Predicted (in red) and observed (in blue) rotational spectrum of NC–CH=C=O between 236.9 and 241.3 GHz. The low-frequency region of the spectrum shown here is dominated by a series of R-type transitions, J = 44–43, while the high-frequency region is dominated by a series of Q-type transitions, K_a = 5–4. The slight inconsistency between predicted and observed spectrum intensities is due to source power and detector sensitivity variations.