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 pKa 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(k2/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.