Hidden Hemibonding in the Aqueous Hydroxl Radical

The existence of a two-center, three-electron hemibond in the first solvation shell of OH(aq) has long been a matter of debate. The hemibond manifests in ab initio molecular dynamics simulations as a small-r feature in the oxygen radial distribution function (RDF) for H2O...OH, but that feature disappears when semilocal density functionals are replaced with hybrids, suggesting a self-interaction artifact. Using periodic simulations at the PBE0+D3 level, we demonstrate that the hemibond is actually still present (as evidenced by delocalization of the spin density onto nearby water molecules) but is obscured by the hydrogen-bonded feature in the RDF, due to a slight elongation of the hemibond. Computed electronic spectra for OH(aq) are in excellent agreement with experiment and confirm that hemibond-like configurations play an outsized role in the spectroscopy due to an intense charge-transfer transition that is strongly attenuated in hydrogen-bonded configurations. Apparently, 25% exact exchange does not eliminate delocalization of unpaired spins.