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.
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]
