February 2023
Ammonium cyanide

Ammonium cyanide, NH4CN, was detected in the coma of comet 67P/Churyumov-Gerasimenko by Altwegg and coworkers, as reported in 2020. It was a logical follow-up to the detection of the ammonium cation (NH+
) in the same comet in 2016. The identification was made via mass spectroscopy using the Rosetta Spectrometer for Ion and Neutral Analysis (ROSINA) on the Rosetta probe by tracking correlations of NH+
with other species.

A number of other ammonium salts are inferred to be present on comet 67P: the presence of ammonium cyanate (NH4OCN), chloride (NH4Cl), formate (NH4HCOO), and acetate (NH4CH3COO) were also noted by Altwegg et al., while ammonium hydrosulfide (NH4SH) and fluoride (NH4F) were reported in 2022 in a subsequent publication. There is also evidence that ammonium carbonate or ammonium chloride salt is present on the dwarf planet Ceres.

Researcher Links
K Altwegg
H Balsiger
N Hänni
M Rubin
M Schuhmann
I Schroeder
T Sémon
SF Wampfler
J-J Berthelier
C Briois
M Combi
TI Gombosi
H Cottin
J De Keyser
F Dhooghe
B Fiethe
SA Fuselier

Ammonium cyanide is unstable salt. Salts are ionic solids that are a mixture of cations and anions, bound together by Coulombic forces. There are at least two ways to transform pairs of neutral atoms or molecules into ion pairs: via electron transfer and proton transfer. Salts formed from alkali metals and halogens involve the transfer of an electron from the metal to halogen. This occurs even when the simplest salt monomer forms, such as in the known diatomic astromolecule KCl. Ammonium cyanide is an example of a proton transfer salt, where ammonia extracts H+ from HCN to form NH+
, leaving CN. In contrast to KCl, an encounter between single NH3 and HCN molecules in the gas phase does not yield NH4CN (NH+
)(CN). It is plausible that interstellar and cometary salts form on ice surfaces, where water enhances the stability of ions with respect to neutrals. As shown in the structure, the simple NH4CN monomer is computationally stable in a small cluster with even a limited amount of water present.

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