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

Abstract

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, 1Σ u + , and 3Σ u + ). The key concept of our model is the existence of two “valence-hole” configurations, for 1,3Π g states and for 1,3Σ u + states, that are derived from 3σ g ← 2σ u 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.

Multireference configuration interaction study of the predissociation of C2 via its F1Π u state

“…The Deslandres-d'Azambuja C 1 Π g − A 1 Π u and Messerle-Krauss C ′ 1 Π g − A 1 Π u bands were only observed in very early studies [29][30][31][32]. However, recent calculations suggested that the Messerle-Krauss band is actually a part of the Deslandres-d'Azambuja band [11,33], which is also verified by our calculation here. Among the three 3 Π u states in this region, the e 3 Π g state was discovered early [34,35].…”

“…The C 1 Π g state has a notable avoided crossing with 2 1 Π g , which has been well described by a diabatic valence-hole model. [33] The adiabatic D 1 Σ + u state has an avoided crossing with the adiabatic 2 1 Σ + u state at R = 1.75 Å. From the shape of the two adiabatic curves, the diabatic 2 1 Σ + u state in this region has a potential well around R = 1.90 Å, which is very close to the avoided crossing point.…”

“…As discussed later, the additional diffuse functions are important for obtaining accurate electronic energies for Rydberg states. In total, the basis set comprises 270 orbitals, with (50,33,33,19,50,33,33,19) symmetry-adapted functions in D 2h . Tests were also performed using additional Dunning's augmented core-valence basis sets aug-cc-pCV5Z and augcc-pCV6Z [58,59]; as expected, for Rydberg states these basis sets had minimal effect on the calculated energy but increased the calculation time by a factor of about 2.5.…”

A New Band System of the Dicarbon Molecule in the Vacuum Ultraviolet Region

“…In addition to the important applications relevant in astrochemistry, the bonding nature of C 2 in its ground electronic state has long been a challenging while at the same time interesting topic in the study of chemical bonding. There have been substantial debates about whether C 2 has a real quadruple bond in its ground electronic state. − Because of its relevance in astrochemistry and chemical bonding, spectroscopic research on C 2 has a history spanning more than two centuries, and it remains a subject of significant current experimental and theoretical interest. , …”

Spectroscopic Study of a New Electronic Band System 33Δg-a3Πu of C2

“…In the recent work by Jiang et al, it was shown that the previously neglected valence-hole states, which intrinsically have large binding energy and correlated with high lying atomic fragment channels, can lead to large and systematic disruptions to the highly excited electronic states of C 2 . 17 Π g and 1,3 Σ u + and thus should not significantly perturb the 3 3 Δ g state here.…”