1. Department of Instrumentation Science, University of Science & Technology Beijing, Beijing 100083, China 2. Beijing Engineering Research Center of Industrial Spectrum Imaging, Beijing 100083, China 3. Institute of Applied Physics, University of Science & Technology Beijing, Beijing 100083, China
Abstract：With simulation of absorption spectra in THz region based on quantum-chemical calculation, the THz absorption features of target materials can be assigned with theoretical normal vibration modes. This is necessary for deeply understanding the origin of THz absorption spectra. The reliabilities of simulation results mainly depend on the initial structures and theoretical methods used throughout the calculation. In our study, we utilized THz-TDS to obtain the THz absorption spectrum of solid-state L-glutamine. Then three quantum-chemical calculation schemes with different initial structures commonly used in previous studies were proposed to study the inter-molecular interactions’ contribution to the THz absorption of glutamine, containing monomer structure, dimer structure and crystal unit cell structure. After structure optimization and vibration modes’ calculation based on density functional theory, the calculation results were converted to absorption spectra by Lorentzian line shape function for visual comparison with experimental spectra. The result of dimmer structure is better than monomer structure in number of absorption features while worse than crystal unit cell structure in position of absorption peaks. With the most reliable simulation result from crystal unit cell calculation, we successfully assigned all three experimental absorption peaks of glutamine ranged from 0.3 to 2.6 THz with overall vibration modes. Our study reveals that the crystal unit cell should be used as initial structure during theoretical simulation of solid-state samples’ THz absorption spectrum which comprehensively considers not only the intra-molecular interactions but also inter-molecular interactions.
Key words：Terahertz absorption spectra;Glutamine;Quantum-chemical calculation;Monomer;Dimer;Crystal unit cell
[1] Qiao L, Wang Y, Zhao Z, et al. Optical Engineering, 2014, 53(7): 074102. [2] Zhao X, Su H, Zhang Z, et al. Chinese Science Bulletin, 2014, 59(24): 2987. [3] Yu B, Zeng F, Yang Y, et al. Biophysical Journal, 2004, 86(3): 1649. [4] WANG Wei-ning, LI Yuan-bo, YUE Wei-wei（王卫宁, 李元波, 岳伟伟）. Acta Physica Sinica（物理学报）, 2007, 56(2): 781. [5] Takahashi M, Kawazoe Y, Ishikawa Y, et al. Chemical Physics Letters, 2009, 479(4): 211. [6] Yan H, Fan W H, Zheng Z P. Optics Communications, 2012, 285(6): 1593. [7] Zheng Z P, Fan W H. Journal of Biological Physics, 2012, 38(3): 405. [8] SU Hai-xia, ZHANG Zhao-hui, ZHAO Xiao-yan, et al（苏海霞, 张朝晖, 赵小燕, 等）. Spectroscopy and Spectral Analysis（光谱学与光谱分析）, 2013，33(4): 921. [9] Withayachumnankul W, Naftaly M. Journal of Infrared, Millimeter, and Terahertz Waves, 2014, 35(8): 610. [10] Li Z, Zhang Z, Zhao X, et al. Applied Optics, 2012, 51(20): 4879. [11] Wong M W. Chemical Physics Letters, 1996, 256(4): 391. [12] Clark S J, Segall M D, Pickard C J, et al. Zeitschriftfür Kristallographie, 2005, 220(5/6/2005): 567. [13] Perdew J P, Ruzsinszky A, Csonka G I, et al. Physical Review Letters, 2008, 100(13): 136406. [14] Koetzle T F, Frey M N, Lehmann M S, et al. Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry, 1973, 29(11): 2571. [15] Tian Lu, Feiwu Chen. Journal of Computational Chemistry, 2012, 33(5): 580.
