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

Hydroxyl radicals react with thiourea (and its tetramethyl derivative) yielding dimeric radical cations which are characterized by strong absorptions at 400 nm (450 nm). An analysis of the kinetics of the buildup of these absorptions gives evidence for the intermediacy of OH-adducts and the monomeric radical cations. The dimeric radical cations are also generated in the reactions of triplet-excited maleimide with those thioureas. Moreover, in acidic solutions even reducing radicals such as the H atom and α-hydroxy alkyl radicals give rise to these intermediates in full yields, albeit displaying different kinetics. Potential mechanistic implications are discussed. The dimeric thiourea radical cations are strong oxidants and readily oxidize the anions of phenol and 2'-deoxyguanosine. The solvated electron gives rise to an intermediate which is rapidly protonated by water (pKa > 11). Quantum mechanical calculations support the assignment of the 400 nm (450 nm) absorption to the respective dimeric thiourea radical cation. (author)