Konstantinos C. Christoforidis

Effect of metal ions on the indigenous radicals of humic acids: high field electron paramagnetic resonance study.

The interaction of indigenous radicals of humic acid (HA) with metal cations has been studied using high magnetic field (10.5T-285 GHz) electron paramagnetic resonance (HFEPR) spectroscopy. Strong [HA]-[metal] interaction was observed in the case of heavy metals, Cd(2+), Pb(2+), and Sr(2+), leading to formation of covalent bonds with the radicals of HA. On the contrary, alkaline earth metal ions, such as Mg(2+), generate only electrostatic interaction. The two types of indigenous radicals that exist in all HAs are influenced by the metal cations in a unified manner. This provides evidence that the two types of indigenous radicals in HAs originate from a unique, phenolic, moiety in HA. Mg(2+) ions dramatically changed the pH profile of the two radical types of HA, downshifting their interconversion pK(a) by ca. 3 pH units. This is the first experimental observation of the effect of metals on the H-dissociation of the radical centers in HAs.

EPR study of a novel [Fe–porphyrin] catalyst

This paper reports on an EPR and catalytic study of a novel [Fe-porphyrin] catalyst, [FeR4P], used for the catalytic decomposition of pentacholorophenol (PCP). The porphyrin complex [FeR4P] bears 2,6-di-tert-butylphenols (where R = di-ButPhen) at each meso-aryl position of the porphyrin ring. The FeR4P-SiO2 catalyst was found to be very efficient for the degradation of PCP, using NaIO4 as oxidant plus imidazole as cocatalyst. EPR spectroscopy shows that in the presence of the oxidant NaIO4, at a redox potential of Eh = 250 mV, [FeR4P] forms a high valent [FeIV = O Por+· ] state with an EPR spectrum characterized by effective g-values g ⊥ eff  = 3.41 and g || eff  = 2, similar to Compound I of Horseradish Peroxidase. The [FeIV = O Por+· ] state can described by a S = 1 state of the oxoferryl FeIV = O weakly coupled by exchange interaction J to a S′ = 1/2 porphyrin cation radical. The zero field splitting of the FeIV = O is D = +16.5 K and the coupling parameters are J/D = 0.31, J = +5.2 K. We suggest that the formation of [FeIV = O Por+· ] specie to be the key intermediate responsible for the observed efficient catalytic decomposition of PCP.