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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2002-10, Vol.106 (40), p.9251-9260
• Main Authors: Korzeniowska-Sobczuk, Anna, Hug, Gordon L, Carmichael, Ian, Bobrowski, Krzysztof
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp021039o

Hydroxyl radicals (•OH) react with thioanisole (Ph−S−CH3) 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+•−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 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-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 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+•−CH3 radical cations and other key transients.