K. Kawaguchi, M. Ohishi, S.-I. Ishikawa, and N. Kaifu
reported on the
Detection of Isocyanoacetylene HCCNC in TMC-1
Astrophys. J. 386, L51 (1992).
The Nobeyama 45 m telescope was used to detect the J = 4 – 3, 5 – 4, and 9 – 8 transitions near 39.7, 49.7, and 89.4 GHz. The HNC₃ isomer was found shortly thereafter, whereas the HC₃N isomer had been known for a considerable time.

P. D. Gensheimer reported on
Observations of HCCNC and HNCCC in IRC+10216
Astrophys. Space Sci. 251, 199 (1997).
The IRAM 30 m telescope was used to detect the J = 9 – 8, 14 – 13, and 21 – 20 transitions near 89.4, 139.1, and 208.6 GHz. The HC₃N to HCCNC ratio is around 150, somewhat higher than in TMC-1.

A. Belloche, H. S. P. Müller, K. M. Menten, P. Schilke, and C. Comito
investigated
Complex Organic Molecules in the Interstellar Medium: IRAM 30 m Line Survey of Sagittarius B2(N) and (M)
Astron. Astrophys. 559, Art. No. A47 (2013).
The J = 10 – 9 and 11 – 10 transitions near 99.4 and 109.3 GHz were detected, the latter possibly blended with part of a methyl formate transitions. Overall, this detections is tentative.

C. Vastel, K. Kawaguchi, D. Quénard, M. Ohishi, B. Lefloch, R. Bachiller, and H. S. P. Müller
used the IRAM 30 m telescope for
High Spectral Resolution Observations of HNC₃ and HCCNC in the L1544 Pre-stellar Core
Mon. Not. R. Astron. Soc. 474, L76 (2018).
The J = 8 – 7 to 11 – 10 transitions were detected near 79.5, 89.4, 99.4, and 109.3 GHz. The column density is only slightly smaller than that in TMC-1, similar to HNC₃, even though HC₃N is less abundant by about one order of magnitide in L1544 compared to TMC-1.
The J = 9 – 8 transition was observed shortly before in the study of
The Observed Chemical Structure of L1544
Astron. Astrophys. 606, Art. No. A82 (2017)
by S. Spezzano, P. Caselli, L. Bizzocchi, B. M. Giuliano, and V. Lattanzi.

J. Cernicharo, N. Marcelino, M. Agúndez, C. Bermúdez, C. Cabezas, B. Tercero, and J. R. Pardo
reported on the
Discovery of HC₄NC in TMC-1: A study of the isomers of HC₃N, HC₅N, and HC₇N
Astron. Astrophys. 642, Art. No. L8 (2020).
The authors report the detections of the ¹³C isotopomers of HCCNC as well as that of DCCNC. The observations were obtained with the Yebes 40m radio telescope in the course of a line survey covering 31.0–50.3 GHz.

J. Cernicharo, B. Tercero, C. Cabezas, M. Agúndez, E. Roueff, R. Fuentetaja, N. Marcelino, and P. de Vicente
carried out more recently a
Study of the HCCNC and HNCCC isotopologs in TMC-1
Astron. Astrophys. 682, Art. No. L13 (2024).
They reported in the framework of improved data of the above survey the detection of HCC¹⁵NC with a ¹⁴N/¹⁵N ratio of roughly 240, slightly lower than ~317 obtained for HC₃N. However, this value may be affected by a probable but at that time unrecognized blending of the J = 5 − 4 transition of HCC¹⁵NC (J. Cernicharo, private communication). They also redetermined the ¹²C/¹³C ratios. The values are ~95 with scatter that is insignificant with respect to the uncertainties. These values are somewhat lower than ~65 usually found in the the local ISM, but very similar to values of HNC₃ ¹³C isotopomers and to two of the three HC₃N ¹³C isotopomers. An H/D ratio of ~31 was reported, substantially lower than ~55 observed for HNC₃ and HC₃N.

Contributor(s): H. S. P. Müller; 12, 2017; 11, 2020; 04, 2024