由于氰胺类化合物在有机合成领域的重要应用, 对各种氰胺类化合物的设计和机理研究成为国内外的研究热点。 结构是功能材料设计的基础, 为了更深入的探索一种典型的氰胺类化合物-双氰胺的结构稳定性, 研究了24 GPa压强下双氰胺的高压拉曼光谱。 在压强作用下, 绝大多数的拉曼谱线向高波数方向移动, 表明双氰胺的官能团折合键长被压缩, 并且有四个特征拉曼带的强度发生了很大的变化(502,524, 934和2 157 cm^-1), 它们分别归属为N—H, N—C≡N, C=N—C以及C≡N键的振动。 同时在光谱上有新拉曼峰的出现, 原有拉曼峰的消失或者某些拉曼峰的劈裂, 这都表明了在压强作用下双氰胺出现了新的结构, 并且电子云密度发生了重构。 通过双氰胺的拉曼频移-压强关系曲线, 观察到绝大多数曲线的斜率在5 GPa下存在突变, 得出其可能在5 GPa附近发生一阶结构相变。 在结构相变前后, 观察到C=N和C≡N的频移-压强关系曲线斜率无显著变化, 表明这两种官能团在相变前后对压强的响应不变。 与此相反, N—H键的弯曲振动的曲线斜率变化显著, 表明这种官能团对压强存在复杂的响应, 这归因于双氰胺分子中存在复杂的氢键作用。 N—H键的伸缩振动谱线强度随着压强的增加而逐渐减弱, 并且拉曼频率出现了反常的蓝移, 这表明在新的相结构中N—H键长随压强增加而变长, 双氰胺分子间的氢键作用增强。 常规情况下, 双氰胺具有亚氨式和氨式两种互变异构体, 亚氨式的特征峰为2 157 cm^-1, 氨式的特征峰为2 203 cm^-1。 根据两种异构体的特征拉曼谱线强度随压强的演变规律, 发现随着压强的增加双氰胺的氨式结构比例逐渐减少, 最终在11 GPa下全部变成亚氨式结构。 研究表明高压拉曼光谱方法是研究氰胺类化合物结构相变和异构体调控的有效手段, 为功能材料的设计和合成提供一定的研究基础。

Due to the important application of cyanamide compounds in organic synthesis, the design and mechanism research of various cyanamide compounds have become a hot topic. The structure is the basis for functional material design. In order to explore the structural stability of a prototype of cyanamide compounds-dicyandiamide, we study the high-pressure Raman investigation of dicyandiamide at pressures of 24GPa.Under compression, most of the Raman lines move toward to high wave-number region, indicating that the reduced bond length of the functional group in dicyandiamide is shortened. Furthermore, the intensity of four Raman bands located at 502, 524, 934 and 2 157 cm^-1 respectively change greatly with pressure (N—H, N—C≡N, C=N—C C≡N). At the same time, spectral phenomena such as new appearance of Raman peaks, disappearance of original Raman peaks and splitting of some Raman peaks are observed, indicating a pressure-induced electronic density rearrangement occurs in the dicyandiamide. Through the analysis of Raman frequency-pressure curves of dicyandiamide. It is observed that the slope of most curves suddenly changes at 5 GPa, and it could be concluded that a first-order phase-transition happens at about 5 GPa. Furthermore, there is no significant change in the slope of the C=N and C≡N frequency-pressure curves, indicating that the two functional groups have similar pressure responses before and after the phase transition. In contrast, the slope of N—H bending vibration changes significantly, indicating that this functional group has a complex response to pressure, which is attributed to the inter-molecular hydrogen bond between dicyandiamide molecules. In addition, the intensity of the N—H stretching mode decreases gradually with the increase of pressure, and the frequency shows an abnormal blue shift, which indicates that the N-H bond length extends and the intermolecular hydrogen bond of dicyandiamide is enhanced in the new structural phase. At ambient conditions, dicyandiamide has two kinds of isomers, imino and amino forms. The characteristic peak of the imino form is 2 157 cm^-1 and that of the amino form is 2 203 cm^-1. According to the characteristic Raman line intensity evolution of the two isomers, it is found that the amino form of dicyandiamide transforms into an imino form gradually and disappears at 11 GPa. This study shows that high pressure Raman spectroscopy is an effective method to study the structural phase-transition and isomer identification of cyanamides, which provides an experimental basis for the design and synthesis of functional materials.