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Concept
Isocyanure d'hydrogène
Résumé
Hydrogen isocyanide is a chemical with the molecular formula HNC. It is a minor tautomer of hydrogen cyanide (HCN). Its importance in the field of astrochemistry is linked to its ubiquity in the interstellar medium.
Both hydrogen isocyanide and azanylidyniummethanide are correct IUPAC names for HNC. There is no preferred IUPAC name. The second one is according to the substitutive nomenclature rules, derived from the parent hydride azane () and the anion methanide ().
Hydrogen isocyanide (HNC) is a linear triatomic molecule with C∞v point group symmetry. It is a zwitterion and an isomer of hydrogen cyanide (HCN). Both HNC and HCN have large, similar dipole moments, with μHNC = 3.05 Debye and μHCN = 2.98 Debye respectively. These large dipole moments facilitate the easy observation of these species in the interstellar medium.
As HNC is higher in energy than HCN by 3920 cm−1 (46.9 kJ/mol), one might assume that the two would have an equilibrium ratio at temperatures below 100 Kelvin of 10−25. However, observations show a very different conclusion; is much higher than 10−25, and is in fact on the order of unity in cold environments. This is because of the potential energy path of the tautomerization reaction; there is an activation barrier on the order of roughly 12,000 cm−1 for the tautomerization to occur, which corresponds to a temperature at which HNC would already have been destroyed by neutral-neutral reactions.
In practice, HNC is almost exclusively observed astronomically using the J = 1→0 transition. This transition occurs at ~90.66 GHz, which is a point of good visibility in the atmospheric window, thus making astronomical observations of HNC particularly simple. Many other related species (including HCN) are observed in roughly the same window.
HNC is intricately linked to the formation and destruction of numerous other molecules of importance in the interstellar medium—aside from the obvious partners HCN, protonated hydrogen cyanide (HCNH+), and cyanide (CN), HNC is linked to the abundances of many other compounds, either directly or through a few degrees of separation.
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