On The Nature of FeIV =Oaq in Aqueous Media: A DFT analysis

Abstract

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 (H₂ O)₄ (OH)Fe^IV =O⁺ and (H₂ O)₂ (OH)₂ Fe^IV =O. The oxidation potential of Fe^IV =O_aq in an acidic solution, E⁰ {(H₂ O)₅ Fe^IV =O²⁺ /Fe^III (H₂ O)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 Fe^VI O₄ ^2- even in neutral media.

Keywords: E0(Fe(IV)O2++2H+/Fe3+); aqueous iron; density functional calculations; iron; pKa.