DFT/PCM investigation of the Mn(II) chemical speciation in aqueous solution

摘要

Chemical speciation of Mn(II) in aqueous solution has been investigated by means of density functional methods. Different geometries and electronic states of all species arising from the Mn(II) hydrolysis have been calculated. The Gibbs free energy of the hydrolysis reaction and the respective equilibrium constants have been estimated and compared with the available experimental data. Polarizable continuum model (PCM-UAHF/6-31+G(d)) was used to estimate the solvation energy. The ionization potential of the predominant species are linearly dependent on the number of hydroxyls in their structure. The Mn(II)/Mn(III) oxidation potential estimate is in good agreement with the experimental value, about 0.2 V smaller. The estimated hydrolysis reaction energies are in good agreement with the experimental values with an error of about 3 kcal mol (1) (PBE/TZVP). The unique exception is the [Mn(OH)(3)](-), hydrolysis constant which is not in good agreement with the experimental value. This is attributed to inaccuracy of the presently available experimental value and the PCM solvation free energy estimate. The results shown in this work reinforce the importance of the chemical speciation in order to understand at a molecular level the mechanism of processes involving transition metal ions. (C) 2008 Wiley Periodicals, Inc.