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 C (Π, Π, Σ, and Σ). Here, this model is extended to the electronically excited states of four more molecules: CN (Σ), N (Π), SiC (Π), and Si (Π). 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 model. The key concept of the model is valence-hole electron configurations: 3σ4σ1π5σ in CN (Σ), 2σ2σ1π3σ1π in N (Π), 5σ6σ7σ2π in SiC (Π), and 4σ4σ5σ2π in Si (Π), all of which have a triply occupied "valence-core" (i.e., 2σ2σ 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 ( = 2, 3) promotion in 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.