Diabatic Valence-Hole States in the C2 Molecule: “Putting Humpty Dumpty Together Again”

Despite the long history of spectroscopic studies of the C2 molecule, fundamental questions about its chemical bonding are still being hotly debated. The complex electronic structure of C2 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 C2 (1Πg, 3Πg, 1Σu+ , and 3Σu+ ). The key concept of our model is the existence of two “valence-hole” configurations, 2σg22σu11πu33σg2 for 1,3Πg states and 2σg22σu11πu43σg1 for 1,3Σu+ states, that are derived from 3σg ← 2σu electron promotion. The lowest-energy state from each of the four C2 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 2s22p2 + 2s2p3 separated-atom configurations, the presence of these valence-hole configurations has a profound impact on the global electronic structure and unimolecular dynamics of C2.