Kinetic and spectral studies on transient species formed in pulse radiolysis of 2,6-diacetylpyridine in aqueous solutions

Abstract

Reactions of e − aq, OH radicals, H-atoms and reducing (CH 3) 2C OH/CO 2 − radicals with 2,6-diacetylpyridine were studied using pulse radiolysis technique in aqueous solutions. At pH 9.2, e − aq reaction with 2,6-DAP ( k = 2.1 × 1 0 10 dm 3 mol - 1 s - 1 ) was found to generate a transient species having absorption maxima at 320 and 420 nm. Reaction of e − aq with 2,6-DAP was also studied at pH∼13.3. Semi-reduced species formed at pH∼13.3 were found to transfer electron to methyl viologen with 100% efficiency ( k = 2.9 × 1 0 9 dm 3 mol - 1 s - 1 ), whereas at pH 9.2 an equilibrium was attained. Making use of this equilibrium, the one-electron reduction potential of 2,6-DAP has been estimated to be −0.513 V vs. NHE at pH 9.2. Following the change in absorption spectra of semi-reduced species at pH 9.2 and ∼13.3, p K a for the protonation of 2,6-DAP radical anion (2,6-DAP − /2,6-DAPH ) has been determined to be ∼11.9. Acetone ketyl radicals were found to transfer electron to 2,6-DAP with rate constants of 2×10 9, 6.3×10 7 and 1.2 × 1 0 8 dm 3 mol - 1 s - 1 at pHs∼13.3, 9.2 and 1, respectively. By making use of the changes in the absorbance of species formed in reaction with acetone ketyl radicals with 2,6-DAP, p K a for the protonation of 2,6-DAPH (2,6-DAPH /2,6-DAPH 2 + ) has been estimated to be ∼1.5. H-atoms were found to react with 2,6-DAP by addition pathway rather than electron transfer. OH radical reaction with 2,6-DAP ( k = 1.8 × 1 0 9 dm 3 mol - 1 s - 1 ) at pH 9.2 was found to generate transient species having absorption maxima at 310 and 400 nm. At pH 1 also, the spectrum of the species formed in OH radical reaction with 2,6-DAP was similar to that formed at pH 9.2 whereas reaction of O − radicals with 2,6-DAP ( k = 1.8 × 1 0 9 dm 3 mol - 1 s - 1 ) generated a transient species whose absorption spectrum was different.