Pulse radiolysis of thionicotinamide in aqueous solutions: formation of resonance stabilized species on one electron oxidation

Abstract

Pulse radiolysis studies on thionicotinamide (TNA) have been carried out in aqueous solutions at pH 6.8 and 1. The initial species formed by OH radical reaction with TNA at pH 6.8 was found to react with the parent molecule to give a dimer radical species (λmax = 420-440 nm). Reaction of Br2-• radicals with TNA was found to give a transient species having λmax at 380 nm. This spectrum has been assigned to a resonance stabilized species with free electron delocalised over the -N-C-S bond. Reactions of OH and Cl22-• radicals with TNA at pH 1 were found to give identical transient species with λmax at 420 nm, which decayed by first-order kinetics at a rate of about 8.0 × 103 s-1. This species is suggested to be the protonated form of the resonance-stabilized species formed at pH 6.8 in the reaction of Br2-• with TNA. The rate constant for the reaction was 4 × 109 dm3 mol-1 s-1. Semi-reduced species formed by the reaction of e-aq with TNA (k = 1.6 × 1010 dm3 mol-1 s-1) was found to be a good reductant which could transfer electron to methyl viologen. CO2-• radicals also reacted with TNA to give a reducing species. Although, the absorption peaks in the two cases were at the same wavelengths viz. 380 and 480 nm, the ratios of the peak heights were different suggesting the formation of different species. Hydrogen atoms and (CH3)2C•OH radicals were found to transfer an electron to TNA at pH 1, as seen by quantitative electron transfer to methyl viologen from the transient species.