Protonated hydrogen cyanide

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Protonated hydrogen cyanide
Protonated hydrogen cyanide.svg
Protonated hydrogen cyanide cation 3D spacefill.png
IUPAC names
Methylidyneammonium, [1] Methylidyneazanium [2]
Systematic IUPAC name
Methylidyneammonium [1]
Other names
Methanimine, Iminomethylcation; 1-Azoniaethyne [2]
3D model (JSmol)
PubChem CID
  • InChI=1S/CHN/c1-2/h1H/p+1 [1]
  • linear form (HC≡N+H):InChI=1S/CHN/c1-2/h1H/p+1 [2]
  • HC+=NH [4] :InChI=1S/CH3N/c1-2/h2H,1H2/q+1
  • linear form (HC≡N+H):C#[NH+]
  • HC+=NH:[CH+]=N
  • CNH+
  • 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) [1]
Related compounds
Related isoelectronic
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

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.

Related Research Articles

Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure H−C≡N. It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at 25.6 °C (78.1 °F). HCN is produced on an industrial scale and is a highly valued precursor to many chemical compounds ranging from polymers to pharmaceuticals. Large-scale applications are for the production of potassium cyanide and adiponitrile, used in mining and plastics, respectively. It is more toxic than solid cyanide compounds due to its volatile nature.

<span class="mw-page-title-main">Astrochemistry</span> Study of molecules in the Universe and their reactions

Astrochemistry is the study of the abundance and reactions of molecules in the universe, and their interaction with radiation. The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form.

<span class="mw-page-title-main">Protostar</span> Early stage in the process of star formation

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Rotational–vibrational spectroscopy is a branch of molecular spectroscopy concerned with infrared and Raman spectra of molecules in the gas phase. Transitions involving changes in both vibrational and rotational states can be abbreviated as rovibrational transitions. When such transitions emit or absorb photons, the frequency is proportional to the difference in energy levels and can be detected by certain kinds of spectroscopy. Since changes in rotational energy levels are typically much smaller than changes in vibrational energy levels, changes in rotational state are said to give fine structure to the vibrational spectrum. For a given vibrational transition, the same theoretical treatment as for pure rotational spectroscopy gives the rotational quantum numbers, energy levels, and selection rules. In linear and spherical top molecules, rotational lines are found as simple progressions at both higher and lower frequencies relative to the pure vibration frequency. In symmetric top molecules the transitions are classified as parallel when the dipole moment change is parallel to the principal axis of rotation, and perpendicular when the change is perpendicular to that axis. The ro-vibrational spectrum of the asymmetric rotor water is important because of the presence of water vapor in the atmosphere.

<span class="mw-page-title-main">Rotational spectroscopy</span> Spectroscopy of quantized rotational states of gases

Rotational spectroscopy is concerned with the measurement of the energies of transitions between quantized rotational states of molecules in the gas phase. The spectra of polar molecules can be measured in absorption or emission by microwave spectroscopy or by far infrared spectroscopy. The rotational spectra of non-polar molecules cannot be observed by those methods, but can be observed and measured by Raman spectroscopy. Rotational spectroscopy is sometimes referred to as pure rotational spectroscopy to distinguish it from rotational-vibrational spectroscopy where changes in rotational energy occur together with changes in vibrational energy, and also from ro-vibronic spectroscopy where rotational, vibrational and electronic energy changes occur simultaneously.

<span class="mw-page-title-main">Morse potential</span> Model for the potential energy of a diatomic molecule

The Morse potential, named after physicist Philip M. Morse, is a convenient interatomic interaction model for the potential energy of a diatomic molecule. It is a better approximation for the vibrational structure of the molecule than the quantum harmonic oscillator because it explicitly includes the effects of bond breaking, such as the existence of unbound states. It also accounts for the anharmonicity of real bonds and the non-zero transition probability for overtone and combination bands. The Morse potential can also be used to model other interactions such as the interaction between an atom and a surface. Due to its simplicity, it is not used in modern spectroscopy. However, its mathematical form inspired the MLR (Morse/Long-range) potential, which is the most popular potential energy function used for fitting spectroscopic data.

<span class="mw-page-title-main">Trihydrogen cation</span> Polyatomic ion (H₃, charge +1)

The trihydrogen cation or protonated molecular hydrogen is a cation with formula H+
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<span class="mw-page-title-main">Ethynyl radical</span> Hydrocarbon compound (•CCH)

The ethynyl radical (systematically named λ3-ethyne and hydridodicarbon(CC)) is an organic compound with the chemical formula C≡CH (also written [CCH] or C
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<span class="mw-page-title-main">Diazenylium</span> Chemical compound

Diazenylium is the chemical N2H+, an inorganic cation that was one of the first ions to be observed in interstellar clouds. Since then, it has been observed for in several different types of interstellar environments, observations that have several different scientific uses. It gives astronomers information about the fractional ionization of gas clouds, the chemistry that happens within those clouds, and it is often used as a tracer for molecules that are not as easily detected (such as N2). Its 1–0 rotational transition occurs at 93.174 GHz, a region of the spectrum where Earth's atmosphere is transparent and it has a significant optical depth in both cold and warm clouds so it is relatively easy to observe with ground-based observatories. The results of N2H+ observations can be used not only for determining the chemistry of interstellar clouds, but also for mapping the density and velocity profiles of these clouds.

<span class="mw-page-title-main">Cyano radical</span> Chemical compound

The cyano radical (or cyanido radical) is a radical with molecular formula CN, sometimes written CN. The cyano radical was one of the first detected molecules in the interstellar medium, in 1938. Its detection and analysis was influential in astrochemistry. The discovery was confirmed with a coudé spectrograph, which was made famous and credible due to this detection. ·CN has been observed in both diffuse clouds and dense clouds. Usually, CN is detected in regions with hydrogen cyanide, hydrogen isocyanide, and HCNH+, since it is involved in the creation and destruction of these species (see also Cyanogen).

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<span class="mw-page-title-main">Iodosilane</span> Chemical compound

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<span class="mw-page-title-main">Calcium monohydride</span> Chemical compound

Calcium monohydride is a molecule composed of calcium and hydrogen with formula CaH. It can be found in stars as a gas formed when calcium atoms are present with hydrogen atoms.

<span class="mw-page-title-main">Vela Molecular Ridge</span> Molecular cloud complex in the constellations Vela and Puppis

Vela Molecular Ridge is a molecular cloud complex in the constellations Vela and Puppis. Radio 12CO observations of the region showed the ridge to be composed of several clouds, each with masses 100,000–1,000,000 M. This cloud complex lies on the sky in the direction of the Gum Nebula (foreground) and the Carina–Sagittarius Spiral Arm (background). The most important clouds in the region are identified by the letters A, B, C and D, and in fact belong to two different complexes: the clouds A, C and D are located at an average distance of about 700-1000 parsecs and are related to the OB association Vela R2, while cloud B is located at a greater distance, up to 2000 parsecs away, and is physically connected to the extended Vela OB1 association.

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<span class="mw-page-title-main">1,4-Pentadiyne</span> Chemical compound

1,4-Pentadiyne (penta-1,4-diyne) is a chemical compound belonging to the alkynes. The compound is the structural isomer to 1,3-pentadiyne.


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