PETER BOTSCHWINA
Institut für Physikalische Chemie
Universität Göttingen, Tammannstraße 6
D37077 Göttingen, Germany

H. S. P. Müller, F. Schlöder, J. Stutzki and G. Winnewisser, J. Mol. Struct. 742, 215 (2005).

Among the ca. 140 different molecules found in the interstellar medium (ISM) carbon chains present the dominating structural motif.
These are often very reactive and difficult to investigate in the laboratory. During the past three decades, the identification and characterisation of interstellar molecules has often benefitted from a fruitful interplay between theoretical chemistry, laboratory spectroscopy and (radio) astronomy.

2n+1
n

2n+1
• Almost ubiquituous in the ISM and CSM
• Provide largest (in terms of number of atoms) interstellar molecule unambiguously detected by radio astronomy HC
11
N
• Through the presence of low-lying bending vibrational states observable by radio astronomy in excited vibrational states
 important information on dynamical processes

Chemically, cyanopolyynes are linear molecules with conjugated triple bonds, an energetically very stable situation (once formed). Organic chemists call the cyano group a strong “electron withdrawing group“, which has the astronomically important consequence that cyanopolyynes have rather large electric dipole moments.
Already for cyanoacetylene (HC
3
N), the experimental ground-state dipole moment is as large as

0
= 3.72 D

Cyanopolyynes: demanding cases for accurate equilibrium structure determinations recommended method: combination of experimental and theoretical data exp.: B
0 values for various (as many as possible) isotopomers theor.:

B
0
= B e
- B
0 calculated from high-quality ab initio cubic force fields (e.g., CCSD(T) with large basis set)

B
0

1
2
 i
 i
 d i
( α i from 2 nd order perturbation theory)
 i d i
: vibration-rotation coupling constant
: degeneracy factor of vibrational mode i

3
[1] P. Botschwina, M. Horn, S. Seeger and J. Flügge,
Mol. Phys. 78, 191 (1993).
[2] P. Botschwina, Mol. Phys. 103, 1441 (2005).

Millimeter-wave spectroscopy of rare isotopomers of
HC
5
N and DC
5
N: determination of a mixed experimental-theoretical equilibrium structure for cyanobutadiyne
D 12 C
5
15 N
J = 43

42 (*)
L. Bizzocchi, C. Degli Esposti and P. Botschwina
J. Mol. Spectrosc. 225, 145 (2004)

HC
5
N isotopomers: spectroscopic constants from MMW spectroscopy

5
* P. Botschwina, Ä. Heyl, M. Oswald and T. Hirano,
Spectrochim. Acta A 53, 1079 (1997).

r (HC
(1)
)/Å
R
1
(C
(1)
C
(2)
)/Å
R
2
(C
(2)
C
(3)
)/Å
R
3
(C
(3)
C
(4)
)/Å
R
4
(C
(4)
C
(5)
)/Å
R
5
(C
(5)
C
(6)
)/Å
R
6
(C
(6)
C
(7)
)/Å
R
7
(C
(7)
C
(8)
)/Å
R
8
(C
(8)
C
(9)
)/Å
R
9
(C
(9)
C
(10)
)/Å
R
10
(C
(10)
C
(11)
)/Å
R
11
(C
(11)
N)/Å
Geometric structures for linear HC
11
N r
0 structure a
1.057(1)
1.210(1)
1.360(1)
1.218(2)
1.351(3)
1.217(5)
1.360(8)
1.219(6)
1.350(3)
1.217(2)
1.365(1)
1.161(1) r e estimate b
1.0627
1.2104
1.3637
1.2178
1.3564
1.2187
1.3571
1.2153
1.3695
1.2153
1.3695
1.1620
CCSD(T)/cc-pVTZ c
1.0643
1.2169
1.3695
1.2246
1.3616
1.2267
1.3601
1.2261
1.3624
1.2219
1.3753
1.1689
r e structure c
(recommended)
1.0625
1.2105
1.3635
1.2182
1.3556
1.2203
1.3541
1.2197
1.3564
1.2155
1.3693
1.1620
a M. C. McCarthy, E. S. Levine, A. J. Apponi and P. Thaddeus, J. Mol. Spectrosc. 203 (2000) 75.
Statistical uncertainties (1

) in terms of the last significant digit are given in parentheses.
b See above reference.
c P. Botschwina, Phys. Chem. Chem. Phys. 5 (2003) 3337.

HC
11
N: Variation of CC equilibrium bond lengths

11
• 1982 and 1985 : weak radio lines observed in IRC+10216 and TMC-1 attributed to HC
11
N (without accurate laboratory data at hand)
• For more than 10 years no confirmation of assignments successful
• 1996 : FT-MW spectroscopy of HC
11
N by Thaddeus and coworkers
(Harvard University); 20 rotational transitions measured spectroscopic constants not compatible with previous assignments of radio lines
M. J. Travers, M. C. McCarthy, P. Kalmus, C. A. Gottlieb and
P. Thaddeus, Astrophys. J. 469 (1996) L65.
• 1997 : detection of rotational transitions J = 39

38 and 38

37 by means of NRAO 43 m telescope
M. B. Bell, P. A. Feldman, M. J. Travers, M. C. McCarthy, C. A.
Gottlieb and P. Thaddeus, Astrophys. J, 483 (1997) L61.

Dipole moments and column densities of cyanopolyynes
(HC
2n+1
N) in TMC-1 a
Molecule
HC
5
N
HC
7
N
HC
9
N
HC
11
N

(D)
4.33
4.82
5.20
5.47 b c c c
N (10 11
330
110
19
2.8
cm -2 ) a c
M. B. Bell et al., Astrophys. J. 483, L61 (1997).
b A. J. Alexander et al., J. Mol. Spectrosc. 62, 175 (1976).
P. Botschwina (1997), unpublished.
See also: P. Botschwina, in:
Jahrbuch der Akademie der Wissenschaften zu Göttingen 2002

Vibrationally excited molecules in “hot cores”: centres of star formation:
HC
3
N as a probe for highly excited gas rotational transitions within 11 different excited states observed
F. Wyrowski, P. Schilke and C. M. Walmsley,
Astron. Astrophys. 341, 882 (1999).

Characterisation of vibrationally excited states of HC
3
N
Spectroscopic CCSD(T) constant

1
/cm -1

2
/cm -1

3
/cm -1

4
/cm -1

5
/cm -1

6
/cm -1

7
/cm -1

1
/MHz

2
/MHz

3
/MHz

4
/MHz cc-pVQZ
3452
2316
2111
879
671
501
223
7.030
21.589
13.767
10.447
exp.
7.331
b
21.572
13.895
11.100
b
Spectroscopic constant

5
/MHz

6
/MHz

7
/MHz q
5
/MHz q
6
/MHz q
7
/MHz q J
5 q J
6 q J
7
/Hz
/Hz
/Hz
D J e
/kHz
CCSD(T) cc-pVQZ
-1.714
-9.233
-14.389
2.419
3.498
6.394
-1.052
-1.770
-15.516
0.506
exp.
-1.563
b
-9.256
-14.455
2.538
3.582
6.538
-1.331
-2.063
-16.291
0.544
a a Ground-state value b Deperturbed values from approximate deperturbation procedures

5
-1
K. M. T. Yamada, C. Degli Esposti,
P. Botschwina, P. Förster, L. Bizzocchi,
S. Thorwirth, and G. Winnewisser
Astron. Astrophys. 425 (2004) 767.

Calculated a and experimental spectroscopic constants for low-lying singly excited bending vibrational states of HC
5
N v theor.
11
= 1 exp.
v
10
= 1 theor.
exp.
theor.
v
9
= 1 exp.

(cm -1 ) 106.8
254.0
462.9

(MHz) -2.705
-2.786
-2.453
-2.452
-1.594
-1.593
q t
(MHz) 1.125
1.163
0.490
0.500
0.320
0.329
q t
J (Hz) -0.993
-1.063
-0.176
-0.173
-0.032
-0.039
a CCSD(T)/cc-pVQZ. Standard 2nd order perturbation theory in normal coordinate space is employed in the calculation of

, q t and q t
J values.

J. Cernicharo, A. M. Heras, J. R. Pardo, A. G. G. M. Tielens, M. Guélin, E. Dartois, R. Neri and L. B. F. M. Waters,
Astrophys. J. 546 (2001) L127.

• HC
3
N is so far the only interstellar molecule for which two more isomers (HCCNC and HNC
3
) could be detected in the ISM
• For one isomer of each HC
5
N and HC
7
N, namely HC
4
NC and HC
6
NC, precise data suitable for radioastronomy are available through FT-MW spectroscopy carried out at
Harvard.

Interstellar isomers of cyanoacetylene, detected in TMC-1
• Linear HCCNC
K. Kawaguchi, M. Ohishi, S.-I. Ishikawa and N. Kaifu,
Astrophys. J. 386, L51 (1992).
• quasilinear HNC
3
K. Kawaguchi, S. Takano, M. Ohishi, S.-I. Ishikawa, K. Miyazawa,
N. Kaifu, K. Yamashita, S. Yamamoto, S. Saito, Y. Ohshima and Y. Endo,
Astrophys. J. 396, L49 (1992).
High-energy isomer HCNCC observed through matrix-isolation
IR spectroscopy
Z. Guennoun, I. CouturierTamburelli, N. Piétri and J. P. Aycard,
Chem. Phys. Lett. 368, 574 (2003).
R. Kolos and J. C. Dobrowolski, Chem. Phys. Lett. 369, 75 (2003).

P. Botschwina, Phys. Chem. Chem. Phys. 5 (2003) 3337.

4
6
P. Botschwina, Ä. Heyl, W. Chen, M. C. McCarthy, J.-U. Grabow,
M. J. Travers and P. Thaddeus, J. Chem. Phys., 109, 3108 (1998)
Fourier transform microwave spectroscopy in a supersonic jet

4
Isomerisation energy with respect to HC
5
N (0 K): 114 kJ mol -1
B e
(HC
4
NC): 1399.7 MHz from corrected equilibrium structure.

B
0
= B e
-B
0
 ½  i
 i d i
. → B
0
= 1401.20 MHz.
B
0
(exp.) = 1401.18227(7) MHz.

B
0 predictions for less abundant isotopomers of
HC
4
NC isotopomer B
0
(MHz) isotopomer B
0
(MHz)
DCCCCNC 1336.05
HCCC 13 CNC 1399.62
H 13 CCCCNC 1364.01
HCCCC 15 NC 1386.91
HC 13 CCCNC 1386.82
HCCCCN 13 C 1364.69
HCC 13 CCNC 1399.89
B
0 values for 13 C and 15 N substituted species are expected to have uncertainties of ca. 0.005 MHz;
B
0 value for DC
5
NC is probably less accurate.

2n+1
C
3
N: found in IRC+10216 already in 1977 [1], six years prior to its laboratory investigation by millimeter-wave spectroscopy [2].
[1] M.
Guélin and P. Thaddeus, Astrophys. J. 212
(1977) L81.
[2] C. A. Gottlieb et al., Astrophys. J. 275 (1983) 916.
Mixed experimental / theoretical work
M. C. McCarthy, C. A. Gottlieb, P. Thaddeus, M. Horn and P. Botschwina, J. Chem. Phys. 103 (1995) 7820.

M. C. McCarthy, G. W. Fuchs, J. Kucera, G. Winnewisser and P. Thaddeus, J. Chem. Phys. 118 (2003) 3549.

5
Theoretical predictions
F. Pauzat, Y. Ellinger and A. D. McLean,
Astrophys. J. 369, L13 (1991)
UHF-SCF calculations yield 2
 ground state with small dipole moment
P. Botschwina, Chem. Phys. Lett. 259, 627 (1996)
RCCSD(T) yields 2
 ground state with large dipole moment
Laboratory detection by FTMW
Y. Kasai, Y. Sumiyoshi, Y. Endo and K. Kawaguchi,
Astrophys. J. 477, L65 (1997) radical generated by discharge in a mixture of HC
5
N and HC
3
N diluted in Ar
Radioastronomical detection
M. Guélin, N. Neininger and J. Cernicharo, Astron. Astrophys. 335, L1 (1998)

(RCCSD(T) + corrections) upper lines: lower lines:
2
 states
2
 states
P. Botschwina, Phys. Chem. Chem. Phys. 5 (2003) 3337.

Calculated equilibrium excitation energies (in cm -1 ) for the 2
 states of radicals of type C
2n+1
N (n = 1-3) a
2
3 n
1
RHF
508
-324
-612
RCCSD RCCSD-T RCCSD(T)
2320 2285 2316
698
124
455
-357
491
-304 a Basis set: cc-pVQZ. Throughout, the calculations were carried out at the recommended equilibrium structures.

2n+1
a radical state RHF RCCSD RCCSD-T RCCSD(T)
C
3
N X 2

-3.255
-2.901
A 2

-0.551
0.046
C
5
N X 2

-3.865
-3.423
A 2

-0.532
0.335
C
7
N X 2

-0.431
0.660
A 2

-4.328
-3.809
-2.865
0.200
-3.409
0.567
0.958
-3.824
-2.867
0.200
-3.412
0.566
0.957
-3.826
a Basis set: aug-cc-pVTZ.

Although ion-molecule reactions are believed to play a central role in the synthesis of interstellar molecules, the number of unambiguously detected chemically different cations is still rather small, currently not exceeding 15.
Theoretical work at Kaiserslautern (until 1989) and
Göttingen (since 1990) provided various predictions for:
H
3
+ , HN
2
+ , HCO + /HOC + , HCS + , HCNH + ,
H
3
O + , H
2
COH + and HC
3
NH +

3
+
An ion playing a key role in the oxygen chemistry network

1986: tentative assignment of a line found in OMC-1 and Sgr B2 near
307.2 GHz to transition
P (2,1) (J, K = 1,1 – 2,1) of H
3
O +
A. Wotten et al., Astron. Astrophys. 166 (1986) L15.

1990: Confirming line at 364.8 GHz observed with Caltech Submillimetre
Observatory at Mauna Kea in the above two sources
A. Wotten et al., Astrophys. J. 380 (1991) L79.

1991: above two lines found in W3 IRS 5 cloud, together with new line at
396.3 GHz
T. G. Phillips et al., Astrophys. J. 399 (1992) 533

What has been measured?
H
3
O + has a pyramidal equilibrium structure with a low barrier height to inversion and consequently an unusually large inversion splitting.
Energy level diagram
T. G. Pillips et al., Astrophys. J. 399 (1992)533.

3
+
1983: 2-dimensional anharmonic variational treatment of

1 and

2 vibrations, using CEPA-1 potential surface
P. Botschwina, P. Rosmus and A. E. Reinsch,
Chem. Phys. Lett. 102 (1983) 299.
predicted 0 - 0 + splitting: 46 cm -1 best uncorrected ab initio value for quite some time transition dipole moment: 1.44 D

First far-infrared detection of H
3
O + in Sagittarius B2
J. R. Goicoechea and J. Cernicharo, Astrophys. J. 554 (2001) L213
Using the Infrared Space Observatory (ISO) Long-
Wavelength Spectrometer three lines arising from the
 ground-state inversion mode (0 +
2

0 ) at 55.3 cm -1 could be observed toward the Sagittarius B2 molecular cloud, near the Galactic center. All transitions were observed in absorption against the optically thick infrared continuum emission of the dust.
Again, the theoretical value for the (0 +

0 ) transition dipole moment published in 1984 by BRR was employed to arrive at column densities.

3
+
Following CEPA-1 calculations (Botschwina, 1987) and laser-spectroscopic studies of the

1 and

3 bands (Lee,
Amano, 1987; Kawaguchi et al., 1990) two lines of
HC
3
NH + (J = 5-4 and J = 4-3) were detected in TMC-1 with the Nobeyama 45 m radio telescope.
K. Kawaguchi et al., Astrophys. J. 420 (1994) L95.
Using the CEPA-1 dipole moment of Botschwina, the column density of HC
3
NH + was determined to be
1.0 (0.2) · 10 12 cm -2
In TMC-1, HC
3
NH + is thus 160 times less abundant than
HC
3
N and 2.6 times more abundant than HNCCC.

Another frequent structural motif within the series of known interstellar molecules is provided by cumulenic chains with one or two hetero end groups
( “heterocumulenes“)
Individual series and known examples with n ≥ 3:
C n
O: C
3
O
C n
S: C
3
S , (potentially C
5
S )
SiC n
: (SiC
3
), SiC
4
, ( potentially longer chains )
H
2
C n
: H
2
C
3
, H
2
C
4
, H
2
C
5
, H
2
C
6

3
1987: three strong lines at 23.123, 40.465 and 46.246 GHz detected with Nobeyama 45 m telescope in TMC-1 [1]; assigned to J = 4-3, 7-6 and 8-7 transitions after laboratory MW data became available [2].
[1]
[2]
N. Kaifu et al., Astrophys. J,317 (1987) L111.
Y. Yamamoto et al., Astrophys. J. 317 (1987) L119.
Theoretical work at Göttingen
S. Seeger et al., J. Mol. Struct. 3003 (1994) 213.
P. Botschwina, Phys. Chem. Chem. Phys. 5 (2003) 3337.

1.2810
1.2927
1.5374
exp./theor.
Spectroscopic constant

1

4
(MHz)
(MHz) q
4
(MHz) q J
4

5
(Hz)
(MHz) q
5
(MHz) q
5
J (Hz) exp.
14.83
-5.65
1.51
-0.48
-12.36
3.96
-11.4
CCSD(T)/ cc-pVQZ
14.41
-5.40
1.48
-0.57
-11.89
3.82
-10.5
For details see:
P. Botschwina, Phys. Chem. Chem. Phys. 5 (2003) 337.

5
MW spectra in 5-20 GHz region
Y. Kasai et al., Astrophys. J. 410 (1993) L45.
Tentative assignment of J = 13-12 transition in IRC+ 10216
(probably wrong)
M. B. Bell et al., Astrophys. J. 417 (1993) L37.
1.2838
1.2926
1.2662
1.2815
1.5448
 e
= 5.32 D
CCSD(T)/cc-pVQZ + corrections (taken over from C
3
S)
P. Botschwina, Phys. Chem. Chem. Phys. 5 (2003) 3337.

n n: even n: odd closed-shell singlet ground-states (X 1 Σ + ) triplet ground-states (X 3 Σ )
SiC
2 and SiC
3 detected in the ISM in their ring forms
Linear SiC
4 detected in IRC+10216
M. Ohishi et al., Astrophys. J. 345 (1989) L83
Joint experimental/theoretical work (Harvard/Göttingen) on SiC
4 and SiC
6
:
V. D. Gordon et al., J. Chem. Phys. 113 (2000) 5311
1.6928
1.2726
1.2986
1.2809
SiC
4 and SiC
6 are rather normal semi-rigid linear molecules

Calculated equilibrium dipole moments (in D) for SiC
4 and SiC
6 a
Method SiC
4
SiC
6 basis A b basis B c basis A basis B
-7.035
-7.042
-9.439
-9.500
SCF
MP2
CCSD
-6.713
-6.734
-8.475
-8.533
-7.004
-7.023
-9.312
-9.381
CCSD-T -6.408
-6.427
-8.196
-8.248
CCSD(T) -6.401
-6.421
-8.195
-8.249
a Evaluated at the recommended equilibrium structures from this work. c
The positive end of the dipole is located at the silicon site.
b aug-cc-pVTZ basis.
aug-cc-pVQZ basis exclusive of g functions.
V. D. Gordon, E. S. Nathan, A. J. Apponi, M. C. McCarthy, P. Thaddeus and P. Botschwina,
J. Chem. Phys. 113 (2000) 5311.

Recommended equilibrium structures
P. Botschwina, Mol. Phys. 103 (2005) 1441.

Calculated equilibrium electric dipole moments
(
 e
, in D) for cyanopolyynes HC
2n+
1N a n SCF MP2 CCSD CCSD-T CCSD(T)
1 -4.178 -3.782 -3.821
-3.781
-3.783
2 -4.882 -4.409 -4.427
-4.409
-4.411
3 -5.431 -4.923 -4.892
-4.909
-4.910
4 -5.855 -5.343 -5.247
-5.305
-5.305
5 -6.181 -5.683 -5.518
-5.618
-5.618
a Basis set: aug-cc-pVTZ. Sign of dipole moment corresponds to polarity + HC
2n+1
N .
Throughout, the calculations were carried out at the recommended equilibrium structure.

n
PETER BOTSCHWINA
Institut für Physikalische Chemie
Universität Göttingen, Tammannstraße 6
D37077 Göttingen, Germany

C
3
 astronomically well-known through its electronic transition at 4051.6
Å, discovered in comet spectra, carbon-rich planetary nebulae and diffuse interstellar clouds towards various reddened stars
 
3
(antisymmetric stretch) and

2
(bend) observed in mid and far IR, respectively
C
5 observed in the circumstellar envelope of
IRC+10216 (Bernath et al., Science 244, 562
(1989)) through its

3 band (antisymmetric stretching vibration with highest wavenumber)

4
tentative assignment to

5 band of astronomical feature at 57.5
 m (174 cm -1 ) found in five different source (Sgr B2, IRC+10216, CRL 618, CRL 2688 and NGC 7027)
6
5
admittedly rather speculative assignments of a molecular band found in the young planetary nebula NGC 7027 at ca. 98
 m (102 cm -1 ) to bending vibrational transitions (

9 and/or

7 of
C
6 and C
5
, respectively)

J. R. Goicoechea, J. Cernicharo, H. Masso and M. L. Senent, Astrophys. J., 609 (2004) 225.

1) Linear carbon chains of type C
2n+1
(odd number of carbon atoms) have a ··· 
4 electronic configuration and an electronic ground-state of
1   g symmetry.
2) Linear carbon chains of type C
2n
(even number of carbon atoms) have a ··· 
2 electronic configuration and an electronic ground-state of
3   g symmetry.

C
3
M. Mladenovi ć, S. Schmatz and P. Botschwina, 101 (1994) 5891.

5
 g
 u
 g
 u

CCSD(T)/cc-pVQZ spectroscopic constants for C
5 and 13 C
5 a
C
5

3

5

3

7
/cm
/cm
-1
-1
2221.4 (2214.6)
112.5
/MHz 12.779 (12.59)
/MHz -9.939 (-10.24)

7
/MHz -9.383 (-9.30)

B
0
-8.143
q
5
/MHz 2.134 (2.36) q
7
/MHz
D J e
/Hz
3.900 (3.99)
119 (161) b
13 C
5
2133.9
108.1
11.329 (11.07)
-8.811
-8.318 (-8.14)
-7.219
1.892
3.457 (3.49)
102 (138) b a Exp. values are given in parentheses. b Ground-state values.
For details and further references see:
P. Botschwina, Phys. Chem. Chem. Phys. 5 (2003) 337.

2n

1

2


'
2

1
'

Symmetry coordinates: S i
=
1
2
(
 i
  i
' )
For details see:
P. Botschwina, Chem. Phys. Lett. 421 (2006) 488.

Parameters (in a.u.) of near-equilibrium cis-bending potential energy functions for linear C
4 a
V – V e
=
3  i

1
C (
1
2 i ) S
1
( 2 i )
PEF term
S
1
2
S
1
4
S
1
6
RHF-SCF cc-pVQZ
0.028077
0.001077
0.000308
RCCSD(T) cc-pVQZ
0.020467
0.002126
0.000521
a Throughout, the RCCSD(T)/cc-pVQZ equilibrium structure is used as expansion point: R
1e
= 1.3135 Å and R
2e
= 1.2936 Å.

First derivative of electric dipole moment with respect to the cis-bending symmetry coordinate (in a.u.) for linear C
4 a basis b spd (avtz) sp (avtz) + df (vtz) avtz spdf (avqz) avqz
RHF
-0.641
-0.622
-0.638
-0.639
-0.640
RCCSD(T)
-0.912
-0.905
-0.918
-0.917
-0.918
a All calculations are carried out around the recommended equilibrium structure:
R
1e
(outer) = 1.3098 Å and R
2e
(inner) = 1.2899 Å.
b An obvious shorthand notation is employed to designate the basis sets.

cis-bending potentials

n Harmonic wavenumbers (in cm -1 ) and IR intensities (in km mol -1 ) of cis-bending vibrations for linear C
2n species a
2 171.1 (44.5) exp. (argon matrix): 172.4 cm -1
3 370.1 (6.2), 99.4 (28.3)
4 480.7 (0.1), 232.1 (17.1), 60.5 (18.6)
5 495.9 (1.0), 354.7 (2.1), 174.6 (20.2), 39.8 (13.3) a Calculated from (RCCSD(T)/vqz) quadratic force constants and RCCSD(T)/avtz dipole moment derivatives.
P. Botschwina, Chem. Phys. Lett. 421 (2006) 488.

Review wisdom
“C
7 possesses a filled
 u
HOMO, which makes this molecule a candidate for extremely large amplitude bending motion.
“
A. Van Orden and R. J. Saykally, Chem. Rev. 98 (1998) 2313.

Linear C
7
: floppy or not?
According to the interpretation of experimental data obtained by
Saykally and coworkers, linear C
7 was described as a highly flexible species with an “extremely large amplitude bending motion about the central carbon atom”.
(J. R. Heath and R .J. Saykally, J. Chem. Phys. 94, 1724 (1991)).

Comparison of CCSD(T)/cc-pVQZ potentials of C
7 and C
3 for bending about the central carbon atom (bond lengths and other angles kept fixed at their equilibrium values).

Spectroscopic constants for linear C
7 a

1
/cm -1

2
/cm -1

3
/cm -1

4
/cm -1

5
/cm -1

6
/cm -1

7
/cm -1

8
/cm -1

9
/cm -1

10
/cm -1

11
/cm -1
2169.5
1565.1
574.6
2203.8
1933.3
1088.6
493.7
156.5
528.6
237.5
70.0

1
/MHz

2
/MHz

3
/MHz

4
/MHz

5
/MHz

6
/MHz

7
/MHz

8
/MHz

9
/MHz

10
/MHz

11
/MHz
2.588
1.602
0.454
3.411
2.018
1.098
-1.012
-1.816
-1.067
-1.930
-1.952
q e
7
/MHz q e
8
/MHz q e
9
/MHz q e
10
/MHz q e
11
/MHz q J
7
/Hz q J
8
/Hz q J
9
/Hz q J
10
/Hz q J
11
/Hz
D J e
/Hz a CCSD(T)/cc-pVQZ. Vibrations 1-3 are totally symmetric (
 g symmetry species
 u
, 7-8 to
 g and 9-11 to
 u symmetry.
), 4-6 belong to
0.140
0.368
0.131
0.248
0.804
-0.01
-0.15
-0.01
-0.07
-0.76
10.1


Linear C
7 is a fairly normal semirigid linear molecule with no evidence of floppiness.

Excitation of the

11 bending vibration changes the rotational constant by only 0.2 % (Heath and Saykally: 9.3 %!!)

No unusually large negative value for centrifugal distortion constant.

Spectroscopic constants of linear C
7
: comparison of theory and experiment



 q q
4
5
8
11
8
11
/MHz
/MHz
/MHz
/MHz
/MHz
/MHz
CCSD(T)/cc-pVQZ
3.411
2.018
-1.816
-1.952
0.368
0.804
exp.
a
3.47 (32) b
1.71 (87) c
-1.56 (26) b
-1.67 (32) b
0.618 (213) b
1.15 (35) b a b
Standard derivations in terms of the last digit in parentheses.
Neubauer-Guenther et al., unpublished (2006).
c J. R. Heath, A. Van Orden, E. Kuo and R. J. Saykally, Chem. Phys. Lett., 182, 17 (1991).

CCSD(T)/cc-pVQZ bending potential curves for C
2n+1 chains
P. Botschwina and R. Oswald, Chem. Phys. 325 (2006) 485.

P. Botschwina, Theor. Chem. Acc. 114 (2005) 350.

Equilibrium bond lengths (CCSD(T)/cc-pVQZ + corrections

CCSD(T) harmonic wavenumbers and IR intensities for
 u vibrations of linear C
15
No.

(cm -1 ) A (km mol -1 )
21 506 12.6
22
23
24
483
453
281
4.2
0.0
1.9
25
26
27
188
88
17
11.3
12.5
7.6

• High-level ab initio calculations, mostly by CCSD(T) with cc-pVQZ basis set, yield rather accurate values for various spectroscopic properties of (potential) interstellar molecules
• important quantities for astronomers: rotational constants and centrifugal distortion constants electric dipole moments
(ro)vibrational frequencies vibration-rotation coupling constants l-type doubling constants

• Present and former Coworkers at Göttingen
Drs. J. Flügge, Ä. Heyl, M. Horn, M. Mladenović, M. Oswald,
R. Oswald, S. Schmatz and S. Seeger
• International coworkers (selection)
Profs. C. Degli Esposti, T. Hirano, P. Thaddeus and
G. Winnewisser
Drs. L. Bizzocchi, M. C. McCarthy and K. T. M. Yamada
• Profs. H.-J. Werner (Stuttgart) and P. J. Knowles (Cardiff) for various versions of MOLPRO
• Financial support through DFG and Fonds der Chemischen
Industrie

Ongoing theoretical work on antisymmetric stretching vibrations of C
2n+1 chains
Absolute IR intensities for strongest stretching vibrations of linear C
2n+1
In parentheses: wavenumber in cm -1 /A max
.
chains.
P. Botschwina, J. Mol. Struct. 795 (2006) 230.