A recent report on the azo coupling of 4-nitrobenzo-15-crown-ether (4NB15C) and 4-nitrothiophenol (4NTP) indicated that the reaction barrier could be reduced greatly with surface plasmonic effects on silver dendritic nanostructures in aqueous solution. Accordingly, an azo coupling reaction mechanism was proposed based on one or two SERS peaks. Toward a profound understanding of this azo coupling reaction mechanism, it is crucial to scrutinize the origin of the full SERS spectrum. Here, we construct a molecular model consisting of 4NTP and 4NB15C on an Ag₇ cluster that simulates a silver dendritic nanostructure, and investigate the SERS spectra of the azo coupling of these two molecules. We propose five different adsorption sites and 13 different orientations of 4NTP on the Ag₇ cluster and optimize the geometries of the five configurations. With each optimized configuration of 4NTP adsorbed on Ag₇, we further consider the azo coupling product with a 4NB15C molecule and simulate the corresponding Raman spectra. Comparing the measured Raman spectra and model analysis, we conclude that the azo coupling reaction depends decisively on a parallel molecular orientation of the adsorbed 4NTP relative to the facets of Ag₇, the orientation of which further directs the subsequent reaction for the product of 4NB15C–4NTP.

中文翻译：

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揭示分子取向对表面等离子体共振驱动的硝基化合物偶氮偶联反应的影响

最近关于 4-硝基苯并-15-冠醚 (4NB15C) 和 4-硝基苯硫酚 (4NTP) 偶氮偶联的报告表明，通过表面等离子体对水溶液中的银树枝状纳米结构的影响，可以大大降低反应势垒。因此，提出了基于一个或两个 SERS 峰的偶氮偶联反应机理。为了深入理解这种偶氮偶联反应机制，仔细研究整个 SERS 光谱的起源至关重要。在这里，我们在模拟银树枝状纳米结构的 Ag ₇簇上构建了一个由 4NTP 和 4NB15C 组成的分子模型，并研究了这两种分子偶氮偶联的 SERS 光谱。我们提出了 5 个不同的吸附位点和 13 个不同方向的 4NTP 在 Ag _{7 上}集群并优化五种配置的几何形状。对于吸附在 Ag ₇上的 4NTP 的每个优化配置，我们进一步考虑偶氮偶联产物与 4NB15C 分子并模拟相应的拉曼光谱。比较测量的拉曼光谱和模型分析，我们得出结论，偶氮偶联反应决定性地取决于吸附的 4NTP 相对于 Ag ₇晶面的平行分子取向，其取向进一步指导后续反应生成 4NB15C– 4NTP。