Diabatic Valence-Hole States in the C 2 Molecule: "Putting Humpty Dumpty Together Again".

Despite the long history of spectroscopic studies of the C 2 molecule, fundamental questions about its chemical bonding are still being hotly debated. The complex electronic structure of C 2 is a consequence of its dense manifold of near-degenerate, low-lying electronic states. A global multi-state diabatic model is proposed here to disentangle the numerous configuration interactions that occur within four symmetry manifolds of excited states of C 2 ( 1 Π g , 3 Π g , 1 Σ u + , and 3 Σ u + ). The key concept of our model is the existence of two "valence-hole" configurations, 2 σ g 2 2 σ u 1 1 π u 3 3 σ g 2 for 1,3 Π g states and 2 σ g 2 2 σ u 1 1 π u 4 3 σ g 1 for 1,3 Σ u + states, that are derived from 3σ g ← 2σ u electron promotion. The lowest-energy state from each of the four C 2 symmetry species is dominated by this type of valence-hole configuration at its equilibrium internuclear separation. As a result of their large binding energy (nominal bond order of 3) and correlation with the 2s 2 2p 2 + 2s2p 3 separated-atom configurations, the presence of these valence-hole configurations has a profound impact on the global electronic structure and unimolecular dynamics of C 2 .