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 greater than or equal to 9.8) and acidic/neutral (pH less than or equal to 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 pK(a) 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) x 10(10) and (1.0+/-0.05) x 10(9) dm(3) 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 e(aq)(-) with MOR was determined from the decay of hydrated electron to be (2.0 +/- 0.4) x 10(8) dm(3) mol(-1) s(-1) at pH 6.8 and < 10(7) dm(3) mol(-1) s(-1) at pH 11. (C) 2003 Elsevier Science Ltd. All rights reserved.