On the trimerization of cyanoacetylene: Mechanism of formation of tricyanobenzene isomers and laboratory detection of their radio spectra

Article abstract of DOI:10.1002/chem.201001648

In support of a deeper understanding of the chemistry of cyanoacetylene-a known constituent of planetary atmospheres and interstellar space-theoretical and experimental studies address the chemical mechanism of dimerization and trimerization, and provide high-resolution rotational spectra of two of the trimeric products, 1,2,3- and 1,2,4-tricyanobenzene. Analysis of the rotational spectra is particularly challenging because of quadrupolar coupling from three ¹⁴N nuclei. The laboratory rotational spectra provide the basis for future searches for these polar aromatic compounds in interstellar space by radio astronomy.

Full text of DOI:10.1002/chem.201001648

FULL PAPER
DOI: 10.1002/chem.201001648
On the Trimerization of Cyanoacetylene: Mechanism of Formation of
Tricyanobenzene Isomers and Laboratory Detection of Their Radio Spectra
Henning Hopf,*[a] Cornelia Mlynek,[a] Robert J. McMahon,*[b] Jessica L. Menke,[b]
Alberto Lesarri,[d] Michael Rosemeyer,[c] and Jens-Uwe Grabow*[c]
Abstract: In support of a deeper understanding of the chemistry of cyanoacety-
lene—a known constituent of planetary atmospheres and interstellar space—theo-
retical and experimental studies address the chemical mechanism of dimerization
and trimerization, and provide high-resolution rotational spectra of two of the tri-
meric products, 1,2,3- and 1,2,4-tricyanobenzene. Analysis of the rotational spectra
is particularly challenging because of quadrupolar coupling from three 14N nuclei.
The laboratory rotational spectra provide the basis for future searches for these
polar aromatic compounds in interstellar space by radio astronomy.
Keywords: astrochemistry · cyano-
acetylene · density functional calcu-
lations · quantum chemistry · rota-
tional spectroscopy
Introduction
clouds (densities of 104 cmꢀ3 and temperatures as low as
10 K).[2] HCN is presumably formed by molecular N2 photo-
dissociation as shown by Equations (1) and (2).[3–4]
Cyanocarbons are highly unsaturated organic molecules, the
chemistry of which is largely determined by the presence of
the polarizing cyano substituents.[1] Recently studied repre-
sentatives of this class of compounds are the cyanoacety-
lenes, a group of cyanocarbons known to be quite abundant
in the interstellar medium (ISM)[2] and also in the atmos-
phere of Titan.[3] Ion–molecule reactions have been shown
to be the most likely pathways for the formation of mole-
cules such as acetylene in the dense cores of molecular
hv
C
C
N2
N þ N
ð1Þ
ð2Þ
ƒ!
C
C
N þ CH3 ! HCN þ H2
Cyanoacetylene (1) is subsequently produced through pho-
tolysis of HCN followed by a radical reaction between the
cyano radical and acetylene [Eqs. (3), (4), and (5)]:[5]
hv
C
C
HCN
H þ CN
ð3Þ
ð4Þ
ð5Þ
ƒ!
[a] Prof. Dr. H. Hopf, C. Mlynek
C
C
CN þ HCN ! NCCN þ H
Institut fꢀr Organische Chemie, Technische Universitꢁt Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
Fax : (+49)531-391-5388
C
C
C2H2 þ CN ! HCꢁCꢀCN þ H
E-mail: H.Hopf@tu-bs.de
[b] Prof. Dr. R. J. McMahon, J. L. Menke
Department of Chemistry, University of Wisconsin
1101 University Ave., Madison, WI, 53706-1322 (USA)
Fax : (+1)608-263-5549
Similar reactions can occur with longer alkynes to give
larger cyanopolyynes, of which the longest presently known
in the interstellar medium is HC11N.[6]
E-mail: mcmahon@chem.wisc.edu
Many of these molecules have been discovered by the use
of radio and optical spectroscopy, all with the assistance of
rotational and vibrational (microwave and infrared, respec-
tively) experiments conducted here on Earth. Both experi-
mental work in which molecules are synthesized or photo-
chemically obtained and then studied spectroscopically, and
computational work in which reaction pathways, molecular
dipoles, and rotational constants can be computed are ex-
tremely helpful in interpreting spectra of the ISM.
[c] Dr. M. Rosemeyer, Prof. Dr. J.-U. Grabow
Institut fꢀr Physikalische Chemie und Elektrochemie
Leibniz-Universitꢁt Hannover, Callinstrasse 3A
30167 Hannover (Germany)
Fax : (+49)511-762 4009
E-mail: jens-uwe.grabow@pci.uni-hannover.de
[d] Prof. Dr. A. Lesarri
Departamento de Quꢂmica Fꢂsica y Quꢂmica Inorgꢃnica
Facultad de Ciencias, Universidad de Valladolid
47011 Valladolid (Spain)
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Preparative cyanocarbon chemistry is a field that was
heavily researched at du Pont in the 1950s and 1960s. How-
ever, further exploration has been limited. Due to the
strong electron-withdrawing capability and the cylindrical
shape of the nitrile group, cyanocarbons provide some inter-
esting reactivity, especially with electron-rich molecules.
Much of this reactivity has been nicely reviewed by one of
the lead researchers at du Pont, Owen W. Webster.[7]
In experiments designed to investigate the behavior of cy-
anoacetylene (1) as an addend to strained aromatic com-
pounds,[8] we made the interesting observation that, when
[2.2]paracyclophane (4) and 1 were heated in benzene, cy-
cloadducts were indeed formed, but that these did not have
the structure expected if 4 reacts as a diene and 1 as a dien-
ophile.[9] Rather than the expected 1:1 adduct, we isolated
different 2:1 adducts of 1 to 4, the structures of which were
determined by spectroscopic methods and X-ray structural
analysis to be the dinitriles 5–8 (Scheme 1).[8]
carbon 4. That a 1,2- rather than a 1,3-difunctionalized cy-
clobutadiene is generated in the initial step we have ration-
alized by the following: a radical mechanism in which the
diradical formed in the presumably rate-determining step is
more stable with two unsaturated substituents in vicinal po-
sition than in 1,3-orientation with opposing nitrile moieties.
Interestingly, in all our cycloadditions there were also ob-
tained two isomeric trimers of cyanoacetylene (1): 1,2,3-tri-
cyanobenzene (13) and its 1,2,4-isomer, 14. In no experi-
ment did we observe the formation of 1,3,5-tricyanobenzene
(17) (Scheme 1, Scheme 2).
This result agrees with the putative formation of 2/3 in
the first step. If these isomeric cyclobutadienes are inter-
cepted not by 4 but by an excess amount of 1 in a competi-
tive route, the four isomeric Dewar benzene derivatives 9–
12 could be produced. These highly strained adducts (tri-
AHCTUNGTREGmNNNU ers) would not be expected to survive under the drastic re-
action conditions (reaction temperature: 1608C, sealed am-
poule) and would isomerize by thermal cleavage of one of
their four-membered rings. As can be derived from
Scheme 1, only the two (observed) trimers 13 and 14, but
never 17, could be produced by this route. A conceivable
route to 17 could involve the head-to-tail dimer 15, which
by addition of a third equiva-
To rationalize this result, we proposed that 1 dimerizes
thermally to a cyclobutadiene intermediate, which can exist
in two valence-tautomeric forms, 2 and 3. The products 5–8
are then the expected 2:1 adducts if the highly reactive cy-
clobutadiene is intercepted by the strained aromatic hydro-
lent of 1 could yield 17 through
the intermediate Dewar ben-
zene 16. Of course, 15 could
also furnish 14 through 12
(Scheme 2).
The present paper has two
major aims. First, we wished to
support or refute the above
mechanistic considerations for
the formation of 2/3, 13, and 14
through computational studies
of the potential energy surfaces.
Second, we sought to obtain
laboratory rotational spectra of
the two aromatic products 13
and 14, thus establishing the
prerequisite to identify these
cyanoacetylene derivatives in
interstellar space. Although cy-
anoacetylene and the family of
larger
cyanopolyynes
are
known constituents of interstel-
lar space, cyano-substituted aro-
matic compounds are not. The
rotational spectroscopy of 13
and 14 is of considerable inter-
est in its own right because of
the spectral complexity that
arises in these highly polar
compounds as a consequence of
three quadrupolar 14N nuclei.
Our combined computational/
spectroscopic studies also set
Scheme 1. Observed dimerization, trimerization, and trapping chemistry involving 1,4- and 1,2-dicyano-1,3-cy-
clobutadiene (2 and 3) according to reference [8].
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Trimerization of Cyanoacetylene
FULL PAPER
their spectroscopic and analyti-
cal data are identical with those
reported in the literature.[11]
Computations
Dimerization of cyanoacetylene:
The thermal [2+2] dimerization
of cyanoacetylene to afford di-
cyano-1,3-cyclobutadiene is for-
bidden by orbital symmetry.
The formation of the dimer, as
inferred from trapping with
Scheme 2. Hypothetical dimerization and trimerization chemistry involving 1,3-dicyano-1,3-cyclobutadiene
(15).
[2.2]paracyclophane
(see
above), has been rationalized in
the stage for subsequent experimental investigations of the
terms of a stepwise mechanism that involves radical and/or
diradical intermediates.[8] In the current study, we sought a
deeper understanding of the details of the dimerization
dicyanocyclobutadiene isomers (2 and 3).[10]
ꢀ
mechanism. Stepwise C C bond formation in the dimeriza-
Results and Discussion
tion of cyanoacetylene may proceed through the 1,4-diradi-
cals 23–25 depicted in Scheme 4. A key issue involves the
relative energies of these diradical intermediates—a direct
manifestation of the influence of the cyano substituents.
Computational studies of conjugated radicals and diradi-
cals of this type are challenging for a variety of reasons:
1) the inherent multireference character of diradicals, 2) the
inability of perturbation theory to handle spin-contaminated
wavefunctions, and 3) the tendency of density functional
methods to overestimate deloc-
Synthesis of 1,2,3- and 1,2,4-tricyanobenzene (13 and 14):
Although both compounds could be prepared by trimeriza-
tion of cyanoacetylene (1, see above), this route is rather in-
volved and requires extensive separation work.[8a,b,e] Fortu-
nately, both isomers have been prepared previously by the
simple routes summarized in Scheme 3, both of them having
been reported in the literature.[11]
alization.[12] Nevertheless, the
literature provides some gener-
al guidance concerning cyano-
substituted radicals. The radical
stabilization energy of the
cyano group in the 1-cyanovinyl
radical is estimated to be
around 4 kcalmolꢀ1.[13] The 1-
ꢀ
cyanovinyl radical (H2C=C
CN), which bears the cyano
substituent at the radical
center, is approximately 7 kcal
molꢀ1 lower in energy than the
isomeric 2-cyanovinyl radical
Scheme 3. Syntheses of authentic samples of 1,2,3- and 1,2,4-tricyanobenzene (13 and 14) according to refer-
ence [11].
[14]
ꢀ
(H C=C(H)CN).
On the
basis of these data alone, one
may plausibly infer the relative
As the starting material for 13, we used the commercially
available tricarboxylic acid hydrate 18. By treatment with
phosphorus pentachloride, this was converted into the tris(a-
cid chloride) 19, which on aminolysis with aqueous ammonia
gave the amide 20. Dehydration to 1,2,3-tricyanobenzene
was finally accomplished by treatment with POCl3 in pyri-
dine. The route to the 1,2,4-isomer 14 is essentially the
energy of the diradicals as: 23<24<25. Although diradicals
of this type have been computed for the parent C4H4
series,[15] we encountered considerable difficulty in our ef-
forts to optimize structures for the dicyano series (23–25)
using simple DFT methods. (The s-cis isomers cyclized
during geometry optimization.) We therefore computed the
corresponding monoradicals 26–29, which we consider to be
suitable surrogates in revealing the influence of the cyano
substituents. The structures and relative energies are depict-
ed in Scheme 4. These results clearly depict an influence of
cyano stabilization of a vinyl radical intermediate (an effect
same, except that it starts from the trisACHTUNGTREN(NGNU ester) 21 and con-
verts this first to the tris(amide) 22, the dehydration of
AHCTUNGTRENNUNG
which to 14 proceeds without problems in good yields. Both
isomers are available in gram quantities by these routes, and
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H. Hopf, R. J. McMahon, J.-U. Grabow et al.
substituents are forced to ꢅshareꢆ the same alkene moiety, is
slightly higher in energy than the 1,3- and 1,4-dicyano iso-
mers (and less polar than the 1,4-isomer).
Cycloaddition to produce Dewar benzenes: A common
mode of reactivity of cyclobutadienes involves [4+2] cyclo-
addition.[16] With an alkyne as the dienophile, [4+2] cycload-
dition affords a Dewar benzene derivative. In the current in-
vestigation, we computed structures for all of the regioiso-
meric tricyano-Dewar benzene products that may be formed
upon [4+2] cycloaddition of 1,2-, 1,3-, and 1,4-dicyanocyclo-
butadiene with cyanoacetylene (Scheme 5). The computed
relative energies of the five Dewar benzenes span a rather
wide range, from 74 to 84 kcalmolꢀ1. The tricyano-Dewar
benzene derivative that is computed to be the lowest in
energy, isomer 10, has been experimentally isolated as one
of the trimerization products of cyanoacetylene.[8e]
Ring opening of Dewar benzenes to tricyanobenzenes: Theo-
retical analyses[17] and molecular dynamics simulations[17c]
reveal that the thermal transformation of Dewar benzene to
benzene involves several subtle mechanistic features. Two
transition states for ring opening (conrotatory and disrotato-
ry), as well as a trans-benzene (Mçbius benzene) isomer, lie
within 2–4 kcalmolꢀ1 of one another at an energy of approx-
Scheme 4. Diradical intermediates (23, 24, 25) in the dimerization of cy-
ACHTUNGTRENNUNGanoacetylene (1), and monoradicals (26, 27, 28, 29) that serve as model
compounds. Computed relative energies in italics (kcalmolꢀ1; B3LYP/6-
31G*, ZPVE included).
that is likely to be overestimat-
ed at the B3LYP/6-31G* level).
Closure of the dicyano-1,4-
butadienyl diradicals (23–25)
will afford the corresponding
dicyanocyclobutadienes.
The
calculations provide interesting
insights concerning the cyclic
isomers. Without taking small
differences literally, all three
isomers are equal in energy
(Scheme 5). Thus, the apparent
absence of the 1,3-dicyano
isomer (15) in the trapping ex-
periment cannot be rationalized
by an argument that invokes a
difference in thermodynamic
stability among the dicyanocy-
clobutadiene isomers. The 1,4-
and 1,2-dicyanocyclobutadienes
(2 and 3), which arise as a con-
sequence of bond-shift isomer-
ism in the rectangular forms of
singlet cyclobutadiene, are not
equivalent and would not be
expected to be equal in energy.
The computed energy differ-
ence (around 0.7 kcalmolꢀ1) is,
however,
(Scheme 5). The 1,2-dicyano
quite
modest
Scheme 5. Dimerization and trimerization products of cyanoacetylene (1). Computed relative energies in ital-
ics (kcalmolꢀ1; B3LYP/6-31G*, ZPVE included). Relative energies reported for dimers are dicyanocyclobuta-
isomer, in which both cyano diene and cyanoacetylene. Computed dipole moments (m; Debye).
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Trimerization of Cyanoacetylene
FULL PAPER
imately 30 kcalmolꢀ1 above the Dewar benzene isomer. Al-
though we have not attempted to compute each of these
three stationary points for each of the five Dewar benzene
isomers shown in Scheme 5, we computed several of the
Mçbius benzene isomers as surrogates for the lowest energy
transition states that connect the Dewar isomers to the ring-
opened tricyanobenzene derivatives. As noted recently,[17c]
the energies of the Mçbius benzene isomers exhibit little de-
pendence on the nature of substituents. We also find this to
be the case, as the computed energies for five Mçbius tricy-
structural rigidity of the tricyano-Dewar benzenes make
them intriguing candidates for investigation by rotational
spectroscopy. In support of future investigations of this type,
computed rotational constants are included as Supporting
Information.
Spectroscopy
Microwave rotational spectra: The tricyanobenzenes are
rigid asymmetric rotors with a single conformation. For
1,2,3-tricyanobenzene (13, C2v), the high symmetry of the
molecule reduces the observable transitions to only mb selec-
tion rules. On the other hand, the spectrum benefits from an
expected large dipole moment due to the favorable orienta-
tion of the three cyano groups. Accordingly, a set of mb tran-
sitions could be readily assigned by following the initial
ACHTUNGTRENNUNGanobenzene isomers, relative to 1,3,5-tricyanobenzene (17)
(Scheme 5), are tightly clustered near (101ꢂ1) kcalmolꢀ1
(see the Supporting Information).
The tricyanobenzene isomers (13, 14, 17) lie within 3 kcal
molꢀ1 of one another, with the most stable isomer (1,3,5-tri-
cyanobenzene, 17) being the isomer that is not observed, ex-
perimentally, in the trimerization of cyanoacetylene.
DFT models (B3LYP/6-311++GACTHNUTRGNEUGN(d,p)) collected in the Sup-
porting Information. As observed in Figure 1, each rotation-
al transition is split into a very complicated hyperfine struc-
ture (hfs) pattern that arises from the nuclear quadrupole
coupling effects originated by the three 14N nuclei of the
molecule (spin I=1). In these molecules, the electric inter-
action between the quadrupolar nuclei and the molecular
electric field gradient at the quadrupolar site provides a
mechanism to couple the spin and rotational angular mo-
menta,[18] and makes the hyperfine analysis a case of consid-
erable difficulty seldom considered.[19]
Overall mechanism of tricyanobenzene formation: Our com-
putational studies provide no basis to rationalize the ob-
served regioselectivity of cyanoacetylene trimerization in
terms of either the formation or destruction of the tricyano-
Dewar benzene isomers. If 1,3-dicyano-1,3-cyclobutadiene
(15) was present as an intermediate, it would reasonably be
expected to form a tricyano-Dewar benzene isomer (com-
pound 16), and this Dewar isomer would reasonably be ex-
pected to undergo ring opening to the most stable tricyano-
benzene isomer (1,3,5-tricyanobenzene (17)). Thus, we are
forced to conclude (as we have done previously)[8] that 1,3-
dicyano-1,3-cyclobutadiene (15) is apparently not formed
under the reaction conditions. The basis for this selectivity is
not thermodynamic, since the isomeric 1,3- and 1,4-dicyano-
cyclobutadienes are computed to have the same energy. The
rationalization for the experimentally observed regiochemis-
try rests with the kinetics of cyanoacetylene dimerization.
As we described above, the diradical intermediate that leads
to 1,4-dicyanocyclobutadiene (diradical 23) benefits from
cyano stabilization of both vinyl radicals, whereas the diradi-
cal intermediate that leads to 1,3-dicyanocyclobutadiene
(diradical 24) benefits from cyano stabilization of only one
vinyl radical. Estimates for this energy difference range
from 7 kcalmolꢀ1 (the energy difference between 1-cyano-
vinyl radical and 2-cyanovinyl radical) to 14 kcalmolꢀ1 (the
energy difference between model monoradicals 26 and 28).
Even the smaller of these values is sufficient to render the
dimerization as virtually regiospecific under the experimen-
tal reaction conditions (selectivity >1000 at 1608C).
The analysis of the spectrum was based on a Watsonꢆs
semirigid-rotor Hamiltonian[20] (A reduction) with a nuclear
quadrupole coupling scheme, I2 +I3 =Ia, Ia +I1 =IT, J+I1 =
F1, F1 +Ia =J+IT =F, which was then directly diagonal-
ized.[21] The rotational constants and quartic centrifugal dis-
tortion parameters of 1,2,3-tricyanobenzene, derived from
b
b
an extensive experimental data set of 245 R- and Q-branch
transitions hfs components (J<15), are shown in Table 1.
The nuclear quadrupole coupling tensors (c=cab; a, b=a, b,
c) for the three 14N atoms of the molecule are collected in
Table 2. Due to the molecular symmetry, the tensors for
N(1) and N(3) are identical, and the only nonzero off-diago-
nal element is cab (the ab symmetry plane that contain the
quadrupolar nuclei). For N(2), no off-diagonal elements (the
ab and bc symmetry planes contain the quadrupolar nu-
cleus) of the coupling tensor are nonzero.
For 1,2,4-tricyanobenzene (14, Cs), both ma and mb selec-
tion rules are allowed by symmetry, so the assignment was
simplified by the presence of the characteristic patterns of
the aR-branch transitions. The analysis of the rotational
spectrum and hyperfine effects, exemplified in Figure 2, was
similar to the previous molecule. The final fit of the rota-
tional and nuclear quadrupole coupling parameters used a
data set of 308 transitions (J<17) to yield the values shown
in Table 1 and Table 3. All experimental measurements and
residuals, together with the DFT predictions, are collected
in the Supporting Information.
Computed spectroscopic data for tricyanobenzenes: The
computed molecular structures, rotational constants, and
dipole moments for 1,2,3- and 1,2,4-tricyanobenzene (13 and
14) helped to guide the initial search for rotational transi-
tions in the FT-microwave experiments (see below). In
future experiments, we hope to generate and investigate an
equilibrating mixture of 1,2- and 1,4-dicyanocyclobutadienes
by rotational spectroscopy. Rotational constants are provid-
ed for these isomers. We also note that the polarity and
Molecular structure: The standard approach to derive the
molecular structure, based on the rotational spectrum of
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H. Hopf, R. J. McMahon, J.-U. Grabow et al.
Table 1. Rotational and centrifugal distortion parameters of 1,2,3-tricya-
nobenzene (13) and 1,2,4-tricyanobenzene (14).
1,2,3-Tricyanobenzene (13)
Experiment
Theory
A [MHz][a]
B [MHz]
C [MHz]
DJ [kHz]
DJK [kHz]
DK [kHz]
dJ [kHz]
1413.898772(14)[b]
893.252893(12)
547.3164030(58)
0.033883(71)
0.06505(34)
ꢀ0.01586(44)
0.014295(39)
0.10160(27)
1421.7[c]
888.0
546.6
0.029
0.061
ꢀ0.015
0.012
dK [kHz]
0.089
1,2,4-Tricyanobenzene (14)
Experiment
Theory
A [MHz]
B [MHz]
C [MHz]
DJ [kHz]
DJK [kHz]
DK [kHz]
dJ [kHz]
dK [kHz]
1891.26820366(43)
631.74808224(29)
473.47493138(11)
0.00904190(34)
0.0321118(83)
0.856939(16)
0.00320496(21)
0.056081(16)
1872.3
634.1
473.7
0.0084
0.033
0.72
0.0029
0.055
[a] Rotational constants (A, B, C) and Watsonꢆs quartic centrifugal distor-
tion constants (DJ, DJK, DK, dJ, dK). [b] Standard error in parentheses in
units of the last digit. [c] B3LYP/6-311++GACTHNUGRTNEUNG(d,p).
Table 2. Nuclear quadrupole coupling parameters for 1,2,3-tricyanoben-
zene (13).
1,2,3-Tricyanobenzene (13)
Experiment
Theory
N(1), N(3)
N(2)
N(1), N(3) N(2)
caa [MHz][a]
cbb [MHz]
ccc [MHz]
cab [MHz]
ꢀ2.93716(20)[h] 2.28913(31)
ꢀ3.25[i]
2.46
0.80706(57)
2.13010(37)
2.79(12)
ꢀ4.47658(90) 0.97
ꢀ4.82
2.18745(59)
0
2.29
2.93
2.29
0.03
cxx [MHz][b]
cyy [MHz]
czz [MHz]
2.29(10)
2.28913(31)
2.18745(59)
ꢀ4.47658(90)
0.10168(90)
ꢀ0.02271(20)
ꢀ0.00605(16)
2.13010(37)
ꢀ4.42(10)
cxxꢀcyy [MHz] 0.16(10)
h[c]
ꢀ0.037(23)
pxꢀpy[d]
ꢀ0.0098(64)
28.06(60)
61.94(60)
0.2279(92)
0.701(18)
Figure 1. A single rotational transition of 1,2,3-tricyanobenzene (13)
!
(JKꢀ1,K1 =33,1 22,0) that illustrates the complex hyperfine pattern due to
a [8][e]
0
three-nuclei nuclear quadrupole coupling (each hyperfine component fur-
ther split into a doublet by the instrumental Doppler effect). The experi-
mental spectrum in the center has been deconvoluted in the upper trace.
The hyperfine pattern predicted ab initio is displayed in the lower trace.
[f]
ip
0.2228(40)
0.691(8)
ic[g]
[a] Nuclear quadrupole coupling constants cab refer to the principal iner-
tial axis system (a, b =a, b, c). [b] Nuclear quadrupole coupling con-
stants in their principal axis system. [c] Asymmetry parameter (cxxꢀcyy)/
multiple isotopic species,[18] is not convenient in this case be-
cause of the signal intensity being distributed among the
components of the complicated hfs pattern. Nevertheless,
the orientation of the three cyano groups can be established
by using the nuclear quadrupole coupling tensors. For this
calculation, we assume that the principal axis of the cou-
pling tensors, obtained by diagonalization of the inertial axis
coupling tensor, is coincident with each of the CN bonds.
Thus, the angle a that transforms the inertial axis tensor
into its principal axis system, gives the orientation of the CN
bond with respect to the inertial axis. The values of the cou-
pling tensors in their principal axis are given in Table 2. As-
suming colinearity with the CC bond axes, the orientations
czz. [d] Double-bond character 2czzh/3eQq210
,
eQq210 =ꢀ11.2(2) MHz.
[e] Orientation of the principal axis of the nuclear quadrupole coupling
tensor with respect to the principal inertial axis. [f] Ionic p character of
the CN bond. [g] Total ionic character of the CN bond. [h] Standard
error in parentheses in units of the last digit. [i] B3LYP/6-311++GACHTUNGTRENNUNG(d,p).
of the CN bonds within the inertia principal axes system are
used in Figure 3 to derive bond angles with respect to the
coordinates of the aromatic ring C atoms as predicted by
the ab initio calculations. For 1,2,3-tricyanobenzene (13), we
note that the bond angles of the 1,3-cyano groups deviate by
about 1.58 from 1208, which can be rationalized by steric or
electrostatic repulsion from the neighboring 2-cyano group.
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Trimerization of Cyanoacetylene
FULL PAPER
Figure 3. Molecular orientation of the cyano groups in 1,2,3- and 1,2,4-tri-
cyanobenzene. The structural angles reflect the orientation of the CN
bond axis derived from the quadrupole coupling tensor with respect to
the coordinates of the aromatic ring C atoms from the quantum chemical
calculation. See text for details.
!
Figure 2. The JKꢀ1,K+1 =51,5 40,4 rotational transition of 1,2,4-tricyano-
benzene (14), with the experimental signal (center), the deconvoluted
spectrum (upper trace), and the ab initio (lower trace) hyperfine patterns
due to the three-nuclei nuclear quadrupole coupling (additional doubling
caused by the instrumental Doppler effect).
s contribution (is) and a p contribution (ip and, accordingly,
ic =is +mip, m=2). The limiting case of a complete ioniza-
tion leads to a spherical charge distribution at the nuclei,
which corresponds to a vanishing electric field gradient that
results from all p electrons, at the nucleiꢆs position. Thus,
the z component of the field gradient qz also vanishes, itself
being directly proportional to the principal value czz of the
quadrupole coupling tensor [Eq. (6)]:
A similar situation is found for 1,2,4-tricyanobenzene (14).
Here the 4-cyano group, which has no immediate neighbors,
exhibits deviations of less than 18.
czz ¼ eQqz ¼ eQ½ꢀðUpÞzqn10ꢃ ¼ eQfꢀ½ðnx þ nyÞ=2ꢀnzꢃqn10
g
Electronic structure: The ionic character is of the CN s bond
can be calculated as is =jDEN/2j=0.245 from the electrone-
gativity difference DEN between the carbon (EN(C)=2.52)
and nitrogen atom (EN(N)=3.01), with the values of the
latter being obtained from the number of valence electrons
n and the covalent radius r of the homonuclear single bond
ð6Þ
Therefore, the quadrupole coupling constant eQqz is a direct
probe for properties of the chemical bond. The molecular
value czz(N) is related to the atomic constant q210(N)=
ꢀ11.2(2) MHz[23] and denotes the coupling of the N nucleus
to the electrons in the p orbitals of all spatial directions in
its valence shell n=2, weighed by the occupations nx, ny, and
as EN=0.31ACHTUNGTRENNUNG(n+1)/r+0.50, which was empirically found to
be surprisingly accurate.[22] The total ionic character ic of the
CN bond, formed by one s and two p orbitals, consists of a
Chem. Eur. J. 2010, 16, 14115 – 14123
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14121

H. Hopf, R. J. McMahon, J.-U. Grabow et al.
Table 3. Nuclear quadrupole coupling parameters for 1,2,4-tricyanobenzene (14).
Conclusion
1,2,4-Tricyanobenzene (14)
Experiment
N(2)
Theory
N(2)
As known from earlier studies,
N(1)
N(3)
N(1)
N(3)
cyanoacetylene undergoes di-
merization and trimerization in
sealed-tube experiments (ben-
zene, 1608C). Our computation-
al studies rationalize the ob-
served regioselectivity of these
processes in terms of selectivity
in the initial dimerization
event, which likely proceeds
through a radical mechanism.
Most of the cyanoacetylene
dimers and trimers are comput-
ed to possess large molecular
dipole moments, which will
confer strong rotational transi-
tions. Laboratory rotational
caa [MHz][a]
cbb [MHz]
ccc [MHz]
cab [MHz]
ꢀ4.048847(40)
1.891201(80)
2.157646(40)
ꢀ1.9283(30)
ꢀ0.566751(20)
ꢀ1.565488(45)
2.132239(26)
3.0087 (16)
ꢀ4.093065(37)
1.991751(77)
2.101315(40)
ꢀ1.3114(32)
ꢀ4.33
1.99
2.34
ꢀ1.71
ꢀ0.48
ꢀ1.80
2.28
ꢀ4.41
2.15
2.25
3.56
1.42
cxx [MHz]
cyy [MHz]
czz [MHz]
cxxꢀcyy [MHz]
h
2.4623(16)
2.157646(10)
ꢀ4.6199(15)
0.3046(16)
ꢀ0.06594(32)
ꢀ0.01813(42)
16.497(19)
73.503(19)
0.2100(38)
0.665(8)
1.9837(17)
2.2624(13)
2.101314(39)
ꢀ4.3637(13)
0.1610(14)
ꢀ0.03690(13)
ꢀ0.00959(25)
11.659(26)
78.341(26)
0.2329(42)
0.711(8)
2.132239(26)
ꢀ4.1160(17)
ꢀ0.1485(17)
0.03608(42)
0.008840(58)
40.2881(28)
49.7119(28)
0.2250(47)
pxꢀpy
a [8]
ip
ic
0.755(10)
[a] Parameter definition as in Table 2.
nz. The latter numbers can be collected to give a measure
(Up)z for the occupation state of the p orbitals along the
bond axis. The occupations nx and ny of the respective p or-
bitals depend on the ionic character ip. Assuming the same
degree of ionization for both p bonds, we have nx =ny =1+
spectra for two of the trimers, 1,2,3- (13) and 1,2,4-tricyano-
benzene (14), have been acquired using FT-microwave spec-
troscopy and analyzed to reveal the effects of quadrupolar
coupling from three 14N nuclei. The laboratory rotational
spectra provide the basis for future searches for these polar
aromatic compounds in interstellar space by radio astrono-
my.
ip. The occupation nz is given by nz =(1+is)CAHTUNGTRENNNUG
depends on the ionic character is of the s bond that exhibits
a p character of (1ꢀas2) and the free s-electron pair that ex-
hibits a p character of as2 due to hybridization. Assuming
as2 =0.5, that is, sp hybridization for the CN bond, we can
calculate the ionic character ip of the CN bond [Eq. (7)]:
Experimental Section
Preparative methods: The tricyanobenzenes 13 and 14 were prepared by
the routes described in reference [11] and characterized by the usual
spectroscopic and analytical methods as summarized in reference [8a,b].
ipðCNÞ ¼ ð1 þ isðCNÞÞ=2ꢀczzðNÞ=eQq210ðNÞ
ð7Þ
Computational methods: Optimized structures, harmonic vibrational fre-
quencies, and infrared intensities were computed using the B3LYP densi-
ACTHNUTRGNEUNG
ty functional[24] with the 6-31G* or the 6-311++G(d,p) basis sets,[25] as
Thus, an increase of the ionic character ip of the p bonds
correlates with a decrease of the quadrupole coupling con-
stant czz(N). The ionic characters of the CN bonds are com-
piled in Tables 2 and 3. The total ionic character denotes the
partial charge at the nitrogen atom of the cyano group that
arises from the polarizations of the participating s bond and
two p bonds. As seen, the three bonds contribute almost
equally to the total polarization of the CN group. One
should note, however, that due to the assumptions that have
been made by such a model, the results should be taken
with care. An asymmetry h=(cxxꢀcyy)/czz of the coupling
tensor indicates a deviation from the cylindrical charge dis-
tribution of a triple bond, that is, a partial double-bond char-
acter, which corresponds to an uneven occupation of the p
orbitals. Tables 2 and 3 also give the derived values for the
double-bond character pxꢀpy =nxꢀny. We note that the
charge distribution is rather symmetric and shows a devia-
tion from a pure triple-bond character on the order of about
1%. This indicates the minor importance of the mesomeric
structures to the conjugated p system that exhibit a CN
double bond.
implemented in Gaussian 03.[26] Harmonic vibrational frequencies were
computed to verify that all geometries were energy minima (no imagina-
ry frequencies) and to obtain zero-point vibrational energies (ZPVE).
Unless otherwise noted, relative energies have been corrected for ZPVE
(B3LYP/6-31G* level).
Microwave spectroscopy: The study of the microwave spectrum of the
two tricyanobenzenes was conducted in a supersonic jet. The preparation
of the sample as a supersonic expansion of a highly diluted gas mixture
entrained in a noble carrier gas provides a virtually collision-free envi-
ronment in which the molecular properties can be determined under con-
ditions of effective isolation.[27] The solid compounds were vaporized in
situ by using a heating nozzle (110–1208C). Neon (1–2 bar) was used as
carrier gas. The jet species were probed with the Balle–Flygare-type[28]
Fourier transform microwave (FTMW) spectrometer in Hannover, which
uses a coaxial arrangement of the supersonic beam and resonator axis
(COBRA).[29] The experimental coherence technique is described else-
where.[30] Briefly, a short (1 ms) MW excitation pulse polarizes the molec-
ular ensemble, and the subsequent free-induction decay (FID) of the po-
larization is recorded in the time domain. After Fourier transform, the
spectrum in the frequency domain is obtained, from which the resonance
frequencies of the rotational transitions can be extracted. All transitions
appear as doublets by the Doppler effect of the jet transiting though the
resonator. The accuracy of the frequency measurements is on the order
of 1 kHz. Transitions separated more than 6 kHz are resolvable.
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Trimerization of Cyanoacetylene
FULL PAPER
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We gratefully acknowledge financial support from the following organiza-
tions: Fonds der Chemischen Industrie (H.H.), the U.S. National Science
Foundation (CHE-0715305; R.J.M.), Deutsche Forschungsgemeinschaft
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Received: June 10, 2010
Published online: October 22, 2010
Chem. Eur. J. 2010, 16, 14115 – 14123
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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