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Ethynyl radical
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The ethynyl radical (systematically named λ3-ethyne and hydridodicarbon(''C''—''C'')) is an organic compound with the chemical formula C≡CH (also written () or ). 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.〔Cochran, E. L., Adrian, F. J., and Bowers, V. A., ''ESR Study of Ethynyl and Vinyl Free Radicals'', J. Chem. Phys., 40, 213 (1964).〕 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.〔Tucker, K. D., Kutner, M. L., and Thaddeus, P., ''The Ethynyl Radical C2H--A New Interstellar Molecule'', ApJ, 193, L115-L119 (1974).〕 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).〔Huggins, P. J., Carlson, W. J., and Kinney, A. L., ''The distribution and abundance of interstellar C2H'', A&A, 133, 347-356 (1984).〕 Since there is 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).〔Vrtilek, J. M., Gottlieb, C. A., Langer, W. D., Thaddeus, P., and Wilson, R. W., ''Laboratory and Astronomical Detection of the Deuterated Ethynyl Radical CCD'', ApJ, 296, L35-L38 (1985).〕 One of the important indirect uses for observations of the ethynyl radical is the determination of acetylene abundances.〔Fuente, A., Cernicharo, J. and Omont, A., ''Inferring acetylene abundances from C2H: the C2H2/HCN abundance ratio'', A&A, 330, 232-242 (1998).〕 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.〔Beuther, H., Semenov, D., Henning, T., and Linz, H., ''Ethynyl (C2H) in Massive Star Formation: Tracing the Initial Conditions?'', ApJ, 675, L33-L36 (2008).〕 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).〔Bel, N. and Leroy, B., ''Zeeman splitting in interstellar molecules. II. The ethynyl radical'', A&A, 335, 1025-1028 (1998).〕
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