Abstract The Raman spectrum of liquid dicyanoacetylene, NC C C CN, was reinvestigated and ν 3 found at 617 cm− 1. The Fermi resonance between ν 1 and 2ν 5 is explained, considering the isotopic species NC 13 C C CN. The high resolution infrared spectrum of the lowest frequency fundamental ν 9 has been recorded for the first time, and the rotational constants for several states with v 9⩽ 9 have been determined.
