Despite the long history of spectroscopic studies of the C2 molecule, fundamental questions about its electronic structure are still the subject of fierce debate. The complexity of the electronic structure of C2, in comparison to the other C/N/O diatomic molecules, is a consequence of its dense manifold of near-degenerate, low-lying electronic states. A global multi-state diabatic model is proposed here. This model disentangles the interactions within four symmetry manifolds of C2 (1,3Σu+ and 1,3Πg). The key concept of our model is the existence of two “valence-hole" configurations, 2σg22σu11πu33σg2 for 1,3Πg and 2σg22σu11πu43σg1 for 1,3Σu+, which derive from 3σg or 1πu←2σu electron promotion. The lowest energy state in each of the four symmetry species is dominated by this type of valence-hole configuration at its equilibrium internuclear distance. As a result of the large binding energies of the valence-hole configurations (nominal bond order of 3) and correlation with the 2s22p2+2s2p3 separated atom configurations, the presence of these valence-hole configurations fundamentally changes the zero-order picture of the global electronic structure of C2. We propose the valence-hole concept as a new “arrow” in the “quiver” of intuitive electronic structure models. The diabatic picture, combined with the traditional molecular orbital theory concepts of bond order and correlation diagram, is the key to uncovering the impact of the valence-hole state on the global electronic structure of C2. In light of the success of our global diabatization scheme for the C2 1,3Σu+ and 1,3Πg states, we propose a diabatic model re-analysis of similar interactions in the other C/N/O diatomic molecules, for which the valence-hole states are expected to have a similar impact on their global electronic structure.

Diabatic valence-hole states in the C2 molecule: "Putting Humpty Dumpty together again"

ACS Spring 2022

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