Diabatic Valence-Hole Concept

A global diabatization scheme, based on the “valence-hole” concept, has been previously applied to model webs of avoided crossings that exist in four electronic-state symmetry manifolds of C₂ (¹Π_g, ³Π_g, ¹Σ_u⁺, and ³Σ_u⁺). Here, this model is extended to the electronically excited states of four more molecules: CN (²Σ⁺), N₂ (³Π_u), SiC (³Π), and Si₂ (³Π_g). Many strangenesses in the spectroscopic observations (e.g., energy level structure, predissociation linewidths, and radiative lifetimes) for all four electronic state systems discussed here are accounted for by this unified model. The key concept of the model is valence-hole electron configurations: 3σ²4σ¹1π⁴5σ² in CN (²Σ⁺), 2σ_g²2σ_u¹1π_u⁴3σ_g²1π_g¹ in N₂ (³Π_u), 5σ²6σ¹7σ²2π³ in SiC (³Π), and 4σ_g²4σ_u¹5σ_g²2π_u³ in Si₂ (³Π_g), all of which have a triply occupied “valence-core” (i.e., 2σ_g²2σ_u¹ or the equivalent). These valence-hole configurations have a nominal bond order of three or higher and correlate with high-energy separated-atom limits with an np ← ns (n = 2, 3) promotion in one of the atomic constituents. On its way to dissociation, the strongly bound diabatic valence-hole state crosses multiple weakly bound or repulsive states, which belong to electron configurations with a completely filled valence-core. These curve crossings between diabatic potentials result in a network of many avoided crossings among multiple electronic states, analogous to the well-studied electronic structure landscape of ionic-covalent crossings in strongly ionic molecules. Considering the unique role of valence-hole states in shaping the global electronic structure, the valence-hole concept should be added to our intuitive framework of chemical bonding.