Investigations on the nature and redox properties of the transients formed on pulse radiolysis of aqueous solutions of 2-(phenylthio)ethanol

Abstract

The radical cation of 2-(phenylthio)ethanol (PTE)⁺, generated on reaction of specific one-electron oxidants Cl₂^-, Tl²⁺, SO₄^-, CCl₃OO, Br₂^-, by pulse radiolysis in aqueous solutions of PTE exhibits absorption bands at 315 and 530 nm. Pulse radiolysis of PTE in 1,2-dichloroethane also produced a similar transient absorption spectrum. The hydroxyl radicals are observed to react with a bimolecular rate constant of 7.5×10⁹ d mol^-1 s^-1 and form absorption bands at 300, 365 and 530 nm. While the 530 nm band decayed by first order kinetics with k=2.1×10⁴ s^-1, other bands showed mixed kinetics. O^- reacts exclusively by H atom abstraction forming a transient absorption band in 290–330 nm region, H reacts both by H abstraction and H-adduct formation. Based on these studies, OH radicals are inferred to react by electron transfer, H abstraction and OH-adduct formation. The radical cation reacts with electron donors, I^-, N₃^-, with a high rate constant value. In neutral solutions, e_aq^- reacts with a bimolecular rate constant of 7.1×10⁸ d mol^-1 s^-1 and the transient absorption band at 360 nm (ε=1.4×10³ d mol^-1 cm^-1) is assigned to H-adduct formed on protonation of radical anion (pK_a=7.9). In basic solutions, the radical anion has very small absorption at 360 nm (ε=0.6×10³ d mol^-1 cm^-1). The radical anion formed on reaction of e_aq^- with PTE at pH 12 is able to transfer an electron to MV²⁺ whereas at neutral pH, electron transfer is not possible. The reduction potential for the PTE/PTE^- couple is determined to be -1.23 V.