Nature of transient species formed during pulse radiolysis of 4-mercaptopyridine in aqueous solutions: Formation of a dimer radical species by one-electron reduction reaction

Reactions of one-electron reducing as well as oxidizing radicals with 4-mercaptopyridine (4-MPy) were studied in aqueous solutions at different pH values. One-electron oxidizing radicals such as N-3(.) and Br-2(-.), react with 4-MPy by electron transfer reaction at pH I I to give 4-pyridylthiyl radical. The reduction potential for the couple 4-PyS./4-PyS- was estimated to be 0.93V vs. NHE by equilibrium reaction with I-2(-.)/2I(-) couple. At pH 6.8, where the compound is predominantly present in the thione form, the transient species formed is a cation radical. OH radicals react with 4-MPy by addition to the pyridine ring at pH 6.8 and 11. At pH 0, OH radicals as well as one-electron oxidants like Cl-2(-.) and Br-2-. radicals react with 4-MPy to produce the protonated form of 4-pyridylthiyl radical. At pH 6.8 and 11, e(aq)(-) reaction with 4-MPy gave an initial adducts which reacted with the parent molecule to give dimer radicals. Acetone ketyl radicals were unable to reduce 4-MPy at neutral pH. Reducing radicals like H-atoms and acetone ketyl radicals reacted with 4-MPy at acidic pH by H-abstraction reaction to give the same species as produced by oxidizing radicals.