Redox chemistry of o-and m-hydroxycinnamic acids: A pulse radiolysis study

Abstract

Radiation chemical reactions of .OH, O.-, N3. and eaq- with o- and m-hydroxycinnamic acids were studied. The second-order rate constants for the reaction of .OH with ortho and meta isomers in buffer solution at pH 7 are 3.9 ± 0.2 × 109 and 4.4 ± 0-3 × 109 dm3 mol-1 s-1 respectivel y. At pH 3 the rate with the ortho isomer was halved (1.6 ± 0.4 × 109 dm3 mol-1 s-1) but it was unaffected in the case of meta isomer (k = 4.2 ± 0.6 × 109 dm3 mol-1 s-1). The rate constant in the reaction of N3. with the ortho isomer is lower by an order of magnitude (k = 4.9 ± 0.4 × 108 dm3 mol-1s-1). The rates of the reaction of eaq- with ortho and meta isomers were found to be diffusion controlled. The transient absorption spectrum measured in the .OH with o-hydroxycinnamic acid exhibited an absorption maximum at 360 nm and in meta isomer the spectrum was blue-shifted (330 nm) with a shoulder at 390 nm. A peak at 420 nm was observed in the reaction of O.- with the o-isomer whereas the meta isomer has a maximum at 390 and a broad shoulder at 450 nm. In the reaction of eaq-, the absorption peaks were centred at 370-380 nm in both the isomers. The underlying reaction mechanism is discussed.