Ethynyl radical
Encyclopedia
The ethynyl radical, with chemical formula C2H, is an abundant interstellar
polyatomic molecule that does not occur naturally on Earth. 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
. It was first observed in the gas phase by Tucker and coworkers in November 1973 toward the Orion Nebula
, using the NRAO
11m radio telescope. It has since been detected in a large variety of interstellar environments, including dense molecular cloud
s, bok globule
s, star forming regions
, the shells around carbon-rich evolved stars
, and even in other galaxies
.
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). One of the important indirect uses for observations of the ethynyl radical is the determination of acetylene
abundances. 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. 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 cyanide
(CN).
s) 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 (C2H3+). 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.
, and the molecule has a permanent dipole moment estimated to be
= 0.8 Debye = 2.7×10-30 coul·m. 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, 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. Transitions between two adjacent higher-lying rotational states have 11 hyperfine components. The molecular constants of the ground vibronic state are tabulated below.
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.
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...
polyatomic molecule that does not occur naturally on Earth. It was first observed by electron spin resonance
Electron paramagnetic resonance
Electron paramagnetic resonance or electron spin resonance spectroscopyis a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metal ion...
isolated in a solid argon matrix
Matrix Isolation
Matrix isolation is an experimental technique used in chemistry and physics which generally involves a material being trapped within an unreactive matrix. A host matrix is a continuous solid phase in which guest particles are embedded. The guest is said to be isolated within the host matrix...
at liquid helium temperatures in 1963 by Cochran and coworkers at the Johns Hopkins Applied Physics Laboratory
Applied Physics Laboratory
The Johns Hopkins University Applied Physics Laboratory , located in Howard County, Maryland near Laurel and Columbia, is a not-for-profit, university-affiliated research center employing 4,500 people. APL is primarily a defense contractor. It serves as a technical resource for the Department of...
. It was first observed in the gas phase by Tucker and coworkers in November 1973 toward the Orion Nebula
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...
, using the NRAO
National Radio Astronomy Observatory
The National Radio Astronomy Observatory is a Federally Funded Research and Development Center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc for the purpose of radio astronomy...
11m radio telescope. It has since been detected in a large variety of interstellar environments, including dense molecular cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
s, bok globule
Bok globule
Bok globules are dark clouds of dense cosmic dust and gas in which star formation sometimes takes place. Bok globules are found within H II regions, and typically have a mass of about 2 to 50 solar masses contained within a region about a light year or so across...
s, star forming regions
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
, the shells around carbon-rich evolved stars
Carbon star
A carbon star is a late-type star similar to a red giant whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds,...
, and even in other galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
.
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). Since there is typically insufficient quantities of C2H along a line of sight to make emission or absorption linesSpectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
optically thick, derived column densities can be relatively accurate (as opposed to more common molecules like CO
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, NO
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
, and OH
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
). 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
Fractionation
See also: Fractionated spacecraftFractionation is a separation process in which a certain quantity of a mixture is divided up in a number of smaller quantities in which the composition changes according to a gradient. Fractions are collected based on differences in a specific property of the...
theories (which explain the enhanced abundance of deuterated molecules in the interstellar medium). One of the important indirect uses for observations of the ethynyl radical is the determination of acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
abundances. Acetylene (C2H2) does not have a dipole moment
Electric dipole moment
In physics, the electric dipole moment is a measure of the separation of positive and negative electrical charges in a system of charges, that is, a measure of the charge system's overall polarity with SI units of Coulomb-meter...
, and therefore pure rotational transitions (typically occurring in the microwave region
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
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. Finally, high spectral resolution observations of Zeeman splitting
Zeeman effect
The Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field...
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 cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
(CN).
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.Formation
In the laboratory, C2H can be made via photolysis of acetylene (C2H2) or C2HCF3, or in a glow discharge of a mixture of acetylene and helium. 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 windStellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
s) into the outer envelope
Circumstellar envelope
Circumstellar envelope is the part of the star,having roughly spherical shape and not gravitationally bound to the star core.Usually circumstellar envelopes are formed from the dense stellar wind or presentbefore formation of the star...
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 (C2H3+). The neutral-neutral reaction of propynylidyne
Propynylidyne
Propynylidyne is a chemical compound that has been identified in interstellar space.- Simulated spectrum :A rotational spectrum of the 2Π electronic ground state of l-C3H can be made using the PGopher software and molecular constants extracted from the literature...
(C3H) and atomic oxygen also produces ethynyl (and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, CO), though this is typically not a dominant formation mechanism. The dominant creation reactions are listed below.
- C2H3+ + e- → C2H + H + H
- C2H3+ + e- → C2H + H2
- CH3CCH+ + e- → C2H + CH3
- C3H + O → C2H + CO
Destruction
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 H3+. The dominant destruction reactions are listed below.- C2H + O2 → HCO + CO
- C2H + N → C2N + H
- C2H + HCO+ → C2H2+ + CO
- C2H + N → C2H2+ + H2
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 collinearLine (geometry)
The notion of line or straight line was introduced by the ancient mathematicians to represent straight objects with negligible width and depth. Lines are an idealization of such objects...
, and the molecule has a permanent dipole moment estimated to be
= 0.8 Debye = 2.7×10-30 coul·m. The ground vibrational and electronic (vibronic) state exhibits a simple rigid rotor-type rotational spectrum. However, each rotational state exhibits fineFine structure
In atomic physics, the fine structure describes the splitting of the spectral lines of atoms due to first order relativistic corrections.The gross structure of line spectra is the line spectra predicted by non-relativistic electrons with no spin. For a hydrogenic atom, the gross structure energy...
and hyperfine structure
Hyperfine structure
The term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron...
, 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, 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. Transitions between two adjacent higher-lying rotational states have 11 hyperfine components. The molecular constants of the ground vibronic state are tabulated below.
Isotopologues
Three isotopologuesIsotopologue
Isotopologues are molecules that differ only in their isotopic composition. Simply, the isotopologue of a chemical species has at least one atom with a different number of neutrons than the parent....
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.
| Isotologue | Year Discovered | Molecular Constants (in MHz) | Transition Frequencies (in MHz) | ||
|---|---|---|---|---|---|
| 12C12CH | 1974 | B D  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 | B D  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 | B D  |
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 | B D  |
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 |