Crossed beam reaction of atomic carbon, C(3Pj), with the propargyl radical, C3H3(X 2B2): Observation of diacetylene, C4H2(X 1Σg+)

Article abstract of DOI:10.1063/1.475025

The reaction of ground-state carbon, C(³P_j), with the propargyl radical, C₃H₃(X ²B₂), is investigated at an average collision energy of 42.0 kJmol^-1 employing the crossed molecular beams technique and a universal mass spectrometric detector. The laboratory angular distribution and time-of-flight spectra of the C₄H₂ product are recorded at mle=50. Forward-convolution fitting of our data reveals the formation of diacetylene, HCCCCH, in its X₁Σ_g⁺ electronic ground state. The reaction dynamics are governed by an initial attack of C(³P_j) to the π-electron density at the acetylenic carbon atom of the propargyl radical, followed by a [1,2]-hydrogen migration to the n-C₄H₃ isomer. A final carbon-hydrogen bond rupture yields atomic hydrogen and diacetylene through a tight exit transition state located 30-60 kJmol^-1 above the products. This first successful crossed molecular beams study of a reaction between an atom and a free radical marks the beginning of the next generation of crossed beams experiments elucidating the formation of molecular species in combustion processes, chemical vapor deposition, in the interstellar medium, outflows of carbon stars, and hydrocarbon-rich planetary atmospheres via radical-radical reactions.

Full text of DOI:10.1063/1.475025

Crossed beam reaction of atomic carbon, C ( 3 P j ), with the propargyl radical, C 3 H 3
(X 2 B 2 ): Observation of diacetylene, C 4 H 2 (X 1 Σ g + )
R. I. Kaiser, W. Sun, A. G. Suits, and Y. T. Lee
Citation: The Journal of Chemical Physics 107, 8713 (1997); doi: 10.1063/1.475025
View online: http://dx.doi.org/10.1063/1.475025
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3
Crossed beam reaction of atomic carbon, C„ Pj…, with the propargyl
؉
2
1
radical, C3H3„X B2…: Observation of diacetylene, C4H2„X ⌺g …
R. I. Kaiser,a) W. Sun,b) A. G. Suits,c) and Y. T. Leed)
Department of Chemistry, University of California, Berkeley, California 94720 and Chemical Sciences
Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
͑Received 22 July 1997; accepted 19 September 1997͒
2
The reaction of ground-state carbon, C(3Pj), with the propargyl radical, C3H3(X B2), is
investigated at an average collision energy of 42.0 kJmolϪ1 employing the crossed molecular beams
technique and a universal mass spectrometric detector. The laboratory angular distribution and
time-of-flight spectra of the C4H2 product are recorded at m/eϭ50. Forward-convolution fitting of
our data reveals the formation of diacetylene, HCCCCH, in its X1⌺gϩ electronic ground state. The
reaction dynamics are governed by an initial attack of C(3Pj) to the ␲-electron density at the
acetylenic carbon atom of the propargyl radical, followed by a ͓1,2͔-hydrogen migration to the
n-C4H3 isomer. A final carbon–hydrogen bond rupture yields atomic hydrogen and diacetylene
through a tight exit transition state located 30–60 kJmolϪ1 above the products. This first successful
crossed molecular beams study of a reaction between an atom and a free radical marks the beginning
of the next generation of crossed beams experiments elucidating the formation of molecular species
in combustion processes, chemical vapor deposition, in the interstellar medium, outflows of carbon
stars, and hydrocarbon-rich planetary atmospheres via radical–radical reactions. © 1997 American
Institute of Physics. ͓S0021-9606͑97͒04144-5͔
I. INTRODUCTION
a better understanding of the importance of neutral–neutral
reactions in contrast to ion–molecule reactions in the forma-
tion of molecules and radicals in extraterrestrial environ-
ments. The chemical dynamics of atom–radical as well as
radical–radical reactions in the hydrogen deficient C–H-
system are completely unknown. Both reaction classes, how-
ever, are expected to have a profound impact on chemistry in
interstellar and hydrocarbon-rich planetary environments at
very low temperatures down to 10 K: Reactive encounters
between carbon atoms with open shell hydrocarbons such as
CH, C2H, C2H3, and C3H3 radicals are thought to resemble
prototype reactions proceeding without any barrier in the en-
trance channel. Therefore, these reactions are strongly ex-
pected to form complex species even in the coldest known
interstellar clouds where the average kinetic energy of reac-
tant molecules are about 0.08 kJmolϪ1. In this communica-
tion, we present the chemical dynamics of atomic carbon
reacting with the propargyl radical to form C4H2 isomers
diacetylene ͓reaction ͑1͔͒, butatrienylidene ͓reaction ͑2͔͒,
and/or the cyclic isomer cyclopropenylidenecarbene ͓reac-
The chemical reaction dynamics of atomic carbon in its
C(3Pj) electronic ground state with unsaturated hydrocarbon
molecules are of major importance in interstellar
chemistry,1,2 combustion processes,3 and chemical vapor
deposition.4 Recently, we initiated these studies in our lab
elucidating the chemical dynamics and reaction products of
exothermic atom–molecule reactions of C(3Pj) with acety-
lene, C2H2, ethylene, C2H4, methylacetylene, CH3CCH, and
propylene, C3H6, employing the crossed beams technique
with electron impact ionizer coupled to a quadrupole mass
spectrometer. These investigations provided collision energy
dependent (8.8–45.0 kJmolϪ1), doubly differential cross
sections to both interstellar tricarbon hydride radicals,5
1/c-C3H, and to hitherto unobserved interstellar propargyl6
C H (X B ) , n-C H (X A ), and methylpropargyl8
7
2
2
͓
͓
͔
Ј
3
3
2
4
3
2
2
C H (X B /X A ) radicals. The explicit identification of
͔
Љ
4
5
2
this carbon–hydrogen exchange under single collision con-
ditions elucidated the importance of a one-step pathway to
free hydrocarbon radicals. This mechanism can be employed
further to predict large scale concentrations of unobserved
interstellar radicals C3H3, C4H3, and C4H5 in extraterrestrial
environments.2
9–11
tion ͑3͔͒
ϩ
g
3
2
1
2
C P ͒ϩC H X B ͒→C H →C H X ⌺ ͒ϩH S
͒
1/2
͑
͑
3
͑
2
͑
j
3
2
4
3
4
⌬RH0ϭϪ385Ϯ30 kJmolϪ1
,
͑1͒
͑2͒
͑3͒
The work described above is just the beginning towards
a͒
Present address: Academia Sinica, Institute of Atomic and Molecular Sci-
1
2
→CCCCH X A ͒ϩH S
͒
1/2
͑
2
͑
1
ences, Nankang, Taipei, 11529, Taiwan, ROC; and Department of Physics,
Technical University Chemnitz-Zwickau, 09107 Chemnitz, Germany.
Electronic mail: kaiser@po.iams.sinica.edu.tw
⌬RH0ϭϪ203Ϯ30 kJmolϪ1
,
b͒
Electronic mail: weizhong@leea.cchem.berkeley.edu
c͒
Electronic mail: agsuits@lbl.gov
1
2
d͒
Present address: Academia Sinica, Institute of Atomic and Molecular Sci-
→cϪC H X A ͒ϩH S
͑
2
͑
͒
4
1
1/2
ences, Nankang, Taipei, 11529, Taiwan, ROC. Electronic mail:
ytlee@gate.sinica.edu.tw
⌬RH0ϭϪ45Ϯ30 kJmolϪ1
.
J. Chem. Phys. 107 (20), 22 November 1997
0021-9606/97/107(20)/8713/4/$10.00
© 1997 American Institute of Physics
8713
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8714
Letters to the Editor
II. EXPERIMENT
Investigating the chemical dynamics of radical–radical
reactions using the crossed molecular beams technique is
quite challenging. Compared to C(3Pj) reactions with neat
hydrocarbons, the secondary neat hydrocarbon beam must be
replaced by ϳ10% beam of the radical precursor seeded in
helium. Even assuming an excellent conversion of 10% into
radicals, the reactive scattering signal drops by two orders of
magnitude to gain a comparable signal to noise ratio to
closed shell systems if the reactive scattering cross sections
are comparable. The experiment is performed with a univer-
sal crossed molecular beam apparatus described in Ref. 12 in
detail. Briefly, a pulsed supersonic carbon atom beam is gen-
erated via laser ablation of graphite in the primary source
chamber.13 The 65 mJ 266 nm output of a Spectra Physics
GCR-270-30 Nd:YAG laser is focused onto a rotating graph-
ite rod, and ablated carbon atoms are seeded into helium gas.
A four slot chopper wheel located between skimmer of the
primary source and the interaction region, slices a 9 ␮s seg-
ment of the pulsed carbon beam to select a velocity
v0
ϭ2765Ϯ14 msϪ1 and a speed ratio Sϭ3.0Ϯ0.3. The pulsed
propargyl radical beam is produced in the secondary source
region via photodissociation of 10% propargyl bromide pre-
cursor seeded in helium carrier gas at 1 atm stagnation pres-
sure. The 80 mJ, 193 nm output of a Lamda Physik Compex
110 excimer laser is focused 5 mm downstream of the pulsed
valve nozzle to a rectangle of 1ϫ4 mm. These operation
FIG. 1. Lower: Newton diagram for the reaction C(3Pj)ϩC3H3 at a colli-
sion energy of 42.0 kJmolϪ1. The circle stands for the maximum center-of-
mass recoil velocity in the center-of-mass reference frame of C4H2 isomers.
From outer to inner: Diacetylene, butatrienylidene, and cyclopropenyl-
idenecarbene. Upper: Laboratory angular distribution of C3H3 at m/eϭ50.
Circles and 1␴ error bars indicate experimental data, the solid line the cal-
culated distribution. C.M. designates the center-of-mass angle of 55.0°.
parameters yield a propargyl beam with a velocity
v0
ϭ1220Ϯ20 msϪ1 and a speed ratio Sϭ14.0Ϯ0.5, i.e., a
spread of about 60–80 ␮s at 50% of its peak intensity. Time-
of-flight spectra of reactively scattered species were moni-
tored using a triply differentially pumped quadrupole mass
spectrometer with an electron-impact ionizer14,15 in 5.0°
steps with respect to the carbon beam at m/eϭ51 and m/e
ϭ50. To gain information on the reaction dynamics, the
time-of-flight ͑TOF͒ spectra and the laboratory angular dis-
a correct time delay should show no signal at m/eϭ50 with
the excimer laser ‘‘off’’, but only with the excimer laser
‘‘on’’. Further, the laser ‘‘on’’ TOFs at the correct delay
time should show no intensity at m/eϭ51. This optimization
procedure results in triggering the second pulsed valve be-
tween 7 and 15 ␮s prior to the primary pulsed valve.
tribution ͑LAB͒ are fit using
a forward-convolution
technique16 yielding the translational energy flux distribution
P(ET) and angular distribution T(␪) in the center-of-mass
reference frame.
Since both the carbon and propargyl beams are pulsed,
extreme care has to be taken to select the correct time delay
between the primary and secondary pulsed valve prior to the
crossing at 90° in the interaction region. An improper timing
would have resulted in reaction of C(3Pj) with the slow part
of the secondary beam containing predominantly propargyl
bromide precursor to form C4H3 detectable at m/eϭ51 and
m/eϭ50 as well as Br atoms. Here, we optimize this delay
by monitoring the excimer laser correlated integrated inten-
sity of the time-of-flight ͑TOF͒ spectra at m/eϭ50 versus
the delay time of the second pulsed valve. Signal at m/e
ϭ50 recorded with the excimer laser ‘‘off’’ originates only
from reaction of atomic carbon with the propargyl bromide
precursor and the fragmentation of the C4H3 product in the
electron impact ionizer. However, signal with the excimer
laser ‘‘on’’ can arise from reaction of carbon atoms with
propargyle bromide as well as propargyl radicals. Therefore,
III. RESULTS
Reactive scattering signal was only observed at m/e
ϭ50, i.e., C4H2, cf. Figs. 1 and 2; no signal was detected at
m/eϭ51, unconditionally supporting the absence of propar-
gyl bromide reacting with atomic carbon in our experimental
conditions. The LAB distribution of the C4H2 product ͑Fig.
1͒ peaks at 55° near the center-of-mass angle of 55Ϯ1°, but
shows a slight forward shape with respect to the carbon beam
if we compare intensities at 45° and 50° with those at 55°
and 60°, respectively. Due to the limiting signal to noise
ratio, no data could be obtained at angles smaller than 45°
and larger than 65°. Best fits of our data as shown in Fig. 3
yield translational energy distributions P(ET)s extending to
300–420 kJmolϪ1, whereas the total available energy gives
427Ϯ30 kJmolϪ1 to form the diacetylene isomer, cf. reaction
͑1͒. Finally, the P(ET)s depict
a maximum about
J. Chem. Phys., Vol. 107, No. 20, 22 November 1997
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Letters to the Editor
8715
FIG. 2. Time-of-flight data at the center-of-mass angle of 55.0°. The dashed
line indicates the experimental data, the solid line the fit.
FIG. 4. Schematic energy level diagram for the reaction C(3Pj)ϩC3H3.
Solid dashed lines: No ab initio calculations have been performed on these
structures. Eav indicates the total available energy.
30–60 kJmolϪ1
,
demonstrating
a
repulsive carbon–
hydrogen bond rupture to result in a tight exit transition state.
The center-of-mass angular flux distribution T(␪) is
slightly forward scattered with respect to the carbon beam.
This finding implies ͑a͒ one microchannel to C4H2 through a
complex holding a lifetime comparable to its rotational pe-
riod ͑osculating complex͒ or ͑b͒ two microchannels giving
an isotropic as well as a forward scattered contribution. Fi-
nally, the weak T(␪) polarization results from a poor cou-
pling between the initial L and final orbital angular momen-
the title reaction. Five reaction pathways on the ground-state
doublet surface are theoretically feasible. Defining the acety-
lenic carbon atom of the propargyl radical as C1, the central
C atom as C2, the olefinic as C3, and the attacking one as
C4, these are addition to C3, C2, or C1 forming the C4H3
isomers ͕1͖, ͕2͖, and ͕3͖, as well as C(3Pj) insertion into the
2
tum L with L Ͻ0.15–0.2 L indicating that most of the total
Ј
Ј
acetylenic C–H bond yielding n-C H (X A ) ͕4͖ or inser-
Ј
4
3
2
angular momentum channels into rotational excitation of the
C4H2 product.
tion into an olefinic C–H-bond to form the iso C H (X A )
Ј
4
3
isomer ͕5͖. Hereafter, ͓1,4͔-H migration in ͕3͖–͕4͖, a ͓3,4͔-H
migration in ͕1͖–͕5͖, ring closure of ͕2͖–͕6͖ followed by a
͓3,4͔-H migration to ͕7͖ are theoretically possible. A final
C–H bond rupture in ͕4͖ or ͕5͖ could form diacetylene ͕8͖,
and a C–H cleavage in ͕4͖ can yield butatrienylidene ͕9͖.
Comparing the scattering range with limit circles of the C4H2
isomers and analyzing the high energy cutoff of the P(ET)
strongly suggest that c-C4H2 ͕10͖ is not the major contribu-
tion. Therefore, we restrict the discussion on C4H2 isomers to
diacetylene ͕8͖ and butatrienylidene ͕9͖.
IV. DISCUSSION
Figure 4 displays a schematic energy level diagram of
The crucial question to be answered is the nature of the
decomposing C4H3 complex, i.e., ͕4͖ and/or ͕5͖. The forward
peaked center of mass angular distribution requires that the
attacking C(3Pj) and the leaving H atom must be located on
opposite sites of the rotation axis of the fragmenting C4H3. A
detailed analyses of the principal rotational axes of ͕4͖ and
͕5͖ employing ab initio geometries from Refs. 17 and 18
clearly shows that ͕5͖ must rotate around its A axis to ac-
count for this T(␪) shape. The final C–H bond cleavage,
however, would yield a linear diacetylene molecule, rotating
about its internuclear axis. Due to the vanishing moment of
inertia around the A axis, this rotation is energetically not
accessible, and ͕5͖ can be safely excluded as the fragmenting
C4H3 complex. The same argument strictly eliminates a de-
composing ͕4͖ excited to A-like rotations, but rotations
around the B/C axes contribute to the observed T(␪), lead-
ing to diacetylene ͕8͖ in the final C–H bond rupture excited
to B-like rotations. The preferred olefinic bond cleavage in
͕4͖ as compared to the acetylenic one goes hand in hand with
FIG. 3. Lower: Center-of-mass angular flux distribution for the reaction
C(3Pj)ϩC3H3. Upper: Center-of-mass translational energy flux distribution
for the reaction C(3Pj)ϩC3H3. Solid lines represent the best fit, dashed
lines limit the range of acceptable fits within 1␴ error bars.
J. Chem. Phys., Vol. 107, No. 20, 22 November 1997
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8716
Letters to the Editor
olefinic C–H bonds about 60–80 kJmolϪ1 weaker than
acetylenic ones.
Professor D. Gerlich ͑Technical University Chemnitz, Ger-
many͒ and Professor Y. T. Lee ͑Academia Sinica, Taiwan͒
for supervision. Careful discussion and reading of this manu-
script by Dr. D. Stranges ͑University La Sapienza, Rome,
Italy͒ as well as Dr. H. M. Bevsek ͑UC Berkeley͒ is grate-
fully acknowledged. Special thanks to the machine and elec-
tronic shops ͑Department of Chemistry, UC Berkeley͒ for
their ‘‘stand-by-mode’’ to assist fixing system failures as
soon as possible. Without their cooperation, this project
could not have been accomplished. This work was further
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.
Based on our data alone, we cannot distinguish between
a single insertion process into the acetylenic C1–H bond, or
an initial addition to C1 followed by a hydrogen migration to
yield ͕4͖. Keeping in mind that C(3Pj) insertions are
symmetry-forbidden and are expected to hold an entrance
barrier, and comparing our findings with the chemical dy-
namics of atomic carbon reacting with C2H2 and CH3CCH,
we find that C(3Pj) does not insert into C–H bonds, but
rather attacks preferentially the carbon atom with the highest
␲-electron density, in our system the acetylenic C1. Further,
acceptable impact parameters leading to reaction support the
chemical dynamics. Here, attack to the olefinic C3 atom is
expected to proceed almost perpendicular to the C3H3 mo-
lecular plane. The acetylenic C1–C2 bond, however, shows
almost cylindrical symmetry, and in plane as well as out of
plane approach geometries can lead to a C4H3 intermediate.
We conclude that the reaction between atomic carbon and
propargyl radical is initiated by an attack to the C1 atom
yielding ͕3͖, followed by a hydrogen migration to ͕4͖ and a
final bond rupture to form atomic hydrogen and diacetylene
͕8͖.
1 E. Herbst, H. H. Lee, D. A. Howe, and T. J. Millar, Mon. Not. R. Astron.
Soc. 268, 335 ͑1994͒.
2 R. I. Kaiser, C. Ochsenfeld, M. Head-Gordon, Y. T. Lee, and A. G. Suits,
Science 274, 1508 ͑1996͒.
3 Short-Lived Molecules, edited by M. J. Almond ͑Ellis Horwood, New
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4 J. S. Foord, K. P. Loh, N. K. Singh, R. B. Jackman, and G. J. Davies, J.
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Besides the chemical reaction dynamics, these results are
of fundamental importance to interstellar as well as solar
system chemistry as in Titan’s stratosphere,19 the Jovian
atmosphere20 as well as Neptune, Uranus, Saturn, and
Triton.21 Since C3H3 can be formed through reaction of
atomic carbon with C2H4 and photodissociation of C3H4
isomers,22 the title reaction is strongly suggested to be in-
cluded in chemical reaction networks modeling these extra-
terrestrial environments. Further, reaction of C(3Pj) with
C3H3 provides an alternative pathway to diacetylene hitherto
5 R. I. Kaiser, C. Ochsenfeld, M. Head-Gordon, Y. T. Lee, and A. G. Suits,
J. Chem. Phys. 106, 1729 ͑1997͒.
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C2HϩC2H2→C4H3→C4H2ϩH.
͑4͒
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Future investigations of reactions ͑1͒–͑3͒ will focus on
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cumstellar envelope of IRCϩ10216 and toward TMC-1,25,26
CCCCH2, by replacing HCCCH2 by HCCCD2. Despite this
open question, the present work clearly demonstrates that the
investigation of the chemical reaction dynamics of radical–
radical as well as atom–radical reactions employing the
crossed molecular beam technique is technologically feasible
and extendible to other systems. For example, preliminary
reactive scattering experiments on the interaction of C(3Pj)
with the vinyl radical, C2H3, show laser correlated signal at
m/eϭ38, i.e., ubiquitous interstellar C3H227—boldly to go
where no one has gone before••• .
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ACKNOWLEDGMENTS
R.I.K. is indebted the Deutsche Forschungsgemeinschaft
͑DFG͒ for a Habilitation fellowship ͑IIC1-Ka1081/3-1͒ and
27 R. I. Kaiser, W. Sun, and Y. T. Lee ͑unpublished͒.
J. Chem. Phys., Vol. 107, No. 20, 22 November 1997
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