An interesting case of symmetry breaking in the absence of symmetry: the bicarbonate radical (HCO3)

Abstract

As part of a computational study of the thermochemical properties of systems related to carbonic acid (HCO), an interesting phenomenon was observed in the bicarbonate radical (HCO). This species belongs to the class of formoxyl radicals, but lacks the geometric symmetry of the more heavily-studied and prototypical HCO and FCO molecules. However, it is not immune from the problems that those molecules present to electronic structure calculations. Specifically, at geometries in the vicinity of the equilibrium structure, three solutions to the unrestricted Hartree-Fock equations can be found: two in which the unpaired spin is largely localized onto one of the (inequivalent) carbonyl oxygen atoms, and one in which the spin is delocalized. While all three of these reference functions comprise orbitals that transform as pure irreducible representations of the point group, the two localized choices qualitatively correspond to the symmetry-broken solution of the formoxyl species and the delocalized solution qualitatively corresponds to the symmetry-preserving (but orbitally unstable) solution. The three solutions are described in this work, as well as the corresponding ambiguities that result in the thermochemical characterization of this species.