Determination of accurate rest frequencies and hyperfine structure parameters of cyanobutadiyne, HC5N
Graphical abstract
Introduction
Cyanopolyynes H(CC)nCN occur abundantly in space, in particular the shorter members. Molecules up to cyanooctatetrayne, HC9N (), were detected [1]. The next longer member, HC11N, has not yet been found in space [2]. Isotopic species with D or with one C were detected up to HC7N [3], N isotopologs up to HC5N [4], even all three isotopomers of HC3N with two C were observed astronomically [5]. Measurements of the HC3N species and the isotopomers with one C were frequently used to determine C/C ratios in various objects, such as the protoplanetary nebula CRL618 [6], [7] or in starless cores, where differences in the C/C 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 () 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 N 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 C or N, 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 is split by spin coupling caused by the N nucleus () into three HFS components. The rotational and spin angular momenta are coupled sequentially: J + I(N) = 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 N 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 N 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 N 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 N 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 N 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
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Present address: Laborastrophysik, Universität Kassel, 34132 Kassel, Germany