R. A. Loomis, C. N. Shingledecker, G. Langston, B. A. McGuire, N. M. Dollhopf, A. M. Burkhardt, J. Corby, S. T. Booth, P. B. Carroll, B. Turner, and A. J. Remijan
emphasized the
Non-detection of HC₁₁N towards TMC-1: Constraining the Chemistry of Large Carbon-chain Molecules
Mon. Not. R. Astron. 463, 4175–4183 (2016).
Using the 100 m GBT, the authors have covered six rotational transitions with J“ from 37 to 42. No single line was above 2 σ, and the stacked recordings yield an unsignificant result at the 2 σ level. Using the data from Bell at al., 1997 (see next paragraph), they should have observed most lines at about the 3 σ level, and even more clearly in the stacked observations. They conclude from the latter a 3 σ upper limit of about 10¹¹ cm^–2.

M. A. Cordiner, S. B. Charnley, Z. Kisiel, B. A. McGuire, and Y.-J. Kuan
reported on the
Deep K-band Observations of TMC-1 with the Green Bank Telescope: Detection of HC₇O, Nondetection of HC₁₁N, and a Search for New Organic Molecules
Astrophys. J. 850, Art. No. 187 (2017).
Two transitions, J = 55 – 54 and 60 – 59 near 18.597 and 20.287 GHz were observed. Stacking of these data together with those from above lowered the upper limit somewhat. The molecule should be viewed as undetected until convincing observations will have been reported.

M. B. Bell, P. A. Feldman, M. J. Travers, M. C. McCarthy, C. A. Gottlieb, and P. Thaddeus
reported on the
Detection of HC₁₁N in the Cold Dust Cloud TMC-1
Astrophys. J. 483, L61–L64 (1997).
Two transitions, J = 39 – 38 and 39 – 38 near 12.849 and 13.187 GHz were identified with the 43 m Green Bank Telescope. The invoked column density is 2.8 × 10¹¹ cm^–2 for a temperature of 10 K.
An earlier report was in error. The erroneous assignments were based on an extrapolation of the rotational constant B by
T. Oka,
The Prediction of the Rotational Constants of Polyacetylene Compounds H–(C≡C)_//n//–C≡N
J. Mol. Spectrosc. 72, 172–174 (1978);
which in the end turned out to be in full agreement with the laboratory data and thus to be much better than implied by the earlier purported astronomical identification.

R. A. Loomis, A. M. Burkhardt, C. N. Shingledecker, S. B. Charnley, M. A. Cordiner, E. Herbst, S. Kalenskii, K. L. K. Lee, E. R. Willis, C. Xue, A. J. Remijan, M. C. McCarthy, and B. A. McGuire
reported on the
An investigation of spectral line stacking techniques and application to the detection of HC₁₁N
Nat. Astron. 5, 188 (2021).
The observations were obtained in the course of the line survey Green Bank Telescope Observations of TMC-1: Hunting for Aromatic Molecules. The data were recorded with very high resolution which resolves, at least in part, the complex velocity structure and the ¹⁴N hyperfine structure splitting. The molecule was indentified through line-stacking with a signal-to-noise ratio of 5, which is probably at the edge and may provoke the question if this is the end of this story.

Contributor(s): H. S. P. Müller; 02, 2006; 10, 2008; 11, 2016; 12, 2017; 04, 2021