Substituent Effects on the Oxidation and Reduction Potentials of Phenylthiyl Radicals in Acetonitrile

Abstract

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.

Photomodulated voltammetry investigations on the benzyl radical

“…From the PMV voltammogram one usually determines the half-wave potentials (E 1/2 ) for the oxidation and reduction of the radicals, as it was done for different types of radicals [14,17,24,25]. The half-wave potentials of the benzyl radical determined here in MeCN (Table 1) are in good agreement with the values mentioned in literature [14,24].…”

“…It can react for example with acetonitrile yielding the nitrilium ion, C 6 H 5 CH 2 + NCCH 3 , or with residual water. Similar reaction pathways are mentioned for (C 6 H 5 ) 2 CH + [13], XC 6 H 4 S + [17] or XC 6 H 4 Se + [29]. The anion is much more stable in aprotic, thoroughly degassed solvents, although in acetonitrile some authors mention [30] the formation of hydrocinnamonitrile in addition to toluene.…”

“…If the effect of the follow up reaction is not negligible, it will modify the relation between the rate of forward and backward electron transfer in Eqs. (17) and (18). The steady-state concentration of the anion, [R − ], at the surface of the electrode will result from…”

“…Less common methods like photomodulated voltammetry can also be applied to obtain kinetic parameters for the self-termination reaction of radicals (3.9 × 10 9 M −1 s −1 in acetonitrile [13] for the benzyl radical) as well as for the electron transfer reaction. trochemical cell, made from PCTFE (A. Krüger, Germany), was designed and built at the University of Aarhus, Denmark [17] and has a cylindrical chamber (Ø9 mm, L = 6 mm). The working electrode consists of a gold net (minigrid, 1000 mesh, Goodfellow) sealed in plastic foil, which is provided with a hole (Ø7 mm) in order to ensure contact between net and solution.…”

Solvent influence on the electrochemical reduction of photochemically generated cis-azobenzene

“…The Teflon cell was designed and built at the University of Aarhus, Denmark [19] and has a cylindrical chamber (/ = 9 mm, L = 6 mm). The working electrode consists of a gold net (minigrid, 1000 mesh, Goodfellow) sealed in plastic foil, with a hole (/ = 7 mm) in order to ensure contact between net and solution.…”