低共熔溶剂(DES)作为一种新式的绿色溶剂, 在多种化学过程中表现出色, 因此在诸多热门范畴都展现出良好的发展潜力。 对DES的光谱分析通常局限在一维光谱技术上, 但其分辨率低、 谱峰重叠严重等缺点, 导致光谱数据存在误差。 运用二维拉曼光谱(2D Raman), 能够明显提高光谱分辨率, 并发现重叠峰位置, 获得在外扰条件下不同谱峰的变化顺序及其相互作用等重要信息, 以实现对复杂体系的精确分析。 以氯化胆碱(ChCl)和ZnCl₂合成的DES为例, 利用显微共焦激光拉曼光谱仪分别对ChCl和DES进行拉曼实验, 发现与ChCl相比, DES中各处峰的整体强度显著下降, 原有的谱峰未消失, 说明Zn²⁺的加入没有破坏ChCl的骨架结构。 287 cm^-1处出现一个新的特征峰, 推测有Zn—Cl配位键的伸缩振动。 对DES进行升温拉曼实验, 发现随着温度的升高, NC₄的不对称伸缩振动峰强度逐渐减小, 峰宽变大, 峰形变缓, Zn—Cl配位键伸缩振动峰强度逐渐降低, 峰位置基本不变, 峰形有明显重叠。 运用2D Raman技术对溶液内NC₄和Zn—Cl特征峰的变化进行研究, 结果表明, 随着温度的升高, 溶液中发生ChCl向Ch⁺的解离过程, Zn²⁺与Cl^-形成了多种配合物, $ZnCl_{3}^{-}$, $Zn_{2}Cl_{5}^{-}$, $Zn_{3}Cl_{7}^{-}$之间存在相互转化。 不同位置的特征峰随温度的变化顺序不同, 将特征峰与团簇进行一一归属, 得出了各个团簇对温度的敏感程度。 基于量子化学中的密度泛函理论, 对推测的物质结构进行构型优化和参数计算, 证实了其存在的可能性, 同时也验证了2D Raman的分析结果准确可行。 这些结果将为DES的后续研究提供理论参考, 拓展了二维光谱技术的应用范围。

A new green solvent, deep eutectic solvent (DES) performs well in many chemical processes. It should have broad application prospects in popular fields. The spectral analysis of DES is usually based on one-dimensional spectroscopy. However, the spectral data are inaccurate due to low resolution and overlapping peaks. Two-dimensional Raman spectroscopy (2D Raman) can significantly improve spectral resolution and find overlapping peaks. In addition, important conclusions such as the changing order of different spectral peaks and their interaction under external disturbance could be obtained to realize the accurate analysis of complex systems. In this article, DES has been synthesized from choline chloride (ChCl) and ZnCl₂. Raman experiments of ChCl and DES have been carried out by micro confocal laser Raman spectrometer. It has been found that the overall intensity of each peak in DES is significantly decreased compared with that of ChCl, and the original spectral peaks remain, indicating that Zn²⁺ dose not damage the skeleton structure of ChCl. A new characteristic peak appears at 287 cm^-1, which is speculated to be the stretching vibration of the Zn—Cl coordination bond, and the Raman experiment of DES is conducted with the temperature rising. The results have shown that, with the increase of temperature, the intensity of asymmetric stretching vibration peak of NC₄ decreases, the peak width increases and the peak shape slows down. Furthermore, the intensity of stretching vibration peak of Zn—Cl decreases gradually, the position of the peak remains unchanged, and the peak shape overlaps obviously. 2D Raman studies the characteristic peaks of NC₄ and Zn—Cl. The results reveal that the dissociation process of ChCl to Ch⁺ occurs in the solution, and Zn²⁺ forms various complexes with Cl^-. There is a mutual transformation between $ZnCl_{3}^{-}$, $Zn_{2}Cl_{5}^{-}$ and $Zn_{3}Cl_{7}^{-}$. The peaks at different positions have different orders of temperature change. The characteristic peaks and clusters are assigned, and the sensitivity of each cluster to temperature is obtained. The structure optimization and parameter calculation of the speculated material structure are carried out, which confirm the possibility of its existence. Meanwhile, the feasibility of 2D Raman analysis results is also verified. These results will provide theoretical reference for the research of DES and further expand the application scope of two-dimensional spectroscopy.