HCNH+, also known as protonated hydrogen cyanide, is a molecular ion of astrophysical interest.
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.  Soon afterward, the same authors reported on their investigation of the ν1 (N−H stretch) band.  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) , the ν4 band (H−C≡N bend) , and the ν5 band (H−N≡C bend) .
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. Recently, the pure rotational spectrum of HCNH+ was measured again in order to more precisely determine the molecular rotational constants B and D.
As mentioned above, the most recent laboratory measurement of the pure rotational spectrum of the ground state of HCNH+ was performed in 2006. This study determined the most precise values of the molecular constants to date. Using these constants in conjunction with the program PGOPHER, one can generate the predicted rotational spectrum for the ground state of HCNH+ at a temperature of 30 K, as shown below. Being a linear molecule, HCNH+ follows the basic rotational selection rule of ΔJ = ±1.
Formation and destruction
According to the database at astrochemistry.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. In the case of formation, the 7 dominant reactions are:
3 + HCN → HCNH+ + H2
3 + HNC → HCNH+ + H2
- HCO+ + HCN → HCNH+ + CO
- HCO+ + HNC → HCNH+ + CO
- H3O+ + HCN → HCNH+ + H2O
- H3O+ + HNC → HCNH+ + H2O
- C+ + NH3 → HCNH+ + H
Using rate coefficients from astrochemistry.net and the UMIST Database for Astrochemistry in conjunction with model interstellar abundances  one can calculate the relative importance of these 7 reactions as shown in the table below.
|Reactant 1||Reactant 2||Product 1||Product 2||Relative importance (%)|
Being an ion, HCNH+ is predominantly destroyed by the electron recombination reactions:
- e− + HCNH+ → HCN + H
- e− + HCNH+ → HNC + H
- e− + HCNH+ → CN + H + H
Using the same sources as above, the relative importance of these destruction reactions are calculated and shown in the table below. Also shown in the table is the ion-neutral reaction HCNH+ + H2CO, in order to demonstrate just how dominant electron recombination is.
|Reactant 1||Reactant 2||Product 1||Product 2||Relative importance (%)|
|e−||HCNH+||CN||H + H||33|
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. 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  as well as DR 21(OH) . The initial detection toward Sgr B2 has also been confirmed. 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 in fact been inferred in the atmosphere of Saturn's largest moon, Titan. This was done using 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 Z/M=28 seems to support this theory.
In 1997, observations were made of the long-period comet Hale–Bopp in an attempt to find HCNH+,  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.
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