Rotational rest frequencies of the low lying vibrational states of n-propyl cyanide from extensive laboratory measurements up to 506 GHz^⋆

¹ IRAP, Université de Toulouse, CNRS, CNES, UPS, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
e-mail: delong.liu@irap.omp.eu
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: hspm@ph1.uni-koeln.de

Received: 12 September 2018
Accepted: 9 December 2018

Abstract

Context. The spectra of four low-lying vibrational states of both anti and gauche conformers of normal-propyl cyanide were previously measured and analyzed in two spectral windows between 36 and 127 GHz. All states were then identified in a spectral line survey called Exploring Molecular Complexity with ALMA (EMoCA) toward Sagittarius B2(N) between 84.1 and 114.4 GHz with the Atacama Large Millimeter/submillimeter Array (ALMA) in its Cycles 0 and 1.

Aims. We wanted to extend the measurements and analysis up to 506 GHz to provide accurate predictions over a much wider range of frequencies, quantum numbers and energies.

Methods. We carried out measurements in two additional frequency windows up to 506 GHz.

Results. For the gauche conformer, a large number of both a- and b-type transitions were identified. For the anti conformer, transitions were predominantly, but not exclusively, a-type. We hence improved molecular parameters for the ground states of both anti- and gauche-n-propyl cyanide and for excited vibrational states of the gauche conformer (v₃₀ = 1, v₂₉ = 1, v₃₀ = 2, v₂₈ = 1) and anti conformer (v₃₀ = 1, v₁₈ = 1, v₃₀ = 2, v₂₉ = 1) with high order coupling parameters determined between v₁₈ = 1 and v₃₀ = 2. Parameters are published for the first time for v₁₈ = v₃₀ = 1 of the anti conformer and for v₂₉ = v₃₀ = 1 of the gauche conformer.

Conclusions. In total 15385 lines have been incorporated in the fits and should allow good predictions for unperturbed lines over the whole operating range of radio-telescopes. Evidence is found for vibrational coupling for some levels above 380 GHz. The coupling between v₁₈ = 1 and v₃₀ = 2 of the anti conformer has been well characterized. An additional list of 740 lines showing potential but as yet unidentified coupling has been provided for astrophysical identification.