Electron spin resonance studies of reactions of some short–lived organic radicals in solution

Abstract

A convenient method for studying short-lived organic radicals in solution is to combine e. s. r. spectroscopy with a fast-flow mixing system: solutions of materials which react to give the radicals are mixed a very short time before the combined solution passes through the cavity of the spectrometer. Dixon & Norman (1963) applied this principle to the reaction between acidified aqueous solutions of titanium (lll) ions and hydrogen peroxide; the reaction between these species rapidly produces the hydroxyl radical [Ti (lll) + H 2 O 2 → Ti (IV) + ·OH + OH-] and, when simple aliphatic alcohols are included in the reactants, the spectra of organic radicals derived by the abstraction of a hydrogen atom from a C—H bond of the alcohol are observed (e. g. CH 3 OH + ·OH → ·CH 2 OH + H 2 O). Dixon, Norman & Buley (1964) showed that other types of saturated aliphatic compound also undergo abstraction of hydrogen to give observable radicals, Dixon & Norman (1964) showed that benzene forms an adduct with the hydroxyl radical, and Fischer (1964) showed that olefins likewise form adducts. The reactions of olefins are of particular interest in that, for those which are polymerizable, it is possible to derive informa­tion about the characteristics of the polymerizations by examining the effects on the e. s. r. spectra of changes in the concentrations of the reactants. Since the original studies of this type, it has become apparent that the titanous-peroxide system itself is more complex than was first thought. When the reactants are mixed in the absence of an organic compound, either one or two singlets are observed in the spectrum, depending on the relative concentrations of the re­actants. The simplest explanation would be that these are the spectra of the radicals ·OH and ·O 2 H, the latter radical being formed by the reaction of the former with excess of hydrogen peroxide and so increasing in concentration relative to the former radical as the ratio [H 2 O 2 ] : [Ti (lll)] is increased. However, Chiang, Craddock, Mickewich & Turkevich (1966) adduced evidence that the two radicals responsible for these e. s. r. singlets are complexes of oxy-radicals and titanium ions, and more recently Fischer (personal communication) has confirmed this by observing satellite lines for each singlet ascribable to interaction of the unpaired spin in an oxy-radical with the (low-abundant) paramagnetic 47 Ti and 49 Ti nuclei. Moreover, we have found at York that, when titanium (lll) ions and hydrogen peroxide react a short time before an organic reactant such as methanol is introduced, only the singlets are observed. This implies that the singlets are due to relatively unreactive radicals, and that, unless organic compounds are available for reaction immediately the hydroxyl radical is generated, this radical is efficiently scavenged by titanium-containing species. None the less, despite this complexity, there is no reason to doubt that, when an organic compound is available at the point of mixing of the titanium and peroxide reactants, the chemistry with which we are concerned is that of the reaction of the hydroxyl radical with the organic compound. The remainder of this paper will describe reaction processes which are initiated in this way.