On the Nature of Feiv=Oaq in Aqueous Media: A Dft Analysis

Fe-IV=O-aq is a key intermediate in many advanced oxidation processes and probably in biological systems. It is usually referred to as Fe-IV=O2+. The pKa's of Fe-IV=O-aq as derived by DFT are: pKa1=2.37 M06 L/6-311++G(d,p) (SDD for Fe) and pKa2=7.79 M06 L/6-311++G(d,p) (SDD for Fe). This means that in neutral solutions, Fe-IV=O-aq is a mixture of (H2O)(4)(OH)Fe-IV=O+ and (H2O)(2)(OH)(2)Fe-IV=O. The oxidation potential of Fe-IV=O-aq in an acidic solution, E-0{(H2O)(5)Fe-IV=O2+/Fe-III(H2O)(6)(3+), pH 0.0} is calculated with and without a second solvation sphere and the recommended value is between 2.86 V (B3LYP/Def2-TZVP, with a second solvation sphere) and 2.23 V (M06 L/Def2-TZVP without a second solvation sphere). This means that Fe-IV=O-aq is the strongest oxidizing agent formed in systems involving (FeO42-)-O-VI even in neutral media.