利用量子化学计算方法模拟物质的太赫兹吸收谱,可以为目标物质的太赫兹吸收特征匹配分子振动模式,对深刻理解谱的形成机理十分必要.模拟结果的可靠性,主要取决于目标物质初始构型的搭建和振动模式计算方法的选择.首先利用太赫兹时域光谱技术获取了谷氨酰胺固态样品的太赫兹吸收谱,为了在理论模拟过程中体现考虑分子间作用的程度,构建了三种常用于有机物太赫兹吸收谱模拟的谷氨酰胺初始构型,单分子、二聚体、晶胞.使用量子化学计算程序基于密度泛函理论对三种初始构型进行了结构优化和振动模式计算.将计算结果通过洛伦兹线型函数拟合为吸收谱与实验吸收谱进行对比发现,二聚体构型的模拟吸收谱从吸收峰个数上优于单分子构型,在此基础上,晶胞构型模拟结果从吸收峰峰位上又较二聚体构型有了明显的改进,随着初始构型考虑分子间作用的程度提高模拟结果逐步逼近实验吸收谱.在得到了可靠的理论模拟吸收谱的基础上,成功对谷氨酰胺固态样品在0.3~2.6 THz范围内的三个吸收峰匹配了晶胞内各分子的集体振动模式.研究表明,在固体样品的太赫兹吸收谱理论模拟中,在计算能力允许的情况下,应尽可能选择全面反映分子间作用力的晶胞构型作为计算的初始构型.

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.