Pavel I, Szeghalmi A, Moigno D, Cîntă S, Kiefer W
Institut für Physikalische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.
Biopolymers. 2003;72(1):25-37. doi: 10.1002/bip.10248.
The surface enhanced Raman spectroscopy (SERS) spectrum of caffeine is recorded on a silver colloid at different pH values. It is discussed on the basis of the SERS "surface selection rules" in order to characterize its vibrational behavior on such a biological artificial model. To improve the previous assignments in the Raman spectrum and for a reliable, detailed analysis of SERS spectra, density functional theory calculations (structural parameters, harmonic vibrational wavenumbers, total electron density, and natural population analysis of the molecule) are performed for the anhydrous form of caffeine and the results are discussed. The predicted geometry and vibrational Raman spectra are in good agreement with the experimental data. The flat orientation of the mainly chemisorbed caffeine attached through the pi electrons and the lone pair of nonmethylated N atoms of the imidazole ring are proposed to occur at neutral and basic pH values. At acid pH values caffeine is probably adsorbed on the Ag surface through one or both oxygen atoms, more probably through the O atom of the conjugated carbonyl group with an end-on orientation. However, the changes in the overall SERS spectral pattern seem to indicate the electromagnetic mechanism as being the dominant one.
在不同pH值下，在银胶体上记录了咖啡因的表面增强拉曼光谱（SERS）。基于SERS的“表面选择规则”进行了讨论，以表征其在这种生物人工模型上的振动行为。为了改进拉曼光谱中先前的归属，并对SERS光谱进行可靠、详细的分析，对无水咖啡因形式进行了密度泛函理论计算（结构参数、谐振动波数、总电子密度和分子的自然布居分析），并对结果进行了讨论。预测的几何结构和振动拉曼光谱与实验数据吻合良好。提出在中性和碱性pH值下，主要通过π电子和咪唑环未甲基化N原子的孤对化学吸附的咖啡因以平面取向存在。在酸性pH值下，咖啡因可能通过一个或两个氧原子吸附在Ag表面，更可能通过共轭羰基的O原子以端对端取向吸附。然而，整体SERS光谱模式的变化似乎表明电磁机制是主要机制。
