Redox behavior of transient species produced during pulse radiolysis of morpholine in aqueous solutions

Hydroxyl radical reactions with morpholine (MOR) in aqueous solution lead to the formation of both oxidizing and reducing radical species at alkaline (pH≥9.8) and acidic/neutral (pH≤7) solutions. Transient absorbance was generally quite weak. However, the absorbance of the species generated at pH 9.8 was more intense as compared to that obtained at pH 6.2. This may be due to pKa of the semi-oxidized species formed by H-abstraction from -CH2- group beta to nitrogen, which was determined to be 7.8. The rate constants for OH radical reactions were determined by competition kinetics method using potassium thiocyanate (KCNS) as a reference solute and were found to be (1.0±0.05)x1010 and (1.0±0.05)x109 dm3 mol-1 s-1, respectively, at pH 9.8 and 6.2. Reactivity of the semi-oxidized species towards MV2+ as well as ascorbic acid at both the pHs indicates the formation of two types of intermediates. The OH radical reaction appears to occur both at -NH- and -CH2- sites. The rate constant for reaction of eaq- with MOR was determined from the decay of hydrated electron to be (2.0±0.4)x108 dm3 mol-1 s-1 at pH 6.8 and <107 dm3 mol-1 s-1 at pH 11