Substituent effects on the oxidation and reduction potentials of phenylthiyl radicals in acetonitrile

Oxidation (E-1/2(ox)) and reduction potentials (E-1/2(red)) of a series of para substituted phenylthiyl radicals XC6H4S. generated from the pertinent disulfides or thiophenols have been measured by means of photomodulated voltammetry in acetonitrile. The values of EF1/2ox are of particular interest as they give access to the hitherto unknown thermochemistry of short-lived phenylsulfenium cations in solution. Both E-1/2(ox) and E-1/2(red) decrease as the electron-donating power of the substituent raises, resulting in linear correlations with the Hammett substituent coefficient sigma (+) with slopes rho (+) of 4.7 and 6.4, respectively. The finding of a larger substituent effect on E-1/2(red) than E-1/2(ox) is a consequence of a corresponding development in the electron affinities and ionization potentials of XC6H4S. as revealed by quantum-chemical calculations. Solvation energies extracted for XC6H4S+ and XC6H4S- from thermochemical cycles show the expected substituent dependency; i.e., the absolute values of the solvation energies decrease as the charge becomes more delocalized in the ions. Acetonitrile is better in solvating XC6H4S+ than XC6H4S- for most substituents, even if there is a substantial delocalization of the charge in the series of phenylsulfenium cations. The substituent effect on E-1/2(red) is smaller in aqueous solution than acetonitrile, which is attributed to the ability of water to stabilize in particular localized anions through hydrogen bonding.