of 7.5 ␮s. Counting times ranged from 0.5–4 h,
TABLE I. Experimental beam conditions and 1␴ errors averaged over the experimental time: Most probable
velocity 0 , speed ratio S, most probable relative collision energy, Ecoll , center-of-mass angle, ␪CM , composi-
v
tion of the carbon beam, and flux factor f ϭn͑C͒ϫn͑C H ͒ϫ in relative units, with the number density of the
vr
v
vr
3
4
ith reactant n and the relative velocity
.
i
Beam
0 , msϪ1
S
Ecoll , kJ molϪ1
␪
C1 :C2 :C3
fv
v
CM
C͑3Pj͒/Ne
C͑3Pj͒/He
C3H4
1950Ϯ40
2560Ϯ50
790Ϯ30
3.9Ϯ0.3
4.7Ϯ0.3
7.7Ϯ0.5
20.4Ϯ1.0
33.2Ϯ1.4
•••
53.5Ϯ1.5
46.2Ϯ1.6
•••
1:0.6:0.7
1:0.4:0.9
•••
1.0
1.8Ϯ0.3
•••
J. Chem. Phys., Vol. 105, No. 19, 15 November 1996
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8724
Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
averaged over several angular scans. The velocity of the su-
personic carbon beam was monitored frequently after 2–5
angles and minor velocity drifts corrected by adjusting the
laser pulse delay within Ϯ1.5 ␮s. A reference angle was
chosen at 55° and 45°, respectively, to calibrate fluctuating
carbon beam intensities and mass dial settings at the quadru-
pole controller.
Information on the reaction dynamics is gained by fitting
the TOF spectra and the product angular distribution in the
laboratory frame using a forward-convolution routine.50–51
This iterative approach initially guesses the angular flux dis-
tribution T͑␪͒ and the translational energy flux distribution
P(ET) in the center-of-mass system ͑CM͒ assuming mutual
independence. Laboratory TOF spectra and the laboratory
angular distributions were then calculated from these T͑␪͒
and P(ET) averaged over a grid of Newton diagrams defin-
ing the velocity and angular spread of each beam, detector
acceptance angle, and the ionizer length. Best TOF and labo-
FIG. 3. Lower: Newton diagram for the reaction C͑3Pj͒ϩCH3CCH(X1A1)
at a collision energy of 33.2 kJmolϪ1. The circles are corrected for differ-
ence in the relative collision energy. Upper: Laboratory angular distribution
of product channel at m/eϭ51. Circles and 1␴ error bars indicate experi-
mental data, the solid lines the calculated distribution for the upper and
lower carbon beam velocity. The solid lines originating in the Newton dia-
gram point to distinct laboratory angles whose TOFs are shown in Fig. 5.
ratory angular distributions were archived by iteratively re-
fining adjustable T͑␪͒ and P(ET) parameters.
III. RESULTS
A. Reactive scattering signal
Reactive scattering signal was only observed at m/e
ϭ51, i.e., C4H3, cf. Figs. 2–5 and Table II. Reaction of
carbon with methylacetylene dimers does not contribute the
data, since the integrated m/eϭ51 signal scales linearly with
the CH3CCH source backing pressure. TOF spectra recorded
at m/eϭ48–50 show identical patterns indicating the signal
originates in cracking of the parent in the detector. Energeti-
cally accessible channels 2 and 3 to diacetylene ͑Table II͒,
C4H2, are absent within detection limits of our system, and
endothermic channels 3 and 4 could not be opened at relative
collision energies up to 33.2 kJmolϪ1 applied in our experi-
ments. In addition, no radiative association to C4H4 isomers
at m/eϭ52 or higher masses were observed. Lower molecu-
lar weight products bearing two or three carbon atoms ͑exo-
FIG. 2. Lower: Newton diagram for the reaction C͑3Pj͒ϩCH3CCH(X1A1)
at a collision energy of 20.4 kJmolϪ1. The circles stand for the maximum
center-of-mass recoil velocity. From outer to inner: n-C4H3 , i-C4H3 , C4H3
isomers ͑3͒Ϫ͑8͒, and C4H3 isomer ͑9͒. Upper: Laboratory angular distribu-
tion of product channel at m/eϭ51. Circles and 1␴ error bars indicate ex-
perimental data, the solid lines the calculated distribution for the upper and
lower carbon beam velocity ͑Table I͒. C.M. designates the center-of-mass
angle. The solid lines originating in the Newton diagram point to distinct
laboratory angles whose TOFs are shown in Fig. 4.
J. Chem. Phys., Vol. 105, No. 19, 15 November 1996
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Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
8725
FIG. 5. Time-of-flight data at m/eϭ51 for indicated laboratory angles at a
collision energy of 33.2 kJmolϪ1. Open circles represent experimental data,
the solid line the fit. TOF spectra have been normalized to the relative
intensity at each angle.
FIG. 4. Time-of-flight data at m/eϭ51 for indicated laboratory angles at a
collision energy of 20.4 kJmolϪ1. Open circles represent experimental data,
the solid line the fit. TOF spectra have been normalized to the relative
intensity at each angle.
product allows us to eliminate isomer ͑9͒ and the endother-
mic channel to isomer ͑10͒. Further, the large width of the
laboratory angular distribution of at least 60° and the
C4H3ϩH product mass ratio of 51 indicates that the average
thermic channels 6–12͒ could not be verified yet. Their de-
tection suffers from the high background level via
methylacetylene fragmentation in the ionizer. Upper limits of
40% ͑channels 6–8͒, 60% ͑channel 9͒, 10% ͑channel 10͒,
40% ͑channels 11͒, and 80% ͑channel 12͒ relative to m/e
ϭ51 signal were derived.
translational energy release E is large and that the center-
͗
͘
T
of-mass translational energy distributions P(ET)s peak well
away from zero, cf. Sec. III C.
B. Laboratory angular distributions (LAB) and TOF
spectra
TABLE II. Thermochemistry of the reaction C͑3Pj͒ϩCH3CCH(X 1A1). En-
thalpies of formations were taken from Refs. 28, 38, 39, 52, and 53. The
symmetry of the n-C4H3 ground-state electronic wave function is omitted.
Figures 2 and 3 display the most probable Newton dia-
grams of the title reaction as well as the laboratory angular
͑LAB͒ distributions of the C4H3 product at collision energies
of 20.4 and 33.2 kJmolϪ1, respectively. Both LAB distribu-
tions peak close to the CM angles at 53.5° and 46.2° and
show a slightly forward peaking distribution at higher colli-
sion energy. This behavior suggests indirect reaction dynam-
ics via a long-lived C4H4 complex with a lifetime exceeding
͑20.4 kJmolϪ1͒ or comparable to its rotational period ͑33.2
kJmolϪ1, osculating complex͒. Since the enthalpy of forma-
tion of low lying C4H3 isomers differs only by about 30–50
kJmolϪ1, Fig. 1, the nature of the C4H3 solely based on lim-
iting circles is not possible. The maximum scattering range
of isomers ͑1͒–͑8͒ falls within 8°, and individual limit circles
are blurred out due to the velocity spread of the carbon beam
͑Table I͒. However, the scattering range of the m/eϭ51
Reaction enthalpy at 0 K,
#
Exit channel
⌬RH ͑0 K͒, kJ molϪ1
1
2
3
3
4
5
6
7
8
n-C4H3͑?͒ϩH͑2S1/2
͒
Ϫ194Ϯ1
Ϫ444Ϯ12
Ϫ12Ϯ12
ϩ78Ϯ10
ϩ68Ϯ15
ϩ35Ϯ12
Ϫ22Ϯ5
Ϫ36Ϯ4
Ϫ28Ϯ4
Ϫ151.5Ϯ1
Ϫ6Ϯ1
Ϫ30Ϯ6
1
1
ϩ
HCCCCH(X
⌺
⌺
ϩg )ϩH2(X
⌺
)
g
1
HCCCCH(X
ϩg )ϩ2H͑2S1/2
͒
2
1
C4H͑X ⌺͒ϩH2(X
⌺
ϩg )ϩH͑2S1/2
͒
3
1
ϩ
g
C4(X
⌺
Ϫg )ϩ2H2(X
⌺
)
C3H3(X 2B2)ϩCH͑X ⌸͒
c-C3H2(X 1A1)ϩCH2(X 3B2)
c-C3H(X2B2)ϩCH3(X 2AЉ2)
2
l-C3H(X 2⌸)ϩCH3(X A2)
Љ
2
1
9
C3(X
⌺
ϩg )ϩCH4(X 1A1)
1
ϩ
g
10
11
12
C2H4(X 1Ag)ϩC2(X
⌺
)
C2H3͑X 2A ͒ϩC H͑X ⌺ϩ͒
2
Ј
2
1
1
ϩ
g
C2H2(X
⌺
ϩg )ϩC2H2(X
⌺
)
Ϫ440Ϯ1
J. Chem. Phys., Vol. 105, No. 19, 15 November 1996
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8726
Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
FIG. 7. Lower: Center-of-mass angular flux distribution for the reaction
C͑3Pj͒ϩCH3CCH(X1A1) at a collision energy of 33.2 kJmolϪ1. Upper:
Center-of-mass translational energy flux distribution for the reaction
C͑3Pj͒ϩCH3CCH(X1A1) at a collision energy of 33.2 kJmolϪ1. Dashed and
solid lines limit the range of acceptable fits within 1␴ error bars.
FIG. 6. Lower: Center-of-mass angular flux distribution for the reaction
C͑3Pj͒ϩCH3CCH(X1A1) at a collision energy of 20.4 kJmolϪ1. Upper:
Center-of-mass translational energy flux distribution for the reaction
C͑3Pj͒ϩCH3CCH(X1A1) at a collision energy of 20.4 kJmolϪ1. Dashed and
solid lines limit the range of acceptable fits within 1␴ error bars.
isomers ͑2͒–͑6͒ fall within the Emax range. Due to the inter-
nal excitation of the C4H3 product, even Eq. ͑1͒ cannot be
ruled out, cf. Sec. III D.
C. Center-of-mass translational energy distributions,
P(ET)
Besides identification of structural isomers, the most
probable translational energy yields the order-of-magnitude
of the barrier height in the exit channel. Both P(ET)s peak
away from zero as expected from the LAB distributions and
depict a broad plateau between 30–60 kJmolϪ1. A exit bar-
rier is further implied by the large fraction of energy chan-
neled into translational motion of the C4H3 and H products,
i.e., 22Ϯ5% and 30Ϯ3% at lower and higher collision en-
ergy, respectively. These findings suggest a tight transition
state with a significant change in electronic structure as the
C4H4 complex decomposes.
Figures 6 and 7 present the translational energy distribu-
tions P(ET) and angular distributions T͑␪͒ in the center-of-
mass frame. Both LAB distributions and TOF data were fit-
ted with a single P(ET) extending to a maximum
translational energy release Emax of 110–170 kJmolϪ1 and
225–255 kJmolϪ1, respectively. If the energetics of distinct
isomers are well separated, Emax can be used to identify in-
dividual C4H3 isomers. The maximal translational energy re-
leases, i.e., the sum of the reaction exothermicity and relative
collision energy, were already presented in Figs. 2 and 3
with a reasonable approximation of rotationally and vibra-
tionally cold methylacetylene molecules prepared in the su-
personic expansion. The production of the nϪC4H3 isomer at
33.2 kJmolϪ1 is evident by comparing the theoretical and
experimental high energy cutoff of P(ET) with Emax͑exp.;
33.2 kJmolϪ1͒ϭ225–255 kJmolϪ1 vs Emax͑theor.; 33.2
kJmolϪ1͒ϭ226Ϯ7 kJmolϪ1. Formation of the 40 kJmolϪ1
less stable iso isomer can be rejected, since the maximum
energy release is restricted to 186 kJmolϪ1. Based entirely on
the high energy cutoff, the reactive scattering product at
lower collision energy is hard to identify, since all C4H3
D. Center-of-mass angular distributions, T(␪)
At lower collision energy, T͑␪͒ is isotropic and symmet-
ric around ␲/2 implying that either the fragmenting C4H4
holds a lifetime longer than its rotational period ␶ or that the
r
exit transition state is symmetric.54–55 With increasing colli-
sion energy, the center-of-mass angular distribution peaks
forward with respect to the carbon beam. These findings in-
dicate a reduced lifetime of the decomposing C4H4 complex
and agrees with our suggested osculating complex: a com-
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Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
8727
plex formation takes place, but the well depth along the re-
action coordinate is too shallow to allow multiple rotations,
and the complex decomposes with a random lifetime distri-
bution before one full rotation elapses. Based on the intensity
ratio of T͑␪͒ at ␪ϭ0° and 180° of 1.7Ϯ0.1, the identification
of the fragmenting complex enables us to use the rotational
period of the complex as a molecular clock to estimate its
lifetime ͑cf. Sec. IV D͒. To explain the forward-peaking, the
carbon atom and the leaving hydrogen atom must further be
situated on opposite sites of the rotation axis of the fragment-
ing complex.
The weak polarization of all T͑␪͒s can be explained
solely based on total angular momentum conservation and
angular momentum disposal.17,54,55 In terms of a classical
treatment, the total angular momentum J is given by
FIG. 8. Contour flux map for the C4H3 product from the reaction
C͑3Pj͒ϩCH3CCH(X1A1) at a collision energy of 20.4 kJmolϪ1
.
tion and correcting for the reactant flux as well as relative
reactant velocity, we find a total, relative cross section ratio
of ␴͑20.4 kJmolϪ1͒/␴͑33.2 kJmolϪ1͒ϭ1.7Ϯ0.4. This finding
together with recent bulk experiments56 suggest a barrier-
less, attractive long-range dispersion forces dominating the
C–CH3CCH interaction as well as a loose, reactant-like tran-
sition state located at the centrifugal barrier to the triplet
C4H4 PES at about 3 Å.
JϭLϩjϭL ϩj ,
͑15͒
Ј
Ј
with the initial and final orbital angular momentum L and LЈ
perpendicular to the initial and final relative velocity vectors
v and vЈ, and j and jЈ the rotational angular momenta of
reactants and products. Since bulk experiments indicate that
the reaction of C(3Pj) with CH3CCH proceeds within orbit-
ing limits56 and our relative cross sections rise with decreas-
ing collision energy ͑Sec. III E͒, an upper limit of the impact
parameter, bmax , is determined via the classical capture
theory.56,57 Approximating the Lennard-Jones coefficient C6
according to Hirschfelder et al.58 and using the ionization
potentials EC(3Pj) ϭ 11.76 eV, EC3H4ϭ10.36 eV, and polariz-
abilities␣C(3Pj) ϭ 1.76ϫ 10Ϫ30 m3, ␣C3H4ϭ6.18ϫ10Ϫ30 m3,52
bmax gives rise to bmax͑20.4 kJmolϪ1͒ϭ3.8 Å and bmax ͑33.2
kJmolϪ1͒ϭ3.2 Å. The maximum orbital angular momentum
F. Energy partition of total available energy
The identification of the n-C4H3 isomer allows us to es-
timate partition of the total available energy, Etot into product
translation, E , Ϸ E . Even if the butatrienyl structure re-
͗
͘
T
tr
sembles only an inversion transition state, the 3 kJmolϪ1
barrier can be easily overcome at experimental conditions
applied here. The quasilinear n-C4H3 radical holds the rota-
tional constants Aϭ10.17 cmϪ1, Bϭ0.139 cmϪ1, and
Cϭ0.137 cmϪ1 and classify it as a highly prolate asymmetric
top with asymmetry parameter ␬ϭϪ0.9996, Fig. 10. Hence,
we approximate the rotational levels to those of a rigid sym-
metric top17 using the rotational quantum number Jϭ116
͑Ecollϭ20.4 kJmolϪ1͒ and Jϭ125 ͑Ecollϭ33.2 kJmolϪ1͒ cf.
Sec. III D, and calculate the component of the rotational an-
gular momentum about the principal axis K with Kϭ0 for no
rotation about the figure axis, but perpendicular to it, and
yields
Lmax͑20.4
kJmolϪ1͒ϭ116ប
and
Lmax͑33.2
kJmolϪ1͒ϭ125ប. Since CH3CCH is produced in a supersonic
expansion and j peaks at 2–4ប for typical rotational tem-
peratures between 20 and 40 K, j contributes less than 2.5%
to the total angular momentum J, and Eq. ͑15͒ reduces to
LϷJϭL ϩj .
͑16͒
Ј
Ј
To justify the weak T͑␪͒ polarization, L and LЈ must be
uncoupled with jЈӷLЈ, and the initial orbital angular mo-
mentum becomes the final rotational angular momentum.
This weak LϪLЈ correlation is a direct result of the inability
of the departing H atom to carry significant orbital angular
momentum. On the other hand, a strong LϪLЈ correlation
would have indicated that the complex decomposed with
Ϫ1
LЈуjЈ, but the expected ͑sin ␪͒
shaped T͑␪͒ at 20.4
kJmolϪ1 is clearly not observed.
E. Flux contour maps and total relative cross
sections
Figures 8 and 9 show center-of-mass flux contour maps
I(␪,ET)ϳT(␪)ϫP(ET) for collision energies at 20.4 and
33.2 kJmolϪ1. Data at lower collision energy depict a
forward–backward symmetric flux profile as expected from
the center-of-mass angular distribution. With increasing col-
lision energy, the pronounced forward peaking on the rela-
tive velocity vector is evident. Integrating this flux distribu-
FIG. 9. Contour flux map for the C4H3 product from the reaction
C͑3Pj͒ϩCH3CCH(X1A1) at a collision energy of 33.2 kJmolϪ1
.
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8728
Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
to two ␲-orbitals at one carbon atom. The observed CM
angular and translational energy distributions are then com-
pared to what is expected based on these channels. Pathways
incompatible with experimental data are dismissed. Since no
C4H4 intermediate fulfills requirements for intersystem
crossing,17 the discussion is restricted to the triplet surface.
However, neither ab initio nor experimental enthalpies of
formations of triplet C4H4 isomers are available, and their
energetics are approximated based on corresponding triplet
C3H2 isomers.59–65
A. C4H4 potential energy surface
Addition of C͑3Pj͒ to two perpendicular ␲-orbitals at the
methylacetylene ␣-C atom ͑the neighboring carbon atom
to the methyl group͒ yields triplet s-cis/trans
2-methylpropendiylidene ͑11͒/͑12͒, Fig. 11, whereas
attack to the ␤-C atom forms triplet s-trans/cis
1-methylpropendiylidene ͑13͒/͑14͒. Since trans-propendiyl-
idene is energetically favored by about 80 kJmolϪ1 as com-
pared to the cis isomer on the triplet C3H2 surface, this dif-
ference is adapted to cis ͑11͒/͑14͒ vs trans ͑12͒/͑13͒ isomers.
Further, we approximate identical enthalpies of formations
of ͑11͒/͑14͒ ͑⌬fHϭ815 kJmolϪ1͒ and ͑12͒/͑13͒ ͑⌬fHϭ735
kJmolϪ1͒. ͑13͒/͑14͒ undergo ͓2,1͔-H-migration to triplet
1-methylpropadienylidene ͑15͒, ͓2,3͔-H-rearrangement to
triplet 1-methylpropargylene ͑16͒, ring closure to triplet me-
thylcyclopropenylidene ͑17͒, or direct C–H fragmentation to
the linear C4H3 isomer ͑5͒. Two remaining channels are en-
ergetically not accessible: H loss of the methyl group yields
a 1,3,3-triradical which—if it existed—suffers ring closure to
a tri or tetra cycle which is less stable than the already closed
channel to ͑10͒; the ͓1,2͔ methyl group migration to triplet
2-methylpropanediylidyne ͑19͒ is endothermic by 150
kJmolϪ1. Similar to ͑13͒/͑14͒, ͑11͒/͑12͒ might react via ͓2,3͔
or ͓2,1͔ CH3-migration to ͑16͒ and ͑15͒, respectively. Be-
sides addition to ␣-C-atom, C͑3Pj͒ might add to both ␣- and
␤-C-atoms of the methylacetylene molecule, generating me-
thylcyclopropenylidene ͑17͒.
FIG. 10. Principal rotation axis of the butatrienyl radical. The C-axis is
perpendicular to the paper plane.
KϷJ for a fast rotation about the principal axis, with a slow
end-over-end one. Since no data on the K-distributions are
available, we use the same procedure as applied for the
C(3Pj)ϩC2H4 system17 and calculate first the rotational en-
ergy assuming Kϭ0 ͑‘‘low K approximation’’͒. This gives
us the maximal vibrational energy release Evib in the n-C4H3
radical. Hereafter, the highest energetically accessible K
states Kmax are computed assuming Evibϭ0 kJmolϪ1 to esti-
mate an upper limit of the product rotational excitation as
well as an order of magnitude of the lowest tilt angle ␣min of
the n-C4H3 principal inertial axis with respect to jЈ in terms
of the classical vector model.17
The low K approximation yields a nearly constant parti-
tioning of total energy into rotational degrees of freedom at
both collision energies, i.e., 21Ϯ2 kJmolϪ1 ͑10Ϯ1%͒ vs
26Ϯ3 kJmolϪ1 ͑11Ϯ2%͒. Further, the fraction of the maxi-
mum vibrational energy release stays constant within the er-
ror limits ͑68Ϯ20% and 59Ϯ10%; 145 kJmolϪ1 at lower and
133 kJmolϪ1 at higher collision energy͒ and might suggest a
lifetime long enough to randomize the energy into the vibra-
tional modes of the C4H4 complex. Finally, even in the limit
of zero vibrational excitation of the n-C4H3 product and a
maximum K value, the principal axis is tilted 73°–76° with
respect to jЈ and clearly demonstrates a predominant end-
over-end-rotation of the n-C4H3 radical.
Furthermore, C͑3Pj͒ insertion into the acetylenic C–H as
well as the C–C single bond might lead to triplet methylpro-
pargylene ͑16͒, whereas insertion into the aliphatic C–H
bond of the methyl group forms a triplet carbene ͑20͒. The
fate of ͑15͒–͑17͒ is governed by C–H fragmentation and/or
H-migration: ͑17͒ decomposes via C–H bond rupture to ͑4͒
or ͑3͒, then rearranges to triplet methylenecyclopropene ͑21͒,
which is followed by H-loss to ͑3͒ or ͑6͒; the only energeti-
cally feasible fragmentation of ͑15͒ yields C4H3 isomer ͑5͒,
whereas ͑16͒ decomposes either to ͑5͒ or n-C4H3 ͑1͒. Fi-
nally, ͑15͒ might rearrange via hydrogen migration to triplet
vinylidenecarbene ͑22͒.
The reaction pathway to the identified n-C4H3 radical
͑Sec. III C͒ can only proceed via hydrogen loss from the CH3
group of triplet 1-methylpropargylene ͑16͒. Here, the prefer-
ence of the methyl H-atom loss compared to the acetylenic
C–H bond cleavage even at higher collision energy corre-
lates with the ϳ140 kJmolϪ1 weaker aliphatic carbon-
hydrogen bond energy and excludes decomposition of ͑16͒
to C4H3 isomer ͑5͒. Additionally, the identification
IV. DISCUSSION
In this section, we outline feasible reaction pathways on
the triplet C4H4 PES to produce C4H3 isomers ͑1͒–͑6͒ via
insertion of the electrophile carbon atoms into the C-H and
C-C bonds of methylacetylene, addition to two ␲-molecular
orbitals at both distinct carbon atoms, and, finally, addition
J. Chem. Phys., Vol. 105, No. 19, 15 November 1996
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Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
8729
FIG. 11. ͑a͒ Schematic representation of the lowest energy pathways on the triplet C4H4 PES and structures of potentially involved collision complexes.
Triplet methylenecyclopropene and vinylacetylene are not included, since their singlet-triplet gaps have not been investigated yet. ͑b͒ Additional structures for
possible intermediates and products relevant to the discussion. Three potential electronic structures of propargylene are presented: 1,3-diradical ͑16a͒ and
carben-like structures ͑16b/c͒. Isomers ͑11͒–͑14͒ are named with respect to atoms/groups at the former acetylenic triple bond.
J. Chem. Phys., Vol. 105, No. 19, 15 November 1996
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8730
Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
of Eq. ͑16͒ as the decomposing complex eliminates the pos-
sibility the symmetric T͑␪͒ originates from a symmetric
complex ͑Sec. III D͒, since rotation around any principal axis
cannot fulfill this requirement. The remaining question to be
solved is the reaction pathway to ͑16͒. Insertion of C͑3Pj͒
into the acetylenic C–H bond can be rejected, since only a
narrow range of impact parameters between 1.19 and 2.24 Å
would contribute to reactive scattering signal. The over-
whelming contribution of large impact parameters to the cap-
ture process up to 3.8 Å was already mentioned in Sec. III D
and III E. Additionally, no evidence of insertion was found
in the crossed beam reaction of C͑3Pj͒ with unsubstituted
acetylene69 indicating that the symmetry-forbidden insertion
into the acetylenic C–H bond involves a barrier of at least
33.2 kJmolϪ1. Insertion into the C–C single bond can be
excluded as well: The forward peaking center of mass angu-
lar distribution requires the inserted carbon atom and the
leaving hydrogen to be located on opposite sides of the ro-
tation axis of fragmenting ͑16͒. However, this condition is
not satisfied. In addition, hot atom tracer experiments of
11C͑3Pj͒ with C2H6 and even strained cyclobutane rings
show a screening of the C–C bond by hydrogen atoms, and
only insertion into C–H bonds is observed.66 Therefore, any
insertion process can be excluded from the discussion.
Remaining pathways to ͑16͒ involve triplet C4H4 inter-
mediates ͑11͒–͑14͒. Using the concept of regioselectivity of
electrophilic radical attacks on substituted olefines and ex-
tending it to alkynes,67 we can eliminate further collision
complexes. The framework predicts the radical attack to be
directed at the carbon center which holds the highest spin
density. Since partial delocalization of the methyl ␲-group
orbitals increases the spin density on the ␤-C-atom at the
expense of the ␣-position, C͑3Pj͒ attacks preferentially at the
␤-C. Additionally, the sterical hindrance of the CH3 group
reduces the cone of acceptance at the ␣-C-atom and the
range of reactive impact parameters. Both effects together
direct the electrophilic carbon addition to ͑13͒/͑14͒. Even if
͑11͒ and ͑12͒ were formed to a minor extent, rearrangement
to ͑16͒ would involve a CH3-group ͑mϭ15͒ migration which
is unfavorable compared to rearrangement of the light H
atom to ͑16͒ via ͑12͒/͑13͒. Similar arguments eliminate a
simultaneous attack of C͑3Pj͒ to ␣- and ␤-C-atom with
maximum impact parameters of about 0.6 Å to ͑17͒. Both
prevailing pathways to ͑16͒ via ͑13͒ and ͑14͒ cannot be dis-
criminated based on our experimental data. The chemical
dynamics to alternative C4H3 isomers at lower collision en-
ergy involve isomers ͑13͒–͑15͒, ͑17͒, or ͑20͒. The last one
can be ruled out, since insertion of C͑3Pj͒ into the aliphatic
C–H bond does not play a role. Results of crossed beam
experiments C͑3Pj͒ϩCH4 at relative collision energies up to
40 kJmolϪ1 show no reactive scattering signal of insertion
into the aliphatic C–H bond.68 Therefore—if ͑2͒
contributes—hydrogen loss of triplet vinylacetylene ͑22͒
represents the only open channel.
FIG. 12. Approach geometry of the carbon atom toward the methylacety-
lene molecule conserving Cs symmetry and induced rotation around the
B-axis.
B. Rotation axis of the triplet 1-methylpropargylene
complex
Conserving the C-C-C-C-plane as a plane of symmetry,
the singly occupied px and pz orbitals of the carbon atom
might add to the ␲x- and ␲z-orbitals under Cs symmetry on
the 3AЉ surface to form isomers ͑13͒/͑14͒, Fig. 12. This path-
way supports a maximum orbital overlap to the C-C-␴- and
C-C-␲-bond via interaction of px with ␲x- as well as pz with
␲z-orbitals. Oblique approach geometries are supported as
well and open larger impact parameters for the reaction as
discussed in Sec. III E. Since LϷjЈ, the four carbon atoms
rotate in a plane approximately perpendicular to L around
the C-axis of the prolate 1-methylpropendiylidene. The con-
secutive hydrogen shift to 1-methylpropargylene conserves
either the symmetry plane ͓assuming Cs symmetry of ͑16b͒
or ͑16c͒, Fig. 12͔ or follows C1 symmetry ͑geometry ͑16a͔͒.
Since the adduct still rotates around the C-axis, the added
carbon atom in the C4 position and the methyl hydrogens are
located on opposite sites of the rotation axis as required to
explain the forward-peaking C4H3 product with respect to the
carbon beam. This almost in-plane rotation yields extremely
low K values as well as a minor J component about the
J. Chem. Phys., Vol. 105, No. 19, 15 November 1996
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Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
8731
D. Exit transition state
figure axis of the 1-methylpropargylene and can be related to
dominating low K states populated in the n-C4H3 product
͑Sec. III F͒.
The partitioning of the total available energy into the
translational, rotational, and vibrational degrees of freedom
of the n-C4H3 radical as well as the collision energy depen-
dent P(ET) shape reveal information on the exit transition
state. The framework of an ideal RRKM system requires that
exit channel interactions, i.e., the coupling between the reac-
tion coordinate ͑translation͒ and internal motion beyond the
critical configuration, must be small. This condition is only
fulfilled in loose transition states, implying the reverse reac-
tion of Hϩn-C4H3 to 1-methylpropargylene holds no en-
trance barrier. As shown in Sec. III C, the exit transition state
is located at least 30–60 kJmolϪ1 above the products,
indicating that the C–H bond rupture in triplet
1-methylpropargylene does not follow the patterns of an
ideal RRKM system with a loose transition state. On the
other hand, Marcus’ tight transition state theory quantifies a
rising fraction of total available energy into vibration with
increasing collision energy, if the decomposing triplet C4H4
complex has many degrees of freedom. Both P(ET)s clearly
underline a tight transition state, but within our error limits, a
complete energy randomization cannot be proved or dis-
proved. These deviations from the loose transition state are
based on dynamical effects during the separation of frag-
ments into products together with a significant geometry
change from the triplet 1-methylpropargylene complex.
Comparing the bond orders (BO) of n-C4H3, Fig. 1 ͑1a/b͒
with those of methylpropargylene, Fig. 11 ͑16a–c͒, supports
this approach: In the case of a propargylene-like, 1-3 diradi-
cal ͑16a͒, the C–C bond orders change from two times 2.5
and 1.0 to an acetylenic ͑BOϭ3͒, olefinic ͑BOϭ2͒ and
shifted aliphatic C–C bond ͑BOϭ1͒. If ͑16͒ exists as a trip-
let carbene ͑16b/c͒, the conversion of the C–C single bond
located at the carbene center to a partially delocalized CϭC
bond increases the bond strength by ϳ250 kJmolϪ1. Finally,
an isotropic T͑␪͒ distribution as compared to the forward
An alternative C͑3Pj͒ trajectory under C1 symmetry on
3
the A surface might induce rotations about the A, B, and C
axes of 1-methylpropendiylidene and—after H migra-
tion—of 1-methylpropargylene, but does not support a maxi-
mum overlap of both perpendicular p- with ␲-orbitals. Fur-
thermore, a freely rotating CH3 group in ͑16͒ undermines
A-like rotations, since its hydrogen atoms rotate to the same
side as the incorporated carbon atom. Hence, the required
forward-peaked T͑␪͒ cannot be supplied.
C. Lifetime of the triplet 1-methylpropargylene
complex
The rotational period of the 1-methylpropargylene com-
plex can act as a clock in the molecular beam experiment and
can be used to estimate the lifetime ␶ of the decomposing
complex at a relative collision energy of 33.2 kJmolϪ1. The
osculating model relates the intensity ratio of T͑␪͒ at both
poles to ␶ via Eq. ͑17͒
trot
I 180°͒/I 0°͒ϭexp Ϫ
,
͑17͒
͑
͑
ͩ
ͪ
2␶
where trot represents the rotational period with:
trotϭ2␲Ii /Lmax
.
͑18͒
Ii represents the moment of inertia of the complex rotating
around the i-axis, and Lmax the maximum orbital angular
momentum. Using the ab initio geometries of propargylene
and a C–CH3 distance in ͑16͒ of 1.47 Å, we can estimate the
rotational period of the methylpropargylene complex: around
the A axis we find trot(A)ϭ0.01–0.02 ps, and around the
B/C axis we obtain trot(B,C)ϭ1–2 ps. Plugging in all data
in Eq. ͑17͒ yields
a
lifetime of the triplet
peaked
distribution
seen
in
the
reaction
C͑3Pj͒ϩC2H2→C3HϩH ͑Sec. III D and Ref. 17͒ implies the
additional modes of the CH3 group induce the long-lived
complex behavior and that the energy randomization in the
collision complex might be complete.
1-methylpropargylene complex equal to one rotational pe-
riod. The absolute value of ␶ depends dramatically on the
rotation axis, i.e., B, C vs A. Since reactions with a collision
time Ӷ0.1 ps follow direct scattering dynamics, the T͑␪͒ at
33.2 kJmolϪ1 relative collision energy should be strongly
forward peaked, if the complex rotated around the A-axis.
Our data show only a moderate peaked center-of-mass angu-
lar distribution at 33.2 kJmolϪ1 and an isotropic one at 22.4
kJmolϪ1. Therefore, rotation about the A axis can be elimi-
nated as already suggested in Sec. IV B, and end-over-end
rotation around the B- or C-axis of ͑16͒ takes place. Due to
the optimal orbital overlap ͑Sec. IV B͒, C-like rotations
should dominate. Compared to the forward peaked T͑␪͒ as
found in the crossed beam reaction C͑3Pj͒ϩC2H2→C3HϩH
at a relative collision energy of 8.8 kJmolϪ1, the enhanced
complex lifetime is a direct consequence of the additional 9
vibrational modes of the CH3 group. A similar behavior con-
tributes to the increased lifetime of the triplet 1-methylallene
E. Alternative isomers at lower collision energy
Based on our experimental results, any of the pathways
to C4H3 isomers ͑2͒–͑6͒ might show additional contributions
at lower collision energy. The electron density change of
each triplet C4H4 complex fragmenting to ͑3͒–͑6͒ suggest a
tight transition state as expected from the center of mass
translational energy distribution. Since the relative collision
energy increases by only 10 kJmolϪ1, formation of only one
isomer of ͑2͒–͑6͒ with no n-C4H3 formation is hard to ex-
plain. The isotropic center-of-mass angular distribution
might open a potential two channel fit of n-C4H3 and a sec-
ond C4H3 isomer as well. However, neither transition state
frequencies are available, and the experimental data alone
cannot resolve this question.
complex
͓crossed
beam
reaction
C͑3Pj͒ϩC3H6
→C4H6→C4H5ϩH ͑Ref. 69͔͒ vs triplet allene ͓crossed beam
reaction C͑3Pj͒ϩC2H4→C3H4→C3H3ϩH ͑Ref. 69͔͒.
J. Chem. Phys., Vol. 105, No. 19, 15 November 1996
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8732
Kaiser et al.: Reaction of carbon atoms with hydrocarbons. II.
V. IMPLICATIONS TO INTERSTELLAR CHEMISTRY
AND COMBUSTION PROCESSES
1-methylpropargylene. Within 1–2 ps, the complex decom-
poses via hydrogen emission to n-C4H3. The exit transition
state is found to be tight and located at least 30–60 kJmolϪ1
above the products. The explicit identification of the n-C4H3
radical under single collision represents a further example of
a carbon–hydrogen exchange in reactions of ground-state
carbon atoms with unsaturated hydrocarbons. This channel
opens a versatile pathway to synthesize extremely reactive
hydrocarbon radicals relevant to combustion processes as
well as interstellar chemistry.
The crossed beam setup represents a versatile tool to
study reaction products as well as chemical dynamics of
neutral–neutral reactions relevant to combustion processes
and interstellar chemistry under well-defined reactant condi-
tions. Here, the explicit identification of the n-C4H3 radical
under single collision conditions depicts a third example of
the carbon-hydrogen exchange channel in the reaction of
C͑3Pj͒ with unsaturated hydrocarbons studied recently in our
Note added in proof. The assignment of the symmertry
of the electronic wave function for C2v molecules follows
the convention that the molecular plane is defined as the
mirror plane, e.g., C3H3(X2B2) instead of C3H3(X2B1).
lab7,17
ϩ
g
3
2
C P ͒ϩC H X1⌺ ͒→C HϩH S ͒,
͑19͒
͑20͒
͑
͑
2
͑
j
2
3
1/2
C P ͒ϩC H X1A ͒→1-C H X2B ͒ϩH S ͒.
3
2
͑
͑
4
͑
3
͑
j
2
1
3
2
1/2
ACKNOWLEDGMENTS
This reaction class presents an alternative to ion-
molecule reactions to synthesize carbon-chain molecules in
the interstellar medium7,17 and strictly excludes the forma-
tion of any C4H2 isomer via
R.I.K. is indebted the Deutsche Forschungsgemeinschaft
for a post-doctoral fellowship. This work was supported by
the Director, Office of Energy Research, Office of Basic En-
ergy Sciences, Chemical Sciences Division of the U.S.
Department of Energy under Contract No. DE-AC03-
76SF00098.
3
C P ͒ϩC H →C H ϩ2H,
͑21͒
͑
j
3
4
4
2
as postulated based on thermochemistry and spin
conservation14 underlining the need of systematic laboratory
studies to establish a well-defined data base for neutral–
neutral reaction products. A rising cross section with de-
creasing translation energy underlines the potential contribu-
tion of these processes in interstellar clouds and should
encourage astronomers to search for hitherto undetected
C4H3 isomers perhaps among unidentified microwave transi-
tions in the spectrum toward the extended ridge of OMC-1.
Since deuterated methylacetylenes ͑CH3CCD and
CH2DCCH͒ were identified in OMC-1 and TMC-1, forma-
tion of partially deuterated C4H2D is expected to take place
as well. Terrestrial based microwave spectra of C4H3 radicals
could be simply recorded during RF discharges of
CH3CCH/He/CO-mixtures.
Likewise, the identification of the n-C4H3 radical under
single collision conditions as well as via trapping experi-
ments in oxygen rich hydrocarbon flames26 validates inclu-
sion of hydrocarbon radicals even in oxidative flames. Fur-
ther investigations of C͑3Pj͒ reactions with unsaturated
hydrocarbons are in progress and will supply a new set of
reactions as well as products to be incorporated into combus-
tion models.
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