Is HO3− multiple-minimum and floppy? Covalent to van der Waals isomerization and bond rupture of a peculiar anion

The singlet ground-state of the HO3− anion is studied with high level single- and multireference methods, and the scheme termed complete-active-space-dynamical-correlation, which has been previously used to study the neutral HO3 radical. It is found to have a planar cis isomeric structure with a long intermediate O–O bond (≈1.75 Å), as is now consensual in the literature. It also has a pyramidal-type branch-isomer, but its minimum lies ≈33 kcal mol−1 above the cis minimum. Interestingly, another isomer is predicted here, with a planar geometry that can be even more stable than cis-HO3− at some levels of theory. It shows a hydrogen-bond (van der Waals) type structure, with an intermediate O–O bond of ≈2.59 Å. All such minima lie on the lowest adiabatic potential energy surface, with the two lowest planar ones (cis and vdW) connected by a saddle point whose structure, also planar, is unveiled. All these lie on the first third of the optimum path for bond-rupture in [HO–OO]−, which is predicted to yield ground state HO plus O2−, an asymptote lying 30 kcal mol−1 above the cis-HO3− minimum. Unprecedented in the literature on the key title anion, such features should bear strong implications for its preparation, spectroscopy, and role in chemistry.