The millimeter-wave spectrum and astronomical search of succinonitrile and its vibrational excited states^⋆

¹ Instituto de Física Fundamental (IFF-CSIC), Group of Molecular Astrophysics, C/Serrano 121, 28006 Madrid, Spain
e-mail: carlos.cabezas@csic.es
² Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
³ Observatorio Astronómico Nacional (OAN-IGN), C/Alfonso XII 3, 28014 Madrid, Spain
⁴ Instituto de Estructura de la Materia (IEM-CSIC), Molecular Physics Department, C/Serrano 123, 28006 Madrid, Spain

Received: 16 May 2019
Accepted: 10 July 2019

Abstract

Context. Dinitriles with a saturated hydrocarbon skeleton and a −C≡N group at each end can have large electric dipole moments. Their formation can be related to highly reactive radicals such as CH₂CN, C₂N, or CN. Thus, these saturated dinitriles are potential candidates to be observed in the interstellar medium (ISM).

Aims. Our goal is the investigation of the rotational spectrum of one of the simplest dinitriles N≡C−CH₂−CH₂−C≡N, succinonitrile, whose actual rotational parameters are not precise enough to allow its detection in the ISM. In addition, the rotational spectra for its vibrational excited states will be analysed.

Methods. The rotational spectra of succinonitrile was measured in the frequency range 72−116.5 GHz using a new broadband millimeter-wave spectrometer based on radio astronomy receivers with Fast Fourier Transform backends. The identification of the vibrational excited states of succinonitrile was supported by high-level ab initio calculations on the harmonic and anharmonic force fields.

Results. A total of 459 rotational transitions with maximum values of J and K_a quantum numbers 70 and 14, respectively, were measured for the ground vibrational state of succinonitrile. The analysis allowed us to accurately determine the rotational, quartic, and sextic centrifugal distortion constants. Up to eleven vibrational excited states, resulting from the four lowest frequency vibrational modes ν₁₃, ν₁₂, ν₂₄, and ν₂₃ were identified. In addition to the four fundamental modes, we observed overtones together with some combination states. The rotational parameters for the ground state were employed to unsuccessfully search for succinonitrile in the cold and warm molecular clouds Orion KL, Sgr B2(N), B1-b, and TMC-1, using the spectral surveys captured by IRAM 30 m at 3 mm and the Yebes 40 m at 1.3 cm and 7 mm.