Free-radical chemistry of thiourea in aqueous solution, induced by OH radical, H atom, alpha-hydroxyalkyl radicals, photoexcited maleimide, and the solvated electron

Hydroxyl radicals react with thiourea (and its tetramethyl derivative) yiel ding dimeric radical cations which are characterized by strong absorptions at 400 nm (450 nm). An analysis of the kinetics of the buildup of these abs orptions gives evidence for the intermediacy of OH-adducts and the monomeri c radical cations. The dimeric radical cations are also generated in the re actions of triplet-excited maleimide with these thioureas. Moreover, in aci d solutions even reducing radicals such as the H atom and cr-hydroxyalkyl r adicals give rise to these intermediates in full yields, albeit displaying different kinetics. Potential mechanistic implications are discussed. The d imeric thiourea radical cations are strong oxidants and readily oxidize the anions of phenol and 2'-deoxyguanosine. The solvated electron gives rise t o an intermediate which is rapidly protonated by water (pK(a) > 11). Quantu m-mechanical calculations support the assignment of the 400 nm (450 nm) abs orption to the respective dimeric thiourea radical cation.