Hexapentaenylidene in its 1A1 ground electronic
state has been detected with the 70 m DSN antenna in emission in
two rotational transitions toward the dark cloud TMC-1 by
W. D. Langer, T. Velusamy, T. B. H. Kuiper, R. Peng, M. C. McCarthy, M. J. Travers, A. Kovács, C. A. Gottlieb, and P. Thaddeus,
First Astronomical Detection of the Cumulene Carbon Chain Molecule H2C6 in TMC-1,
Astrophys. J. 480, L63L66 (1997)
The ortho transitions 71,7 61,6 and 81,8 71,7 near 18.8 and 21.5 GHZ, respectively, were detected. The lighter homolog C4H2 is about 25 times more abundant.
The molecule was also detected by
M. Araki, S. Takano, N. Sakai, S. Yamamoto, T. Oyama, N. Kuze, and K. Tsukiyama,
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 = 16 15 transitions with Ka = 0 and 1 were detected near 43 GHz with the 100 m GBT.