Diatomic carbon

Diatomic carbon is a diatomic molecule of carbon (C₂), which occurs when in an electric arc (along with some buckyballs), in comets, and in the blue light we see in flames.^{[ 1 ]}

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Chemistry

Though valence bond theory would predict a quadruple bond as the only way to satisfy the octet rule in diatomic carbon, this is an incorrect prediction and is indicative of fundamental flaws in valence bond theory.

Molecular Orbital theory shows that there are two sets of paired electrons in the sigma system (one bonding, one antibonding), and two sets of paired electrons in a degenerate pi bonding set of orbitals. This adds up to give a bond order of 2, meaning that there exists a double bond between the two carbons. This is interesting because the MO diagram of diatomic carbon shows that there are two pi bonds and no sigma bonds.

Bond dissociation energies of B₂, C₂, and N₂ show an increasing BDE, indicating single, double, and triple bonds, respectively.

C₂ is not a commonly encountered molecule, as carbon is far more stable as diamond, graphite, and fullerenes.

Comets

The light of fainter comets mainly originates from the emission of diatomic carbon. There are several lines of C₂ light, mostly in the visual wavelengths, forming the Swan bands. ^{[ 2]}

^  Roald Hoffmann (1995). "C2 In All Its Guises". American Scientist 83: 309–311.

^ Herman Mikuz, Bojan Dintinjana. CCD Photometry of Comets. Retrieved on 2006-10-26.