CrN, CuB, and AuB: A Tale of Two Dissociation Limits.

Predissociation thresholds corresponding to dissociation at ground state separated atom limits (SALs) have been recorded in this group for more than 100 - and -block metal-containing molecules. The metal atom electronic degeneracies in these molecules generate a dense manifold of electronic states that allow high-lying vibronic levels to couple to pathways leading to dissociation. However, CrN, CuB, and AuB fail to dissociate at their ground SAL. Instead, the molecules remain bound at energies that far surpass their bond dissociation energies (BDEs), and their bonds break only when excited at or above an excited SAL. Sharp predissociation thresholds at excited SALs nevertheless allowed BDEs to be derived: D(CrN): 3.941(22) eV; D(CuB): 2.26(15) eV; D(AuB): 3.724(3) eV. A previous measurement of D(AlCr) is re-evaluated as dissociating to a higher energy limit, giving a revised value of D(AlCr) = 1.32(2) eV. A discussion of this physical behavior is presented.