Protonated hydrogen cyanide


Protonated hydrogen cyanide
Protonated hydrogen cyanide.svg
Protonated hydrogen cyanide cation 3D spacefill.png
IUPAC names
Methylidyneammonium,[2] Methylidyneazanium[1]
Systematic IUPAC name
Other names
Methanimine, Iminomethylcation; 1-Azoniaethyne[1]
  • 38263-97-7 ☒N[3]
3D model (JSmol)
  • linear form (HC≡N+H): Interactive image
  • HC+=NH: Interactive image
  • CNH+
    : Interactive image
  • H2CN+: Interactive image
  • cis-HCNH+: Interactive image
  • trans-HCNH+: Interactive image
  • 10446358 linear form (HC≡N+H)
  • 18948137 HC+=NH
  • 22952220 linear form (HC≡N+H)
  • 274441977
  • InChI=1S/CHN/c1-2/h1H/p+1[2]
  • linear form (HC≡N+H): InChI=1S/CHN/c1-2/h1H/p+1[1]
  • HC+=NH[4]: InChI=1S/CH3N/c1-2/h2H,1H2/q+1
  • linear form (HC≡N+H): C#[NH+]
  • HC+=NH: [CH+]=N
  • CNH+
    : [CH0]=[NH2+]
  • H2CN+: C=[NH0+]
  • cis-HCNH+: [H]/[C]=[N+]\[H]
  • trans-HCNH+: [H]/[C]=[N+]/[H]
Molar mass 28.033 g·mol−1
Conjugate base Hydroisocyanic acid
C∞v (linear form (HC≡N+H))
linear: HC≡N+H
Flash point −21.3 to −43.7 °C (−6.3 to −46.7 °F; 251.8 to 229.5 K)[2]
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

HCNH+, also known as protonated hydrogen cyanide, is a molecular ion of astrophysical interest. It also exists in the condensed state when formed by superacids.


In the ground state, HC+NH is a simple linear molecule, whereas its excited triplet state is expected to have cis and trans isomeric forms. The higher-energy structural isomers H2CN+ and C+NH2 have also been studied theoretically.[5]

Laboratory studies

As a relatively simple molecular ion, HCNH+ has been extensively studied in the laboratory. The very first spectrum taken at any wavelength focused on the ν2 (C−H stretch) ro-vibrational band in the infrared. [6] Soon afterward, the same authors reported on their investigation of the ν1 (N−H stretch) band. [7] Following these initial studies, several groups published manuscripts on the various ro-vibrational spectra of HCNH+, including studies of the ν3 band (C≡N stretch),[8] the ν4 band (H−C≡N bend),[9] and the ν5 band (H−N≡C bend) .[10]

While all of these studies focused on ro-vibrational spectra in the infrared, it was not until 1998 that technology advanced far enough for an investigation of the pure rotational spectrum of HCNH+ in the microwave region to take place. At that time, microwave spectra for HCNH+ and its isotopomers HCND+ and DCND+ were published.[11] Recently, the pure rotational spectrum of HCNH+ was measured again in order to more precisely determine the molecular rotational constants B and D.[12]

Formation and destruction

According to the database at, the most advanced chemical models of HCNH+ include 71 total formation reactions and 21 total destruction reactions. Of these, however, only a handful dominate the overall formation and destruction.[13] In the case of formation, the 7 dominant reactions are:

+ HCN → HCNH+ + H2
+ HNC → HCNH+ + H2
H3O+ + HCN → HCNH+ + H2O
H3O+ + HNC → HCNH+ + H2O
C+ + NH3 → HCNH+ + H

Astronomical detections

Initial interstellar detection

HCNH+ was first detected in interstellar space in 1986 toward the dense cloud Sgr B2 using the NRAO 12 m dish and the Texas Millimeter Wave Observatory.[14] These observations utilized the J = 1–0, 2–1, and 3–2 pure rotational transitions at 74, 148, and 222 GHz, respectively.

Subsequent interstellar detections

Since the initial detection, HCNH+ has also been observed in TMC-1[15] [16] as well as DR 21(OH)[15] .[17] The initial detection toward Sgr B2 has also been confirmed.[15][18] All 3 of these sources are dense molecular clouds, and to date HCNH+ has not been detected in diffuse interstellar material.

Solar System bodies

While not directly detected via spectroscopy, the existence of HCNH+ has been inferred to exist in the atmosphere of Saturn's largest moon, Titan,[19] based on data from the Ion and Neutral Mass Spectrometer (INMS) instrument aboard the Cassini space probe. Models of Titan's atmosphere had predicted that HCNH+ would be the dominant ion present, and a strong peak in the mass spectrum at m/z = 28 seems to support this theory.

In 1997, observations were made of the long-period comet Hale–Bopp in an attempt to find HCNH+, [20] but it was not detected. However, the upper limit derived from these observations, along with the detections of HCN, HNC, and CN, is important in understanding the chemistry associated with comets.


  1. ^ a b c d "Methanimine". PubChem. Retrieved 27 January 2019.
  2. ^ a b c d e "Methylidyneammonium | CH2N". ChemSpider. Retrieved 27 January 2019.
  3. ^ "HCNH+".
  4. ^ "methanimine | CH2N". ChemSpider. Retrieved 27 January 2019.
  5. ^ Allen, T. L., Goddard, J. D., & Schaefer, H. F. III (1980). "A possible role for triplet H2CN+ isomers in the formation of HCN and HNC in interstellar clouds". Journal of Chemical Physics. 73 (7): 3255–3263. Bibcode:1980JChPh..73.3255A. doi:10.1063/1.440520.CS1 maint: multiple names: authors list (link)
  6. ^ Altman, R. S., Crofton, M. W., & Oka, T. (1984). "Observation of the infrared ν2 band (CH stretch) of protonated hydrogen cyanide, HCNH+". Journal of Chemical Physics. 80 (8): 3911–3912. Bibcode:1984JChPh..80.3911A. doi:10.1063/1.447173.CS1 maint: multiple names: authors list (link)
  7. ^ Altman, R. S., Crofton, M. W., & Oka, T. (1984). "High resolution infrared spectroscopy of the ν1 (NH stretch) and ν2 (CH stretch) bands of HCNH+". Journal of Chemical Physics. 81 (10): 4255–4258. Bibcode:1984JChPh..81.4255A. doi:10.1063/1.447433.CS1 maint: multiple names: authors list (link)
  8. ^ Kajita, M., Kawaguchi, K., & Hirota, E. (1988). "Diode laser spectroscopy of the ν3 (CN stretch) band of HCNH+". Journal of Molecular Spectroscopy. 127 (1): 275–276. Bibcode:1988JMoSp.127..275K. doi:10.1016/0022-2852(88)90026-4.CS1 maint: multiple names: authors list (link)
  9. ^ Tanaka, K., Kawaguchi, K., & Hirota, E. (1986). "Diode laser spectroscopy of the ν4 (HCN bend) band of HCNH+". Journal of Molecular Spectroscopy. 117 (2): 408–415. Bibcode:1986JMoSp.117..408T. doi:10.1016/0022-2852(86)90164-5.CS1 maint: multiple names: authors list (link)
  10. ^ Ho, W.-C., Blom, C. E., Liu, D.-J., & Oka, T. (1987). "The infrared ν5 band (HNC bend) of protonated hydrogen cyanide, HCNH+". Journal of Molecular Spectroscopy. 123 (1): 251–253. Bibcode:1987JMoSp.123..251H. doi:10.1016/0022-2852(87)90275-X.CS1 maint: multiple names: authors list (link)
  11. ^ Araki, M., Ozeki, H., & Saito, S. (1998). "Laboratory Measurement of the Pure Rotational Transitions of HCNH+ and Its Isotopic Species". Astrophysical Journal Letters. 496 (1): L53. arXiv:astro-ph/9801241. Bibcode:1998ApJ...496L..53A. doi:10.1086/311245. S2CID 17868534.CS1 maint: multiple names: authors list (link)
  12. ^ Amano, T., Hashimoto, K., & Hirao, T. (2006). "Submillimeter-wave spectroscopy of HCNH+ and CH3CNH+". Journal of Molecular Structure. 795 (1–3): 190–193. Bibcode:2006JMoSt.795..190A. doi:10.1016/j.molstruc.2006.02.035.CS1 maint: multiple names: authors list (link)
  13. ^ Millar, T. J., Farquhar, P. R. A., & Willacy, K. (1997). "The UMIST Database for Astrochemistry 1995". Astronomy & Astrophysics Supplement Series. 121 (1): 139–185. arXiv:1212.6362. Bibcode:1997A&AS..121..139M. doi:10.1051/aas:1997118.CS1 maint: multiple names: authors list (link)
  14. ^ Ziurys, L. M.; Turner, B. E. (1986). "HCNH+: A New Interstellar Molecular Ion" (PDF). The Astrophysical Journal Letters. 302: L31–L36. Bibcode:1986ApJ...302L..31Z. doi:10.1086/184631. PMID 11542069.
  15. ^ a b c Schilke, P., Walmsley, C. M., Millar, T. J., & Henkel, C. (1991). "Protonated HCN in molecular clouds". Astronomy & Astrophysics. 247: 487–496. Bibcode:1991A&A...247..487S.CS1 maint: multiple names: authors list (link)
  16. ^ Ziurys, L. M., Apponi, A. J., & Yoder, J. T. (1992). "Detection of the Quadrupole Hyperfine Structure in HCNH+". The Astrophysical Journal Letters. 397: L123–L126. Bibcode:1992ApJ...397L.123Z. doi:10.1086/186560.CS1 maint: multiple names: authors list (link)
  17. ^ Hezareh, T., Houde, M., McCoey, C., Vastel, C., & Peng, R. (2008). "Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR 21(OH)". The Astrophysical Journal. 684 (2): 1221–1227. arXiv:0805.4018. Bibcode:2008ApJ...684.1221H. doi:10.1086/590365. S2CID 6284545.CS1 maint: multiple names: authors list (link)
  18. ^ Nummelin, A., Bergman, P., Hjalmarson, Å., Friberg, P., Irvine, W. M., Millar, T. J., Ohishi, M., & Saito, S. (2000). "A Three-Position Spectral Line Survey of Sagittarius B2 between 218 and 263 GHz. II. Data Analysis". The Astrophysical Journal Supplement Series. 128 (1): 213–243. Bibcode:2000ApJS..128..213N. doi:10.1086/313376.CS1 maint: multiple names: authors list (link)
  19. ^ Cravens, T. E., Robertson, I. P., Waite, J. H., Yelle, R. V., Kasprzak, W. T., Keller, C. N., Ledvina, S. A., Niemann, H. B., Luhmann, J. G., McNutt, R. L., Ip, W.-H., De La Haye, V., Mueller-Wodarg, I., Wahlund, J.-E., Anicich, V. G., & Vuitton, V. (2006). "Composition of Titan's atmosphere" (PDF). Geophysical Research Letters. 33 (7): L07105. Bibcode:2006GeoRL..3307105C. doi:10.1029/2005GL025575. hdl:2027.42/94758.CS1 maint: multiple names: authors list (link)
  20. ^ Ziurys, L. M., Savage, C., Brewster, M. A., Apponi, A. J., Pesch, T. C., & Wyckoff, S. (1999). "Cyanide Chemistry in Comet Hale-Bopp (C/1995 O1)". The Astrophysical Journal Letters. 527 (1): L67–L71. Bibcode:1999ApJ...527L..67Z. doi:10.1086/312388. PMID 10567001.CS1 maint: multiple names: authors list (link)