Radiolytic oxidation of 1,2,4-benzenetriol. An application of time-resolved resonance Raman spectroscopy to kinetic studies of reaction intermediates

Abstract

In acidic solution, 1,2,4-benzenetriol is rapidly oxidized by OH or N/sub 3/ to form a mixture of neutral 2,4- and 3,4-dihydroxyphenoxyl radicals. At higher pH these radicals deprotonate (pK/sub a/(1) = 4.75) to form the 2-hydroxy-p-benzosemiquinone radical anion which exhibits a prominent resonance Raman band at 1625 cm/sup -1/ attributable to the Wilson 8a ring stretching mode. In basic solutions this radical subsequently reacts with OH/sup -/ to form the radical dianion (pK/sub a/(2) = 8.85) in which the 8a band is shifted to an appreciably lower frequency (1587 cm/sup -1/). While the absorption spectra of these latter radicals are very similar and do not allow ready examination of their interconversion by absorption spectrophotometry, the difference between these 8a frequencies is sufficiently great that the Raman method can be used to examine the acid-base equilibrium between the two forms of the radical and to follow the deprotonation kinetics. It is shown that even at high pH the radical monoanion is initially formed on oxidation by N/sub 3/ and that deprotonation subsequently occurs by its reaction with base with a rate constant of (9.6 +/- 1.5) x 10/sup 9/ M/sup -1/ d/sup -1/. These studies illustrate very well the application ofmore » time-resolved resonance Raman spectroscopy as a complement to kinetic spectrophotometry in sorting out the details of secondary processes in pulse radiolysis studies.« less