Detection of the Linear Radical HC 4 N in IRC +10216

We report the detection of the linear radical HC 4 N in the C-rich envelope of IRC +10216. After HCCN, HC 4 N is the second member of the allenic chain family HC 2 n N observed in space. The column density of HC 4 N is found to be 1.5 × 10 12 cm -2 . The abundance ratio HC 2 N/HC 4 N is 9, a factor of 2 larger than the decrement observed for the cyanopolyynes (HC 2 n +1 N/HC 2 n +3 N). Linear HC 4 N has a 3 Σ electronic ground state and is one of the three low-energy isomeric forms of this molecule. We have searched for the bent and ringed HC 4 N isomers but could only derive an upper limit to their column densities: ≤3 × 10 12 cm -2 (at 3 σ).