Intra- and inter-molecular electron transfer reactions in 3,3′-thiodipropionic acid

Abstract

The transient optical absorption band (λ_max 390 nm, t_½ 38 µs, ε 3.13 × 10³ dm³ mol^–1 cm^–1) produced on pulse radiolysis of N₂O-saturated aqueous solution of 3,3′-thiodipropionic acid (pH 6.0–10.0) has been assigned to intramolecular 3-electron-bonded species formed on p-orbital overlap of sulphur and oxygen. The absolute rate constant for the reaction of OH radicals forming a 390 nm band has been determined to be 1.1 × 10¹⁰ dm³ mol^–1 s^–1. An intermolecular dimer radical cation (λ_max 500 nm, t_½ 17 µs, ε 7.20 × 10³ dm³ mol^–1 cm^–1) is formed on pulse radiolysis of an O₂-saturated aqueous solution of 3,3′-thiodipropionic acid (pH 1.0–3.5, concentration >3.0 × 10^–4 mol dm^–3). Tl²⁺ and Cl₂^– are able to oxidize 3,3′-thiodipropionic acid with a bimolecular rate constant of 3.8 × 10⁹ and 5.5 × 10⁹ dm³ mol^–1 s^–1 respectively. The dimer radical cation of 3,3′-thiodipropionic acid (TDPA)₂⁺˙, is able to oxidize I^– to I₂^– with a bimolecular rate constant of 5.5 × 10⁹ dm³ mol^–1 s^–1. An equilibrium is established between dimer radical cation (TDPA)₂⁺˙ and Br^– from which the one-electron redox potential for the TDPA ⁺/TDPA couple has been determined to be 1.81 V versus normal hydrogen electrode (NHE).