Hidden Hemibonding in the Aqueous Hydroxyl Radical

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• The nature of the hemibond in aqueous hydroxyl radical (•OH(aq)) has been a long-standing debate.
• Previous simulations suggested the hemibond might be a self-interaction artifact due to limitations in density functionals.
• The oxygen radial distribution function (RDF) for H₂O···•OH shows a feature attributed to the hemibond, but its presence is sensitive to the computational method.

Purpose of the Study:

• To resolve the debate surrounding the existence and characteristics of the hemibond in •OH(aq).
• To investigate the role of computational methods, specifically density functionals, in observing the hemibond.
• To understand the contribution of hemibonded configurations to the electronic spectra of •OH(aq).

Main Methods:

• Employed periodic simulations utilizing the PBE0+D3 level of theory.
• Analyzed the oxygen radial distribution function (RDF) for H₂O···•OH.
• Examined spin density delocalization to identify the hemibond.
• Computed electronic spectra for •OH(aq) and compared with experimental data.

Main Results:

• The hemibond in •OH(aq) persists even with hybrid density functionals like PBE0, evidenced by spin density delocalization.
• The hemibond feature in the RDF is obscured by the hydrogen-bonded feature due to a slight elongation of the hemibond.
• Computed electronic spectra align well with experimental results, highlighting the significance of hemibond-like configurations.
• An intense charge-transfer transition in hemibonded configurations strongly influences spectroscopy.

Conclusions:

• The two-center, three-electron hemibond in •OH(aq) is a real phenomenon, not an artifact.
• Even 25% exact exchange in PBE0 is insufficient to completely eliminate unpaired spin delocalization.
• Hemibonded configurations play a crucial role in the observed electronic spectra of aqueous hydroxyl radicals.