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 (1Πg, 3Πg, 1Σu+, and 3Σu+). Here, this model is extended to the electronically
excited states of four more molecules: CN (2Σ+), N2 (3Πu), SiC (3Π), and Si2 (3Πg). 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+), 2σg22σu11πu43σg21πg1 in N2 (3Πu), 5σ26σ17σ22π3 in SiC (3Π), and 4σg24σu15σg22πu3 in Si2 (3Πg), all of which have a triply occupied
“valence-core” (i.e., 2σg22σu1 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.