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

Abstract

Radiation chemical reactions of ^•OH, O^•-, N₃^• and e _aq ^t- with o- and m-hydroxycinnamic acids were studied. The second-orderrateconstantsforthereaction of ^•OH with ortho and meta isomers in buffer solution at pH7 are 3.9± 0.2 × 109 and 4.4 ± 0.3 × 10⁹ dm³ mol^-1 s^-1 respectively. 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 × 10⁹dm³mol^-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 e _aq ^t- 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^bb-with the o-isomer whereas the meta isomer has a maximum at 390 and a broad shoulder at 450 nm. In the reaction of the absorption peaks were centred at 370-380 nm in both the isomers. The underlying reaction mechanism is discussed.