Superficial and Fundamental Correspondences in the Terahertz/IR (6–15 THz) Absorption Spectra of Aspirin and Benzoic Acid

The terahertz absorption spectra of aspirin and benzoic acid have been measured in the range 200–500 cm–1 (6–15 THz). Density-functional theory (DFT) modeling has assigned fundamental vibrational modes to the observed absorption bands. Hydrogen bonds between the crystalline planes of aspirin resulted in better agreement between the experimental and modeled spectra than for benzoic acid. The similar structure of these two molecules suggests a similar absorption spectrum, which indeed was obtained experimentally. However, the detailed crystal structure and molecular differences result in some of the apparently common absorption bands being assigned to different vibrational modes through the DFT modeling. Thus, our study importantly reveals that even though crystalline forms of two similar molecules may have similar experimental terahertz spectra, the resemblance may be superficial rather than fundamental.本研究测定了阿司匹林与苯甲酸在200–500 cm⁻¹（6–15 THz）范围内的太赫兹吸收光谱。密度泛函理论（Density-functional theory, DFT）建模将观测到的吸收谱带归属为基频振动模式。阿司匹林晶面间的氢键使得其实验光谱与模拟光谱的吻合程度优于苯甲酸体系。二者分子结构相似，理论上其吸收光谱应具备相似性，实验结果也确实印证了这一点。然而，细致的晶体结构差异与分子固有差异，使得通过DFT建模将部分看似共有的吸收谱带归属为不同的振动模式。因此，本研究的重要启示在于：即便两种结构相似的分子的晶态形式虽可呈现相似的实验太赫兹光谱，但这种相似性可能仅为表面层面，而非本质层面的共性。