Kinetics of the reaction of ferrous ions with hydroxyl radicals in the temperature range 25-300 C

The kinetics and mechanism of the reaction between OH radicals and ferrous ions in the temperature range 25-300 degrees C were studied using pulse radiolysis. At temperatures <150 degrees C the rate of reaction is essentially independent of temperature, while at temperatures >150 degrees C the activation energy is 45.8 +/- 3.0 kJ mol(-1). The change in activation energy is attributed to a change in the dominant mechanism from hydrogen atom transfer (HAT) to dissociative ligand interchange. The kinetic isotope effect (KIE) was measured by repeating experiments in heavy water. A value of 2.9 was measured at room temperature where HAT is the dominant mechanism. The KIE decreases to zero at temperatures > 150 degrees C as ligand interchange becomes dominant and the O-H bond is no longer involved in the reaction.