An improved study of cyanoacetylene and diacetylene including the strong 2ν5 band of HC3N and ν4 band of HC4H

This paper presents an extension of the previous spectroscopic investigations for the linear molecules HC 3 N and HC 4 H. For both species, new high-resolution data have been recorded and analyzed. As far as HC 3 N is concerned, the investigation of the ro-vibrational features has been based on the Fourier transform far-infrared spectra from our previous studies and on the mid-infrared spectra targeting the prominent 2 ν 5 overtone band recently recorded at the synchrotron facility SOLEIL with a resolution of 0.003 and 0.002 cm −1 . Moreover, several hundreds of pure rotational transitions belonging to excited vibrational states were recorded in Bologna. The resonance network wrapping around the v 5 = 2 level and involving the ( v 4 = 1 , v 7 = 2 ), ( v 6 = 2 , v 7 = 2 ), ( v 5 = 1 , v 7 = 3 ), and v 7 = 6 excited states has been thoroughly analyzed. A global ro-vibrational fit was then achieved for all the vibrational levels up to 1300 cm −1 , therefore improving our previous analysis. For diacetylene, the ν 4 stretching band around 3333 cm −1 was recorded by high-resolution FTIR spectroscopy at the synchrotron facility SOLEIL at a resolution of 0.005 cm −1 . The accidental resonance affecting this band has been successfully analyzed and the number of observed and assigned ro-vibrational transitions was considerably extended. The present investigation allowed the integration of the existing line lists in the HITRAN database for both molecules. In particular, no information about the ν 4 band of diacetylene was present in HITRAN before this study. • 2 ν 5 overtone band of HC 3 N studied at high-resolution. • Anharmonic resonances fully accounted for the 3rd polyad system of HC 3 N around 1300 cm − 1 . • 11 newly observed ro-vibrational bands for HC 3 N. • Full treatment of the HC 4 H ν 4 band with proper modelling of perturbations. • Improvement of HC 3 N and HC 4 H entries to the HITRAN2024 edition.