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

Radiation chemical reactions of •CH, O•-, N3•and-eaq with o- and mhydroxycinnamic 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 plus-minus 0,2 cross109 and 4,4 plus-minus0,3cross109 dm3 mol–1 s–1 respectively. At pH 3 the rate with the ortho isomer was halved (1,6 plus-minus0,4 cross109 dm3 mol–1 s–1) but it was unaffected in the case of meta isomer (k = 4,2 plus-minus0,6cross109 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 plus-minus0,4 cross108 dm3 mol–1 s–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 ohydroxycinnamic 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.