The carbon dimer or dicarbon (C2) has been known for nearly 150 years through its presence in
comets, as initially reported by Donati in 1864, who
observed the Swan bands that arise from an electronic exitation of
C2. Much later, Souza and Lutz reported the
first detection of C2 in an interstellar source in 1977. Their initial identification was made at the
Smithsonian Institution's Mount Hopkins Observatory
(now the Fred Lawrence Whipple Observatory) toward the hypergiant star
Since C2 is symmetric and does not have a dipole moment, the detection was based upon the molecule's
vibrational spectrum, using the Phillips bands.
Some of the additional interstellar detections include: ζ Ophiuci by
Chaffee and Lutz in 1978, ζ Persei and ο
Persei by Hobbs in
1981, respectively, and χ Ophiuchi, HD 154368,
147889, and 149404 by van Dishoeck and de Zeeuw in 1984.
The nature of the bonding in the ground state of C2 continues to be debated. While the
prevalent view is that it essentially has a double bond (mostly due to pi bonding), an argument has been advanced by
Shaik et al. that C2 has a quadruple bond.