HCN is abundant in all kinds of environments, from dark clouds to star-forming regions and circumstellar envelopes. The Detection of Interstellar HCN was reported in detail by
L. E. Snyder and D. Buhl,
Observations of Radio Emission from Interstellar Hydrogen Cyanide
Astrophys. J. 163, L47–L52 (1971).
The detection of H¹³CN was also reported.

Note: Often one finds confusion about the two stretching modes of HCN. According to the generally accepted recommendation, we designate v₁ = 1 as the H-C stretching state at 3311.5 cm^–1 or 4764 K, and v₃ = 1 as the C-N stretching state at 2096.8 cm^–1 or 3017 K. Rather frequently, the numbering of the two stretching states is exchanged. And on occasion people use the wrong state designations.
States are designated according to the number of quanta excited: (v₁v₂v₃).

L. M. Ziurys and B. E. Turner,
Detection of Interstellar Vibrationally Excited HCN
Astrophys. J. 300, L19–L23 (1986),
reported on the detection of HCN transitions within (010) and (020). Emission lines of both states were seen in the circumstellar envelope of CW Leo, aka IRC +10216. Lines of (010) were also detected toward Orion KL.

T. D. Groesbeck, T. G. Philips, and G. A. Blake,
The Molecular Emission-Line Spectrum of IRC +10216 Between 330 and 358 GHz
Astrophys. J. Suppl. Ser. 94, L147–L162 (1994),
report on the detection of HCN transitions within (001) and (100) as well as (010) of H¹³CN.

The observation of even higher states, up to (120), was reported by
L. W. Avery et al.,
Submillimeter molecular line observations of IRC +10216: Searches for MgH, SiH₂, and HCO⁺, and detection of hot HCN
Astrophys. J. 426, 737–741 (1994).
In this paper, the stretching states are designated as indicated above. However, the authors claimed they were using the traditional labelling.

A plethora of excited vibrational states, several still higher than those described above (up to almost 11000 K), were detected, again toward CW Leo by
J. Cernicharo, M. Agúndez, C. Kahane, M. Guélin, J. R. Goicoechea, N. Marcelino, E. De Beck, and L. Decin,
Probing the Dust Formation Region in IRC +10216 with the High Vibrational States of Hydrogen Cyanide
Astron. Astrophys. 529, Art. No. L3 (2011).

Maser emission within (010) and (020) was reported by
S. Guilloteau, A. Omont, and R. Lucas,
A New Strong Maser – HCN
Astron. Astrophys. 176, L24–L26 (1987); and
R. Lucas and J. Cernicharo,
Discovery of Strong Maser Emission from HCN in IRC+10216
Astron. Astrophys. 218, L20–L22 (1989).

Maser emission involving the Coriolis-coupled (040) and (001) states was reported by
P.Schilke, D. M. Mehringer, and K. M. Menten,
A Submillimeter HCN Laser in IRC +10216
Astrophys. J. 528, L37–L40 (2000); and
P.Schilke, D. M. Mehringer, and K. M. Menten
Detection of a Second, Strong Submillimeter HCN Laser Line toward Carbon Stars
Astrophys. J. 583, 446–450 (2003).

The detection of direct-l-type transitions (ΔJ = 0) within (010) paved the way for observing vibrationally excited HCN at centimeter wavelengths !
S. Thorwirth, F. Wyrowski, P. Schilke, K. M. Menten, S. Brünken, H. S. P. Müller, and G. Winnewisser,
Detection of HCN Direct-//l//-Type Transitions Probing Hot Molecular Gas in the Proto-Planetary Nebula CRL 618
Astrophys. J. 586, 338–343 (2003).

Early observations of DCN were reported by
K. B. Jefferts, A. A.Penzias, and R. W. Wilson,
Deuterium in the Orion Nebula
Astrophys. J. 179, L57–L59 (1973); and by
R. W. Wilson, A. A.Penzias, K. B. Jefferts, and P. M. Solomon,
Interstellar Deuterium: the Hyperfine Structure of DCN
Astrophys. J. 179, L107–L110 (1973).
More recently, DCN was observed even in the photodissociation region Orion Bar:
S. Leurini, R. Rolffs, S. Thorwirth, B. Parise, P. Schilke, C. Comito, F. Wyrowski, R. Güsten, P. Bergman, K. M. Menten and L.-Å. Nyman,
APEX 1 mm Line Survey of the Orion Bar
Astron. Astrophys. 454, L47–L50 (2006).

Early observations of HC¹⁵N were reported by
R. A. Linke, P. F. Goldsmith, P. G. Wannier, R. W. Wilson, and A. A.Penzias,
Isotopic Abundance Variations in Interstellar HCN
Astrophys. J. 214, 50–59 (1977).

Transitions pertaining to (010) of H¹³CN had been seen earlier, see above; transitions up to (030) were reported by
J. Cernicharo, M. Agúndez, C. Kahane, M. Guélin, J. R. Goicoechea, N. Marcelino, E. De Beck, and L. Decin,
Probing the Dust Formation Region in IRC +10216 with the High Vibrational States of Hydrogen Cyanide
Astron. Astrophys. 529, Art. No. L3 (2011).

Rovibrational transitions have been also detected:
S. T. Ridgway, D. F. Carbon, and D. N. B. Hall,
Polyatomic Species Contributing to the Carbon-star 3 micron Band
Astrophys. J. 225, 138–147 (1978).

Contributor(s): H. S. P. Müller; 04, 2004; 03, 2007; 06, 2011