The cyanobutadiynylide ion has been detected toward the carbon rich star IRC +10216 in 12 rotational transitions from J = 8 – 7 to 40 – 39. Most of these occur in the 3 mm region and have been detected with the IRAM 30 m telescope; one additional transition was detected with the Effelsberg 100 m telescope at 13.5 mm:
J. Cernicharo, M. Guélin, M. Agúndez, M. C. McCarthy, and P. Thaddeus,
Detection of C₅N^– and Vibrationally Excited C₆H in IRC +10216
Astrophys. J. 688, (2008), L83–L86.
No laboratory data are available for this molecule at present. The assignments were based on high level ab initio calculations by
P. Botschwina and R. Oswald,
Carbon Chains of Type C_2//n//+1N^– (//n// = 2 – 6): A Theoretical Study of Potential Interstellar Anions
J. Chem. Phys. 129, Art. No. 044305 (2008).
Its abundance in that source in combination with those of related molecules leave little doubt about the assignments.
An additional line, U33332.3, reported by
K. Kawaguchi, Y. Kasai, S.-I. Ishikawa, and N. Kaifu,
A Spectral-Line Survey Observation of IRC +10216 between 28 and 50 GHz
Publ. Astron. Soc. Japan 47, 853–876 (1995),
is, at least in part, due to C₅N^–.
This anion is isolelectronic to C₆H^– which was the first molecular anion to be detected in space. In addition, it is a sibling of the recently detected C₃N^–.

Meanwhile,
J. Cernicharo, N. Marcelino, J. R. Pardo, M. Agúndez, B. Tercero, P. de Vicente, C. Cabezas, and C. Bermúdez
reported on
Interstellar nitrile anions: Detection of C₃N^– and C₅N^– in TMC-1
Astron. Astrophys. 641, Art. No. L9 (2020).

Contributor(s): H. S. P. Müller; 10, 2008; 09, 2020