One-Electron Reduction Potential and the β-Fragmentation of Acetylthiyl Radical, Comparisons with Benzoylthiyl Radical and the Oxygen Counterparts

Abstract

One-electron oxidation of acetyl thiolate (CH3C(O)S-) was achieved by hydroxyl (OH•) and azide (N3•) radicals in aqueous solution. The resulting acetylthiyl radical (CH3C(O)S•) absorbs in the wavelength region 300−550 nm, with a maximum extinction coefficient of 3900 M-1 cm-1 at 440 nm. With N3•/N3 - as a reference couple, the reduction potential E°(CH3C(O)S•/CH3C(O)S-) was measured to be 1.22 V vs NHE. Using a pK a of 3.35 for thioacetic acid (CH3C(O)SH), the standard reduction potential E°(CH3C(O)S•, H+/CH3C(O)SH) is calculated to be 1.42 V vs NHE. This reduction potential implies that the S−H bond energy of CH3C(O)S−H is 88.6 kcal/mol (370.8 kJ/mol). The β-fragmentation of the CH3C(O)S• radical, i.e., CH3C(O)S• → CH3• + COS, was observed. Its kinetics was found to follow the Arrhenius equation, log(k 2/s-1) = (12.3 ± 0.1) − (10.1 ± 0.2)/θ, where θ = 2.3RT kcal/mol. At 22 °C, the CH3C(O)S• radical decays with a rate constant of 6.6 × 104 s-1. The thermochemical properties of the CH3C(O)S• radical and its β-fragmentation reaction are compared with those of the benzoylthiyl radical (PhC(O)S•), as well as the corresponding oxygen counterparts, the acetyloxyl (CH3C(O)O•) and benzoyloxyl (PhC(O)O•) radicals.

Reactions of OH and OD radicals with simple thiols and sulfides studied by infrared chemiluminescence of isotopic water products: Reaction OH + CH3SH revisited

“…The reported experimental D(S‐H) values are larger than D(CH 3 S‐H) by 0.7‐1.2 kcal mol −1 with the error limits of 2 kcal mol −1 (Refs. 34–36) and < E av > = 33.9 kcal mol −1 was assigned to the major channel of both reactions. The spectra 18 from reactions (4) and (5) were refitted using the new model bands and the distributions were analyzed neglecting channel (b).…”

Rate constants and vibrational distributions for water‐forming reactions of OH and OD radicals with thioacetic acid, 1,2‐ethanedithiol and tert‐butanethiol

“…The reaction enthalpy –Δ H o 0 = D 0 (HO‐H) – D 0 (R‐H) was obtained from the bond dissociation energies (BDE) of sulfur compounds and D 0 (HO‐H) = 118.08 ± 0.03 kcal mol −1 . The D 0 (CH 3 C(O)S‐H) and D 0 ((CH 3 ) 3 CS‐H) obtained by electrochemical methods are given in Table . The C–H BDEs in the methyl groups of these compounds are not known, but are expected to be at least 5–6 kcal mol −1 higher than those for S–H (as, for example ,94 kcal mol −1 in CH 3 SH or 95.3 in CH 3 COOH,); hence, abstraction from the methyl groups is a minor channel and was ignored.…”

Computational electrochemistry: prediction of liquid-phase reduction potentials

“…The calculations were carried out using ADF, 228 Gaussian 09, 168 and Molpro. 229,230 Further details including experimental reference data 53,84,85,[231][232][233][234][235][236][237][238][239][240][241][242][243][244][245][246][247] used in the assessment study are given in the ESI. † Fig.…”