HCNH+, also known as protonatedhydrogen cyanide, is a molecular ion of astrophysical interest. It also exists in the condensed state when formed bysuperacids.
As a relatively simple molecular ion, HCNH+ has been extensively studied in the laboratory. The very first spectrum taken at any wavelength focused on theν2 (C−H stretch) ro-vibrational band in theinfrared.[6]Soon afterward, the same authors reported on their investigation of theν1 (N−H stretch) band.[7]Following these initial studies, several groups published manuscripts on the various ro-vibrational spectra of HCNH+, including studies of theν3 band (C≡N stretch),[8]theν4 band (H−C≡N bend),[9]and theν5 band (H−N≡C bend).[10]
While all of these studies focused on ro-vibrational spectra in theinfrared, it was not until 1998 that technology advanced far enough for an investigation of the pure rotational spectrum of HCNH+ in themicrowave 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 constantsB andD.[12]
According to the database atastrochemistry.net, 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:
HCNH+ was first detected ininterstellar space in 1986 toward the dense cloudSgr B2 using theNRAO 12 m dish and theTexas Millimeter Wave Observatory.[14]These observations utilized theJ = 1–0, 2–1, and 3–2 pure rotational transitions at 74, 148, and 222 GHz, respectively.
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 towardSgr 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.
While not directly detected via spectroscopy, the existence of HCNH+ has been inferred to exist in the atmosphere ofSaturn's largest moon,Titan,[19] based on data from the Ion and Neutral Mass Spectrometer (INMS) instrument aboard theCassini 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 atm/z = 28 seems to support this theory.
In 1997, observations were made of the long-period cometHale–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 ofHCN,HNC, andCN, is important in understanding the chemistry associated withcomets.
^Altman RS, Crofton MW, 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.
