Spectral, Kinetics, and Theoretical Studies of Radical Cations Derived from Thioanisole and Its Carboxylic Derivative

Hydroxyl radicals ( • OH) react with thioanisole (Ph−S−CH 3 ) via two competitive addition pathways: with the thioether functionality and with the aromatic ring. At neutral pH, • OH addition leads to the prompt formation of monomeric sulfur radical cations (Ph−S +• −CH 3, addition to the thioether group) and hydroxycyclohexadienyl radicals (Ph • −(OH)−S−CH 3, addition to the aromatic ring). The latter radicals subsequently decay into products, which do not include the corresponding radical cations with delocalized positive charge on the aromatic ring (Ph +• −S−CH 3 ). On the other hand, at low pH, • OH addition, both to the thioether functionality and to the aromatic ring, leads promptly only to Ph−S +• −CH 3 radical cations. These observations are rationalized in terms of the highly unstable nature of Ph +• −S−CH 3 radical cations (formed via proton-catalyzed water elimination from Ph • −(OH)−S−CH 3 radicals) and their rapid conversion into Ph−S +• −CH 3 radical cations. Additional experimental support for the instability of radical cations derived from aromatic thioethers with delocalized positive charge on the aromatic ring has been obtained from the • OH-induced oxidation studies of phenylthioacetic acid (Ph−S−CH 2 −COOH). At low pH, Ph−S−CH 2 −COOH undergoes nearly complete (relative to the available • OH radicals) quantitative decarboxylation, in contrast to neutral pH, at which the yield of decarboxylation accounts for only half of the available • OH radicals. To support our conclusions, quantum mechanical calculations were performed using density functional theory (DFT) that provided predictions of the electronic structure and optical excitation energies of the Ph−S +• −CH 3 radical cations and other key transients.