Hydrogen isocyanide
Encyclopedia
Hydrogen isocyanide is a chemical with the molecular formula HNC. It is a minor tautomer
Tautomer
Tautomers are isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond...

 of hydrogen cyanide, HCN). Its importance in the field of astrochemistry
Astrochemistry
Astrochemistry is the study of the abundance and reactions of chemical elements and 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...

 is linked to its ubiquity in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

.

Nomenclature

Both 'hydrogen isocyanide' and 'methanidylidyneazanium' are correct IUPAC names for HNC. Currently there is no Preferred IUPAC name
Preferred IUPAC name
In chemical nomenclature, a preferred IUPAC name is a unique name, assigned to a chemical substance and preferred among the possible names generated by IUPAC nomenclature. The "preferred IUPAC nomenclature" provides a set of rules for choosing between multiple possibilities in situations where it...

. The second one is according to the substitutive nomenclature rules, derived from the parent hydride
Parent hydride
In IUPAC nomenclature, a parent hydride is an unbranched acyclic or cyclic structure to which only hydrogen atoms are attached. Parent hydrides are parent structures that contain one or more hydrogen atoms...

azane (NH3) and the anion methanide (C-).

Molecular properties

Hydrogen isocyanide (HNC) is a linear triatomic molecule with C∞v point group symmetry
Molecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...

. It is a zwitterion
Zwitterion
In chemistry, a zwitterion is a neutral molecule with a positive and a negative electrical charge at different locations within that molecule. Zwitterions are sometimes also called inner salts.-Examples:...

 and an isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

 of hydrogen cyanide (HCN).
Both HNC and HCN have large, similar dipole moments, with respectively μHNC=3.05 Debye
Debye
The debye is a CGS unit of electric dipole momentElectric dipole moment is defined as charge times displacement: Historically the debye was defined as the dipole moment resulting from two charges of opposite sign but an equal magnitude of 10-10 statcoulomb10-10 statcoulomb is approximately 0.2083...

 and μHCN=2.98 Debye. These large dipole moments facilitate the easy observation of these species in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

.

HNC−HCN tautomerism

As HNC is higher in energy than HCN by 3920 cm−1 (46.9 kJ/mol), the naïve expectation would be that the two would have an equilibrium ratio at T<100K of ([HNC]/[HCN])eq,T<100K<10−25. However, observations show a very different conclusion; ([HNC]/[HCN])observed 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.

Spectral properties

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.

Significance in the interstellar medium

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, HCNH+
HCNH+
HCNH+, also known as protonated hydrogen cyanide, is a molecular ion of astrophysical interest.- Structure :The ground state structure of HCNH+ is a simple linear molecule...

, and CN, HNC is linked to the abundances of many other compounds, either directly of through few degrees of separation. As such, an understanding of the chemistry of HNC leads to an understanding of countless other species - HNC is an integral piece in the complex puzzle representing interstellar chemistry.

Furthermore, HNC (alongside HCN) is a commonly used tracer of dense gas in molecular clouds, as referenced in this paper. Aside from the potential to use HNC to investigate gravitational collapse as the means of star formation, HNC abundance (relative to the abundance of other nitrogenous molecules) can be used to determine the evolutionary stage of protostellar cores. This is demonstrated in the aforementioned paper by Tennekes et al. In the same paper, the authors also elaborate on the HNC/HCN abundance ratio as a means of determining the temperature of the environment.

This paper demonstrates a myriad of uses for knowledge of the abundance of HNC. In it, the HCO+/HNC line ratio is used to good effect as a measure of density of gas. This information provides great insight into the mechanisms of the formation of (Ultra-)Luminous Infrared Galaxies ((U)LIRGs), as it provides data on the nuclear environment, star formation, and even black hole fueling. Furthermore, the HNC/HCN line ratio is used to distinguish between photon-dissociation regions (PDRs) and X-ray-dissociation regions (XDRs) on the basis that [HNC]/[HCN] is roughly unity in PDR sources, but greater than unity in XDR sources.

The study of HNC is a relatively simple pursuit, and this is one of the greatest motivations for its study. Aside from having its J=1→0 transition in a clear portion of the atmospheric window, as well as having numerous isotopomers also available for easy study, and in addition to having a large dipole moment that makes observations particularly simple, HNC is, in its molecular nature, a quite simple molecule. This makes the study of the reaction pathways that lead to its formation and destruction a good means of obtaining insight to the workings of these reactions in space. Furthermore, the study of the tautomerization of HNC to HCN (and vice versa), which has been studied extensively, has been suggested as a model by which more complicated isomerization reactions can be studied.

Chemistry in the interstellar medium

HNC is found primarily in dense molecular clouds, though it is ubiquitous in the interstellar medium. Its abundance is closely linked to the abundances of other nitrogen containing compounds in a complex relationship partially demonstrated in the chart available on page 256 of this article. HNC is formed primarily through the dissociative recombination of HNCH+ and H2NC+, and it is destroyed primarily through ion-neutral reactions with H3+ and C+. These facts are corroborated in both this article and this article. Rate constants are taken from udfa.net, and data on fractional abundances is taken from this article. Rate calculations were done at 3.16x105 years, which is considered early time, and at 20K, which is a typical temperature for dense molecular clouds.
Formation Reactions
Reactant 1 Reactant 2 Product 1 Product 2 Rate constant Rate/[H2]2 Relative Rate
HCNH+ e- HNC H 9.50e-8 4.76e-25 3.4
H2NC+ e- HNC H 1.80e-7 1.39e-25 1.0
Destruction Reactions
Reactant 1 Reactant 2 Product 1 Product 2 Rate constant Rate/[H2]2 Relative Rate
H3+ HNC HCNH+ H2 8.10e-9 1.26e-24 1.7
C+ HNC C2N+ H 3.10e-9 7.48e-25 1.0


These four reactions are merely the four most dominant, and thus the most significant in the formation of the HNC abundances in dense molecular clouds; there are dozens more reactions for the formation and destruction of HNC. Though these reactions primarily lead to various protonated species, HNC is linked closely to the abundances of many other nitrogen containing molecules, for example, NH3 and CN. The pathways leading between these species can be found in the paper by Turner et al. that is linked above. The abundance HNC is also inexorably linked to the abundance of HCN, and the two tend to exist in a specific ratio based on the environment, as noted in the paper by Hiraoka et al. that is linked above. This is because the reactions that form HNC can often also form HCN, and vice versa, depending on the conditions in which the reaction occurs, and also that there exist isomerization reactions for the two species. A simplified pathway showing many of the methods of HNC formation and destruction is available as Fig. 10 from Turner et al.

Astronomical detections

HNC was first detected in June 1970 by L. E. Snyder and D. Buhl using the 36-foot radio telescope of the National Radio Astronomy Observatory (NRAO). The main molecular isotope, H12C14N, was observed via its J=1→0 transition at 88.6 GHz in six different sources: W3 (OH), Orion A, Sgr A(NH3A), W49, W51, DR 21(OH). A secondary molecular isotope, H13C14N, was observed via its J=1→0 transition at 86.3 GHz in only two of these sources: Orion A and Sgr A(NH3A). HNC was then later detected extragalactically in 1988 by C. Henkel, R. Mauersberger, and P. Schilke using the IRAM 30-m
IRAM 30-m
The IRAM 30-m Millimeter Radio Telescope is a radio telescope for astronomical observations in the millimeter range of wavelengths, operated by the Institute for Radio Astronomy in the Millimeter Range ) and located on the Sierra Nevada, in Spain, close to the Pico Veleta peak. It is the largest...

 telescope at the Pico de Veleta
Veleta (Sierra Nevada)
Veleta or Pico del Veleta is the third highest peak of the Iberian peninsula and the second highest in the Sierra Nevada. Its height is given variously as , and ....

in Spain. It was observed via its J=1→0 transition at 90.7 GHz toward IC342.

A number of detections have been made towards the end of confirming the temperature dependence of the abundance ratio of [HNC]/[HCN]. A strong fit between temperature and the abundance ratio would allow observers to spectroscopically detect the ratio and then extrapolate the temperature of the environment, thus gaining great insight into the environment of the species. In 1986, Goldsmith et al. measured the abundances of rare isotopes of HNC and HCN along the OMC-1 and determined that the abundance ratio varies by more than an order of magnitude in warm regions versus cold regions. In 1992, Schilke et al. measured abundances of HNC, HCN, and deuterated analogs along the OMC-1 ridge and core and confirmed the temperature dependence of the abundance ratio. Helmich and van Dishoeck performed a survey of the W 3 Giant Molecular Cloud in 1997 in which they detected over 24 different molecular isotopes, comprising over 14 distinct chemical species, including HNC, HN13C, and H15NC. This survey further confirmed the temperature dependence of the abundance ratio, [HNC]/[HCN], this time ever confirming the dependence of the isotopomers.

These are not the only detections of importance of HNC in the interstellar medium. In 1997, Pratap et al. observed HNC along the TMC-1 ridge and found that its abundance relative to HCO+ to be constant along the ridge – this led credence to the reaction pathway that posits that HNC is derived initially from HCO+. One significant astronomical detection that demonstrated the practical use of observing HNC occurred in 2006 by Tennekes et al., in which the authors detected and then used the abundances of various nitrogenous compounds (including HN13C and H15NC) to determine the stage of evolution of the protostellar core Cha-MMS1 based on the relative magnitudes of the abundances.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK