The ethynyl radical (systematically named λ
3-ethyne and hydridodicarbon(''C''—''C'')) is an
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...
with the
chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbol ...
C≡CH (also written
CHor ). 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
Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the sp ...
isolated in a
solid argon matrix at liquid helium temperatures in 1963 by Cochran and coworkers at the
Johns Hopkins Applied Physics Laboratory
The Johns Hopkins University Applied Physics Laboratory (Applied Physics Laboratory, or APL) is a not-for-profit university-affiliated research center (UARC) in Howard County, Maryland. It is affiliated with Johns Hopkins University and emplo ...
.
It was first observed in the gas phase by Tucker and coworkers in November 1973 toward the
Orion Nebula, using the
NRAO
The National Radio Astronomy Observatory (NRAO) 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 a ...
11-meter radio telescope.
It has since been detected in a large variety of interstellar environments, including dense
molecular cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydroge ...
s,
bok globule
In astronomy, Bok globules are isolated and relatively small dark nebulae, containing dense cosmic dust and gas from which star formation may take place. Bok globules are found within H II regions, and typically have a mass of about 2 to 50 solar ...
s,
star forming regions, the shells around
carbon-rich evolved stars, and even in other
galaxies.
Astronomical Importance
Observations of C
2H 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 are typically insufficient quantities of C
2H 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 C
2H can result in estimates of the local density and temperature. Observations of the deuterated molecule, C
2D, can test and extend
fractionation
Fractionation is a separation process in which a certain quantity of a mixture (of gases, solids, liquids, enzymes, or isotopes, or a suspension) is divided during a phase transition, into a number of smaller quantities (fractions) in which 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 abundances.
Acetylene (C
2H
2) 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 C
2H 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. C
2H 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
The Zeeman effect (; ) is the effect of splitting of a spectral line into several components in the presence of a static magnetic field. It is named after the Dutch physicist Pieter Zeeman, who discovered it in 1896 and received a Nobel prize ...
in C
2H 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 is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
(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 () understanding, but other formation and destruction pathways may be possible, or even dominant, in certain situations.
Formation
In the laboratory, C
2H can be made via
photolysis of acetylene (C
2H
2) or C
2HCF
3,
or in a
glow discharge
A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the st ...
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 winds) into the outer
envelope
An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card.
Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a sh ...
of these stars. In the cold, dense cores of molecular clouds (prior to star formation) where ''n'' > 10
4 cm
−3 and ''T'' < 20 K, ethynyl is dominantly formed via an electron recombination with the
vinyl radical ().
The neutral-neutral reaction of
propynylidyne (C
3H) and atomic oxygen also produces ethynyl (and
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
, CO), though this is typically not a dominant formation mechanism. The dominant creation reactions are listed below.
* + e
− → C
2H + H + H
* + e
− → C
2H + H
2
*CH
3CCH
+ + e
− → C
2H + CH
3
*C
3H + O → C
2H + CO
Destruction
The destruction of ethynyl is dominantly through neutral-neutral reactions with O
2 (producing carbon monoxide and
formyl, HCO), or with atomic nitrogen (producing atomic hydrogen and C
2N). Ion-neutral reactions can also play a role in the destruction of ethynyl, through reactions with HCO
+ and
. The dominant destruction reactions are listed below.
*C
2H + O
2 → HCO + CO
*C
2H + N → C
2N + H
*C
2H + HCO
+ → + CO
*C
2H + → + H
2
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
In geometry, collinearity of a set of points is the property of their lying on a single line. A set of points with this property is said to be collinear (sometimes spelled as colinear). In greater generality, the term has been used for aligned o ...
, and the molecule has a permanent dipole moment estimated to be ''μ'' = 0.8
D = .
The ground vibrational and electronic (vibronic) state exhibits a simple
rigid rotor
In rotordynamics, the rigid rotor is a mechanical model of Rotation, rotating systems. An arbitrary rigid rotor is a 3-dimensional Rigid body, rigid object, such as a top. To orient such an object in space requires three angles, known as Euler an ...
-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.
Isotopologues
Three
isotopologue
In chemistry, isotopologues are molecules that differ only in their isotopic composition. They have the same chemical formula and bonding arrangement of atoms, but at least one atom has a different number of neutrons than the parent.
An exampl ...
s of the
12C
12CH 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.
:
See also
*
List of molecules in interstellar space
This is a list of molecules that have been detected in the interstellar medium and circumstellar envelopes, grouped by the number of component atoms. The chemical formula is listed for each detected compound, along with any ionized form that has a ...
References
{{Hydrides by group
Alkynyl groups
Free radicals