Protonated cyanoacetylene (HC3NH+) was discovered in 1994 by
Kawaguchi et al. toward
TMC-1. The detection was based on
two consecutive rotational transitions, J=5–4 and J=4–3, observed with the
Nobeyama 45m telescope. Two previous
efforts in 1990 to detect HC3NH+, by
Turner et al. and
Amano et al., had proven to be unsuccessful.
The initial detection of the J=5–4 and J=4–3 lines was confirmed in the survey of
TMC-1 by Kaifu et al. in 2004 but did not
add any more lines. In 2017, Quénard et al.
reported the detection of HC3NH+ toward the pre-stellar core L1544
(Lynds 1544) in Taurus via its
J=11–10, 10–9, and 9–8 lines.