啶虫脒太赫兹光谱的实验研究和理论模拟

啶虫脒是一种氯吡啶类新烟碱类杀虫剂，由于其对昆虫烟碱乙酰胆碱受体的结合选择性而成为最常用的杀虫剂之一。为研究啶虫脒在太赫兹波段的指纹特征与其结构信息之间的关系，利用太赫兹时域光谱(THz-TDS)技术并结合密度泛函理论（DFT）对啶虫脒晶体的太赫兹吸收光谱进行了实验研究和理论模拟。利用THz-TDS技术测量了室温下啶虫脒在0.3～3.3 THz频段的特征吸收谱，发现啶虫脒晶体在该频段内有多处强度不同的特征吸收峰，分别位于1.08，1.38，1.97，2.54, 2.89 THz处，这些特征吸收峰构成了用于检测啶虫脒的THz指纹谱。为更好地理解啶虫脒太赫兹实验特征谱产生的理论机理，基于密度泛函理论分别对啶虫脒孤立分子模型和晶胞模型进行了理论模拟计算。单分子模型的理论计算在Gaussian09软件中进行，采用基于密度泛函理论的B3LYP杂化泛函方法，并选取6-311G（d, p）基组进行几何优化，并在相同水平上进行振动频率计算，模拟结果与实验数据存在一定的差异，说明单分子模拟存在一定的局限性。晶胞模型的理论计算在Materials Studio 8.0中适合计算周期性结构的CASTEP模块中进行, 采用基于平面波赝势和广义梯度近似(GGA)的PW91，PBE，PBEsol和WC四种交换相关泛函对啶虫脒晶胞模型进行几何优化和晶格动力学计算。将啶虫脒单分子和晶胞的理论模拟结构参数(键长、键角)分别与其X射线衍射(XRD)实验测量的结构参数进行了详细的比较分析，发现基于PBE方法获得的固态仿真结果中分子的结构参数与X射线衍射实验数据的一致性最好。同时用PBE方法获得的理论仿真谱与实验吸收谱也最为吻合。基于PBE的计算结果对实验特征吸收峰进行了振动模式指认。研究结果表明，啶虫脒晶体在太赫兹频段的特征吸收主要来源于晶体中以C-H…N氢键为主的分子间弱相互作用带动的集体振动模式，以及由分子内吡啶环的平动和甲基基团的扭动所引发的骨架振动。

Experimental Measurement and Theoretical Simulation on Terahertz Spectra of Crystal Acetamiprid

Acetamiprid is a chloropyridine neonicotinoid insecticide that is one of the most commonly used pesticides due to its binding selectivity toward the nicotinic acetylcholine receptor of insects. To understand the relationship between the fingerprint feature and the corresponding structural information in the terahertz region, experimental and theoretical investigations of the terahertz absorption spectrum of acetamiprid crystal were carried out using the terahertz time-domain spectroscopy (THz-TDS) and density functional theory(DFT). The terahertz absorption spectrum of acetamiprid was measured in 0.3~3.3 THz frequency range at room temperature by the THz-TDS system. A number of characteristic absorption peaks in this range were observed at 1.08，1.38，1.97，2.54 and 2.89 THz, respectively, which can be the THz fingerprint spectrum for detecting acetamiprid. To better understand the experimental absorption spectrum’s theoretical mechanism, calculations based on density functional theory were performed to analyze the isolated molecule and unit cell of acetamiprid. The geometry optimization and frequency calculation of the isolated molecule model was performed using DFT with periodic boundary conditions employing the B3LYP hybrid functional with the 6-311G(d,p) basis set. Some differences were observed between the simulation results and the experimental data, which means that the isolated molecule simulation has some limitations. The theoretical calculations of the unit cell were performed based on the solid-state DFT using the CASTEP program, a part of Materials Studio 8.0 from Accelrys. The calculations were performed on the crystalline state within the generalized gradient approximation (GGA) at PW91, PBE, PBEsol and WC correlation functions. The calculated structural data of bond lengths and bond angles for acetamiprid molecule and unit cell were compared with -X-ray diffraction values(XRD). Among these calculations, PBE simulations provide a significantly similar tendency with the XRD experimental values. And the calculated THz spectrum of the PBE provides better agreements with observed THz spectral characters. Consequently, the observed spectral features were assigned according to the results of the PBE simulation. The study indicates that acetamiprid’s characterized features primarily originated from intermolecular collective vibrational modes, which were dominated by hydrogen bonds such as C-H…N.