The 1,3-pentadiyne molecule CH₃C₄H, aka methyldiacetylene, had been identified in spectra recorded toward TMC-1 by two groups independently and at almost the same time
J. M. MacLeod, L. W. Avery, N. W. Broten,
The detection of interstellar methyldiacetylene (CH₃C₄H)
Astrophys. J. 282, L89–L92 (1984);
C. M. Walmsley, P. R. Jewell, L. E. Snyder, and G. Winnewisser,
Detection of interstellar methyldiacetylene (CH₃C₄H) in the dark dust cloud TMC 1
Astron. Astrophys. 184, L11–L14 (1984).

The molecule was also detected by
M. Araki, S. Takano, N. Sakai, S. Yamamoto, T. Oyama, N. Kuze, and K. Tsukiyama,
who studied
Long Carbon Chains in the Warm Carbon-chain-chemistry Source L1527: First Detection of C₇H in Molecular Clouds,
Astrophys. J. 847 Art. No. 51 (2017).
The three J = 11 – 10 transitions with K = 0, 1, and 2 were detected near 44.8 GHz with the 100 m GBT.

C. Cabezas, R. Fuentetaja, E. Roueff, M. Agúndez, B. Tercero, N. Marcelino, J. R. Pardo, P. de Vicente and J. Cernicharo
reported on
New Deuterated Species in TMC-1: Detection of CH₂DC₄H with the QUIJOTE line survey,
Astron. Astrophys. 657 Art. No. L5 (2022).
Twelve transition with J = 8 to 12 and K_a ≤ 1 were detected with the Yebes 40 m radio telescope. The CH₃C₄H to CH₂DC₄H abundance ratio of 24 ± 2 is very similar to that of CH₃C₃N to CH₂DC₃N ratio of 22 ± 2.

More recently,
C. Cabezas, M. Agúndez, E. Roueff, R. Fuentetaja, B. Tercero, N. Marcelino, P. de Vicente, and J. Cernicharo
announced the
Detection of Six Isotopologues of CH₃CCCCH in TMC-1 with the QUIJOTE Line Survey,
Astron. Astrophys. 703 Art. No. L22 (2025).
With an increase of the signal-to-noise ratio in comparison to the previous item, the authors were able to identify transition with J = 8 to 12 and K_a ≤ 1 of all five singly substituted ¹³C isotopologs and of CH₃C₄D with reasonable to good S/N and mostly not or only slightly blended. The lowest J pair of CH₃C₄D transitions was not covered. The ¹²C/¹³C ratio is 93 ± 11, while the H/D ratio in the case of CH₃C₄D is 54 ± 8.

Contributor(s): H. S. P. Müller; 7, 2004; 10, 2017; 03, 2022; 11, 2025