An electron spin resonance study of the photochemistry of aromatic carboxylic acids, phenols and their derivatives

A large number of aromatic carboxylic acids, ester, anhydrides, imides, amides, cyanobenzenes and naphthalene carboxylic acids were photolyzed under steady-state and pulse conditions. The ESR spectra of the transient free radicals formed during steady-state photolysis were recorded. In the presence of a hydrogen donor, two kinds of hydrogen abstraction reactions take place. The hydrogen atom adds to the carbonyl group to form a ketyl-type radical and under suitable pH conditions, the anion radical of the corresponding compound is formed. The hydrogen atom also adds to the benzene ring in selective positions to yield a cyclohexadienyl radical. The preferential addition of the hydrogen atom to specific positions on the benzene ring is attributed to the higher spin density at those positions of the precursor, the {dollar}pipi{dollar}* triplet state. The steady-state ESR spectra of most of the cylohexadienyl radicals are highly polarized, where the low-field lines are in emission and the high-field lines in absorption. The time-resolved ESR results for the terephthalic acid system suggest that a contribution from the triplet mechanism is possible for the anion radical and the radical pair mechanism involving the cyclohexadienyl radical and the anion radical is active for the cyclohexadienyl radical. Phenoxides and thiophenoxides can be photoionized and the hydrated electron can be easily observed even under steady-state conditions. The hydrated electrons are used to generate anion radicals from a host of aromatic carboxylic and sulfonic acids. The steady-state ESR spectra of the anion radicals of the methyl and methoxy substituted benzoic acids showed Absorption/Emission type polarization in the presence of either the phenoxyl or the phenylthiyl radical. The time-resolved ESR results for the hydrated electron in the presence of either the phenoxyl or the phenylthiyl radical obtained by photolysis and radiolysis indicate that the exchange energy (J) is positive for the corresponding radical pair. The J is also found to be positive for the radical pair of the anion radical of the methyl and methoxy substituted benzoic acids with either the phenoxyl or the phenylthiyl radical.