Spectral, kinetics, and theoretical studies of radical cations derived from thioanisole and its carboxylic derivative

Hydroxyl radicals ((OH)-O-.) react with thioanisole (Ph-S-CH3) via two competitive addition pathways:. with the thioether functionality and with the aromatic ring. At neutral pH, (OH)-O-. addition leads to the prompt formation of monomeric sulfur radical cations (Ph-S+.-CH3, addition to the thioether group) and hydroxycyclohexadienyl radicals (Ph-.-(OH)-S-CH3, 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-CH3)- On the other hand, at low pH, (OH)-O-. addition, both to the thioether functionality and to the aromatic ring, leads promptly only to Ph-S+.-CH3 radical cations. These observations are rationalized in terms of the highly unstable nature of Ph+.-S-CH3 radical cations (formed via proton-catalyzed water elimination from Ph.--(OH)-S-CH3 radicals) and their rapid conversion into Ph-S+.-CH3 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)-O-.-induced oxidation studies of phenylthioacetic acid (Ph-S-CH2-COOH). At low pH, Ph-S-CH2-COOH undergoes nearly complete (relative to the available (OH)-O-. radicals) quantitative decarboxylation, in contrast to neutral pH, at which the yield of decarboxylation accounts for only half of the available (OH)-O-. 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+.-CH3 radical cations and other key transients.