Abstract：Anthraquinone is an organic compound with a planar structure and a macrocyclic conjugated system, which has a wide range of applications in various industries such as dyes, papermaking, biology, and medicine. To investigate the relationship between the characteristic absorption of anthraquinone in the terahertz region and its molecular crystal structure, theoretical simulation and experimental research of terahertz absorption spectrum was carried out by using density functional theory (DFT) and terahertz time-domain spectroscopy (THz-TDS). Firstly, the characteristic absorption spectrum of anthraquinone in the frequency range of 0.5～3.0 THz at room temperature was measured using the THz-TDS system. It was found that anthraquinone crystals have six distinct characteristic absorption peaks in this frequency range, located at 0.95, 1.05, 2.09, 2.25, 2.49, and 2.78 THz, respectively. For deeply analyzing the generation mechanism of the characteristic spectra of anthraquinone in the terahertz frequency band, theoretical simulation calculations were conducted on the single molecule model and the cell model of anthraquinone based on density functional theory. Theoretical calculations were conducted on the anthraquinone single molecule model using Gaussian09 software-based DFT theory with the B3LYP hybrid functional method and 6-311G (d, p) basis set. Geometric structure optimization and vibration frequency calculation were carried out at the same level. The simulation results showed significant differences from experimental measurement data, indicating that single-molecule simulation has certain limitations. Theoretical calculations were carried out on the anthraquinone crystal cell model using the CASTEP module, which is suitable for calculating periodic structures in the MS 8.0 software package. Five exchange-related functionals, PBE, PW91, WC, PBEsol, and RPBE, based on plane wave pseudo potential and generalized gradient approximation (GGA), were used. Geometric structure optimization and lattice dynamics calculations were performed at the same level. A detailed comparative analysis was conducted between the simulated structural parameters (bond length, bond angle) of anthraquinone single molecules and crystal cells and the structural parameters measured by X-ray diffraction experiments. It was found that the consistency between the molecular structural parameters and X-ray diffraction experimental data was the best in the solid-state simulation results obtained based on the PBE method. The theoretical simulation spectra obtained by PBE and RPBE methods agree with the experimental absorption spectra. Therefore, the vibration mode assignment of the experimental characteristic absorption peak was carried out based on PBE and RPBE calculation results. The study indicates that the characteristic absorption of anthraquinone crystals primarily originated from the overall vibration of the anthraquinone ring and benzene ring groups dominated by C—H…O intermolecular hydrogen bonds in the crystal, as well as the collective vibration mode caused by weak intermolecular interactions.
Key words：Anthraquinone; Terahertz; Density functional theory;Unit cell;Vibrational mode
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