The linear propynylidynium, C₃H⁺, was detected with the IRAM 30 m telescope in nine transitions (J“ = 3 to 11) in the course of a molecular line survey of the Horsehead Nebula in Orion, covering the 3, 2, and 1.3 mm regions, by
J. Pety, P. Gratier, V. Guzmán, E. Roueff, M. Gerin, J. R. Goicoechea, S. Bardeau, A. Sievers, F. Le Petit, J. Le Bourlot, A. Belloche, and D. Talbi,
The IRAM-30 m Line Survey of the Horsehead PDR. II. First detection of the //l//-C₃H⁺ Hydrocarbon Cation,
Astron. Astrophys. 548, Art. No. A68 (2012).
In the absence of laboratory data, the identification was secured through quantum chemical calculations.

R. C. Fortenberry, X. Huang, T. D. Crawford, and T. J. Lee,
however, proposed
High-accuracy Quartic Force Field Calculations for the Spectroscopic Constants and Vibrational Frequencies of 1 ¹//A'// //l//-C₃H^–: A Possible Link to Lines Observed in the Horsehead Nebula Photodissociation Region,
Astrophys. J. 772, Art. No. 39 (2013).
In an earlier publication, some of the authors found a discrepancy between their calculated equilibrium quartic distortion parameter and the ground state quartic distortion parameter derived from astronomical observations. As a results, they proposed to reject l-C₃H⁺ as carrier of the emission features mentioned above. Instead, they now propose the anion in its excited singlet state as the carrier of the emission lines.

Considering that the Horsehead PDR may well be far from LTE conditions, one should not reject the idea of the anion as carrier entirely. But the occurance of anions in a PDR as well as other regions (see below) in general or of C₃H^– in particular needs to be verified.
Moreover, the rejection of the assignment to the cation based on the deviations in the calculated equilibrium versus observed ground state distortion parameters is not compelling as vibrational averaging in other molecules has created much larger deviations.
Finally, laboratory experiments carried out at the CfA by K. Crabtree and M. C. McCarthy favor the cation as carrier of these lines.

B. A. McGuire, P. B. Carroll, R. A. Loomis, G. A. Blake, J. M. Hollis, F. J. Lovas, P. R. Jewell, A. J. Remijan
reported on
A Search for //l//-C₃H⁺ and //l//-C₃H in Sgr B2(N), Sgr B2(OH), and the Dark Cloud TMC-1,
Astrophys. J. 774, Art. No. 56 (2013).
The J” = 0 and 1 transitions of l-C₃H⁺ (or an alterantive carrier) were abserved in absorption in their line survey of Sgr B2(N) carried out with the GBT 100 m telescope. A suitable, albeit blended emission at the J“ = 6 transition was found in archival survey data. Archival survey data toward Sgr B2(OH) and TMC-1 were not unambiguous, but would yield l-C₃H column density ratios similar to those found for the Horsehead PDR and for Sgr B2(N). These findings make it more interesting and important to establish or reject by laboratory spectroscopy that the carrier of these lines is l-C₃H⁺ or to present an alternative carrier.

S. Brünken, L. Kluge, A. Stoffels, O. Asvany, and S. Schlemmer
reported on the
Laboratory Rotational Spectrum of //l//-C₃H⁺ and Confirmation of its Astronomical Detection,
Astrophys. J. 783, Art. No. L4 (2014).
A novel, charge-selective form of action spectroscopy was employed to measure accurately the J” = 1 to 4 transition frequencies.

J. Cernicharo, M. Agúndez, C. Cabezas, R. Fuentetaja, B. Tercero, N. Marcelino, Y. Endo, J. R. Pardo, and P. de Vicente
reported on the
Discovery of C₅H⁺ and Detection of C₃H⁺ in TMC-1 with the QUIJOTE Line Survey,
Astron. Astrophys. 657, Art. No. L16 (2022).
C₅H⁺ was found to be a factor of 3.7 ± 0.5 more abundant than C₃H⁺, which was explained by the reactivity of the latter with H₂ through chemical modeling. The authors refuted the earlier identification of C₃H⁺ in archival TMC-1 data by McGuire et al. in 2013.

Contributor(s): H. S. P. Müller; 11, 2012; 05, 2013; 07, 2013; 02, 2014; 03, 2022