Ma Zhen, Lee Ilkeun, Zaera Francisco
Department of Chemistry, University of California, Riverside, California 92521, USA.
J Am Chem Soc. 2007 Dec 26;129(51):16083-90. doi: 10.1021/ja076011a. Epub 2007 Nov 29.
The room-temperature adsorption of four closely related cinchona alkaloids and three reference quinoline-based compounds from CCl4 solutions onto a polycrystalline platinum surface was characterized by in situ reflection-absorption infrared spectroscopy (RAIRS). The adsorption equilibrium constants (Kads) were found to follow the sequence cinchonine > quinidine > cinchonidine > quinine > 6-methoxyquinoline > lepidine > quinoline. Some of this ordering can be explained by differences in solubility, but quinidine displays a much larger Kads than expected on the basis of its large relative solubility; bonding to the surface must also play a role in determining its behavior. It was determined that each alkaloid binds differently on Pt at saturation coverages. While the quinoline ring of cinchonidine tilts along its long axis to optimize pi-pi intermolecular interactions, in cinchonine it tilts along the short axis and bonds through the lone electron pair of the nitrogen atom instead, and both quinine and quinidine exhibit additional bonding via the methoxy oxygen atom at intermediate concentrations. Perhaps a more surprising result from this work is the fact that cinchonine displays a higher Kads than cinchonidine, quinine, or quinidine even though, according to previous work, it can be easily displaced from the surface by any of those other cinchona alkaloids. A full explanation of these observations requires consideration of the solvent above the adsorbed species.
通过原位反射吸收红外光谱（RAIRS）对四种密切相关的金鸡纳生物碱和三种基于喹啉的参考化合物从四氯化碳溶液在多晶铂表面的室温吸附进行了表征。发现吸附平衡常数（Kads）遵循以下顺序：辛可宁＞奎尼丁＞辛可尼定＞奎宁＞6-甲氧基喹啉＞勒皮啶＞喹啉。这种排序的一部分可以用溶解度的差异来解释，但奎尼丁的Kads比基于其较大相对溶解度预期的要大得多；与表面的键合在决定其行为方面也一定起到了作用。已确定每种生物碱在饱和覆盖度下在铂上的结合方式不同。虽然辛可尼定的喹啉环沿其长轴倾斜以优化π-π分子间相互作用，但在辛可宁中它沿短轴倾斜并通过氮原子的孤电子对键合，并且奎宁和奎尼丁在中等浓度下都通过甲氧基氧原子表现出额外的键合。这项工作可能更令人惊讶的结果是，尽管根据先前的工作，辛可宁很容易被其他任何一种金鸡纳生物碱从表面取代，但它的Kads却比辛可尼定、奎宁或奎尼丁更高。对这些观察结果的完整解释需要考虑吸附物种上方的溶剂。
