Diabatic valence-hole states in the C$_2$ molecule: "Putting Humpty Dumpty together again"

Abstract

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 within four symmetry manifoldsThe key concept of our model is the existence of two "valence-hole" configurations, 2σ 2 g 2σ 1 u 1π 3 u 3σ 2 g for 1,3 Π g states and 2σ 2 g 2σ 1 u 1π 4 u 3σ 1 g for 1,3 Σ + u states that derive 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 separatedatom configurations, the presence of these valence-hole configurations has a profound impact on the global electronic structure and unimolecular dynamics of C 2 .