Ethynyl radical

Last updated
Ethynyl radical
Structural formula of the ethynyl radical Ethynyl-radical-2D.png
Structural formula of the ethynyl radical
Spacefill model of ethynyl radical Ethynyl-radical-3D-vdW.png
Spacefill model of ethynyl radical
Preferred IUPAC name
3D model (JSmol)
PubChem CID
  • InChI=1S/C2H/c1-2/h1H Yes check.svgY
  • InChI=1/C2H/c1-2/h1H
  • C#[C]
  • [C]#C
Molar mass 25.030 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

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
). It is a simple molecule that does not occur naturally on Earth but is abundant in the interstellar medium. It was first observed by electron spin resonance isolated in a solid argon matrix at liquid helium temperatures in 1963 by Cochran and coworkers at the Johns Hopkins Applied Physics Laboratory. [1] It was first observed in the gas phase by Tucker and coworkers in November 1973 toward the Orion Nebula, using the NRAO 11-meter radio telescope. [2] It has since been detected in a large variety of interstellar environments, including dense molecular clouds, bok globules, star forming regions, the shells around carbon-rich evolved stars, and even in other galaxies.


Astronomical Importance

Observations of C2H can yield a large number of insights into the chemical and physical conditions where it is located. First, the relative abundance of ethynyl is an indication of the carbon-richness of its environment (as opposed to oxygen, which provides an important destruction mechanism). [3] Since there are typically insufficient quantities of C2H along a line of sight to make emission or absorption lines optically thick, derived column densities can be relatively accurate (as opposed to more common molecules like CO, NO, and OH). Observations of multiple rotational transitions of C2H can result in estimates of the local density and temperature. Observations of the deuterated molecule, C2D, can test and extend fractionation theories (which explain the enhanced abundance of deuterated molecules in the interstellar medium). [4] One of the important indirect uses for observations of the ethynyl radical is the determination of acetylene abundances. [5] Acetylene (C2H2) does not have a dipole moment, and therefore pure rotational transitions (typically occurring in the microwave region of the spectrum) are too weak to be observable. Since acetylene provides a dominant formation pathway to ethynyl, observations of the product can yield estimates of the unobservable acetylene. Observations of C2H in star-forming regions frequently exhibit shell structures, which implies that it is quickly converted to more complex molecules in the densest regions of a molecular cloud. C2H can therefore be used to study the initial conditions at the onset of massive star formation in dense cores. [6] Finally, high-spectral-resolution observations of Zeeman splitting in C2H can give information about the magnetic fields in dense clouds, which can augment similar observations that are more commonly done in the simpler cyano radical (CN). [7]

Formation and destruction

The formation and destruction mechanisms of the ethynyl radical vary widely with its environment. The mechanisms listed below represent the current (as of 2008) understanding, but other formation and destruction pathways may be possible, or even dominant, in certain situations.


In the laboratory, C2H can be made via photolysis of acetylene (C2H2) or C2HCF3, [8] or in a glow discharge of a mixture of acetylene and helium. [9] In the envelopes of carbon-rich evolved stars, acetylene is created in the thermal equilibrium in the stellar photosphere. Ethynyl is created as a photodissociation product of the acetylene that is ejected (via strong stellar winds) into the outer envelope of these stars. In the cold, dense cores of molecular clouds (prior to star formation) where n > 104 cm−3 and T < 20 K, ethynyl is dominantly formed via an electron recombination with the vinyl radical (C
). [10] The neutral-neutral reaction of propynylidyne (C3H) and atomic oxygen also produces ethynyl (and carbon monoxide, CO), though this is typically not a dominant formation mechanism. The dominant creation reactions are listed below.


The destruction of ethynyl is dominantly through neutral-neutral reactions with O2 (producing carbon monoxide and formyl, HCO), or with atomic nitrogen (producing atomic hydrogen and C2N). Ion-neutral reactions can also play a role in the destruction of ethynyl, through reactions with HCO+ and H+
. The dominant destruction reactions are listed below.

Method of observation

The ethynyl radical is observed in the microwave portion of the spectrum via pure rotational transitions. In its ground electronic and vibrational state, the nuclei are collinear, and the molecule has a permanent dipole moment estimated to be μ = 0.8  D = 2.7×10−30 C·m. [2] The ground vibrational and electronic (vibronic) state exhibits a simple rigid rotor-type rotational spectrum. However, each rotational state exhibits fine and hyperfine structure, due to the spin-orbit and electron-nucleus interactions, respectively. The ground rotational state is split into two hyperfine states, and the higher rotational states are each split into four hyperfine states. Selection rules prohibit all but six transitions between the ground and the first excited rotational state. Four of the six components were observed by Tucker et al. in 1974, [2] the initial astronomical detection of ethynyl, and 4 years later, all six components were observed, which provided the final piece of evidence confirming the initial identification of the previously unassigned lines. [11] Transitions between two adjacent higher-lying rotational states have 11 hyperfine components. The molecular constants of the ground vibronic state are tabulated below.


Three isotopologues of the 12C12CH molecule have been observed in the interstellar medium. The change in molecular mass is associated with a shift in the energy levels and therefore the transition frequencies associated with the molecule. The molecular constants of the ground vibronic state, and the approximate transition frequency for the lowest 5 rotational transitions are given for each of the isotopologues in the table below.

Rotational transitions for ethenyl isotopologues
Molecular constants
Transition frequencies
12C12CH1974 [2] B
N = 1→0
N = 2→1
N = 3→2
N = 4→3
N = 5→4
12C12CD1985 [4] [12] B
N = 1→0
N = 2→1
N = 3→2
N = 4→3
N = 5→4
13C12CH1994 [13] B
N = 1→0
N = 2→1
N = 3→2
N = 4→3
N = 5→4
12C13CH1994 [13] B
N = 1→0
N = 2→1
N = 3→2
N = 4→3
N = 5→4

See also

Related Research Articles

In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation H3O+, the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous H+ and conjugate base. Three main structures for the aqueous proton have garnered experimental support: the Eigen cation, which is a tetrahydrate, H3O+(H2O)3, the Zundel cation, which is a symmetric dihydrate, H+(H2O)2, and the Stoyanov cation, an expanded Zundel cation, which is a hexahydrate: H+(H2O)2(H2O)4. Spectroscopic evidence from well-defined IR spectra overwhelmingly supports the Stoyanov cation as the predominant form. For this reason, it has been suggested that wherever possible, the symbol H+(aq) should be used instead of the hydronium ion.

<span class="mw-page-title-main">Interstellar medium</span> Matter and radiation in the space between the star systems in a galaxy

In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field. Although the density of atoms in the ISM is usually far below that in the best laboratory vacuums, the mean free path between collisions is short compared to typical interstellar lengths, so on these scales the ISM behaves as a gas (more precisely, as a plasma: it is everywhere at least slightly ionized), responding to pressure forces, and not as a collection of non-interacting particles.

<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">Hydroxyl radical</span> Neutral form of the hydroxide ion (OH−)

The hydroxyl radical is the diatomic molecule
. The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also important in the field of radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and SCC in coolant systems subjected to radioactive environments.

<span class="mw-page-title-main">Diffuse interstellar bands</span>

Diffuse interstellar bands (DIBs) are absorption features seen in the spectra of astronomical objects in the Milky Way and other galaxies. They are caused by the absorption of light by the interstellar medium. Circa 500 bands have now been seen, in ultraviolet, visible and infrared wavelengths.

<span class="mw-page-title-main">Theoretical astronomy</span> Applied and interdisciplinary physics

Theoretical astronomy is the use of analytical and computational models based on principles from physics and chemistry to describe and explain astronomical objects and astronomical phenomena. Theorists in astronomy endeavor to create theoretical models and from the results predict observational consequences of those models. The observation of a phenomenon predicted by a model allows astronomers to select between several alternate or conflicting models as the one best able to describe the phenomena.

<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+
, consisting of three hydrogen nuclei (protons) sharing two electrons.

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.

Propynylidyne is a chemical compound that has been identified in interstellar space.

Interstellar formaldehyde (a topic relevant to astrochemistry) was first discovered in 1969 by L. Snyder et al. using the National Radio Astronomy Observatory. Formaldehyde (H2CO) was detected by means of the 111 - 110 ground state rotational transition at 4830 MHz. On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).

<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">Protonated hydrogen cyanide</span> Chemical compound

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.

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

Cyclopropenylidene, or c-C3H2, is a partially aromatic molecule belonging to a highly reactive class of organic molecules known as carbenes. On Earth, cyclopropenylidene is only seen in the laboratory due to its reactivity. However, cyclopropenylidene is found in significant concentrations in the interstellar medium (ISM) and on Saturn's moon Titan. Its C2v symmetric isomer, propadienylidene (CCCH2) is also found in the ISM, but with abundances about an order of magnitude lower. A third C2 symmetric isomer, propargylene (HCCCH), has not yet been detected in the ISM, most likely due to its low dipole moment.

<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).

<span class="mw-page-title-main">William Klemperer</span> American chemist

William A. Klemperer (October 6, 1927 – November 5, 2017) was an American chemist who was one of the most influential chemical physicists and molecular spectroscopists in the second half of the 20th century. Klemperer is most widely known for introducing molecular beam methods into chemical physics research, greatly increasing the understanding of nonbonding interactions between atoms and molecules through development of the microwave spectroscopy of van der Waals molecules formed in supersonic expansions, pioneering astrochemistry, including developing the first gas phase chemical models of cold molecular clouds that predicted an abundance of the molecular HCO+ ion that was later confirmed by radio astronomy.

In organic chemistry, cyanopolyynes are a family of organic compounds with the chemical formula HCnN (n = 3,5,7,…) and the structural formula H−[C≡C−]nC≡N (n = 1,2,3,…). Structurally, they are polyynes with a cyano group (−C≡N) covalently bonded to one of the terminal acetylene units (H−C≡C).

<span class="mw-page-title-main">Imidogen</span> Inorganic radical with the chemical formula NH

Imidogen is an inorganic compound with the chemical formula NH. Like other simple radicals, it is highly reactive and consequently short-lived except as a dilute gas. Its behavior depends on its spin multiplicity.

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

Phosphorus monoxide is an unstable radical inorganic compound with molecular formula PO.

<span class="mw-page-title-main">HD 73882</span> Eclipsing binary system in constellation Vela

HD 73882 is a visual binary system with the components separated by 0.6″ and a combined spectral class of O8. One of stars is an eclipsing binary system. The period of variability is listed as both 2.9199 days and 20.6 days, possibly due to the secondary being a spectroscopic binary star.


  1. Cochran, E. L.; Adrian, F. J.; Bowers, V. A. (1964). "ESR Study of Ethynyl and Vinyl Free Radicals". Journal of Chemical Physics. 40 (1): 213. Bibcode:1964JChPh..40..213C. doi:10.1063/1.1724865.
  2. 1 2 3 4 Tucker, K. D.; Kutner, M. L.; Thaddeus, P. (1974). "The Ethynyl Radical C2H – A New Interstellar Molecule". Astrophysical Journal. 193: L115–L119. Bibcode:1974ApJ...193L.115T. doi:10.1086/181646.
  3. Huggins, P. J.; Carlson, W. J.; Kinney, A. L. (1984). "The distribution and abundance of interstellar C2H". Astronomy & Astrophysics. 133: 347–356. Bibcode:1984A&A...133..347H.
  4. 1 2 Vrtilek, J. M.; Gottlieb, C. A.; Langer, W. D.; Thaddeus, P.; Wilson, R. W. (1985). "Laboratory and Astronomical Detection of the Deuterated Ethynyl Radical CCD". Astrophysical Journal. 296: L35–L38. Bibcode:1985ApJ...296L..35V. doi: 10.1086/184544 .
  5. Fuente, A.; Cernicharo, J.; Omont, A. (1998). "Inferring acetylene abundances from C2H: the C2H2/HCN abundance ratio". Astronomy & Astrophysics. 330: 232–242. Bibcode:1998A&A...330..232F.
  6. Beuther, H.; Semenov, D.; Henning, T.; Linz, H. (2008). "Ethynyl (C2H) in Massive Star Formation: Tracing the Initial Conditions?". Astrophysical Journal. 675 (1): L33–L36. arXiv: 0801.4493 . Bibcode:2008ApJ...675L..33B. doi:10.1086/533412. S2CID   15820346.
  7. Bel, N.; Leroy, B. (1998). "Zeeman splitting in interstellar molecules. II. The ethynyl radical". Astronomy & Astrophysics. 335: 1025–1028. Bibcode:1998A&A...335.1025B.
  8. Fahr, A. (2003). "Ultraviolet absorption spectrum and cross-sections of ethynyl (C2H) radicals". Journal of Molecular Spectroscopy. 217 (2): 249. Bibcode:2003JMoSp.217..249F. doi:10.1016/S0022-2852(02)00039-5.
  9. Müller, H. S. P.; Klaus, T.; Winnewisser, G. (2000). "Submillimeter-wave spectrum of the ethynyl radical, CCH, up to 1 THz". Astronomy & Astrophysics. 357: L65. Bibcode:2000A&A...357L..65M.
  10. Woodall, J.; Agúndez, M.; Markwick-Kemper, A. J.; Millar, T. J. (2007). "The UMIST database for astrochemistry 2006". Astronomy & Astrophysics. 466 (3): 1197. arXiv: 1212.6362 . Bibcode:2007A&A...466.1197W. doi:10.1051/0004-6361:20064981.
  11. Tucker, K. D.; Kutner, M. L. (1978). "The Abundance and Distribution of Interstellar C2H". Astrophysical Journal. 222: 859. Bibcode:1978ApJ...222..859T. doi:10.1086/156204.
  12. Combes, F.; Boulanger, F.; Encrenaz, P. J.; Gerin, M.; Bogey, M.; Demuynck, C.; Destombes, J. L. (1985). "Detection of interstellar CCD". Astronomy & Astrophysics. 147: L25. Bibcode:1985A&A...147L..25C.
  13. 1 2 Saleck, A. H.; Simon, R.; Winnewisser, G.; Wouterloot, J. G. A. (1994). "Detection of interstellar 13C12CH and 12C13CH". Canadian Journal of Physics. 72: 747. Bibcode:1994CaJPh..72..747S. doi:10.1139/p94-098.