Aqueous transition-metal cations as impurities in a wide gap oxide: the Cu(2+)/Cu(+) and Ag(2+)/Ag(+) redox couples revisited.

Abstract

The interactions of the d electrons of transition-metal aqua ions with the solvent are usually divided in short-range electronic interactions with ligand water molecules and long-range electrostatic interactions with molecules beyond the first coordination shell. This is the rationale behind the cluster continuum and QM/MM methods developed for the computation of the redox potentials. In the density functional theory based molecular dynamics (DFTMD) method, the electronic states of the complex are also allowed to mix with the extended band states of the solvent. Returning to the Cu(+) and Ag(+) oxidation reaction, which has been the subject of DFTMD simulation before, we show that coupling to the valence band states of water is greatly enhanced by the band gap error in the density functional approximation commonly used in DFTMD (the generalized gradient approximation). This effect is analyzed by viewing the solvent as a wide gap oxide and the redox active ions as electronic defects. The errors can be reduced significantly by application of hybrid functionals containing a fraction of Hartree-Fock exchange. These calculations make use of recent progress in DFTMD technology, enabling us to include sp core polarization and Hartree-Fock exchange in condensed-phase model systems.