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Some interstellar molecules: ab initio theory, laboratory spectroscopy and (radio) astronomy

Some interstellar molecules: ab initio theory, laboratory spectroscopy and (radio) astronomy PETER BOTSCHWINA Institut für Physikalische Chemie Universität Göttingen, Tammannstraße 6 D-37077 Göttingen, Germany.

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Some interstellar molecules: ab initio theory, laboratory spectroscopy and (radio) astronomy

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  1. Some interstellar molecules: ab initio theory, laboratory spectroscopy and (radio) astronomy PETER BOTSCHWINA Institut für Physikalische Chemie Universität Göttingen, Tammannstraße 6 D-37077 Göttingen, Germany

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

  3. 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.

  4. Contents of lectures I. Overview of work on cyanopolyynes (HC2n+1N) and related species II. Interstellar cations III. Heterocumulenic chains IV. Pure carbon chains Cn

  5. CYANOPOLYYNES (HC2n+1N) • Almost ubiquituous in the ISM and CSM • Provide largest (in terms of number of atoms) interstellar molecule unambiguously detected by radio astronomy HC11N • Through the presence of low-lying bending vibrational states observable by radio astronomy in excited vibrational states  important information on dynamical processes

  6. 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 (HC3N), the experimental ground-state dipole moment is as large as 0 = 3.72 D

  7. Cyanopolyynes: demanding cases for accurate equilibrium structure determinations recommended method: combination of experimental and theoretical data exp.: B0 values for various (as many as possible) isotopomers theor.: B0 = Be- B0 calculated from high-quality ab initio cubic force fields (e.g., CCSD(T) with large basis set) (αi from 2nd order perturbation theory) i : vibration-rotation coupling constant di : degeneracy factor of vibrational mode i

  8. Equilibrium structure for HC3N [1] P. Botschwina, M. Horn, S. Seeger and J. Flügge, Mol. Phys. 78, 191 (1993). [2] P. Botschwina, Mol. Phys. 103, 1441 (2005).

  9. D12C515N J = 43  42 (*) Millimeter-wave spectroscopy of rare isotopomers of HC5N and DC5N: determination of a mixed experimental-theoretical equilibrium structure for cyanobutadiyne L. Bizzocchi, C. Degli Esposti and P. Botschwina J. Mol. Spectrosc. 225, 145 (2004)

  10. HC5N isotopomers: spectroscopic constants from MMW spectroscopy

  11. Equilibrium structures for HC5N * P. Botschwina, Ä. Heyl, M. Oswald and T. Hirano, Spectrochim. Acta A 53, 1079 (1997).

  12. Geometric structures for linear HC11N 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.

  13. HC11N: Variation of CC equilibrium bond lengths

  14. HC11N: a story of lost and found • 1982 and 1985: weak radio lines observed in IRC+10216 and TMC-1 attributed to HC11N (without accurate laboratory data at hand) • For more than 10 years no confirmation of assignments successful • 1996: FT-MW spectroscopy of HC11N 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.

  15. Dipole moments and column densities of cyanopolyynes (HC2n+1N) in TMC-1 a a M. B. Bell et al., Astrophys. J. 483, L61 (1997). b A. J. Alexander et al., J. Mol. Spectrosc. 62, 175 (1976). c P. Botschwina (1997), unpublished. See also: P. Botschwina, in: Jahrbuch der Akademie der Wissenschaften zu Göttingen 2002

  16. Vibrationally excited molecules in “hot cores”: centres of star formation:HC3N as a probe for highly excited gasrotational transitions within 11 different excited states observed F. Wyrowski, P. Schilke and C. M. Walmsley, Astron. Astrophys. 341, 882 (1999).

  17. Characterisation of vibrationally excited states of HC3N a Ground-state value b Deperturbed values from approximate deperturbation procedures

  18. Millimeter-wave spectroscopy of HC5N in vibrationally excited states below 500 cm-1 K. M. T. Yamada, C. Degli Esposti, P. Botschwina, P. Förster, L. Bizzocchi, S. Thorwirth, and G. Winnewisser Astron. Astrophys. 425 (2004) 767.

  19. Calculateda and experimental spectroscopic constants for low-lying singly excited bending vibrational states of HC5N a CCSD(T)/cc-pVQZ. Standard 2nd order perturbation theory in normal coordinate space is employed in the calculation of , qt and qtJ values.

  20. 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.

  21. Cyanopolyynes: what about isomers? • HC3N is so far the only interstellar molecule for which two more isomers (HCCNC and HNC3) could be detected in the ISM • For one isomer of each HC5N and HC7N, namely HC4NC and HC6NC, precise data suitable for radioastronomy are available through FT-MW spectroscopy carried out at Harvard.

  22. 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 HNC3 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. Couturier-Tamburelli, N. Piétri and J. P. Aycard, Chem. Phys. Lett. 368, 574 (2003). R. Kolos and J. C. Dobrowolski, Chem. Phys. Lett. 369, 75 (2003).

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

  24. HC4NC and HC6NC 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

  25. HC4NC Isomerisation energy with respect to HC5N (0 K): 114 kJ mol-1 Be (HC4NC): 1399.7 MHz from corrected equilibrium structure. B0 = Be-B0  ½ iidi. → B0 = 1401.20 MHz. B0 (exp.) = 1401.18227(7) MHz.

  26. B0 predictions for less abundant isotopomers of HC4NC B0 values for 13C and 15N substituted species are expected to have uncertainties of ca. 0.005 MHz; B0 value for DC5NC is probably less accurate.

  27. Radicals of type C2n+1N C3N: 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.

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

  29. 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 HC5N and HC3N diluted in Ar Radioastronomical detection M. Guélin, N. Neininger and J. Cernicharo, Astron. Astrophys. 335, L1 (1998)

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

  31. Calculated equilibrium excitation energies (in cm-1) for the 2 states of radicals of type C2n+1N (n = 1-3)a a Basis set: cc-pVQZ. Throughout, the calculations were carried out at the recommended equilibrium structures.

  32. Calculated equilibrium dipole moments (in D) for radicals of type C2n+1Na a Basis set: aug-cc-pVTZ.

  33. II. Interstellar cations 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: H3+, HN2+, HCO+/HOC+, HCS+, HCNH+, H3O+, H2COH+ and HC3NH+

  34. Interstellar H3O+ 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 H3O+ • 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

  35. What has been measured? H3O+ 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.

  36. H3O+: ab initio predictions 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

  37. First far-infrared detection of H3O+ 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 2 ground-state inversion mode (0+  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.

  38. HC3NH+ 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 HC3NH+ (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 HC3NH+ was determined to be 1.0 (0.2) · 1012 cm-2 In TMC-1, HC3NH+ is thus 160 times less abundant than HC3N and 2.6 times more abundant than HNCCC.

  39. Another frequent structural motif within the series of known interstellar molecules is provided by cumulenic chains with one or two hetero end groups (“heterocumulenes“) III.Heterocumulenic chains Individual series and known examples with n ≥ 3: CnO: C3O CnS: C3S, (potentially C5S) SiCn: (SiC3), SiC4, (potentially longer chains) H2Cn: H2C3, H2C4, H2C5, H2C6

  40. C3S 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] N. Kaifu et al., Astrophys. J,317 (1987) L111. [2] 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.

  41. For details see: P. Botschwina, Phys. Chem. Chem. Phys. 5 (2003) 337.

  42. C5S 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. e = 5.32 D CCSD(T)/cc-pVQZ + corrections (taken over from C3S) P. Botschwina, Phys. Chem. Chem. Phys. 5 (2003) 3337.

  43. Linear silicon carbides SiCn n: even closed-shell singlet ground-states (X 1Σ+) n: odd triplet ground-states (X 3Σ-) SiC2 and SiC3 detected in the ISM in their ring forms Linear SiC4 detected in IRC+10216 M. Ohishi et al., Astrophys. J. 345 (1989) L83 Joint experimental/theoretical work (Harvard/Göttingen) on SiC4 and SiC6: V. D. Gordon et al., J. Chem. Phys. 113 (2000) 5311 SiC4 and SiC6 are rather normal semi-rigid linear molecules

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