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

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.