Looking for heteroaromatic rings and related isomers as interstellar candidates

Article abstract of DOI:10.1039/b924574j

Finding complex organic molecules in the interstellar medium (ISM) is a major concern for understanding the possible role of interstellar organic chemistry in the synthesis of prebiotic species. The present interdisciplinary report is a prospective study aimed at helping detection of heteroaromatic compounds or at least of some of their isomers in the ISM. The thermodynamic stabilities of the C₄H₅N, C₄H₄O, C₄H₄S families were calculated using density functional theory (DFT). It was found that pyrrole, furan and thiophene are unambiguously the most stable isomers at the 10-50 K temperatures of the ISM. Several of the less stable isomers were synthesized and flash vacuum thermolysis experiments were performed on these species. Although the detection of pyrrole in the pyrolysis of many compounds has been reported in the literature, we observed that none of its isomers led to pyrrole in these conditions, which suggests that other formation routes are to be considered. On the other hand, these three aromatic compounds present a very high stability, few % been decomposed at 1500 K by flash vacuum thermolysis; these experiments also show a great stability of crotonitrile that is the most stable compound that can be formed in these conditions. The rotational constants, dipole moments and IR frequencies of the low-lying isomers are given to encourage laboratory experiments on these prototype molecules.

Full text of DOI:10.1039/b924574j

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Looking for heteroaromatic rings and related isomers
as interstellar candidates
M. Lattelais,a Y. Ellinger,*a A. Matranebc and J.-C. Guillemin*b
Received 23rd November 2009, Accepted 9th February 2010
First published as an Advance Article on the web 16th March 2010
DOI: 10.1039/b924574j
Finding complex organic molecules in the interstellar medium (ISM) is a major concern for
understanding the possible role of interstellar organic chemistry in the synthesis of prebiotic
species. The present interdisciplinary report is a prospective study aimed at helping detection
of heteroaromatic compounds or at least of some of their isomers in the ISM. The
thermodynamic stabilities of the C4H5N, C4H4O, C4H4S families were calculated using density
functional theory (DFT). It was found that pyrrole, furan and thiophene are unambiguously the
most stable isomers at the 10–50 K temperatures of the ISM. Several of the less stable isomers
were synthesized and flash vacuum thermolysis experiments were performed on these species.
Although the detection of pyrrole in the pyrolysis of many compounds has been reported in the
literature, we observed that none of its isomers led to pyrrole in these conditions, which suggests
that other formation routes are to be considered. On the other hand, these three aromatic
compounds present a very high stability, few % been decomposed at 1500 K by flash vacuum
thermolysis; these experiments also show a great stability of crotonitrile that is the most stable
compound that can be formed in these conditions. The rotational constants, dipole moments and
IR frequencies of the low-lying isomers are given to encourage laboratory experiments on these
prototype molecules.
over the corresponding enol) has been identified in hot
A. Introduction
cores.8,9 Consequently, when the most stable isomer is a cyclic
system, it is a strong incentive for its search in space.
Following benzene, pyrrole and its homologues (furan and
thiophene), all stabilized by aromaticity, are good candidates
with resonance energies10 of 36 (benzene) > 29 (thiophene) >
21 (pyrrole) > 16 kcal molÀ1 (furan). They have a permanent
dipole moment, which is a necessity for radio observations and
the computational sizes of 9–10 atoms are within reasonable
limits for accurate structural determinations.
Of the B150 species identified in the interstellar medium
(ISM) over the past 25 years, only six are cyclic species, five
of these being three-membered rings. Three of them are
hydrocarbons, i.e. cyclopropenylidene1 (c-C3H2), dehydroge-
nated cyclopropenylidene2 (c-C3H) and benzene3 (C6H6), two
are oxygen containing species, oxirane4 (C2H4O) and cyclo-
propenone5 (c-H2C3O), the last one being a silicon carbide6
(c-SiC2). Except for C6H6 and c-SiC2, all these species have
non-cyclic isomers that have also been observed in the ISM.
A recent study of the series of interstellar molecules
identified with two or more isomeric forms shows a strong
relationship between the observed abundances and the energy
differences relative to the most stable isomer. What can be seen
as a minimum energy principle seems to govern the abundance
ratio between isomers, pointing to a local thermal equilibrium
at one time or another along the chemical history of these
species.7 It then seems logical that the most stable isomers are
observed since they are the most abundant. It does not
preclude, however, that higher energy isomers be observed.
For example, oxirane, which ranks in third position on the
C2H4O energy scale (B28 kcal molÀ1 over acetaldehyde, B18 kcal
In this report, the search for these heteroaromatic molecules
is addressed by an interdisciplinary approach coupling
theoretical calculations and experimental syntheses.
The theoretical part consisted of the calculation of the
structure, stability and spectral signatures of the C4H5N,
C4H4O and C4H4S families, namely pyrrole, furan, thiophene
and their most probable isomers. The search for isomers was
limited to an arbitrary B45 kcal molÀ1 above the minimum
energy structures, a value beyond which higher energy isomers
are rarely found in the ISM.7
The experimental part consisted in the synthesis of several of
these compounds followed by flash vacuum thermolysis (FVT)
in the gas phase in order to determine whether they give the
aromatic species or not. This approach is not relevant for all
the environments present in the ISM, but is expected to
provide information as to the possible hot chemistry in high
temperature regions.
a
´
´
Laboratoire de Chimie Theorique (LETMEX), Universite Pierre et
Marie Curie, Paris VI, UMR-CNRS 7616, 4, Place Jussieu,
75252 Paris cedex, France. E-mail: ellinger@lct.jussieu.fr
b Sciences Chimiques de Rennes, UMR 6226 CNRS-ENSCR, F-35700
Rennes, France. E-mail: jean-claude.guillemin@ensc-rennes.fr
The attention is mainly focused on pyrrole whose dipole
moment of B2 Debye is at least three times larger than that of
furan and thiophene.
c
´
Laboratoire de Chimie Bio-Organique et Macromoleculaire, Faculte
des Sciences et Techniques Marrakech, Universite Cadi Ayyad,
´
´
BP 549, Marrakech, Morocco
ꢀc
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Table 1 Relative energies (kcal molÀ1), rotational constants (GHz)
and dipole moments (Debye) at the B3LYP/cc-pVQZ for the
low-energy isomers of pyrrole
B. Theoretical part
In a study that implied the calculation of more than thirty
molecules of ten atoms on average, it was deemed necessary to
reach a compromise between the level of theory and computa-
tional efficiency. It is the reason why the calculations were
performed using density functional theory (DFT) in the
B3LYP formalism. Geometry optimizations were carried out
using the standard split-valence polarized 6-311G** basis set
of triple-zeta quality for a first approach. Then, the structures
were refined using the more flexible correlation consistent
cc-pVQZ basis that gave the final results reported here. Each
structure, fully optimized, was verified to be a stationary point
by vibrational analysis.
Isomers
DE
A
B
C
m
1
2
0.0
9.198
9.073
4.568
1.9
7.9
9.5
39.087
9.340
2.302
4.197
2.203
2.948
4.8
4.0
3
Vibrational frequencies were calculated at the B3LYP/
cc-pVQZ level for characterization of the energy minima and
evaluation of zero-point energies that are taken into account
for computing relative energies (anharmonic corrections are
not considered; frequencies are not scaled at this level).
Including entropic terms to determine Gibbs free energies of
formation at the temperature of the interstellar medium has a
marginal impact on the values of the energy differences. It will
be considered only for the discussion of the thermolysis
results.
4
5
13.7
15.6
11.625
9.139
3.666
8.478
2.835
4.519
3.9
2.1
6
7
17.2
19.8
8.979
8.629
3.456
4.522
3.274
2.5
4.4
16.083
8
29.1
9.290
4.628
3.149
3.6
The calculations were performed using the methods and
basis sets as implemented in the GAUSSIAN03 packages.11
B.1 Isomers of C4H5N formula
9
29.4
35.3
12.653
11.785
3.640
3.924
2.876
2.998
3.8
3.8
Three different connectivities were investigated for the five
heavy atoms: five-membered rings, open chains and substi-
tuted three-membered rings. The results reported in Table 1
for the 16 low-lying isomers show that pyrrole, stabilized by
aromaticity, is unambiguously the most stable species. Its
dipole moment close to 2 Debye makes it a reasonable target
for observations.
10
11
12
35.4
36.8
36.848
12.943
2.368
3.506
2.259
2.762
2.3
2.4
The next compounds on the energy scale are two conjugated
methyl acrylonitriles, B8 and B10 kcal molÀ1 above. The
value of their dipole moments, i.e. B4–5 Debye is encouraging
for radio detection. It is worth noting that a molecule of this
type, acrylonitrile (H2CQCHCN), has already been observed
in the ISM.12
13
14
15
38.5
42.3
42.7
41.798
16.115
40.629
2.123
3.720
2.170
2.046
3.499
2.063
2.3
4.1
2.3
A third nitrile, but non-conjugated, ranks fourth. No such
molecule is known in the ISM where unsaturations are always
conjugated. In the fifth and sixth position, one finds two
non-aromatic five-membered ring isomers, close in energy.
The last nitrile, cyclopropane cyanide is found B20 kcal molÀ1
above pyrrole.
16
44.6
11.862
3.620
2.821
2.3
B.2 Isomers of C4H4O formula
The isocyanides are B30 kcal molÀ1 above pyrrole, that is
B20 kcal molÀ1 above their cyano isomers. Such an energy
difference does not rule out the possibility of detecting these
species, as demonstrated by the identification of CH3NC13 and
HCRCNC,14 which are B24 and 27 kcal molÀ1 above
CH3CN and HCRC–CN,7 respectively. A similar energy
difference is noted between the cyano- and isocyano-
substituted cyclopropanes.
The relative energies of the 11 isomers found below the
B45 kcal molÀ1 threshold are reported in Table 2. This
extensive survey shows that furan, the only aromatic structure
on the list, is the most stable compound. Its dipole moment of
B0.6 Debye is weak. The same conclusion can be reached for
the conjugated vinylketene positioned B9 kcal molÀ1 above
on the energy scale.
In the third position one finds cyclobutenone, B18 kcal molÀ1
above furan, with a significant dipole moment of 3.5 Debye.
To our knowledge, no four-membered ring has ever
been detected in space. It is B1 kcal molÀ1 below allenic
aldehyde, the latter being more stable than 2-butyn-1-aldehyde
The allenic imine is number 11 on the energy scale; it is
3 kcal molÀ1 below the corresponding acetylenic isomer. In
fact, the isomers with a –CRC bond are much less stable than
their –CRN counterparts.
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Table 2 Relative energies (kcal molÀ1), rotational constants (GHz)
and dipole moments (Debye) at the B3LYP/cc-pVQZ for the
low-energy isomers of furan
Table 3 Relative energies (kcal molÀ1), rotational constants (GHz)
and dipole moments (Debye) at the B3LYP/cc-pVQZ for the
low-energy isomers of thiophene
Isomers
DE
A
B
C
m
Isomers
DE
A
B
C
m
1
2
0.0
9.507
9.340
4.711
0.6
1
2
0.0
8.057
5.426
3.242
0.5
8.5
40.888
12.648
2.388
5.328
2.256
3.840
0.9
3.5
28.7
37.9
37.867
7.650
1.526
4.502
1.467
2.834
0.7
1.3
3
18.1
3
4
5
19.3
23.7
36.199
43.631
2.405
2.124
2.291
2.051
3.7
4.0
4
5
38.6
39.4
12.948
26.704
2.883
1.486
2.394
1.420
3.3
3.6
6
24.4
10.288
4.050
2.959
3.1
6
7
41.2
42.3
9.043
2.483
1.418
1.948
1.365
0.3
0.8
7
8
29.1
29.8
24.565
13.072
2.563
3.635
2.417
2.844
2.2
0.2
36.496
8
9
42.8
49.8
6.478
3.134
1.675
2.140
1.619
2.6
1.8
9
40.7
14.101
5.507
3.960
1.3
20.435
10
11
41.6
44.7
44.133
21.101
2.172
3.250
2.070
3.133
1.6
3.8
The next four isomers are within a B4 kcal molÀ1 range,
about 40 kcal molÀ1 above thiophene. They are all acetylenic
derivatives. Among them, only 2-butyn-1-thioaldehyde has a
relatively attractive dipole moment, i.e. 3.6 Debye.
by B4 kcal molÀ1. In positions 6–8 one finds three acetylenic
compounds, two of which conjugated to a carbonyl group, the
last one being the corresponding enolic species.
C. Flash vacuum thermolysis experiments
Above 40 kcal molÀ1 of furan, one has another four-
membered ring, a cyclic ether, followed by a conjugated ynol.
The first three-membered ring is a cyclopropene substituted by
a formyl group, B45 kcal molÀ1 over furan.
Seven nitrogen derivatives (pyrrole 1, crotonitrile 2, methacrylo-
nitrile 3, allyl cyanide 4, cyclopropyl cyanide 7, allyl isocyanide
10, cyclopropyl isocyanide 14), furan and thiophene have been
thermolyzed at temperatures of 1073, 1273 and 1500 K in FVT
conditions (Table 4). Experiments carried out at temperatures
lower than 1073 K showed no significant changes, except for
allyl isocyanide 10 that is totally rearranged in allyl cyanide 4
from 873 K.
B.3 Isomers of C4H4S formula
The relative energies of the 9 isomers below the B45 kcal molÀ1
threshold are given in Table 3. Here again the aromatic
structure, thiophene, is the most stable. Compared to pyrrole,
its dipole moment of 0.5 Debye is four times smaller, which is
not encouraging for observation by microwave detection.
As for furan, the second isomer on the energy scale is an
allenic derivative. However this isomer is almost 30 kcal molÀ1
higher than thiophene, instead of B9 in the case of furan. Its
dipole is weaker than that of the oxygenated analogue. In the
third and fourth positions one finds two four-membered rings,
but here also, the energetic gap of B38 kcal molÀ1 with the
most stable isomer (thiophene) is twice that of the lowest
energy four-membered in the furan series. Their dipole
moments are smaller than those of the corresponding cyclic
ethers of the furan series.
Starting from pyrrole, more than 94% of the original
compounds were recovered at 1273 K, with small amounts
of crotonitrile 2, methacrylonitrile 3 and allyl cyanide 4. At
1500 K, 20% of the material was rearranged in crotonitrile
(11%) and allyl cyanide (3%) or decomposed in acrylo-
nitrile (6%).
In the same experimental conditions, only 3% of crotonitrile
2 was isomerized in allyl cyanide 4 at 1273 K. At a temperature
of 1500 K, crotonitrile 2 (4%) and allyl cyanide 4 (2%) were
detected in the thermolysis of methacrylonitrile. At the same
temperature, 43% of crotonitrile 2 and 8% of methacrylo-
nitrile 3 were observed in the thermolysis products of the allyl
cyanide 4. All these results clearly establish the rearrangements
between compounds 2, 3 and 4.
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Table 4 FVT of pyrrole and several isomers (yields %)
Table 5 Relative Gibbs energies of formation (DG(1500 K)) of low-
lying isomers of pyrrole, furan and thiophene
Products (%)
Pyrrole isomers
DG
Furan isomers
DG
1
2
3
4
7
10 14 Acrylonitrile
Isomer T/K
1
1073 99 tr. tr.
0
1
0
0
0
0
0
0
0
0
0
0
0
0
63
8
tr.
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
98
93
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
71
tr.
0
0
0
0
6
—
tr.
11
0
1
8
2
0.0
2
1
0.0
1273 94
1500 82 11
3
tr.
0
3.2
tr.
3
11
0
tr.
2
99
86
46
8
2
1073
1273
1500
1073
1273
1500
1073
1273
1500
1073
1273
1500
473
673
873
1073
1273
1500
873
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
99
96
54 13
0 100
tr. 97
0
0
3
1
1.6
3.5
3.2
3
4
80
4
tr. tr.
11 tr.
43
28
51 tr.
58
0
—
tr.
8
4
3
10.2
16.7
8
0
7
—
10
12
—
—
—
—
6
24
19
0
4
7
6.2
5
0
0
tr.
tr.
7
10
17.0
Thiophene isomers
0
1
4
97
93
60
24
5
12
16
12
5
31
26 10
11
52
56
51
0
0
0
0
22
12
0
5
6
19.3
20.4
1
2
0.0
29
3
2
14
4
2
1
6
1073
1273
1500
18.0
1
8a
a
With many unidentified minor compounds.
most stable and into allyl cyanide 4). The cyclopropane
derivatives open and transform by a majority into crotonitrile;
isocyanides rearrange into the corresponding cyanides.
On the other hand, cyclopropyl cyanide 7 was completely
rearranged in crotonitrile 2 (58%), methacrylonitrile 3 (5%)
and allyl cyanide 4 (19%) at 1500 K. Cyclopropyl isocyanide
14 gave mainly crotonitrile 2 and cyclopropyl cyanide 7 from a
temperature of 1073 K.
D. The signatures to look for
Levering on the astrophysical observations, it is reasonable
to consider that, the more stable is a molecule, the larger its
chances are of being detected. In these conditions, the short list
of potential targets is composed of the molecules within
10 kcal molÀ1 of the most stable species of each series. So
doing one has the three aromatic heterocycles, pyrrole, furan
and thiophene to which are added the lowest two cyano
isomers of pyrrole (which seems reasonable considering of
the large number of such compounds already discovered in
space) and the vinylketene in the furan series.
For the oxygen series, we only performed the FVT of furan
at 1073, 1273 and 1500 K. No decomposition was observed at
a temperature of 1073 or 1273 K. On the contrary, a huge
amount of propyne (32% yield) was formed in the FVT at
1500 K, in agreement with previous experiments.15
Carbon monoxide was probably also formed. Such small
molecules formed by fragmentation of the starting material are
usually observed in the high temperature FVT products of
many compounds.
We then performed FVT of thiophene at 1073, 1273
and 1500 K. Even at the highest temperature only little
decomposition was observed.
Looking for a candidate by radio is strongly dependent on
the dipole moment since line intensities scale with the square
of its value. The experimental rotational constants:
The reported experiments can be better understood by
considering the Gibbs free energies of formations of the low-
lying isomers at 1500 K, reported in Table 5. The major result
is that pyrrole and furan are no longer the most stable
compounds that can be formed in their respective families.
For that reason, pyrrole was never observed among the
thermolysis products of all these compounds containing a
nitrogen atom. Crotonitrile, instead, is the main product in
all the thermolysis at high temperature. It is interesting to
observe that nitriles 2–4 and acrylonitrile, a decomposition
product, were formed in all the experiments performed at
1500 K.
Pyrrole:16 A = 9.131; B = 9.001; C = 4.532 (GHz)
Furan:17 A = 9.447; B = 9.247; C = 4.671 (GHz)
Thiophene:18 A = 8.042; B = 5.418; C = 3.236 (GHz) are
very close to the theoretical values (Tables 1–3), with ex/th
ratios of 0.992 (pyrrole, furan) and 0.998 (thiophene), so that
we do not consider that any empirical correction should be
applied for the unknown species.
In all cases, pyrrole, with m = 1.8019 Debye is a better
prospect than furan and thiophene whose dipole moments are
m = 0.6620 and 0.5521 Debye, respectively. This study shows
the general trend that the dipole moment decreases in the
average from the pyrrole to the furan and thiophene series.
Finding furan and thiophene in space will be a difficult task
since their dipole moments are the smallest in their respective
series of isomers (below 40 kcal molÀ1).
Looking individually at the C4H5N isomers shows that
pyrrole opposes a strong resistance to thermolysis but in the
end rearranges into linear chain isomers (crotonitrile 2 that is
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In such conditions, the only technique that remains to
identify these molecules is IR spectroscopy. One should
remember, indeed, that the simplest aromatic system was
detected by using its IR signature.3 Benzene was identified in
the IR for the obvious reason that it has no permanent dipole
moment. It has 30 vibrational modes of which only four are
active in the infrared. The observed frequency n4 = 674 cmÀ1
is the signature of the synchronous bending of the six CH
bonds, all of them moving in the same direction with respect to
the six-membered ring. It is the strongest IR band of benzene
with an intensity of 105 km molÀ1. The other infrared active
bands observed in laboratory spectra22 are n12, n13 and n14
(3048, 1484 and 1038 cmÀ1, respectively) were not observed
in space.
furan24 and thiophene26 whose gas phase spectra have been
reported recently.27 Only the frequencies with a calculated
intensity larger than B10% of the largest value to which they
are normalized, are reported in Table 6.
From a computational point of view, the main result
displayed in Table 6 is the remarkable agreement between
the observed frequencies and the best estimates obtained by
the scaling procedure; considering the type of vibration as the
determining factor for adjusting the frequencies is entirely
justified.
From the astrophysical point of view, there is, practically,
only one band to search for. It is the most intense band in the
four aromatic cycles, i.e. that corresponding to the out-of-
plane of the CH bonds. This band has an intensity similar to
that in benzene and should be detectable provided the
abundances are not too weak. The best prospect seems to be
pyrrole that shows two additional bands, the NH stretch and
the NH bend that have relatively large intensities (Bhalf of the
CH out-of-plane bending) and should also be present in
the spectrum. No such vibrations are present in furan and
thiophene.
The same approach is also to be considered for its homo-
logues. Calibration is a necessary procedure to obtain accurate
frequencies for molecules of that size. Appropriate scaling
factors are generally determined, globally (for general
estimations), or per type of vibration,23 (for more precise
adjustments) by fitting the calculated bands to the spectro-
scopic values of a representative molecule. So doing we obtain
scaling factors intended to account for the anharmonic
contribution missing in this type of calculation: 0.958 for
CH stretching, 0.978 for in-plane bending and 0.975 for
out-of-plane CH bending. All other types of vibrations that
do not imply hydrogen atoms are given the usual general
scaling factor of 0.98. The procedure is applied to pyrrole,24,25
Concerning the non-cyclic isomers, our results show that
the cyano derivatives are interesting prospects for radio
astronomy because of the magnitude of their dipole moments:
4.8 and 4.0 Debye for crotonitrile and methacrylonitrile,
respectively. The rotational constants given in Table 1 should
Table 7 Vibrational analysis of cyano derivatives: assignment,
frequencies (cmÀ1
)
before and after scaling in parentheses and
)
Table 6 Vibrational analysis of the aromatic rings: assignment,
frequencies (cmÀ1) before and after scaling in parentheses and
intensities (km molÀ1
intensities (km molÀ1
)
Frequency
Intensity
Exp
Frequency
Intensity
Exp
CH2 asym str
CH str
CN str
CQC str
CH2 wag
C–C str
3245
3169
2334
1676
996
1
1
8
1
48
2
3125
3042
2239
1620
955
CH str
3181
1518
1061
691
36
7
5
105
3048
1484
1038
674
CH bend in plane
CH bend in plane
CH bend out of plane
883
872
Frequency
Intensity
Exp
Frequency
Intensity
Exp
NH str
3672(3525)
1092(1064)
1069(1045)
1034(1011)
733(715)
64
8
25
30
145
70
3531
1075
1049
1017
722
CH bend in plane
CH bend in plane
CH bend in plane
CH bend out of plane
NH bend
C–H asym str.
CH3 asym str.
CH3 asym str.
CH3 sym str.
CN str.
CQC str.
CH3 bend
CH wag.
3161(3035)
3150(3018)
3105(2975)
3063(2919)
2329(2234)
1697(1641)
1486(1433)
992(951)
7
11
9
11
26
23
18
39
3060
2965
492(471)
475
2232
1642
1450
958
Frequency
Intensity
Exp
CQC sym str
C–O str
C–C str + C–O–C sym str
CH bend in plane
C–O–C bend
1510(1480)
1199(1175)
1085(1063)
1014(992)
888(870)
18
18
12
41
16
101
23
1490
1181
1067
995
870
745
Frequency
Intensity
Exp
CH bend out of plane
C–O–C wag
761(742)
624(612)
CH3 str
CH3 str
CH3 str
CN str
CQC str
CH3 bend
C–C–C str
CH2 wag
3120(2989)
3085(2955)
3033(2890)
2328(2233)
1684(1628)
1496(1442)
1287(1270)
970(930)
10
8
10
11
9
16
7
36
2999
2976
603
Frequency
Intensity
Exp
2227
1642
1453
1270
928
CH bend in plane
C–S str
CH bend out of plane
1284(1256)
842(825)
726(708)
9
22
122
1256
839
712
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Table 8 Vibrational analysis of vinylketene: assignment, frequencies
(cmÀ1
before and after scaling in parentheses and intensities
(km molÀ1
crotonitrile, methacrylonitrile and allyl cyanide when
submitted to these very harsh experimental conditions. It is
interesting to note that the percentage of decomposition or
isomerisation products of the heterocycles follows the decrease
of the resonance energy.
)
)
H2OQCQO
Frequency
2216
Intensity
652
Exp
CQCQO asym str
2151
The fact that pyrrole is not formed in these conditions,
whatever the isomer considered as starting material suggests
that other formation routes involving smaller fragments, are
to be considered. In this respect, the identification of acrylo-
nitrile as a decomposition product in all experiments may be a
valuable indication. The amount of each species that remains
stable at the temperatures probed in the experiments means
also that the activation barriers for conversion between the
isomers should be large enough to allow their detection in the
ISM conditions.
Frequency
Intensity
Exp
CQCQO asym str
CQC–C asym str
CH2 wag
2198(2143)
1674(1619)
903(866)
1049
98
51
2137
1626
875
CH2 twist + CH bend
566(543)
47
540
be accurate enough to help in laboratory studies. The IR
spectra of these low-energy cyano isomers are presented in
Table 7. Only the most intense bands relevant to our study
have been reported, together with the experimental values of
Cerceau et al.28
Concerning an eventual detection, the search for pyrrole
appears to be worth the try; it has a dipole moment B2 Debye
suitable for radio-astronomy and a well characterized IR
frequency at 722 cmÀ1 with an intensity larger than that used
for the identification of benzene.
Calibration, here, relies on acrylonitrile for all but the –CH3
vibrations for which we have taken the values reported for
propylene29 (average scaling factors of 0.958, 0953 and 0964
follow for the CH asymmetric, symmetric stretching and CH3
deformation, respectively).
Besides the cyclic species, these calculations show that
crotonitrile and methacrylonitrile, with dipole moments of
4.8 and 4.0 Debye are also promising candidates for detection
in space. To this end, the atmosphere of Titan that is known to
be the theatre of rich nitrogen chemistry is most probably as
valuable a place as any interstellar region. Concerning the
furan isomers, the best prospect appears to be vinylketene
whose IR signature should be detectable even with a low
abundance.
The nitrile isomers of pyrrole have no really intense
and characteristic band that can be used for a clear-cut
identification by IR in space. Only if acrylonitrile, already
identified by its radio spectrum,12 can also be identified by its
IR spectrum will it be possible to search for the cyano
derivatives.
For vinylketene (m = B0.9 Debye), one should first
mention the remarkable agreement of the predicted rotational
constants Bth = 2.388 GHz and Cth = 2.256 GHz with those
F. Experimental part
F.1 Materials
determined from the microwave spectrum,30,31 i.e. Bth
=
2.393 GHz and Cth = 2.256 GHz. In the infrared, a very
strong band that is characteristic of the ketene group
dominates the spectrum (Table 8). Scaling the calculated value
by the ratio of the observed32 to the calculated frequency of
the simple ketene gives a wave number of 2143 cmÀ1 in perfect
agreement with the experimental spectrum (2137 cmÀ1). The
calculated absolute intensity over 1000 km molÀ1 is more
than ten times that of the band used to identify benzene from
the ISO survey. It is a promising signature that should be
searched for.
Crotonitrile, methacrylonitrile, allyl cyanide, pyrrole, furan
and thiophene were purchased from the Aldrich company.
Allyl isocyanide and cyclopropyl isocyanide were synthesized
by dehydration of the corresponding formamide.33
F.2 Flash vacuum thermolysis experiments
The compound (1 mmol) was vaporized in a vacuum line
(10À1 mbar) equipped with a short oven (15 cm) heated
to a temperature of 1073, 1273 or 1500 K. The gaseous
flow was condensed on a cold finger (77 K) equipped at
the bottom with an NMR tube. At the end of the thermolysis,
a solvent (CDCl3, 700 mL) was added. After disconnection
from the vacuum line by stopcocks, the apparatus was
filled with dry nitrogen and liquid nitrogen was subsequently
removed. The products were collected in the NMR tube and
analyzed by 1H NMR spectroscopy, and by comparison of the
spectra with those of authentic samples.
E. Concluding remarks
The aim of the study was to provide new information as to the
presence of heteroaromatic analogues of benzene in the
interstellar medium, and their possible detection.
DFT calculations showed that pyrrole, furan and thiophene
are unambiguously the most stable isomers at the 10–50 K
temperatures of the cold molecular clouds of the ISM. It is
no longer the case for pyrrole and furan at high temperatures
(B1500 K). The calculated Gibbs free energies of formation
show that crotonitrile and methacrylonitrile on one side and
vinylketene on the other side become more stable.
Acknowledgements
This research was supported by CNRS ‘‘Programme National
PCMI’’ and ‘‘PID-OPV’’. Parts of the calculations presented
in this contribution were performed using HPC resources from
GENCI-CINES (Grant 2009-085128).
The FVT experiments confirm the high stability of the three
aromatic systems. They also show the particular stability of
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C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. G. Johnson, W. Chen, M. W. Wong, C. Gonzalez and
J. A. Pople, GAUSSIAN 03 (Revision C.02), Gaussian, Inc.,
Wallingford, CT, 2004.
References
1 P. Thaddeus, J. M. Vrtilek and C. A. Gottlieb, Astrophys. J., 1985,
299, L63.
12 F. F. Gardner and G. Winnewisser, Astrophys. J., 1975, 195, L127.
2 S. Yamamoto, S. Saito, M. Ohishi, H. Suzuki, S.-I. Ishikawa,
N. Kaifu and A. Murakami, Astrophys. J., 1987, 322, L55.
3 J. Cernicharo, A. M. Heras, A. G. G. M. Tielens, J. R. Pardo,
´
13 J. Cernicharo, C. Kahane, M. Guelin and J. Gomez-Gonzalez,
Astron. Astrophys., 1988, 189, L1.
14 K. Kawaguchi, M. Ohishi, S.-I. Ishikawa and N. Kaifu, Astrophys.
J., 1992, 386, L51.
´
F. Herpin, M. Guelin and L. B. F. M. Waters, Astrophys. J., 2001,
546, L123.
15 J. K. Winkler, W. Karow and P. Rademacher, J. Anal. Appl.
Pyrolysis, 2001, 57(1), 133–144.
16 A. Mellouki, J. van der Auwera and M. Herman, J. Mol.
Spectrosc., 1999, 193, 195.
4 J. E. Dickens, W. M. Irvine, M. Ohishi, M. Ikeda, S. Ishikawa,
A. Nummelin and A. Hjalmarson, Astrophys. J., 1997, 489, 753.
5 J. M. Hollis, A. J. Remijan, P. R. Jewell and F. J. Lovas,
Astrophys. J., 2006, 642, 933.
17 A. Mellouki, M. Herman, J. Demaison, B. Lemoine and
L. Margules, J. Mol. Spectrosc., 1999, 198, 348.
6 P. Thaddeus, S. E. Cummins and R. A. Linke, Astrophys. J., 1984,
283, L45.
18 B. Bak, D. Christensen, L. Hansen-Nygaard and J. Rastrup-
Andersen, J. Mol. Spectrosc., 1961, 7, 58.
7 M. Lattelais, F. Pauzat, Y. Ellinger and C. Ceccarelli, Astrophys.
J., 2009, 696, L133.
19 H. Kofod, L. E. Sutton and J. Jackson, J. Chem. Soc., 1952, 1467.
20 H. Sirvetz, J. Chem. Phys., 1951, 19, 1609.
21 T. Ogata and K. Kozima, J. Mol. Spectrosc., 1972, 42, 38.
22 M. Dnag-Nhu and J. Pliva, J. Mol. Spectrosc., 1989, 138, 423.
23 J. Sczcepanski, M. Vala, D. Talbi, O. Parisel and Y. Ellinger,
J. Chem. Phys., 1999, 193, 195.
8 A. Nummelin, J. E. Dickens, P. Bergman, A. Hjalmarson, W. M.
Irvine, M. Ikeda and M. Ohishi, Astron. Astrophys., 1998, 337, 275.
9 M. Ikeda, M. Ohishi, A. Nummelin, J. E. Dickens, P. Bergman,
A. Hjalmarson and W. M. Irvine, Astrophys. J., 2001, 560, 792.
10 G. W. Wheland, Resonance in Organic Chemistry, Wiley,
New York, 1955, pp. 98–99.
24 A. Mellouki, J. Lie
239.
´
vin and M. Herman, Chem. Phys., 2001, 271,
11 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr.,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li,
J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. Ochterski, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain,
O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
25 F. Hegelund, R. Wugt Larsen and M. H. Palmer, J. Mol.
Spectrosc., 2008, 252, 93.
26 T. D. Klots, R. D. Chirico and W. V. Steele, Spectrochim. Acta,
1994, 50A, 765.
27 A general discussion of IR spectra of five membered rings can be
found in: M. H. Palmer, J. Mol. Struct., 2007, 834–836, 113.
28 F. Cerceau, F. Raulin, R. Courtin and D. Gautier, Icarus, 1985, 62,
207.
29 R. C. Lord and P. Venkateswarlu, J. Opt. Soc. Am., 1953, 43, 1079.
30 R. D. Brown, P. D. Godfrey and M. Woodruff, Aust. J. Chem.,
1979, 32, 2103.
31 E. Bjarnov, Z. Naturforsch., 1979, 34a, 1269.
32 C. B. Moore and G. C. Pimentel, J. Chem. Phys., 1963, 38, 2816.
33 J. Schuster, E. Scott and J. Casanova, Jr., Org. Synth., 1973, 5, 772.
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