Harmonic and anharmonic studies on THz spectra of two vanillin polymorphs

Yin Li; Li Xu; Jinbo Ouyang; Jiangtao Lei; Jun Hu; Xiaohong Xing; Peng Chen; Jiaqing Li; Changqing Zhong; Bo Yang; Heng Li

doi:10.1016/j.saa.2024.123869

Harmonic and anharmonic studies on THz spectra of two vanillin polymorphs

Spectrochim Acta A Mol Biomol Spectrosc. 2024 Mar 15:309:123869. doi: 10.1016/j.saa.2024.123869. Epub 2024 Jan 7.

Authors

Yin Li¹, Li Xu², Jinbo Ouyang³, Jiangtao Lei⁴, Jun Hu⁵, Xiaohong Xing², Peng Chen², Jiaqing Li⁶, Changqing Zhong⁶, Bo Yang⁶, Heng Li⁷

Affiliations

¹ School of Physics and Materials Science, Nanchang University, Xuefu Avenue 999, Nanchang City 330031, China. Electronic address: liyin@ncu.edu.cn.
² School of Chemistry, Biology and Materials Science, East China University of Technology, Guanglan Avenue 418, Nanchang City 330013, China.
³ School of Chemistry, Biology and Materials Science, East China University of Technology, Guanglan Avenue 418, Nanchang City 330013, China. Electronic address: oyjb1001@163.com.
⁴ Institute of Space Science and Technology, Nanchang University, Xuefu Avenue 999, Nanchang City 330031, China.
⁵ School of Mechatronics & Vehicle Engineering, East China Jiaotong University, Nanchang, Jiangxi 330013, China.
⁶ School of Physics and Materials Science, Nanchang University, Xuefu Avenue 999, Nanchang City 330031, China.
⁷ Fujian Provincial Key Laboratory of Semiconductors and Applications, Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Department of Physics, Xiamen University, Xiamen 361005, China; Jiujiang Research Institute, Xiamen University, Jiujiang 332000, China.

PMID: 38198992
DOI: 10.1016/j.saa.2024.123869

Abstract

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.

Keywords: Anharmonicity; Low-frequency vibrations; Mode coupling; Polymorphism; Terahertz spectroscopy.