Oxohydroxymethylium, aka protonated carbon dioxide, HOCO⁺, was among the molecules that had been identified in space (toward the star-forming region Sgr B2(OH)) before they had been characterized in the laboratory. Three transitions J+1_0,J+1 – J_0,J with J = 3 – 5 were reported by
P. Thaddeus, M. Guélin, and R. A. Linke,
Three new 'nonterrestrial' molecules
Astrophys. J. 246, L41–L45 (1981). HOCO⁺ or HOCN were seen as possible carriers of these lines. Subsequent laboratory spectroscopic investigations proved the carrier to be HOCO⁺:
M. Bogey, C. Demuynck, and J. L. Destombes,
Laboratory Detection of the Protonated Carbon Dioxide by Submillimeter Wave Spectroscopy
Astron. Astrophys. 138, L11–L12 (1984).

The molecular ion was also detected toward Sgr A:
Y. C. Mihn, W. M. Irvine, and L. M. Ziurys,
Observations of Interstellar HOCO⁺ – Abundance Enhancements toward the Galactic Center
Astrophys. J. 334, 175–181 (1988).

It is also present in dark clouds, such as TMC-1 and L183, as well as in two out of three translucent clouds (CB 17 and CB 228):
B. E. Turner, R. Terzieva, and E. Herbst,
The Physics and Chemistry of Small Translucent Molecular Clouds. XII. More Complex Species Explainable by Gas-Phase Processes
Astrophys. J. 518, 699–732 (1999).

More recently, it was shown that the ion is not confined to massive star-forming regions:
N. Sakai, T. Sakai, Y. Aikawa, and S. Yamamoto,
Detection of HCO₂⁺ toward the Low-Mass Protostar IRAS 04368+2557 in L1527
Astrophys. J. 675, L89–L92 (2008).

C. Vastel, C. Ceccarelli, B. Lefloch, and R. Bachiller,
reported on the
Abundance of HOCO⁺ and CO₂ in the Outer Layers of the L1544 Prestellar Core
Astron. Astrophys. 691, Art. No. L2 (2016).
They report the detection of the 5₀₅ – 4₀₄ transition of DOCO⁺ near 100.4 GHz in emission with the IRAM 30 m telescope. They viewed the detection as somewhat tentative because of issues in the laboratory rest frequencies which have been resolved in the meantime.
At essentially the same time,
A. Fuente, J. Cernicharo, E. Roueff, M. Gerin, J. Pety, N. Marcelino, R. Bachiller, B. Lefloch, O. Roncero, and A. Aguado,
identified the same transition in B1b in the report on the
Ionization Fraction and the Enhanced Sulfur Chemistry in Barnard 1
Astron. Astrophys., in press (2016),
using the same telescope in an extensive molecular line survey.

Contributor(s): H. S. P. Müller; 03, 2008; 03, 2012; 08, 2016