Oxidation of cinnamic acid derivatives: A pulse radiolysis and theoretical study

Second order rate constants in the range of (k = 1.6-4.5) × 109 dm3 mol−1 s−1 were obtained for the OH induced oxidation of nitro- and methoxycinnamic acid derivatives in neutral solutions using pulse radiolysis. The transient absorption spectra exhibited a broad peak around 360-410 nm in o-methoxy, o- and p-nitrocinnamates or two peaks around 310-330 and 370-410 nm in other isomers. Quantum chemical calculations revealed that addition of OH to olefinic moiety yielded considerably more stable structures than ring addition products and the para system among the latter is the most stable. Spin density analysis suggested that olefinic adducts retained the aromaticity in contrast to its loss in ring OH adducts. An excellent linear correlation between the relative stabilities of the OH adducts (after accounting for the aromatic stabilization in olefinic adducts) and the maximum Sd values is also obtained.