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 XC(6)H(4)S* generated from the pertinent disulfides or thiophenols have been measured by means of photomodulated voltammetry in acetonitrile. The values of E(1/2)(ox) 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 than is a consequence of a corresponding development in the electron affinities and ionization potentials of XC(6)H(4)S* as revealed by quantum-chemical calculations. Solvation energies extracted for XC(6)H(4)S(+) and XC(6)H(4)S(-) 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 XC(6)H(4)S(+) than XC(6)H(4)S(-) for most substituents, even if there is a substantial delocalization of the charge in the series of phenylsulfenium cations. The substituent effect on is smaller in aqueous solution than acetonitrile, which is attributed to the ability of water to stabilize in particular localized anions through hydrogen bonding.
Godsk Larsen
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Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark.
Hjarbaek Holm
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Roberson
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Daasbjerg
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