An improved study of cyanoacetylene and diacetylene including the strong <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si53.svg" display="inline" id="d1e5780"> <mml:mrow> <mml:mn>2</mml:mn> <mml:msub> <mml:mrow> <mml:mi>ν</mml:mi>

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.