Carbonate and Carbonate Anion Radicals in Aqueous Solutions Exist as CO₃(H₂O)₆^2- and CO₃(H₂O)₆^.- Respectively: the Crucial Role of the Inner Hydration Sphere of Anions in Explaining their Properties

An effort to reproduce the chemical and physical properties of carbonate and carbonate anion radicals in aqueous solutions by DFT proves that one has to include an inner hydration sphere of six water molecules for both anions. Application of the PCM model to CO₃(H₂O)₆^2- and CO₃(H₂O)₆^.- enables to reach the experimental value of the redox potential of the CO₃(H₂O)₆^.-/2- couple. This is a result of the direct inclusion of a considerable charge transfer, CT, from CO₃^2- to its inner hydration sphere??? in the calculation of the hydration effect. HOMO of clusters is an analogue of non-bonding σ-type a₂’-HOMO of the parent CO₃ moiety with a σ*(OH) contribution. This is a MO manifestation of the CT to the first hydration shell. The localization of the CT on the first hydration shell also re-produces the very strong OH×××O stretch peak. Furthermore the very large difference in the hydration energies of CO₃^2- and CO₃^.- which cause the very large differences in the length of the C-O^…H-O hydrogen bonds suggests that the oxidations by CO₃^.- proceed via the inner sphere mechanism.