| |||
| Names | |||
|---|---|---|---|
| Preferred IUPAC name Ethynyl | |||
| Identifiers | |||
3D model (JSmol) | |||
| 1814004 | |||
| ChEBI | |||
| ChemSpider |
| ||
| 48916 | |||
| |||
| |||
| Properties | |||
| C2H | |||
| Molar mass | 25.030 g·mol−1 | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Theethynyl radical (systematically namedλ3-ethyne andhydridodicarbon(C—C)) is anorganic compound with thechemical formula C≡CH (also written [CCH] orC
2H). It is a simple molecule that does not occur naturally on Earth but is abundant in theinterstellar medium. It was first observed byelectron spin resonance isolated in asolid argon matrix at liquid helium temperatures in 1963 by Cochran and coworkers at theJohns Hopkins Applied Physics Laboratory.[1] It was first observed in the gas phase by Tucker and coworkers in November 1973 toward theOrion Nebula, using theNRAO 11-meter radio telescope.[2] It has since been detected in a large variety of interstellar environments, including densemolecular clouds,bok globules,star forming regions, the shells aroundcarbon-rich evolved stars, and even in othergalaxies.
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 makeemission or absorption lines optically thick, derived column densities can be relatively accurate (as opposed to more common molecules likeCO,NO, andOH). 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 extendfractionation 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 ofacetylene abundances.[5] Acetylene (C2H2) does not have adipole moment, and therefore pure rotational transitions (typically occurring in themicrowave 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 ofZeeman 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 simplercyano radical (CN).[7]
The formation and destruction mechanisms of the ethynyl radical vary widely with its environment. The mechanisms listed below represent the current (as of 2008[update]) understanding, but other formation and destruction pathways may be possible, or even dominant, in certain situations.
In the laboratory, C2H can be made viaphotolysis of acetylene (C2H2) or C2HCF3,[8] or in aglow 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 strongstellar winds) into the outerenvelope of these stars. In the cold, dense cores of molecular clouds (prior to star formation) wheren > 104 cm−3 andT < 20 K, ethynyl is dominantly formed via an electron recombination with thevinyl radical (C
2H+
3).[10] The neutral-neutral reaction ofpropynylidyne (C3H) and atomic oxygen also produces ethynyl (andcarbon 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 andformyl, 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+ andH+
3. The dominant destruction reactions are listed below.
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 arecollinear, and the molecule has a permanent dipole moment estimated to beμ = 0.8 D =2.7×10−30 C·m.[2] The ground vibrational and electronic (vibronic) state exhibits a simplerigid rotor-type rotational spectrum. However, each rotational state exhibitsfine andhyperfine 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 Tuckeret 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.
Threeisotopologues of the12C12CH 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.
| Isotopologue | Year discovered | Molecular constants (MHz) | Transition frequencies (MHz) | ||
|---|---|---|---|---|---|
| 12C12CH | 1974[2] | B D γ b c | 43674.534 0.1071 −62.606 40.426 12.254 | N = 1→0 N = 2→1 N = 3→2 N = 4→3 N = 5→4 | 87348.64 174694.71 262035.64 349368.85 436691.79 |
| 12C12CD | 1985[4][12] | B D γ b c | 36068.035 0.0687 −55.84 6.35 1.59 | N = 1→0 N = 2→1 N = 3→2 N = 4→3 N = 5→4 | 72135.80 144269.94 216400.79 288526.69 360646.00 |
| 13C12CH | 1994[13] | B D γ | 42077.459 0.09805 −59.84 | N = 1→0 N = 2→1 N = 3→2 N = 4→3 N = 5→4 | 84154.53 168306.70 252454.16 336594.57 420725.57 |
| 12C13CH | 1994[13] | B D γ | 42631.3831 0.10131 −61.207 | N = 1→0 N = 2→1 N = 3→2 N = 4→3 N = 5→4 | 85262.36 170522.29 255777.36 341025.13 426263.18 |
