Post-irradiation decomposition of sulfanylperoxides and peroxycyclohexadienethiones formed from thiyl and peroxyl radicals

Abstract

Photolysis of (E)-azoisobutane and dialkyl and diaryl disulfides in oxygenated 2-methylbutane at temperatures below 230 K results in a very low photostationary state of peroxyl radical, because of the high efficiency of the mixed termination reaction to form sulfanylperoxide and peroxycyclohexa-dienethione.

Surprisingly, after photolysis is stopped there is an immediate threefold rise in the peroxyl radical concentration. This post-itlumination dark reaction is interpreted and accurately modelled using an algorithm, by assuming an irreversible decomposition of the sulfanylperoxide or peroxycyclohexa-dienethione to alkoxyl and other radicals. Spin traps have been used to confirm the nature of the radicals involved in many of the proposed reactions and to give a measure of radical concentrations under various experimental conditions. Values of the rate parameters for a number of these reactions have been measured and Arrhenius parameters E₂= 34 kJ mol^–1, A₂= 10⁶, s^–1, E₂= 14 kJ mol^–1, A₂= 10^2.4 s^–1 determined for the decomposition of phenyl and tert-butyl sulfanylperoxides, respectively. The low A factor for tert-butyl sulfonylperoxide is probably associated with a tight, four-membered ring transition state with a loss of internal rotations about partially formed sulfur–oxygen double bonds and accompanying solvent orientations about the more dipolar SO bond. The decomposition of the phenyl analogue occurs by scission of O–O links in para and ortho substituted peroxycyclohexadienethione.