Our study of the complexation of 3,4-dihydroxy-phenyl-propenoic acid by chromium(III) could give information on the way that this metal ion is available to plants. The reaction between chromium(III) and 3,4-dihydroxy-phenyl-propenoic acid in weak acidic aqueous solutions has been shown to take place by at least three stages. The first stage corresponds to substitution (I(d) mechanism) of water molecule from the Cr(H(2)O)(5)OH(2+) coordination sphere by a ligand molecule. A very rapid protonation equilibrium, which follows, favors the aqua species. The second and the third stages are chromium(III) and ligand concentration independent and are attributed to isomerisation and chelation processes. The corresponding activation parameters are DeltaH(2(obs)) ( not equal) = 28.6 +/- 2.9 kJ mol(-1), DeltaS(2(obs)) ( not equal) = -220 +/- 10 J K(-1)mol(-1), DeltaH(3(obs)) ( not equal) = 62.9 +/- 6.7 kJ mol(-1) and DeltaS(3(obs)) ( not equal) = -121 +/- 22 J K(-1)mol(-1). The kinetic results suggest associative mechanisms for the two steps. The associatively activated substitution processes are accompanied by proton release causing pH decrease.

中文翻译：

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铬 (III) 与 3,4-二羟基-苯基-丙烯酸（咖啡酸）在弱酸性水溶液中反应的动力学和机理。

我们对 3,4-二羟基-苯基-丙烯酸与铬 (III) 络合的研究可以提供有关该金属离子可用于植物的方式的信息。铬 (III) 和 3,4-二羟基-苯基-丙烯酸在弱酸性水溶液中的反应至少经过三个阶段进行。第一阶段对应于配体分子从 Cr(H(2)O)(5)OH(2+) 配位球中置换水分子（I(d) 机制）。随后的非常快速的质子化平衡有利于水族物种。第二和第三阶段与铬 (III) 和配体浓度无关，并归因于异构化和螯合过程。对应的激活参数是 DeltaH(2(obs)) (不等于) = 28.6 +/- 2.9 kJ mol(-1), DeltaS(2(obs)) (不等于) = -220 +/- 10 JK(- 1）摩尔（-1），DeltaH(3(obs))（不等于）= 62.9 +/- 6.7 kJ mol(-1) 和 DeltaS(3(obs))（不等于）= -121 +/- 22 JK(-1)mol(- 1）。动力学结果表明这两个步骤的关联机制。缔合激活的取代过程伴随着质子释放，导致 pH 值下降。