Li Yin, Xu Li, Ouyang Jinbo, Lei Jiangtao, Hu Jun, Xing Xiaohong, Chen Peng, Li Jiaqing, Zhong Changqing, Yang Bo, Li Heng
School of Physics and Materials Science, Nanchang University, Xuefu Avenue 999, Nanchang City 330031, China.
School of Chemistry, Biology and Materials Science, East China University of Technology, Guanglan Avenue 418, Nanchang City 330013, China.
Spectrochim Acta A Mol Biomol Spectrosc. 2024 Mar 15;309:123869. doi: 10.1016/j.saa.2024.123869. Epub 2024 Jan 7.
Polymorphism commonly exists in organic molecular crystals. The fingerprint features in low-frequency vibrational range are important information reflecting different intermolecular interactions of polymorphs. Interpreting these features is very helpful to understand vibrational property of polymorphs and reveal the thermodynamic stability. In this work, the low-frequency vibrations of form I and II of vanillin are investigated using terahertz time-domain spectroscopy. Static DFT calculation and ab initio molecular dynamics (AIMD) are employed to interpret their low-frequency vibrations of both forms in harmonic and anharmonic ways, respectively. Their low-frequency vibration characteristics in harmonic calculations are discussed, and anharmonic mode couplings between OH bond stretch and the stretching and bending motion of hydrogen bonds are uncovered. Moreover, the thermodynamic energies including electronic potential energy and vibrational/kinetic energy arising from nuclear motions are calculated. The result reveals that the stability order of the two forms is mainly dependent on their electric potential energy difference.
多晶型现象普遍存在于有机分子晶体中。低频振动范围内的指纹特征是反映多晶型物不同分子间相互作用的重要信息。解读这些特征对于理解多晶型物的振动性质和揭示其热力学稳定性非常有帮助。在这项工作中，利用太赫兹时域光谱研究了香草醛I型和II型的低频振动。采用静态密度泛函理论（DFT）计算和从头算分子动力学（AIMD）分别以谐波和非谐波方式解释了两种晶型的低频振动。讨论了它们在谐波计算中的低频振动特性，并揭示了OH键伸缩与氢键伸缩和弯曲运动之间的非谐模式耦合。此外，还计算了包括电子势能以及由核运动产生的振动/动能在内的热力学能量。结果表明，两种晶型的稳定性顺序主要取决于它们的电势能量差。
