Determination of accurate rest frequencies and hyperfine structure parameters of cyanobutadiyne, HC5N

https://doi.org/10.1016/j.jms.2020.111303Get rights and content

Highlights

  • Highly accurate microwave frequencies obtained for HC5N.

  • Improved spectroscopic parameters to facilitate radio astronomical observations.

  • First determination of 14N spin-rotation parameter and improved quadrupole value.

  • Some improvement achieved for HC7N.

  • Quantum chemical calculations on hyperfine parameters of HCN to HC7N.

Abstract

Very accurate transition frequencies of HC5N were determined between 5.3 and 21.4 GHz with a Fourier transform microwave spectrometer. The molecules were generated by passing a mixture of HC3N and C2H2 highly diluted in neon through a discharge valve followed by supersonic expansion into the Fabry-Perot cavity of the spectrometer. The accuracies of the data permitted us to improve the experimental 14N nuclear quadrupole coupling parameter considerably and the first experimental determination of the 14N nuclear spin-rotation parameter. The transition frequencies are also well suited to determine in astronomical observations the local speed of rest velocities in molecular clouds with high fidelity. The same setup was used to study HC7N, albeit with modest improvement of the experimental 14N nuclear quadrupole coupling parameter. Quantum chemical calculations were carried out to determine 14N nuclear quadrupole and spin-rotation coupling parameters of HC5N, HC7N, and related molecules. These calculations included evaluation of vibrational and relativistic corrections to the non-relativistic equilibrium quadrupole coupling parameters; their considerations improved the agreement between calculated and experimental values substantially.

Introduction

Cyanopolyynes H(CC)nCN occur abundantly in space, in particular the shorter members. Molecules up to cyanooctatetrayne, HC9N (n=4), were detected [1]. The next longer member, HC11N, has not yet been found in space [2]. Isotopic species with D or with one 13C were detected up to HC7N [3], 15N isotopologs up to HC5N [4], even all three isotopomers of HC3N with two 13C were observed astronomically [5]. Measurements of the HC3N species and the isotopomers with one 13C were frequently used to determine 12C/13C ratios in various objects, such as the protoplanetary nebula CRL618 [6], [7] or in starless cores, where differences in the 12C/13C ratios were found [8].

Cyanobutadiyne, HC5N, also known as cyanodiacetylene or pentadiynenitrile, was detected as early as 1976 toward the high-mass star-forming region Sagittarius B2 close to the Galactic center [9]. It was found soon thereafter in the dark and dense core Heile’s Cloud 2 [10], nowadays better known as Taurus Molecular Cloud 1 or short as TMC-1 [11]. Cyanohexatriyne, HC7N, also known as cyanotriacetylene or heptatriynenitrile, was discovered in that source [12]. Both molecules were also found early in the circumstellar envelope of the famous carbon rich asymptotic giant branch star CW Leonis, also referred to as IRC + 10216 [13].

Molecules up to HC17N (n=8) were investigated by rotational spectroscopy [14]. Alexander et al. were the first to investigate the rotational spectrum of HC5N [15]. They assigned ground state rotational spectra of eight isotopic species in the microwave (MW) region from which they determined structural parameters. They also determined the 14N nuclear quadrupole coupling parameter eQq(N) and the dipole moment of the main isotopic species. Winnewisser et al. improved eQq(N) [16] and expanded the assignments into the millimeter wave (mmW) region [17]. Bizzocchi et al. recorded the spectra of HC5N and DC5N in the mmW and sub-mmW regions [18]. Assignments for these two isotopologs were extended to 460 GHz. They also analyzed spectra of singly substituted isotopic variants of both isotopologs containing one 13C or 15N, and derived from the rotational parameters a semi-experimental equilibrium structure.

Kirby et al. analyzed the ground state rotational spectrum of HC7N in the MW region [19]. McCarthy et al. subjected the main isotopic species as well as all singly substituted ones to a Fourier transform (FT) MW spectroscopic study and determined eQq(N) for all of them [20]. Similar studies were carried out for HC9N and HC11N, and ground state effective structural parameters determined for all three molecules. Soon thereafter, Bizzocchi et al. extended the assignments of the main isotopic species in its ground and several low-lying vibrational states into the mmW region [21].

The aim of the present work is twofold. The first target was the improvement of the hyperfine structure (HFS) parameters of HC5N and potentially of HC7N, and secondly, we wanted to investigate how accurately HFS parameters can be evaluated by high-level quantum-chemical calculations, and in particular trends among related molecules. An earlier study of isotopic species associated with DC3N showed that very good agreement can be achieved for the nuclear quadrupole parameters in high-accuracy coupled-cluster calculations by employing large basis sets together with vibrational corrections [22].

Section snippets

Experimental details

Spectra between 5 and 22 GHz were recorded at the Leibniz Universität in Hannover employing a supersonic-jet Fourier transform microwave (FTMW) spectrometer [23] in the coaxially oriented beam-resonator arrangement (COBRA) [24] which combines a very sensitive setup with an electric discharge nozzle [25]. Cyanobutadiyne was generated by passing a mixture of equal amounts of 1% HC3N in neon and 1% C2H2 in neon at a pressure of 100 kPa through the discharge nozzle and expanding the products into

Observed spectra and determination of spectroscopic parameters

Prediction of the microwave spectra of HC5N and HC7N were very reliable based on earlier data [16], [18], [20], [21]. Pickett’s SPCAT and SPFIT programs [28] were used for prediction and fitting of the spectra, respectively. Each rotational level of HC5N and HC7N with J>0 is split by spin coupling caused by the 14N nucleus (I=1) into three HFS components. The rotational and spin angular momenta are coupled sequentially: J + I(14N) = F. At higher values of J, the strong HFS components are the

Application in astronomical observations

Emission lines of HC5N may be prominent in dense cold molecular clouds such as TMC-1 [30]. Therefore, the very accurate rest frequencies from this and previous studies may be used to determine the local speed of rest in such a cloud with great accuracy, even more so as transitions occur with a spacing of 2660 MHz and because the 14N HFS splitting is usually resolved at low values of J. Table 4 lists some molecules which are often abundant in dense molecular clouds and have an at least

Quantum chemical calculations

Calculations for the equilibrium structure as well as the 14N nuclear quadrupole and spin-rotation coupling parameters were performed at the coupled-cluster (CC) level [47] using the coupled-cluster singles and doubles (CCSD) approach augmented by a perturbative treatment of triple excitations (CCSD(T)) [48], [49], [50], [51] together with correlation consistent core-polarized valence (cc-pCVXZ, X  = T, Q, 5, 6) [52], [53], [54] basis sets. In the calculations of the spin-rotation tensors,

Discussion of hyperfine parameters

The nuclear quadrupole coupling parameters are usually interpreted in terms of bonding of the respective atom [68], [69]. It is not surprising that the 14N value of HCN is considerably different from that of HC3N (and DC3N), as shown in Table 5. Unsurprisingly, the difference is small between HC3N and HC5N, and very close to zero between HC5N and HC7N. The calculated non-relativistic equilibrium values differ slightly from the experimental ground state values. The calculated vibrational

Conclusions

Accurate transition frequencies of HC5N and HC7N were determined employing Fourier transform microwave spectroscopy. These data led to improvements of the spectroscopic parameters. In particular, we improved the accuracy of the 14N nuclear quadrupole coupling parameter of HC5N considerably and that of HC7N slightly. In addition, we determined for the first time an experimental value of the nuclear 14N nuclear spin-rotation parameter of HC5N. Our quantum chemical calculations were able to

CRediT authorship contribution statement

Thomas F. Giesen: Investigation, Methodology, Writing - review & editing. Michael E. Harding: Investigation, Methodology, Writing - original draft, Writing - review & editing. Jürgen Gauss: Resources, Writing - review & editing. Jens-Uwe Grabow: Resources, Writing - review & editing. Holger S.P. Müller: Investigation, Methodology, Formal analysis, Validation, Data curation, Writing - original draft, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are grateful to Peter Förster for initial measurements on HC5N, to Holger Spahn for participation during the final experiments, and to Prof. Axel Klein and his group for preparing the HC3N sample used in the present investigations. We thank the Laboratoire Européen Associé de Spectroscopie Moléculaire ’LEA-HiRes’ for financial support. Additional funding was allocated by the Deutsche Forschungsgemeinschaft (DFG), in Cologne also within the Sonderforschungsbereich (SFB) 494. Further support

References (72)

  • E.A. Alekseev et al.

    The high-precision millimeter-wave spectrum of 32SO2, 32SO2 (ν2), and 34SO2

    J. Mol. Spectrosc.

    (1996)
  • H.S.P. Müller et al.

    Accurate rotational spectroscopy of sulfur dioxide, SO2, in its ground vibrational and first excited bending states, v2=0, 1, up to 2 THz

    J. Mol. Spectrosc.

    (2005)
  • K. Raghavachari et al.

    A fifth-order perturbation comparison of electron correlation theories

    Chem. Phys. Lett.

    (1989)
  • J.D. Watts et al.

    Open-shell analytical energy gradients for triple excitation many-body, coupled-cluster methods: MBPT(4), CCSD+T(CCSD), CCSD(T), and QCISD(T)

    Chem. Phys. Lett.

    (1992)
  • A.K. Wilson et al.

    Gaussian basis sets for use in correlated molecular calculations. VI. Sextuple zeta correlation consistent basis sets for boron through neon

    J. Mol. Struct.: Theochem.

    (1996)
  • M. Tokman et al.

    The nuclear quadrupole moment of 14N obtained from finite-element MCHF calculationson N2+ (2p; 2P3/2) and N+ (2p2; 3P2 and 2p2; 1D2)

    Chem. Phys. Lett.

    (1997)
  • S.E. Novick

    Extended Townes-Dailey analysis of the nuclear quadrupole coupling tensor

    J. Mol. Spectrosc.

    (2011)
  • N.W. Broten et al.

    The detection of HC9N in interstellar space

    Astrophys. J. Lett.

    (1978)
  • R.A. Loomis et al.

    Non-detection of HC11N towards TMC-1: constraining the chemistry of large carbon-chain molecules

    Mon. Not. R. Astron. Soc.

    (2016)
  • A.M. Burkhardt et al.

    Detection of HC5N and HC7N isotopologues in TMC-1 with the Green Bank Telescope

    Mon. Not. R. Astron. Soc.

    (2018)
  • K. Taniguchi et al.

    First detection of HC515N in the interstellar medium

    Publ. Astron. Soc. Jpn.

    (2017)
  • D.R. Schmidt et al.

    Exotic carbon chemistry in a planetary nebula: The unusual case of K4–47

    Astrophys. J. Lett.

    (2019)
  • F. Wyrowski et al.

    Physical conditions in the proto-planetary nebula CRL 618 derived from observations of vibrationally excited HC3N

    Astrophys. J.

    (2003)
  • J.R. Pardo et al.

    Molecular abundances in CRL 618

    Astrophys. J.

    (2007)
  • K. Taniguchi et al.

    13C isotopic fractionation of HC3N in two starless cores: L1521B and L134N (L183)

    Astrophys. J.

    (2017)
  • L.W. Avery et al.

    Detection of the heavy interstellar molecule cyanodiacetylene

    Astrophys. J. Lett.

    (1976)
  • L.T. Little et al.

    Detection of the J = 9–8 transition of interstellar cyanodiacetylene

    Mon. Not. R. Astron. Soc.

    (1977)
  • E. Churchwell et al.

    Molecular observations of a possible proto-solar nebula in a dark cloud in Taurus

    Astron. Astrophys.

    (1978)
  • H.W. Kroto et al.

    The detection of cyanohexatriyne, H(C≡C)3CN, in Heile’s Cloud 2

    Astrophys. J. Lett.

    (1978)
  • G. Winnewisser et al.

    The detection of HC5N and HC7N in IRC +10216

    Astron. Astrophys.

    (1978)
  • M.C. McCarthy et al.

    Laboratory detection of the carbon chains HC15N and HC17N

    Astrophys. J. Lett.

    (1998)
  • G. Winnewisser et al.

    Formation of cyanopolyynes in gas discharges

    Z. Naturforsch. A

    (1978)
  • G. Winnewisser et al.

    The millimeter wave spectrum and discharge chemistry of HC5N

    Astron. Astrophys.

    (1982)
  • L. Bizzocchi et al.

    Laboratory transition frequencies for millimeter-wave lines of vibrationally excited HC7N

    Astrophys. J.

    (2004)
  • T.J. Balle et al.

    Fabry-Perot cavity pulsed Fourier transform microwave spectrometer with a pulsed nozzle particle source

    Rev. Sci. Instrum.

    (1981)
  • J.U. Grabow et al.

    A pulsed molecular beam microwave Fourier transform spectrometer with parallel molecular beam and resonator axes

    Z. Naturforsch. A

    (1990)
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    Present address: Laborastrophysik, Universität Kassel, 34132 Kassel, Germany

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