Crossed beam reaction of cyano radicals with hydrocarbon molecules. IV. Chemical dynamics of cyanoacetylene (HCCCN;X 1Σ+) formation from reaction of CN(X 2Σ+) with acetylene, C2H2(X 1Σg+)

Article abstract of DOI:10.1063/1.1289530

The chemical reaction dynamics to form cyanoacetylene, HCCCN (X ¹Σ⁺), via the radical-neutral reaction of cyano radicals, CN(X ²Σ⁺; ν = 0), with acetylene, C2H2(X ¹Σ_g⁺), are unraveled in crossed molecular beam experiments at two collision energies of 21.1 and 27.0 kJ mol^-1. Laboratory angular distributions and time-of-flight spectra of the HCCCN product are recorded at m/e = 51 and 50. Experiments were supplemented by electronic structure calculations on the doublet C3H2N potential energy surface and RRKM investigations. Forward-convolution fitting of the crossed beam data combined with our theoretical investigations shows that the reaction has no entrance barrier and is initiated by an attack of the CN radical to the ? electron density of the acetylene molecule to form a doublet cis/trans HCCHCN collision complex on the ²A' surface via indirect reactive scattering dynamics. Here 85 percent of the collision complexes undergo C-H bond rupture through a tight transition state located 22 kJ mol^-1 above the cyanoacetylene, HCCCN (X ¹Σ⁺) and H(²S_1/2) products (microchannel 1). To a minor amount (15 percent) trans HCCHCN shows a 1.2-H shift via a 177 kJ mol^-1 barrier to form a doublet H2CCCN radical, which is 46 kJ mol^-1 more stable than the initial reaction intermediate (microchannel 2). The H2CCCN complex decomposes via a rather loose exit transition state situated only 7 kJ mol^-1 above the reaction products HCCCN (X ¹Σ⁺) and H(²S_1/2). In both cases the geometry of the exit transition states is reflected in the observed center-of-mass angular distributions showing a mild forward/sideways peaking. The explicit identification of the cyanoacetylene as the only reaction product represents a solid background for the title reaction to be included in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan.

Full text of DOI:10.1063/1.1289530

Crossed beam reaction of cyano radicals with hydrocarbon molecules. IV. Chemical
dynamics of cyanoacetylene (HCCCN; X 1 Σ + ) formation from reaction of CN (X 2 Σ + )
with acetylene, C 2 H 2 (X 1 Σ g + )
L. C. L. Huang, O. Asvany, A. H. H. Chang, N. Balucani, S. H. Lin, Y. T. Lee, R. I. Kaiser, and Y. Osamura
Citation: The Journal of Chemical Physics 113, 8656 (2000); doi: 10.1063/1.1289530
View online: http://dx.doi.org/10.1063/1.1289530
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 113, NUMBER 19
15 NOVEMBER 2000
Crossed beam reaction of cyano radicals with hydrocarbon molecules. IV.
1
Chemical dynamics of cyanoacetylene „HCCCN; X ⌺¿… formation
¿
2
1
from reaction of CN„X ⌺¿… with acetylene, C2H2„X ⌺g …
L. C. L. Huang, O. Asvany,a) A. H. H. Chang, N. Balucani,b) S. H. Lin, Y. T. Lee,
and R. I. Kaiserc)
Institute of Atomic and Molecular Sciences, 1, Section 4, Roosevelt Rd., 107 Taipei, Taiwan,
Republic of China
Y. Osamurad)
Department of Chemistry, Rikkyo University, 3-34-1 Nishi-ikebukuro, Toshima-ku, Tokyo, 171-8501, Japan
͑Received 27 April 2000; accepted 29 June 2000͒
The chemical reaction dynamics to form cyanoacetylene, HCCCN (X 1⌺ϩ), via the radical–neutral
reaction of cyano radicals, CN(X 2⌺ϩ;␯ϭ0), with acetylene, C2H2(X 1⌺gϩ), are unraveled in
crossed molecular beam experiments at two collision energies of 21.1 and 27.0 kJ molϪ1
.
Laboratory angular distributions and time-of-flight spectra of the HCCCN product are recorded at
m/eϭ51 and 50. Experiments were supplemented by electronic structure calculations on the doublet
C3H2N potential energy surface and RRKM investigations. Forward-convolution fitting of the
crossed beam data combined with our theoretical investigations shows that the reaction has no
entrance barrier and is initiated by an attack of the CN radical to the ␲ electron density of the
2
acetylene molecule to form a doublet cis/trans HCCHCN collision complex on the A surface via
Ј
indirect reactive scattering dynamics. Here 85% of the collision complexes undergo C–H bond
rupture through a tight transition state located 22 kJ molϪ1 above the cyanoacetylene, HCCCN
(X 1⌺ϩ) and H(2S1/2) products ͑microchannel 1͒. To a minor amount ͑15%͒ trans HCCHCN shows
a 1,2-H shift via a 177 kJ molϪ1 barrier to form a doublet H2CCCN radical, which is 46 kJ molϪ1
more stable than the initial reaction intermediate ͑microchannel 2͒. The H2CCCN complex
decomposes via a rather loose exit transition state situated only 7 kJ molϪ1 above the reaction
products HCCCN (X 1⌺ϩ) and H(2S1/2). In both cases the geometry of the exit transition states is
reflected in the observed center-of-mass angular distributions showing a mild forward/sideways
peaking. The explicit identification of the cyanoacetylene as the only reaction product represents a
solid background for the title reaction to be included in reaction networks modeling the chemistry
in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the
atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan. © 2000
American Institute of Physics. ͓S0021-9606͑00͒01136-3͔
I. INTRODUCTION
molecular cloud TMC-1 and the outflow of the carbon star
IRCϩ10126. A recent line survey toward the Taurus mo-
lecular cloud TMC-1 yielded abundance ratios of
͓HCCCN͔:͓HCCNC͔:͓HNCCC͔ϭ1000:8:1.2 Although the
number densities are well established, the formation mecha-
nism of these isomers is far from being resolved. Ion–
molecule reactions have been proposed in chemical models
of the interstellar medium ͑ISM͒, but failed to reproduce the
relative abundances.3 Supported by electronic structure cal-
culations, this model suggested a recombination of interstel-
lar HCCCNHϩ ions with an electron from the cosmic radia-
tion followed by an isomerization from HCCCNH to
HCCNCH and H atom emission. However, this reaction se-
quence yields ratios of ͓HCCCN͔:͓HCCNC͔:͓HNCCC͔
ϭ240:8:1. As a consequence, even though this mechanism
can explain satisfactorily the abundances of both metastable
isomers quantitatively, the majority of the HCCCN mol-
ecules cannot be produced via ion–molecule reactions.
It was therefore suggested that the bimolecular neutral–
neutral reaction of the cyanogen radical, CN(X 2⌺), with
acetylene, C2H2(X 1⌺gϩ), might produce cyanoacetylene in
Investigating the mechanisms to form distinct structural
isomers in extraterrestrial environments is an important
means to test chemical models on the evolution of cold
͑dark͒ molecular clouds. The HC3N isomers cyanoacetylene,
HCCCN, isocyanoacetylene, HCCNC, and HNCCC repre-
sent a particular problem as this system is the only known
example of three structural isomers existing in the interstellar
medium.1 Among them the thermodynamically most stable
HCCCN isomer has the largest abundance; both metastable
HCCNC and HNCCC molecules were identified in the cold
a͒
Also at the Department of Physics, Technical University Chemnitz, 09107
Chemnitz, Germany.
Permanent address: Dipartimento di Chimica, Universita di Perugia,
b͒
`
06123 Perugia, Italy.
c͒
Also at the Department of Physics, Technical University Chemnitz,
09107 Chemnitz, Germany and Department of Physics, National Taiwan
University, Taipei, 106, Taiwan, Republic of China. Electronic mail:
kaiser@po.iams.sinica.edu.tw
d͒
Author to whom correspondence should be addressed. Electronic mail:
osamura@chem.rikkyo.ac.jp
0021-9606/2000/113(19)/8656/11/$17.00
8656
© 2000 American Institute of Physics
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J. Chem. Phys., Vol. 113, No. 19, 15 November 2000
Reaction of cyano radicals. IV
8657
TABLE I. Experimental beam conditions and 1␴ errors: most probable ve-
angle. Integrating these TOF spectra at different laboratory
angles and correcting for the CN beam intensity drift yields
the laboratory angular distribution ͑LAB͒. A forward-
convolution technique is employed to gain information on
the reaction dynamics from the laboratory data.11 This ap-
proach assumes an angular flux T(␪) and a translational en-
ergy P(ET) trial distribution in the center-of-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 intensity as a function of
angle ␪ and velocity u. This plot contains all the basic infor-
mation of the reactive scattering process.
locity p , speed ratio S, most probable collision energy of the CN beam
v
with the acetylene molecules, Ecoll , and center-of-mass angles, ␪
.
CM
Experiment
p ,m sϪ1
S
Ec ,kJ molϪ1
⌰
v
CM
6.8Ϯ0.5
6.0Ϯ0.2
12.0Ϯ0.5
21.1Ϯ0.7
27.0Ϯ0.4
•••
30.0Ϯ0.1
26.2Ϯ0.3
¯
CN/C2H2
CN/C2H2
C2H2
1560Ϯ30
1828Ϯ20
900Ϯ15
the outflow of the circumstellar envelope surrounding the
carbon star IRCϩ10216,4 hot molecular cores,5 and dark
molecular clouds such as TMC-1:6
ϩ
g
2
CN X 2⌺ϩ͒ϩC H X 1⌺ ͒→HCCCN X 1⌺ϩ͒ϩH S ͒.
͑
͑
2
͑
͑
III. ELECTRONIC STRUCTURE AND RRKM
CALCULATIONS
2
1/2
͑1͒
Recent laboratory measurements at ultralow temperatures as
low as 20 K show that the reaction rate constant of ͑1͒ in-
creases as the temperature decreases, reaching a maximum of
5ϫ10Ϫ10 cm3 sϪ1 at 30 K, i.e., in the order of gas phase
collisions. However, these studies can monitor only the de-
cay kinetics of the CN radical, and hence reaction products
could not be determined explicitly.7
The potential energy surface ͑PES͒ of the reaction be-
tween C2H2 and the CN(X 2⌺ϩ;␯ϭ0) radical has been in-
vestigated in terms of ab initio molecular orbital methods.
We have employed the hybrid density functional B3LYP
method, i.e., Becke’s three-parameter nonlocal exchange
functional12 with the correlation functional of Lee, Yang, and
Parr,13 and the 6-311G(d,p) basis set.14 All computation has
been carried out using the GAUSSIAN 98 program package.15
The structures of the intermediates and transition states were
confirmed with the vibrational analysis; all relative energies
shown in this paper are the corrected values of the zero-point
vibrational energies. The coupled cluster CCSD͑T͒ calcula-
tions with the 6-311G(d,p) basis set have also been per-
formed for some of the critical structures in order to refine
the energetics.16
Investigation at the molecular level, where it is possible
to observe the consequences of a single reactive event, can
provide a direct insight into the reaction mechanism. We
have recently reported8 the first investigation of the title re-
action by means of the crossed molecular beam ͑CMB͒ tech-
nique with mass spectrometric detection; the CN/H exchange
channel was found to occur through the formation of a bound
intermediate formed following the addition of a CN radical
to the ␲ system of acetylene. Here, we report a more exten-
sive investigation where the dependence of chemical reaction
dynamics on the collision energy is also examined. In addi-
tion, the experimental results are complemented by the first
complete ab initio molecular orbital calculations of the reac-
tion potential energy surface, where all the possible reaction
pathways were considered; the calculated geometry of the
decomposing transition states allowed us to rationalize the
observed preference for sideways scattering. The branching
ratios among the possible reaction pathways were deter-
mined within the RRKM model.
According to the quasiequilibrium theory or RRKM
theory,17 the rate constant k(E) at a collision energy E for a
]
*
unimolecular reaction A →A →P can be expressed as
␴
W# EϪE#͒
͑
k E͒ϭ
͑
•
,
͑2͒
h
␳ E͒
͑
where ␴ is the symmetry factor, W#(EϪE#) denotes the
total number of states of the transition state ͑activated com-
plex͒ A] with the barrier E#,␳(E) represents the density of
*
states of the energized reactant molecule A , and P is the
product or products. The harmonic oscillator and rigid rotor
approximation were assumed for the species involved
throughout the rate constant calculations. The saddle point
method was applied to evaluate ␳(E) and W(E).
II. EXPERIMENT AND DATA ANALYSES
The experiments were performed with the 35 in. crossed
molecular beams machine.9 Briefly, a pulsed supersonic
cyano radical beam, CN(X 2⌺ϩ;␯ϭ0) in the vibrational
ground state is generated in situ via laser ablation of graphite
at 266 nm and seeding the ablated species in neat nitrogen
carrier gas that acts as a reactant as well. The Spectra Physics
GCR 270-30 Nd-YAG laser operates at 30 Hz and 30 mJ per
pulse. A chopper wheel selects a segment of the CN beam,
cf. Table I, which intersects a second, pulsed acetylene,
C2H2, beam at 90° in the interaction region. Reactively scat-
tered species are detected using a triply differentially
pumped detector consisting of a Brink-type electron-impact
ionizer, quadrupole mass filter, and a Daly ion detector10
recording time-of-flight spectra ͑TOF͒ at different laboratory
angles. Data accumulation times range up to 4 h at every
IV. RESULTS
A. Reactive scattering signal
Reactive scattering signal was observed at m/e
ϭ51 (C3NHϩ) and 50 (C3Nϩ); cf. Figs. 1–4. TOF spectra
recorded at both mass-to-charge ratios show identical pat-
terns revealing that the product of gross formula C3NH is the
only one formed from the reaction and that it fragments to
the daughter ion with m/eϭ50 in the electron impact ionizer.
We did not observe any adduct at mass-to-charge ratio 52;
this indicates that the lifetime of C2H2CN is too short to
survive the flight from the collision region to the ionizer.
Because of the high background level at m/eϭ27, we were
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8658
J. Chem. Phys., Vol. 113, No. 19, 15 November 2000
Huang et al.
2
ϩ
2
ϩ
FIG. 2. Lower: Newton diagram for the reaction CN(X
ϩC2H2(X
⌺ )
FIG. 1. Lower: Newton diagram for the reaction CN(X
ϩC2H2(X
⌺ )
⌺
ϩg ) at a collision energy of 27.0 kJ molϪ1. The circle delimits
1
⌺
ϩg ) at a collision energy of 21.1 kJ molϪ1. The circle delimits
1
the maximum center-of-mass recoil velocity of the HCCCN product. Upper:
Laboratory angular distribution of the HCCCN product at m/eϭ50. Circles
and error bars indicate experimental data, the solid line the calculated dis-
tribution.
the maximum center-of-mass recoil velocity of the HCCCN product. Upper:
Laboratory angular distribution of the HCCCN product at m/eϭ50. Circles
and error bars indicate experimental data, the solid line the calculated dis-
tribution.
unable to detect a possible HCN or HNC reaction product
from the H abstraction reaction to the ethynyl radical C2H.
C. Center-of-mass translational energy distributions,
P„ET…
Figures 5 and 6 depict the best-fit center-of-mass trans-
lational energy distribution functions within our experimen-
tal error limits. All P(ET)’s peak between 20 and 30
kJ molϪ1, suggesting that the exit transition state from the
decomposing C3H2N intermediate to the reaction products is
likely to be tight, involving a repulsive bond rupture and a
significant electron reorganization. The best fit P(ET) energy
tails extend up to an energy maximum Emax of 100–132
kJ molϪ1 ͑Fig. 5͒ and 100–123 kJ molϪ1 ͑Fig. 6͒. The fits of
the laboratory data do not change if the P(ET)’s are ex-
tended or shortened by 10 kJ molϪ1. If we account for the
collision energies, we find that the reaction is exothermic by
about 90Ϯ10 kJ molϪ1. Comparing this data with the exo-
thermicities based on our electronic structure calculations
͑94 kJ molϪ1͒, shows that the thermodynamically most stable
cyanoacetylene isomer is formed; cf. Sec. V. The thermody-
namically unfavorable isocyanoacetylene isomer, HCCNC,
is less stable by 107 kJ molϪ1 and therefore its formation is
not consistent with the experimental findings. Finally, the
fraction of total available energy channeling into transla-
tional energy of the products has been calculated: by assum-
ing HCCCN as the only reaction product, we find fractions
of 33%–36% almost independent on the collision energy.
B. Laboratory angular distributions „LAB… and TOF
spectra
The most probable Newton diagrams of the reaction
CN(X 2⌺ϩ)ϩC2H2(X 1⌺gϩ)→HCCCN(X 1⌺ϩ)ϩH(2S1/2
)
together with the laboratory product angular distributions are
shown in Figs. 1 and 2 for the lower and higher collision
energy, respectively. TOF spectra recorded at selected angles
are depicted in Figs. 3 and 4. The solid lines superimposed
on the experimental data are the calculated curves when em-
ploying the center-of-mass best-fit functions of Figs. 5 and 6.
Both LAB distributions are slightly forward scattered with
respect to the primary cyano radical beam and peak at 27.5°
and 25.0° at lower and higher collision energy, respectively,
that is close to the center-of-mass position angles of 30.0°
and 26.2°. Both LAB distributions are very broad and spread
over 40° and 50° in the scattering plane suggesting that the
P(ET)’s peak well away from zero. If we compare these
scattering ranges with limiting Newton circles of the possible
C3HN isomers, it is obvious that the thermodynamically
most stable cyanoacetylene isomer, HCCCN(X 1⌺ϩ), is
mainly formed.
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J. Chem. Phys., Vol. 113, No. 19, 15 November 2000
Reaction of cyano radicals. IV
8659
FIG. 5. Lower: Center-of-mass angular flux distribution for the reaction
FIG. 3. Time-of-flight data of selected laboratory angles as indicated in Fig.
1. The dots indicate the experimental data, the solid lines the calculated fit.
⌺ ⌺
ϩ)ϩC2H2(X gϩ) at a collision energy of 21.1 kJ molϪ1. Upper:
2
1
CN(X
Center-of-mass translational energy flux distribution for the reaction
2
1
CN(X
⌺ ⌺ .
ϩ)ϩC2H2(X gϩ) at a collision energy of 21.1 kJ molϪ1
This data is similar to complex forming reactions of atomic
carbon and cyano radicals with unsaturated hydrocarbons, as
studied earlier in our group.18
I(0°)/I(180°)ϭ1.24 at lower and I(0°)/I(180°)ϭ1.17 at
higher collision energy, respectively; cf. Figs. 5–8. The
product intensity observed in the whole angular range sug-
gests that the reaction follows indirect scattering dynamics
D. Center-of-mass angular distributions, T„␪…, and
flux contour maps, I„␪,u…
All T(␪)’s and I(␪,u)’s are forward–backward asym-
metric and show intensity ratios at the poles of
FIG. 6. Lower: Center-of-mass angular flux distribution for the reaction
⌺ ⌺
ϩ)ϩC2H2(X gϩ) at a collision energy of 27.0 kJ molϪ1. Upper:
2
1
CN(X
Center-of-mass translational energy flux distribution for the reaction
CN(X
FIG. 4. Time-of-flight data of selected laboratory angles as indicated in Fig.
2. The dots indicate the experimental data, the solid lines the calculated fit.
⌺ ⌺ .
ϩ)ϩC2H2(X gϩ) at a collision energy of 27.0 kJ molϪ1
2
1
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8660
J. Chem. Phys., Vol. 113, No. 19, 15 November 2000
Huang et al.
FIG. 8. Center-of-mass velocity contour flux map distribution for reaction
FIG. 7. Center-of-mass velocity contour flux map distribution for reaction
⌺ ⌺
ϩ)ϩC2H2(X gϩ) at a collision energy of 27.0 kJ molϪ1. Units
2
1
CN(X
are given in m sϪ1
⌺ ⌺
ϩ)ϩC2H2(X gϩ) at a collision energy of 21.1 kJ molϪ1. Units
2
1
CN(X
are given in m sϪ1
.
.
attacks the ␲x /␲ orbital of the acetylene molecule barrier-
via a C3H2N complex formation; the scattering preference
for the forward ͑with respect to the CN beam direction taken
as ␪ϭ0°) hemisphere implies that the complex lifetime is
comparable to its rotational period, according to the osculat-
ing complex model of a chemical reaction. At lower collision
energy, the T(␪) peaks on a broad plateau between 60° and
95°, reflecting a ‘‘bent displacement’’ 19 of the light H atom
during complex decomposition; cf. Sec. V. At the higher
collision energy of 27.0 kJ molϪ1, the peak moves toward
␪ϭ0° ͑see below͒.
y
less via a loose transition state. This process forms a deeply
bound ͑242 kJ molϪ1͒ Cs symmetric cis 1-cyanovinyl-2 radi-
2
cal intermediate, HCCHCN ͑int1c͒ on the A surface; cf.
Ј
Figs. 9–11. Beside the cis structure, int1 exists in an isoen-
ergetic trans form as well ͑int1t͒. Both the cis and trans
forms of int1 can isomerize easily via TS1a located only 14
kJ molϪ1 above the initial collision complexes; since the
isomerization barrier lies well below the total available en-
ergy, int1t and int1c are expected to be present in equal
amounts. Alternatively, the CN radical can attack the acety-
lene molecule with its N side. Similarly to the reaction of the
cyano radical with ethylene, the potential energy surface has
no entrance barrier, and there is no transition state for the
formation of a stable ͑148 and 147 kJ molϪ1͒ cis or trans
1-isocyanovinyl-2 intermediate, HCCHNC ͑nc-int1c/nc-
int1t͒. In strong analogy to int1t/int1c, the isomerization bar-
rier is only about 12/11 kJ mol
V. DISCUSSION
A. Ab initio C3H2N and C3HN potential energy
surfaces „PES…
Electronic structure calculations help a great deal in un-
raveling the underlying chemical reaction dynamics of poly-
atomic neutral–neutral reactions. Therefore, we investigated
computationally the reaction of the cyano radical, CN
(X 2⌺ϩ) with the acetylene molecule, C2H2 (X 1⌺ϩg ). This
investigation is the first complete computation of the C3H2N
PES compared to previous studies.20 Since the unpaired elec-
Ϫ1, and hence the ratio of
nc-int1t to nc-int1c is anticipated to be unity. The initially
formed isomer pairs int1c and nc-int1c as well as int1t and
nc-int1t can isomerize via three-membered cyclic transition
states TS1c and TS1t located 1 and 7 kJ molϪ1 slightly below
the energy of both separated reactants; Figs. 9 and 12. A
2
tron is localized in the ⌺ϩ orbital of the carbon atom, CN
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J. Chem. Phys., Vol. 113, No. 19, 15 November 2000
Reaction of cyano radicals. IV
8661
FIG. 9. Schematic representation of the C3H2N potential energy surface.
higher-energy isomerization process implies the formation of
a bound, tetracyclic intermediate int4 via TS4b and TS4a.
Both transition states together with the tetracyclic intermedi-
ate are higher in energy than the separated reactants. We like
to point out that a ring closure of int1c via TS3 yields a
loosely bound ͑15 kJ molϪ1͒ tricyclic intermediate int3.
Since a H atom loss leads to a highly unstable ͑ϩ357
kJ molϪ1͒ triplet c-C3HN radical ͑prod3, Fig. 13͒, the only
decay pathway of int3 is a ring opening to int1c, hence a
migration of the CN group along the carbon–carbon skeleton
of int1c. A CN group migration exists in nc-int1c as well;
however, the resulting cyclic structure is found to be a first-
order transition state, but not a bound reaction intermediate.
The C3H2N intermediates int1c/int1t and nc-int1c/nc-
int1t can undergo either a H atom elimination to form cya-
noacetylene or isocyanoacetylene or depict a 1,2 H atom
shift. The decay pathways involve tight exit transition states
located 22 kJ molϪ1 ͑TS1b͒ and 23 kJ molϪ1 ͑nc-TS1b͒
the maximum collision energy of 27.0 kJ molϪ1. A weakly
bound ͑about 30–35 kJ molϪ1͒ tricyclic intermediate int5
connects isomers int2 and nc-int2 via transition states TS5a
and TS5b. Both int2 and nc-int2 can decay via a final
carbon–hydrogen bond rupture through rather loose exit
transition states TS2b and nc-TS2b located about 6–7
kJ molϪ1 above the products. The calculated structures of the
two transition states indicate that their geometries are very
close to the product structures and that the H atom is depart-
ing along a direction forming an angle of about 74° with
respect to the principal rotation axis A; cf. Figs. 10–11 and
Fig. 15.
A high-energy singlet cyanovinylidene vin1 ͑ϩ115
kJ molϪ1͒ and singlet isocyanovinylidene nc-vin1 ͑ϩ227
kJ molϪ1͒ isomers also exist; cf. Fig. 13. Both singlet species
are formed without an exit barrier; the corresponding triplet
carbenes vin3 and nc-vin3 are even higher in energy. Since
our maximum collision energy is limited to 27 kJ molϪ1
,
above the reaction products in exothermic ͑Ϫ94 kJ molϪ1
͒
none of the vinylidenes is relevant to our experiments. Fi-
nally, a H atom abstraction to form HCN or HNC is endo-
thermic by 23 and 83 kJ molϪ1, respectively, and involves
transition states located 41 and 96 kJ molϪ1 above the reac-
tants ͑Fig. 14͒.
and endothermic ͑ϩ13 kJ molϪ1͒ reactions. In both transition
states, the H atom leaves along a direction forming an angle
of about 90° with respect to the principal rotation axis A; cf.
Figs. 10–11 and Fig. 15. The alternative 1,2 H atom shifts
via TS2a and nc-TS2a result in int2 and nc-int2 that are
stabilized by 288 and 186 kJ molϪ1. Although the barriers of
both H atom shifts are of the same order of magnitude of
about 180 kJ molϪ1, nc-TS2a does not play a role in our
experiments since its energy of 38 kJ molϪ1 is higher than
B. Energetical considerations and RRKM calculations
The maximum translational energy releases of both
P(ET) graphs are in line with a preferential formation of the
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J. Chem. Phys., Vol. 113, No. 19, 15 November 2000
Huang et al.
FIG. 10. Selected bond distances in angstroms and bond angles in degrees
of reactants, intermediates, and transition states involved in the formation of
cyanoacetylene.
FIG. 11. Selected bond distances in angstroms and bond angles in degrees
of reactants, intermediates, and transition states involved in the formation of
isocyanoacetylene.
thermodynamically most stable cyanoacetylene isomer
HCCCN in its X 1⌺ϩ electronic ground state. Here, our best
fits yield an exothermicity of 90Ϯ10 kJ molϪ1, whereas the
electronic structure calculations show 94 kJ molϪ1. Based on
published enthalpies of formations of the reactants and prod-
stants computed by the RRKM theory; cf. Table II. As a
result, the concentration of each species present in the reac-
tion mechanism was obtained as a function of time. The
concentration of the products at t→ϱ were then taken to
calculate the branching ratios.22 This procedure shows that
HCCCN is the sole reaction product. Even if nc-int1c and
nc-int1t are formed as initial collision complexes, they react
via TS1t and TS1c to int1c and int1t. Finally, we would like
to point out that the formation of other C3HN isomers
c-C3HN and the vinylidenes is not feasible since these reac-
tions are too endothermic to be covered energetically. In ad-
dition, the barriers of H atom abstraction by CN to form
either HCN or HNC is higher than the maximum collision
energy employed in our experiments. Therefore we conclude
that cyanoacetylene, HCCCN, is the sole reaction product.
ucts, we calculate a reaction enthalpy of Ϫ89.7 kJ molϪ1 21
.
The formation of the second most stable isomer isocyano-
acetylene, HCCNC, is endothermic by 13 kJ molϪ1, and
hence cannot account for the majority of the reactive scatter-
ing signal. The involved potential energy surface verifies this
conclusion. The formation of HCCNC must proceed either
via nc-TS2b or nc-TS1b. The latter lies 36.0 kJ molϪ1 above
the energy of the separated reactants; since the highest col-
lision energy in our crossed beam experiments was 27.0
kJ molϪ1, this barrier could not be passed. Therefore, if HC-
CNC is a minor reaction product, nc-int2 is expected to de-
compose via nc-TS2b. Two pathways can lead to nc-TS2b:
from intermediate nc-int1t via nc-TS2a or from int2 via
TS5a, int5, and TS5b. Considering our maximum collision
energy of 27.0 kJ molϪ1, nc-TS2a cannot be passed, and a
cyano–isocyano isomerization from int2 to nc-int2 is the
only remaining pathway. Our experimental data alone cannot
quantify the contribution of the HCCNC isomer, and we em-
ploy RRKM calculations to tackle this problem. The rate
equations for the title reaction were solved with the rate con-
C. The actual reaction pathway
Our investigation shows that the title reaction follows
indirect ͑complex forming͒ reactive scattering dynamics. The
CN(X 2⌺ϩ) radical attacks the ␲x /␲ orbital of the acety-
y
lene molecule barrierless via a loose transition state located
at the centrifugal barrier to yield a Cs symmetric trans or cis
1-cyanovinyl-2 radical intermediate, HCCHCN ͑int1t/int1c͒
on the 2A surface. The electronic structure calculations
Ј
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J. Chem. Phys., Vol. 113, No. 19, 15 November 2000
Reaction of cyano radicals. IV
8663
FIG. 12. Bond distances in angstroms and bond angles in degrees of intermediates and transition states involved in the C2H2CN–C2H2NC isomerization.
show that both intermediates are stabilized by 242 kJ molϪ1
with respect to the reactants. The ultrafast cis-trans isomer-
ization rate constant of kϭ(2.64–2.80)ϫ1013 sϪ1 verifies
that int1t and int1c are present in equal concentrations. Upon
complex formation, all four heavy atoms are rotating in a
plane almost perpendicular to the total angular momentum
vector J around the C axis of int1t/int1c. Both complexes can
react via two microchannels, i.e., a decay via a carbon–
hydrogen bond rupture through a tight transition state TS1b
located 22 kJ molϪ1 above the HCCCN and H products
͑pathway 1͒ or a 1,2 H shift from int1t via TS2a to int2 prior
to a H atom loss via a rather loose TS2b to HCCCN and H
͑pathway 2͒; the anisotropic exit potential excited the linear
HCCCN molecules to B-like rotations. Our RRKM calcula-
tions quantify the branching ratio of microchannel 1 versus
microchannel 2 to 85:15; this ratio is invariant as the colli-
angle between the dissociation direction and the principal
inertia axis, A. The authors also displayed the resulting
center-of-mass angular distributions when the angle ␤ shifts
toward collinear configuration or toward more bent sideways
configurations. For a similar prolate decomposing complex,
dissociation along the B axis (␤ϭ90°) gives a sideways
peaked center-of-mass angular distribution. The relevant de-
composing transition states of our system can approximately
be considered linear rotor; moreover, they encounter the cri-
teria suggested by Grice for a reliable application of the
model, that is the departing moiety is the light hydrogen
atom and an exit potential energy barrier is associated to
them. A closer inspection of the geometry of the two pos-
sible transition states TS1b and TS2b might suggest that
TS1b is actually responsible for the sideways peaking ob-
served at the lower collision energy; cf. Fig. 15. Indeed, the
structure of TS2b should give a double peak in the center-
of-mass angular distribution or, since a range of ␤ is actually
possible, a large plateau from about ␪ϭ70° to ␪ϭ110°. The
geometry of TS1b, instead, is such that both ␪ϭ␤ and ␪
ϭ180°Ϫ␤ give sideways scattering centered at about 90°.
As a matter of fact, the lower Ec center-of-mass angular
distribution is peaked between 60° and 95° rather than
around 90°, while the higher Ec one is mildly peaked at ␪
ϭ0°. The peak shift toward ␪ϭ0°, which augments as the
available energy increases, may be taken as evidence that the
sion energy varies from 0 to 30 kJ molϪ1
.
The shape of the center-of-mass angular distributions
can be understood within the framework of the microcanoni-
cal theory of angular distributions of reactive scattering pro-
posed by the late Roger Grice.18 Grice and Smith success-
fully applied this theory to the reaction OHϩCO→CO2ϩH,
where the decomposing transition state approximates a linear
rotor. In that case, the best fit of the experimentally deter-
mined center-of-mass angular distribution23 was obtained by
using a nominal preferred value of ␤ϭ45°, where ␤ is the
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8664
J. Chem. Phys., Vol. 113, No. 19, 15 November 2000
Huang et al.
FIG. 14. Bond distances in angstroms of H atom abstraction channels to
HCN and HNC.
are either endothermic or are inhibited due to an energeti-
cally not accessible exit barrier. Since the formation of HC-
CCN has no entrance barrier, is exothermic, and all involved
transition states are located well below the energy of the
separated reactants, cyanoacetylene can be formed in very
low temperature extraterrestrial environments such as cold
molecular clouds TMC-1 holding average translational tem-
FIG. 13. Bond distances in angstroms and bond angles in degrees of cyclic
C3HN isomers together with cyanovinylidenes and isocyanovinylidenes.
Relative energies with respect to the separated reactants are given in
kJ molϪ1
.
complex lifetime is actually a fraction of ͑or comparable to͒
the rotational period of the complex, according to the oscu-
lating model of chemical reaction. The further shift toward
␪ϭ0° for the higher Ec experiment could be interpreted both
as ͑a͒ coming from a shortening of the complex lifetime or
͑b͒ a partial quenching of the reaction channel going through
TS1b ͑pathway 1͒ in favor of the one going through TS2b
͑pathway 2͒, which would proceed through the formation of
an osculating complex as well. The latter explanation can
actually be disregarded since RRKM calculations have
shown that the branching ratio between pathway 1 and 2 is
invariant with collision energy.
VI. IMPLICATIONS TO INTERSTELLAR AND SOLAR
SYSTEM CHEMISTRY
Our crossed beam experiments together with ab initio
and RRKM calculations verified for the first time explicitly
that cyanoacetylene, HCCCN, is the sole reaction product of
the radical–neutral reaction between an acetylene molecule,
C2H2, and a cyano radical, CN(X 2⌺ϩ) at relative collision
energies of 21.1 and 27.0 kJ molϪ1. Neither the isocyano-
acetylene isomer, HCCNC, nor the hydrogen abstraction
products HCN or HNC were formed since these processes
FIG. 15. Geometry of the exit transition states TS1b and TS2b together with
the principal rotational axis. The C axis is perpendicular to the paper plane.
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J. Chem. Phys., Vol. 113, No. 19, 15 November 2000
Reaction of cyano radicals. IV
8665
TABLE II. RRKM rate constants in sϪ1 at two collision energies.
reaction has no entrance barrier and is initiated by an attack
of the CN radical to the ␲ electron density of the acetylene
Collision energy
21.1 kJ molϪ1
27.0 kJ molϪ1
molecule to form a doublet cis/trans HCCHCN collision
k(int1c→int1t)
2.80ϫ1013
2.67ϫ1013
2.25ϫ1010
4.58ϫ109
4.52ϫ1011
1.79ϫ1014
6.01ϫ1010
5.75ϫ1010
6.09ϫ1010
8.34ϫ105
2.76ϫ1013
2.64ϫ1013
1.62ϫ1010
3.22ϫ109
1.88ϫ1011
8.21ϫ1013
4.39ϫ1010
4.20ϫ1010
4.58ϫ1010
2.04ϫ104
2
complex on the A surface via indirect reactive scattering
Ј
k(int1t→int1c)
dynamics. The four heavy atoms are rotating in the plane
almost perpendicular to the total angular momentum vector J
around the C axis of the complex. Here 85% of the collision
complexes undergo C–H bond rupture through a tight tran-
sition state located 22 kJ molϪ1 above the cyanoacetylene,
HCCCN (X 1⌺ϩ) and H(2S1/2) products ͑microchannel 1͒.
To a minor amount ͑15%͒ cis HCCHCN shows a 1,2 H shift
via a 178 kJ molϪ1 barrier to form a doublet H2CCCN radi-
cal that is 46 kJ molϪ1 more stable than the initial reaction
intermediate ͑microchannel 2͒. This H2CCCN complex de-
composes via a rather loose exit transition state situated only
7 kJ molϪ1 above HCCCN (X 1⌺ϩ) and H(2S1/2). The mild
forward/sideways peaking of the center-of-mass angular dis-
tributions may well be related to the geometry of the relevant
decomposing transition states within the microcanonical
model of Roger Grice. The explicit identification of the cya-
noacetylene as the only reaction product represents a solid
background for the title reaction to be included in reaction
networks modeling the chemistry in dark, molecular clouds,
outflow of dying carbon stars, hot molecular cores, as well as
the atmosphere of hydrocarbon-rich planets and satellites
such as the Saturnian moon Titan.
k(int1t→int2)
k(int2→int1t)
k(int1c→int3)
k(int3→int1c)
k(int1c→HCCCNϩH)
k(int1t→HCCCNϩH)
k(int2→HCCCNϩH)
k(nc-int2→HCCCNϩH)
peratures as low as 10 K and the atmosphere of Saturn’s
moon Titan. Likewise, this mechanism can account for the
formation of the HCCCN isomer in ultracompact HII
regions.24 Therefore, the title reaction can be identified as the
missing source of interstellar cyanoacetylene molecules that
cannot be formed via postulated ion–molecule reactions. In
addition, our results unravel the Takano et al. observation of
13C isotopomers.25 The authors discovered that the 13C iso-
tope is concentrated in the carbon atom of the HCC 13CN
molecule adjacent to the N atom. If 13C is already enriched in
13CN radicals, a reaction with C2H2 could propagate this iso-
tope enrichment to the newly formed HCC 13CN molecules.
Besides interstellar chemistry, the chemical dynamics of
CN(X 2⌺ϩ) radical reactions with unsaturated hydrocarbons
are of fundamental relevance to the atmospheric chemistry in
Saturn’s moon Titan.26 Titan’s atmospheric composition is
dominated by N2 and CH4 together with the minor compo-
nents C2H6, C3H8, C2H2, HCN, HC3N, CH3CCH, and
C2N2.27,28 Its cyano chemistry is thought to be initiated by
photolysis of HCN and C2N2 by solar radiation to generate
ACKNOWLEDGMENTS
R.I.K. is indebted the Deutsche Forschungsgemeinschaft
͑DFG͒ for a Habilitation fellowship ͑IIC1-Ka1081/3-1͒. The
work was further supported by Academia Sinica and the Tai-
wanese Petrol Organization. Most of the ab initio calcula-
tions were carried out at the computer center of the Institute
for Molecular Science, Japan. This work was performed
within the International Astrophysics Network ͑http://
po.iams.sinica.edu.tw/ϳkaiser/network.htm͒.
reactive CN radicals in their ⌺ϩ electronic ground state.
2
Upon the reaction with C2H2, the cyano radical can form
then the experimentally detected HCCCN isomer cyano-
acetylene. We would like to stress a fundamental difference
between CN radical reactions in the interstellar medium
͑ISM͒ and planetary atmospheres. Due to the low density of
only about 104 –106 molecules cmϪ3 in molecular clouds in
the ISM, only binary collisions are relevant; ternary pro-
cesses do not play a role in the chemical processing of inter-
stellar, cold molecular clouds. Therefore, the involved colli-
sion complexes HCCHCN and H2CCCN cannot be stabilized
but decay to the HCCCN reaction products. In the denser
planetary atmospheres, however, a third body collision can
divert the internal energy to stabilize the HCCHCN and
H2CCCN radicals together with potentially involved isoni-
trile intermediates. Therefore, chemical models of planetary
atmospheres must include these highly reactive and photo-
chemically active radical intermediates as well to give a con-
sistent picture.
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