# Protonated hydrogen cyanide

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.

## Structure

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 H$_2$CN$^+$ and C+NH$_2$ have also been studied theoretically.

## 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 ν$_2$ (C−H stretch) ro-vibrational band in the infrared.
Soon afterward, the same authors reported on their investigation of the ν$_1$ (N−H stretch) band.
Following these initial studies, several groups published manuscripts on the various ro-vibrational spectra of HCNH$^+$, including studies of the ν$_3$ band (C≡N stretch), the ν$_4$ band (H−C≡N bend), and the ν$_5$ 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.

## 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:
    H$^+$ $_3$ + HCN → HCNH$^+$ + H$_2$
    H$^+$ $_3$ + HNC → HCNH$^+$ + H$_2$
    HCO$^+$ + HCN → HCNH$^+$ + CO
    HCO$^+$ + HNC → HCNH$^+$ + CO
    H$_3$O$^+$ + HCN → HCNH$^+$ + H$_2$O
    H$_3$O$^+$ + HNC → HCNH$^+$ + H$_2$O
    C$^+$ + NH$_3$ → 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. 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 been inferred to exist in the atmosphere of Saturn's largest moon, Titan, 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$^+$, 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.