Crossed beam reaction of phenyl radicals with unsaturated hydrocarbon molecules. I. Chemical dynamics of phenylmethylacetylene (C6H5CCCH3;X1A') formation from reaction of C6H5(X2A1) with methylacetylene, CH3CCH(X1A1)

Article abstract of DOI:10.1063/1.481054

The chemical reaction dynamics to form phenylmethylacetylene, C₆H₅CCCH₃(X²A'), via reactive collisions of the phenyl radical C₆H₅(X²A₁) with methylacetylene, CH₃CCH(X¹A₁), are unraveled under single collision conditions in a crossed molecular beam experiment at a collision energy of 140 kJ mol^-1. The laboratory angular distribution and time-of-flight spectra of C₉H₈⁺ at m/e = 116 indicate the existence of a phenyl radical versus hydrogen replacement pathway. Partially deuterated methylacetylene, CH₃CCD(X¹A₁), was used to identify the site of the carbon-hydrogen bond cleavage. Only the loss of the acetylenic hydrogen atom was observed; the methyl group is conserved in the reaction. Electronic structure calculations reveal that the reaction has an entrance barrier of about 17 KJ mol^-1. Forward-convolution fitting of our data shows that the chemical reaction dynamics are on the boundary between an osculating complex and a direct reaction and are governed by an initial attack of the C₆H₅ radical to the π electron density of the Cl carbon atom of the methylacetylene molecule to form a short lived, highly rovibrationally excited (C₆H₅)HCCCH₃ intermediate. The latter loses a hydrogen atom to form the phenylmethylacetylene molecule on the ²A' surface. The phenylallene isomer channel was not observed experimentally. The dynamics of the title reaction and the identification of the phenyl versus hydrogen exchange have a profound impact on combustion chemistry and chemical processes in outflows of carbon stars. For the first time, the reaction of phenyl radicals with acetylene and/or substituted acetylene is inferred experimentally as a feasible, possibly elementary reaction in the stepwise growth of polycyclic aromatic hydrocarbon precursor molecules and alkyl substituted species in high temperature environments such as photospheres of carbon stars and oxygen poor combustion systems.

Full text of DOI:10.1063/1.481054

Crossed beam reaction of phenyl radicals with unsaturated hydrocarbon molecules. I.
Chemical dynamics of phenylmethylacetylene ( C 6 H 5 CCCH 3 ;X 1 A ′ ) formation
from reaction of C 6 H 5 (X 2 A 1 ) with methylacetylene, CH 3 CCH (X 1 A 1 )
R. I. Kaiser, O. Asvany, Y. T. Lee, H. F. Bettinger, P. v. R. Schleyer, and H. F. Schaefer III
Citation: The Journal of Chemical Physics 112, 4994 (2000); doi: 10.1063/1.481054
View online: http://dx.doi.org/10.1063/1.481054
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 112, NUMBER 11
15 MARCH 2000
Crossed beam reaction of phenyl radicals with unsaturated hydrocarbon
molecules. I. Chemical dynamics of phenylmethylacetylene
1
2
„
C H CCCH ;X AЈ… formation from reaction of C H „X A …
6
5
3
6
5
1
1
with methylacetylene, CH CCH„X A …
3
1
a)
b)
R. I. Kaiser, O. Asvany, and Y. T. Lee
Institute of Atomic and Molecular Sciences, 1, Section 4, Roosevelt Road, 107 Taipei, Taiwan,
Republic of China
H. F. Bettinger,c),d) P. v. R. Schleyer,d),e) and H. F. Schaefer IIIf)
Center for Computational Quantum Chemistry, The University of Georgia, Athens, Georgia 30602-2525
͑
Received 31 August 1999; accepted 9 December 1999͒
1
The chemical reaction dynamics to form phenylmethylacetylene, C H CCCH (X AЈ), via reactive
6
5
3
2
1
collisions of the phenyl radical C H (X A ) with methylacetylene, CH CCH(X A ), are unraveled
6
5
1
3
1
under single collision conditions in a crossed molecular beam experiment at a collision energy of
Ϫ1
ϩ
1
40 kJ mol . The laboratory angular distribution and time-of-flight spectra of C H8 at m/e
9
ϭ116 indicate the existence of a phenyl radical versus hydrogen replacement pathway. Partially
1
deuterated methylacetylene, CH CCD(X A ), was used to identify the site of the carbon–hydrogen
3
1
bond cleavage. Only the loss of the acetylenic hydrogen atom was observed; the methyl group is
conserved in the reaction. Electronic structure calculations reveal that the reaction has an entrance
Ϫ1
barrier of about 17 kJ mol . Forward-convolution fitting of our data shows that the chemical
reaction dynamics are on the boundary between an osculating complex and a direct reaction and are
governed by an initial attack of the C H radical to the ␲ electron density of the C1 carbon atom of
6
5
the methylacetylene molecule to form a short lived, highly rovibrationally excited (C H ͒HCCCH
6
5
3
intermediate. The latter loses a hydrogen atom to form the phenylmethylacetylene molecule on the
2
AЈ surface. The phenylallene isomer channel was not observed experimentally. The dynamics of
the title reaction and the identification of the phenyl versus hydrogen exchange have a profound
impact on combustion chemistry and chemical processes in outflows of carbon stars. For the first
time, the reaction of phenyl radicals with acetylene and/or substituted acetylene is inferred
experimentally as a feasible, possibly elementary reaction in the stepwise growth of polycyclic
aromatic hydrocarbon precursor molecules and alkyl substituted species in high temperature
environments such as photospheres of carbon stars and oxygen poor combustion systems. © 2000
American Institute of Physics. ͓S0021-9606͑00͒01409-4͔
2
I. INTRODUCTION
The reactions of phenyl radicals, C H , in their A elec-
thalene like PAH species. The phenyl radicals themselves
are thought to be generated either by dimerization of prop-
2
2
6
5
1
argyl radicals, C H (X B ), via a benzene intermediate ͓re-
3
3
2
tronic ground state with unsaturated hydrocarbons are
strongly considered to be elementary processes leading to the
synthesis of polycyclic aromatic hydrocarbons ͑PAHs͒ and
ultimately to soot formation in oxygen-poor combustion pro-
2
action ͑1͔͒ or from reaction of C H (X AЈ) with acetylene,
C H (X ⌺ ) ͓reaction ͑2͔͒:
4
3
1
ϩ
g
3
2
2
2
2
C H ͑X B ͒ϩC H ͑X B ͒
3
3
2
3
3
2
1
cesses as well as in the outflow of carbon-rich stars. Current
1
2
2
→
C H ͑X A ͒→C H ͑X A ͒ϩH͑ S1/2͒,
͑1͒
͑2͒
6
6
g
6
5
1
chemical models postulate a stepwise PAH formation via
phenyl radicals reacting with acetylene molecules, followed
by a reactive collision of the radical intermediate with a sec-
ond acetylene molecule, and a final ring closure to a naph-
2
1
ϩ
g
2
C H ͑X AЈ͒ϩC H ͑X ⌺ ͒→C H ͑X A ͒.
4
3
2
2
6
5
1
Due to the potential significance of phenyl radicals in
both combustion processes and extraterrestrial environments,
a multitude of kinetic and spectroscopic investigations have
been performed in the past. Hausman and Homann scav-
enged the phenyl radical as a methylthioether in sooting
a͒Also at: Department of Physics, Technical University Chemnitz-Zwickau,
0
9107 Chemnitz, Germany; and Department of Physics, National Taiwan
University, Taipei, 107. Corresponding author. Electronic mail:
kaiser@po.iams.sinica.edu.tw
b͒
flames and thereby demonstrated explicitly that C H exists
Also at: Department of Physics, Technical University Chemnitz-Zwickau,
9107 Chemnitz, Germany.
6 5
4
0
in these high temperature combustion environments. Fur-
ther, the rate constants for the reactions of phenyl radicals
with a variety of molecules have been measured by cavity
c͒
Corresponding author. Electronic mail: bettingr@rice.edu
d͒
Also at: Computer-Chemie-Centrum, Institut f u¨ r Organische Chemie, Uni-
versit a¨ t Erlangen-N u¨ rnberg, Henkestrasse 42, 91054 Erlangen, Germany.
5
e͒
ring-down spectroscopy or other methods; data show very
Electronic mail: schleyer@paul.chem.uga.edu
Electronic mail: hfsiii@arches.uga.edu
f͒
low rate constants at temperature ranges up to 1100 K rang-
0021-9606/2000/112(11)/4994/8/$17.00
4994
© 2000 American Institute of Physics
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1

J. Chem. Phys., Vol. 112, No. 11, 15 March 2000
Reaction of phenyl radicals
4995
FIG. 1. Schematic setup of the pulsed valve and pyrolytic source.
ing between 10Ϫ12 and 10Ϫ14 cm3 sϪ1. These studies further
show an entrance barrier between 25 and 43 kJ mol with
the vacuum system. As the SiC tube has a negative tempera-
ture coefficient, we used a high voltage and low current
power supply ͑200 mA, 400 V͒ for the warm-up phase and
switched thereafter to a low voltage and high current power
supply. During the experiments we used about 20 V and 2 A
depending on the resistance of the SiC tube. The nitrosoben-
zene was kept at 277 K and shielded from light to avoid
decomposition. The pulsed valve was operated at 60 Hz with
pulses of 80 ␮s width. After passing a skimmer, a four slot
chopper wheel rotating at 240 Hz selected a 9 ␮s slice of the
beam.
The beam was characterized on axis employing a triply
differentially pumped detector consisting of an electron im-
pact ionizer followed by a quadrupole mass spectrometer and
a Daly-type scintillation particle detector. Under the above-
described operation conditions, the dissociation is complete,
Ϫ1
olefines and alkynes. But despite valuable kinetic data, the
reaction products—although crucial for a detailed chemical
modeling of combustion processes and PAH formation in
circumstellar shells of carbon starts—were never probed.
Obviously, the products of the reaction of C H with H as
6
5
2
6
studied by Lin and Co-workers should be benzene and
atomic hydrogen, but the problem is more complex in the
case of unsaturated and substituted hydrocarbons.
To obtain a complete understanding of the formation of
PAHs in various interesting environments, it is crucial to set
up a systematic research program for the investigation of the
basic elementary chemical reactions leading to PAHs and
their precursors on the most fundamental, microscopic level.
In this paper, we present the first crossed molecular beam
reaction of phenyl radicals reported so far, i.e., the reaction
and we did not detect signal of the C H NO precursor at
6
5
m/eϭ107. Only C H was observed; not even trace amounts
with methylacetylene, CH CCH ͓reaction ͑3͔͒:
6
5
3
of a potential hydrogen abstraction product C H are present
2
1
6
6
C H ͑X A ͒ϩCH CCH͑X A ͒
12
6
5
1
3
1
in our beam. We estimated a number density of about 10
radicals cm in the interaction region. No biphenyl recom-
bination product, C H –C H , was detected. The velocity of
Ϫ3
1
2
→
→
→
C H CCCH ͑X AЈ͒ϩH͑ S1/2͒
͑3a͒
͑3b͒
͑3c͒
6
5
3
6
5
6
5
1
2
H CCCH͑C H ͒͑X AЈ͒ϩH͑ S1/2͒
2 6 5
the phenyl beam and its speed ratio were determined to vp
Ϫ1
1
2
ϭ3150Ϯ50 ms and Sϭ8.5–9.0, respectively. Due to a pe-
C H CCH͑X A ͒ϩCH ͑X AЉ͒.
6
5
1
3
riodical replacement of the SiC tubes ͑see the following͒, the
day-to-day velocity changed slightly, and the delay time be-
tween the chopper wheel trigger pulse and the pulsed valve
had to be adjusted slightly. The phenyl radical beam crosses
a methylacetylene beam ͑MG Industries͒ in the interaction
region in the main chamber at a collision energy of 140
II. EXPERIMENTAL SETUP
All experiments are performed under single collision
conditions employing the 35Љ crossed molecular beam
7
,8
machine. Briefly, a pulsed supersonic beam of phenyl radi-
cals was generated via flash pyrolysis of a nitrosobenzene,
C H NO ͑Aldrich Chemicals͒, in the primary source cham-
Ϫ1
Ϫ1
Ϯ8 kJ mol (v ϭ850Ϯ10 ms , Sϭ9.0.) Reactively scat-
p
tered species are detected using a triply differentially
6
5
9
ber employing a modified Chen source and a piezoelectric
pumped detector consisting of a Brink-type electron-impact-
pulsed valve, cf. Fig. 1.10 A mixture of Ͻ0.1%C H NO
ionizer, quadrupole mass filter, and a Daly ion detector
11
6
5
seeded in helium was expanded at a stagnation pressure of
60 Torr through a resistively heated SiC tube of 1 cm length
recording time-of-flight spectra ͑TOF͒. Integrating these
TOF spectra at each laboratory angle and correcting for dif-
ferent data accumulation times yields the laboratory angular
distribution ͑LAD͒. After 8–10 h of operation, we observed
a background signal at almost all m/e ratios. This was prob-
ably due to reaction of the C H NO precursor or the formed
8
and 0.7 mm inner diameter; the temperature of the tube was
estimated to be around 1300–1400 K. The electrical heating
and mounting of this tube occurs through two silicon carbide
electrode sleeves and two molybdenum electrode blocks.
The latter are secured through insulating alumina screws and
spacers and are interfaced to a water-cooled copper block.
Both electrodes are connected to the power supplies outside
6
5
C H radicals with the SiC tube. Time-consuming daily dis-
6
5
mantling of the source and changing of the tube was there-
fore necessary. Further, extreme care had to be taken with
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4996
J. Chem. Phys., Vol. 112, No. 11, 15 March 2000
Kaiser et al.
unavoidable 60 Hz oscillations of the pulsed valve trans-
ferred to the pyrolytic source. Operating the pulsed valve
before the warm-up of the SiC tube inevitably led to a mi-
gration of the SiC tube outside the sleeves. Therefore, we
recommend warming up the SiC tube prior to engaging the
pulsed valve. To confirm that the signal at mass to charge
ratio of m/eϭ77 is actually the phenyl radical and not bi-
phenyl which could have been fragmented in the electron
impact ionizer to m/eϭ77, we performed elastic scattering
experiments of the primary beam with argon. Under these
conditions, the scattering signal of the phenyl radicals should
extend up to about 38° with respect to the primary beam,
whereas the biphenyl should give a cutoff angle of 18°. Mea-
surements of the elastic scattering signal at 25.0° confirmed
the presence of phenyl radicals in our beam.
A forward-convolution technique is employed to gain
information on the reaction dynamics from the laboratory
1
2
data. This approach assumes an angular flux T(␪) and a
translational energy P(E ) trial distribution in the center-of-
T
mass coordinate system assuming mutual independence. The
final outcome is the generation of a velocity flux contour
map I(␪,u) in the center-of-mass frame showing the inten-
sity as a function of angle ␪ and velocity u. This plot con-
tains all the basic information of the reactive scattering pro-
cess.
FIG. 2. Laboratory angular distribution together with the most probable
2
1
Newton diagram for the C H (X AЈ)ϩCH CCH(X A ) reaction at a col-
6
5
3
1
Ϫ1
lision energy of 140 kJ mol . The circle centrosymmetric around the center
of mass stands for the maximum center-of-mass recoil velocity of the phe-
nylmethylacetylene product.
III. ELECTRONIC STRUCTURE CALCULATIONS
The geometries of all stationary points were fully opti-
mized at the density functional theory level with the 6-31 G*
and 6-311ϩG** basis sets and Becke’s13 three parameter
hybrid functional in conjunction with the correlation func-
hand, if only a D atom loss occurred, a signal should be
ϩ
present only at m/eϭ116 (C H ). We observed only a
9
8
m/eϭ116 signal. These studies show unambiguously that
under our experimental conditions the H atom is released
1
4
15
tional of Lee, Yang, and Parr ͑B3LYP͒ as implemented in
1
6
GAUSSIAN 94. The spin-unrestricted formalism was em-
ployed for all open-shell species. Intrinsic reaction coordi-
nates were computed at B3LYP/6-31 G* for the addition of
the phenyl radical to the methylacetylene triple bond using
the algorithm of Gonzales and Schlegel.17 Harmonic vibra-
tional frequencies were computed at the B3LYP/6-31 G*
level and the obtained zero-point vibrational energies
from the acetylenic unit, whereas the CH group is conserved
3
in the reaction. Due to the unfavorable kinematics of the
reaction and low signal-to-noise ratio no CH loss channel
3
could be observed.
B. TOF spectra and laboratory angular distribution
„LAB…
͑ZPVE͒ were used for the ZPVE correction of the
The laboratory angular distribution of the product and
the calculated curve are depicted in Fig. 2 together with the
most probable Newton diagram. Figure 3 shows the TOF
spectrum recorded at 8.5° and the best fit. The LAB distri-
bution is very narrow and extends from 3.5° to 11.5°, i.e.,
only 8.0° in the scattering plane. This narrow range is based
on the mass combination of the products of 116 vs 1 and the
dominant translational energy release into the H atom prod-
uct together with a low reaction exothermicity is observed,
cf. Sec. V A. Further, the LAB distribution peaks at 7.5°,
slightly forward with respect to the center-of-mass angle of
B3LYP/6-311ϩG** energies.
IV. RESULTS
A. Reactive scattering signal
In our experiment, the phenyl radical versus hydrogen
exchange pathway was observed, and reactive scattering sig-
nal was detected at mass to charge ratio m/eϭ116(C H ).
ϩ
8
9
All TOF spectra observed at lower m/e ratios down to m/e
ϩ
9
ϭ108(C ) depicted identical patterns. Since they were fit
with the same center-of-mass functions as the parent ions,
these ions originate from cracking of the parent in the elec-
tron impact ionizer. No radiative association was found.
8
.0°Ϯ0.2°.
C. Center-of-mass translational energy distribution,
Since the H atom loss can occur from the CH group as well
3
P„ET…
as from the acetylenic carbon, we performed a crossed beam
experiment with partially deuterated methylacetylene,
CH CCD. If a H atom loss occurred at the CH group, we
The best fits of the translational energy P(E ) and an-
T
gular flux distribution T(␪) are shown in Fig. 4. Both the
LAB distribution and TOF spectra were fit with a single
3
3
ϩ
should have seen signal at m/eϭ117 (C H D ) and m/e
9
7
ϩ
ϩ
ϭ116 (C H D ; fragmentation from C H D ). On the other
P(E ) extending to a maximum translational energy release
9
6
9
7
T
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J. Chem. Phys., Vol. 112, No. 11, 15 March 2000
Reaction of phenyl radicals
4997
D. Center-of-mass angular distribution, T„␪…, and flux
contour map, I„␪,u…
The shape of the flux distribution T(␪) and the contour
map I(␪,u), Fig. 5, contain important information on the
chemical reaction dynamics of the title reaction. In the
C H /CH CCH system, the angular flux distribution is asym-
6
5
3
metric around 90° and shows predominantly flux in the for-
ward hemisphere with respect to the phenyl beam; the initial
and final angular momenta L and LЈ are strongly correlated.
The best fit was achieved with the flux peaking at 0° and
zero intensity at angles larger than 150°. Within the error
limits, a minor intensity at these larger angles can provide an
acceptable fit as well. These results suggest that the reaction
does not proceed through a long-lived complex, but shows
the characteristics of ‘‘direct’’ dynamics: the reaction in-
volves either a highly rovibrationally excited collision com-
plex in a very shallow potential energy well with a lifetime
of less than 0.1 ps, or only one transition state ͓C H ͔* along
8.50
2
FIG. 3. Time-of-flight data at
for the C H (X AЈ)
6 5
1
Ϫ1
ϩCH CCH(X A ) reaction at a collision energy of 140 kJ mol . The
dashed line indicates the experimental data, the solid line the calculated fit.
3
1
9
9
the reaction coordinate without a bound intermediate. We
like to stress that despite the unfavorable kinematics of the
reaction the information gained from the T(␪) distribution
shows unambiguously that this distribution is strongly for-
ward peaked. We did a careful error analysis, and no fit
could be gained with a forward–backward symmetric center-
of-mass angular distribution.
Emax of 160 kJ molϪ1. Within the experimental error limits,
Ϫ1
extending or cutting the P(E ) by 10 kJ mol does not
T
influence the quality of the fit. Further, the P(E ) is very
T
broad and peaks within the error limits at about 60–70
Ϫ1
kJ mol well away from zero translational energy. This
finding together with the large fraction of the total available
energy, i.e., the sum of the reaction energy plus the collision
energy, of 50Ϯ5% released into the translational degrees of
freedom of the products and the forward peaking LAB dis-
tribution suggest that the reaction proceeds via a ͑very͒
short-lived intermediate.
V. DISCUSSION
A. The potential energy surface
The addition of the phenyl radical 1 to the ␲ electron
density of the methylacetylene 2 triple bond can yield the
trans-1-phenylpropene-2-yl as well as the trans-2-
phenylpropene-1-yl structural isomers 3 and 4 via transition
states TS1 and TS2, cf. Figs. 6 and 7 and Table I. The latter
Ϫ1
lie 17.1 and 28.2 kJ mol above the reactants. In agreement
with the Bell–Evans–Polyani principle, the barrier for the
more exothermic addition to the terminal acetylenic C1 atom
Ϫ1
of 2 to form 3 ͑Ϫ149.9 kJ mol ͒ is lower than for addition
Ϫ1
to the C2 atom to give 4 ͑Ϫ126.0 kJ mol ͒. Our calculations
show that the addition of the phenyl radical to methylacety-
lene does not yield the more stable cis-2-phenylpropene-1-yl
or 1-phenylpropene-2-yl conformation isomers 5 or 6 di-
rectly. However, the barriers for cis–trans isomerization of
3
Õ5 and 4Õ6 via TS3 and TS4 are very small ͑18.5 and 14.1
Ϫ1
kJ mol , respectively͒ and well below the total available
energy of the reaction.
The 1-phenylpropene-2-yl radicals 3 and 5 can emit ei-
ther a methyl or a vinyl H atom. The first pathway yields
phenylmethylacetylene 7 via tight transition states TS5 and
Ϫ1
TS6 both of which are about 2 kJ mol above the separated
reactants. Alternatively, TS7 and TS8 which lead to the phe-
nylallene isomer 8 are located slightly below the reactants,
Ϫ1
i.e., Ϫ0.3 and Ϫ1.0 kJ mol , respectively. Further, our cal-
culations show that the reaction between the phenyl radical
1
7
and methylacetylene 2 to give phenylmethylacetylene
Ϫ1
plus a H atom is exothermic by 12.0 kJ mol
at
B3LYP/6-311ϩG** level of theory; the formation of phe-
nylallene isomer 8 and a H atom is the least exothermic
process ͑Ϫ5.7 kJ mol ͒ according to the calculations.
FIG. 4. Center-of-mass angular flux distribution ͑lower͒ and translational
2
1
energy flux distribution ͑upper͒ for the C H (X AЈ)ϩCH CCH(X A ) re-
6
5
3
1
Ϫ1
Ϫ1
action at a collision energy of 140 kJ mol
.
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4998
J. Chem. Phys., Vol. 112, No. 11, 15 March 2000
Kaiser et al.
FIG. 5. Center-of-mass velocity contour flux map distribution for the C H (X AЈ)ϩCH CCH(X A ) reaction at a collision energy of 140 kJ molϪ1; top:
2
1
6
5
3
1
two-dimensional view, bottom: three-dimensional view. The contour lines connect data points with an identical flux. Units of the x and y axis are given in
Ϫ1
ms
.
The fragmentation of 2-phenylpropene-1-yl isomers 4
TS9 for this C–C bond cleavage is slightly higher in energy
than the reactants ͑ϩ1.3 kJ mol ͒. We were not able to
Ϫ1
and 6 into phenylacetylene 9 and a CH radical is the ther-
3
modynamically most favorable pathway of the title reaction
as it is exothermic by Ϫ56.0 kJ mol . The transition state
locate a TS for CH elimination from 6 as all attempts re-
sulted in TS9. All transition states for the fragmentation of
3
Ϫ1
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J. Chem. Phys., Vol. 112, No. 11, 15 March 2000
Reaction of phenyl radicals
4999
FIG. 6. Schematic representation of the C H potential energy surface at the B3LYP/6-311ϩG** level of theory. Computational details are given in the text.
9
9
B. The reaction pathway
the C H radicals lie late on the reaction coordinate and are
9
9
characterized by significantly stretched dissociating bonds.
Finally, we would like to point out that the H atom
elimination from the C1 atom in the 2-phenylpropene-1yl
radicals 4 and 6 results in the formation of phenylmethylvi-
nylidene, ͑CH ͒͑C H ͒CC. As this carbene is a very high
Our experimental and theoretical data suggest that the
reaction of the phenyl radical with methylacetylene proceeds
according to the following reaction dynamics. The indisput-
able identification of the phenyl versus hydrogen replace-
ment together with the assignment of the H loss site as acety-
3
6
5
Ϫ1
lying species ͑ϩ209 kJ mol with respect to the reactants͒,
lenic strongly indicate formation of the phenylmethyl-
its formation is energetically not feasible at our collision
Ϫ1
1
acetylene, C H CCCH (X AЈ) isomer. The high energy cut-
energy of 140 kJ mol
.
6
5
3
FIG. 7. Bond distances in angstrom and bond angles in degrees of reactants and potential products on the C H9 potential energy surface at the
9
B3LYP/6-311ϩG** level of theory. 7a and 7b are isoenergetic and differ only in the orientation of the methyl group.
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5000
J. Chem. Phys., Vol. 112, No. 11, 15 March 2000
Kaiser et al.
TABLE I. Electronic states, dipole moments ␮ ͑in debye͒, and rotational
constants ͑in GHz͒ for reactants, intermediates, and products obtained at the
B3LYP/6-311ϩG** level of theory.
It is worth taking a closer look at the chemical dynamics.
Even if the isomers 3 and/or 5 are bound by about 150–160
Ϫ1
kJ mol , the reaction does not go through a long-lived com-
Electronic
state
Dipole
moment ͑␮͒
plex. Here, the reactive scattering signal was found predomi-
nantly in the forward sphere with respect to the phenyl beam,
cf. I(␪,u). The strong correlation of the initial and final an-
gular momentum L and LЈ is characteristic for a typical
direct reaction following stripping dynamics. This preferen-
tial forward scattering indicates an attractive interaction be-
tween the phenyl radical and the methylacetylene molecule.
Furthermore, the process of reaction is complete before the
colliding pairs of reactants have time for a full rotation
around their rotation axis. Hence the complex 3 and/or 5 is
very short lived. Since an acceptable fit was achieved with
some minor intensity at angles larger than 150° as well, we
propose that the reaction is likely on the boundary between a
direct process and a short-lived osculating complex with a
lifetime of about 0.1 ps. Therefore, the initial collision com-
plex must be highly rovibrationally excited to result in this
short lifetime and translates into an incomplete energy ran-
domization in 1 and/or 2 and hence in a preferential loss of
the acetylenic hydrogen atom although the latter is more
strongly bound than an aliphatic, cf. Sec. V C. In addition,
direct reactions are often found to have an increased fraction
of total available energy channeling into the translational de-
grees of freedom compared to complex forming encounters.
Molecule
2
1
2
3
4
5
6
7
8
9
A1
A1
A
0.9133
0.8506
0.9174
0.8338
1.1154
0.9259
0.5115
0.1199
0.7437
0.0
1
2
2A
2
AЈ
AЈ
AЈ
AЈ
2
1
1
1A
2
1
CH3
AЉ
2
off, i.e., the sum of the reaction energy and the collision
energy of the P(E ) yields a reaction exothermicity of 20
T
Ϫ1
Ϯ10 kJ mol , which is in good agreement with our calcu-
Ϫ1
lated value of 12.0 kJ mol . An alternative reaction channel
which results in an acetylenic hydrogen loss to yield phenyl-
methylvinylidene, CH C H CC, is endothermic by about
3
6
5
Ϫ1
2
09 kJ mol and hence energetically not open at our colli-
Ϫ1
sion energy of 140 kJ mol . These findings suggest an ini-
tial attack of the C H with the unpaired electron in an A
orbital on the ␲ electron density at C1 of the CH CCH mol-
6
5
1
3
This is consistent with our data showing a fraction of 50
ecule to form 3 which could isomerize to 5. A subsequent
homolytic C–H bond rupture releases the vinylic H atom.
We would like to stress that a phenyl radical addition to the
C2 of the methylacetylene giving 4 and/or 6 is energetically
feasible as well, but this pathway cannot yield the experi-
mentally observed site specific H atom loss channel.18 Here,
two factors may direct the carbon–carbon–␴ bond formation
to the C1 atom. First, the ␲-group orbitals of the methyl
group increase the spin density on the C1 atom at the ex-
pense of the C2 position. Second, the sterical hindrance of
3
Ϯ5%; in comparison, all reactions of C( P ) and CN radi-
j
cals with unsaturated hydrocarbons are indirect and depict
fractions of only 30%–35% channeling into translational
energy.
2
¿
C. Comparison with the reaction CN„ ⌺ …¿CH CCH
3
It is interesting to compare the phenyl/methylacetylene
2
ϩ
system to the reaction of cyano radicals, CN( ⌺ ), with
methylacetylene which was studied earlier in our lab.19 In
both systems, the unpaired electron is located in a cen-
trosymmetrical orbital localized predominantly on the radical
carbon atom of the reactant. Directed by a steric ͑screening͒
effect of the bulky methyl group and governed by an in-
creased spin density at the C1 carbon atom of the methy-
lacetylene molecule, this orbital interacts with the ␲ electron
density to form a new carbon–carbon ␴ bond and resulting
in cis/trans CH CCHX (XϭCN, C H ). Whereas the reac-
the CH group reduces the cone of acceptance at the C2
3
position. Both effects together might direct the electrophilic
phenyl addition at C1. This preferential site-specific radical
attack was observed previously in the crossed beam reaction
1
9
20
of methylacetylene with CN radicals, C D radicals, and
2
3
21
C( P ) as well.
j
Since however the unpaired electron of phenyl is local-
ized in an A orbital, the reactive scattering signal depends
1
strongly on the orientation angle of the radical center toward
the acetylenic bond. In combination with the calculated en-
3
6
5
tion of the cyano radical proceeds through an osculating
complex ͑indirect scattering dynamics͒ having a lifetime
around 1–2 ps, the phenyl radical shows a direct reaction via
a short lived ͑Ͻ0.1 ps͒ intermediate. This translates into re-
active scattering signal preferentially in the forward hemi-
sphere with respect to the phenyl radical beam. Likewise, the
Ϫ1
trance barrier of 17.1 kJ mol , the reaction cross section is
expected to be much lower compared to the analogous reac-
tions of atomic carbon and cyano radicals which follow al-
most gas kinetics behavior. Based on the intensity of our
signal, the data accumulation time, different exothermicities,
and beam intensities we estimate that the cross section of our
title reaction is about 2–3 orders of magnitude less than the
energy randomization in the CH CCHCN intermediates is
3
likely to be complete: we observe a H atom loss at the C2
3
one in both C( P )/CH CCH and CN/CH CCH systems.
carbon atom of the CH CCCH reactant—the site with the
j
3
3
3
This finding is well reflected in the temperature-dependent
reaction rate constants k obtained from bulk experiments ͑cf.
Sec. I͒: here, data with unsaturated hydrocarbons range from
newly formed carbon–carbon bond—to yield the substituted
methylacetylene, i.e., CH CCCN, and at the methyl group
3
carbon atom to give the allene isomer H CCCH͑CN͒. The
2
Ϫ12
Ϫ14
Ϫ3
3
10
to 10
cm whereas those of C( P ) and CN are in
shorter lifetime of the CH CCH͑C H ͒ intermediate pre-
j
3
6
5
Ϫ10
Ϫ3
the order of 10
cm
.
cludes an energy distribution into the methyl group. Hence,
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J. Chem. Phys., Vol. 112, No. 11, 15 March 2000
Reaction of phenyl radicals
5001
assignment of the phenyl versus H atom exchange is the first
experimental proof that C H radicals can react with substi-
tuted acetylenes to phenylacetylenes—the latter reaction
thought to be the key elementary process in the formation of
only the CH CCC H isomer is observed experimentally, and
3
6
5
no phenylallene isomer is formed. These findings can be ra-
tionalized comparing the average collision energies of the
6
5
Ϫ1
CN/CH CCH ͑24.7 kJ mol ͒ versus the C H /CH CCH
͑
3
6
5
3
Ϫ1
͑substituted͒ polycyclic aromatic hydrocarbons ͑PAHs͒ in
140 kJ mol ͒ reactive encounters. As a general trend, the
oxygen-deficient combustion flames and outflows of carbon
stars. Finally, we would like to point out that as the collision
energies decrease the lifetime of the collision complex in-
creases. Therefore, the formation of the thermodynamically
less stable phenylallene isomer might be feasible at lower
collision energies and hence in the outflow of carbon stars as
well. This pathway is subject of further theoretical studies.22
lifetime of an intermediate in an indirect reaction decreases
as the collision energy rises, and as an extreme, the reaction
goes from a long lived intermediate through an osculating
complex, and finally via direct scattering dynamics.
VI. IMPLICATIONS TO INTERSTELLAR CHEMISTRY
AND COMBUSTION PROCESSES
Our crossed beam experiments explicitly demonstrated
the formation of a phenyl substituted methylacetylene prod-
uct formed under single collision conditions. This is the very
first unambiguous assignment of a reactive scattering product
of an elementary reaction between a phenyl radical and a
substituted acetylene molecule—an elementary reaction
which is thought to play a central role in the formation of
PAHs in various terrestrial as well as extraterrestrial environ-
ments. Since the title reaction has an entrance barrier of 17
ACKNOWLEDGMENTS
R.I.K. is indebted to the Deutsche Forschungsgemein-
schaft for a Habilitation fellowship ͑IIC1-Ka1081/3-2͒. This
work was supported by Academia Sinica, Taiwan, the Na-
tional Science Council of R.O.C. Partial support from the
Petroleum Research Fund of R.O.C. is also appreciated. Spe-
cial thanks to Dr. Balucani for suggesting the use of ni-
trosobenzene as a phenyl radical precursor. The work in Ath-
ens was supported by the U.S. Department of Energy, Basic
Energy Sciences.
Ϫ1
kJ mol , it is irrelevant for the formation of PAHs in cold
molecular clouds or hydrocarbon-rich atmospheres of Jupi-
ter, Saturn, and Titan because this entrance barrier inhibits
the reaction. However, temperatures close to the photosphere
of carbon stars can reach up to 4000 K, and the reaction
might be important in these environments. The actual effect
on PAH synthesis must be verified in extended and refined
chemical modeling of these scenarios. However, if we extend
previously postulated reaction pathways to substituted me-
thylacetylene, the title reaction is likely to be involved in the
synthesis of methyl substituted naphthalene molecules.
1
I. Glassmanm, Combustion ͑Academic, New York, 1987͒, and references
therein.
2
A. Kazakov and M. Frenklach, Combust. Flame 112, 270 ͑1998͒.
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6
7
VII. CONCLUSIONS
The crossed beam reaction of the phenyl radical
2
1
8
9
0
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6
5
1
3
1
Ϫ1
investigated at an average collision energy of 140 kJ mol
.
1
1
1
The chemical reaction dynamics are direct and proceed
through a very short lived, highly rovibrationally excited
trans-CH CCH͑C H ͒ complex via an initial addition of the
͑
1992͒.
1
2
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unpaired electron to the ␲ orbital of the methylacetylene
unit. As verified in our experiments, the reduced cone of
acceptance of the carbon atom adjacent to the methyl group
favors a carbon–carbon ␴ bond formation at the terminal
acetylenic carbon atom of methylacetylene. As supported in
crossed beam experiments of partially deuterated
13
14
1
5
1
6
M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson,
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Although the isomerization from 4/6 to 3/5 might be viable energetically,
the extremely short lifetime observed for the collision complex rules out
this possibility.
1
d -methylacetylene, CH CCD (X A ), the acetylenic car-
1
3
1
bon hydrogen bond in the cis/trans CH CCH͑C H ͒ interme-
3
6
5
diate͑s͒ is cleaved to form phenylmethylacetylene; the me-
thyl group is conserved throughout the reaction. The
experimentally derived reaction exothermicity of about 20
1
1
7
8
Ϫ1
Ϫ1
kJ mol is in good agreement with the 12 kJ mol calcu-
lated. The site-specific H atom loss can be rationalized in
terms of the subpicosecond lifetime of the reactive interme-
diate and hence a lifetime too short to allow an activation of
the aliphatic carbon–hydrogen bond and/or energy random-
ization; this behavior is well documented in our experiments
since the phenylallene isomer is not formed. The explicit
1
2
2
9
0
1
N. Balucani et al., J. Chem. Phys 111, 2857 ͑1999͒.
R. I. Kaiser, O. Asvany, and Y. T. Lee ͑unpublished͒.
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8
721 ͑1996͒.
22
L. Vereeken, J. Peeters, R. I. Kaiser, and Y. T. Lee ͑unpublished͒.
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1