三聚氰胺-水悬浊液体系的理论研究

Theoretical study of the melamine-water suspension

针对三聚氰胺在水中形成的悬浊液体系能呈现类胶体稳定性的实验现象,采用密度泛函方法在B3LYP/6-311++G**水平上对三聚氰胺及其水解产物进行了量子化学计算,并将计算结果与红外光谱实验结果作比较。热力学计算结果表明,三聚氰酸二酰胺与相应酮式结构之间进行互变异构的△G为-41.62 kJ·mol^(-1),说明酮式结构更加稳定。红外光谱实验结果显示,三聚氰胺样品在2360 cm^(-1)处有较明显的吸收峰,与三聚氰酸二酰胺酮式结构的计算结果比较吻合,因此三聚氰胺在水中发生初步水解生成三聚氰酸二酰胺酮式结构的可能性较大。通过分子振动频率分析与红外光谱实验的对比可知,三聚氰酸二酰胺酮式结构的二聚体在2300 cm^(-1)附近的N-H键的伸缩振动吸收峰发生劈裂,吸收峰位红移约25 cm^(-1),表明二聚体之间形成了N-H…N氢键。可以推测,三聚氰酸二酰胺可以通过氢键连接形成聚集结构,最终导致三聚氰胺悬浊液能够稳定存在。这些研究结果为三聚氰胺-水体系的工业应用和深入探索提供了实验和理论的参考依据。

In order to investigate the stability of the melamine-water suspension, theoretical calculation on melamine and its hydrolysate was carried out at B3LYP/6-311-H-G＊＊ level, and the solvent effect was explored by polarizable continuum model （PCM）, assigning water as the solvent in consideration of the solvent effect. The melamine may be preliminarily hydrolyzed to form ammeline, and the Gibbs free energy change of the tautomedsm from ammeline to its keto form is calculated to be -41.62 kJ.mol＂1, which means the keto form is more stable. Meanwhile, the absorption peak of melamine powder at 2360 cm-1 obtained by infrared spectrums experiment is accordant with the quantum calculation result of the keto form of the ammeline. The coincident results suggest the primary hydrolysate of the melamine. Moreover, the absorption peak of stretching vibration of N-H bond in ammeline dimer splits, and the redshift of the peak is found to be about 25 cm-1, implying the formation of N-H...N hydrogen bond between the monomers. It can be speculated that the hydrolysate may be connected through hydrogen bonds to generate the aggregate structure, resulting in the stableness of the melamine suspension. And the structures of ammeline tetramer and multimer were also optimized at B3LYP/6-31G and PM6 level respectively. These conclusions can provide theoretical and experimental references for the industrial application and further research of melamine solution.