Electronic Energy Levels of Aqueous Hydroxyl Species

We study the structural and electronic properties of the aqueous hydroxyl species in the negatively charged and neutral states employing ab initio molecular dynamics (MD) simulations and advanced electronic structure calculations at the hybrid functional and many-body perturbation theory levels of theory. We first investigate the microsolvation of the solutes ensuing from MD, which show that the long discussed hemibond for the radical species does not form. The analysis of the electronic structure of the two states of the solvated species indicates the presence of in-gap localized states in both cases. Both structural and electronic features can be captured provided the self-interaction error of density functional theory is properly treated. Next, we calculate the electronic energy levels, namely the vertical and adiabatic redox potentials of the OH−/OH• couple, through thermodynamic integration within the grand-canonical formulation of solutes. We demonstrate that properly describing the valence band edge of liquid water and accurately accounting for electrostatic finite-size effects associated with periodic supercells with built-in ionic polarization are fundamental to achieve reliable redox levels. The calculated energy levels at the hybrid-functonal and quasiparticle self-consistent GW level of theory are found to be in excellent agreement with the experiment, thus allowing us to provide reliable estimates for the solvent reorganization upon charge transfer.