草甘膦THz光谱的单分子建模研究

On the isolated-molecule modeling of the terahertz spectrum of the glyphosate

为获得草甘膦在THz波段的光学参数,利用太赫兹时域光谱(THz-TDS)装置对草甘膦标准品进行测试,获得样品在0.4～1.5 THz波段的折射率谱和吸收谱。结果表明,草甘膦样品有5个明显的特征吸收峰,吸收峰的位置分别为0.79 THz、0.89 THz、1.09 THz、1.31 THz和1.43 THz,它的平均折射率为1.512。为了更好地解析试验光谱,利用Gaussian 03程序的B3LYP函数与从头算理论HF在6-311G(d,p)基组水平上,以及DMol^3程序的PW91、VWN-BP和BLYP 3种GGA密度函数在DNP基组水平上模拟草甘膦单分子,其中利用HF和VWN-BP函数计算出的峰位值与其对应的试验值较接近。这两种方法是模拟草甘膦单分子较为适合的方法。利用HF函数计算的键长、键角均与室温下X射线衍射值有相同的一致性趋势,而VWN-BP函数这一趋势不明显。

To obtain optical parameters of the glyphosate in terahertz （THz） region, the paper would like to come up with a new appruaeh to the isolated-molecule modeling of the terahertz specmun of the glyphosate. For this purpose, we have done measurements of the ter- abertz time-domain spectroscopy （THz - TDS） of the glypbosate stan- dard sample at the room temperature and nitrogen environment and measured the refractive index and the absorption coefficient respec- tively in the frequency range between 0.4 - 1.5 THz. The experimen- tal results of ours indicate that the average refractive index of the glyphosate is 1.512, and absorption spectrum with the live character- istic absorption peaks locating at 0.79 THz, 0.89THz, 1.09 THz, 1.31 THz and 1.43 THz, respectively. Based on the experimental results, the paper has analyzed the isolated-molecule vibration modes by using Gaussian 03 program for the glyphosate optimization and norm mode with the B3LYP hybrid density functional or Hartree-Fork （HF） theory and the 6- 311G（d,p） Gaussian-type basis set. We have also employed DMol3 program with the DNP basis sot and PW91, VWN - BP and BLYP generalized gradient approximation （GGA） density function. We have analyzed the ability of their repro- ducing glyphosate isolated-molecule structure by comparing the com- putational results with the room-temperature X-ray diffraction experi- mental data. Besides, the HF with better experimental bond length and bond angle agreement tendency and VWN - BP methods also prove to be in accord with the related results of the room-temperature THz - TDS. It should therefore be noted that Gaussian 03 and DMol3 program were employed to model the single molecule on the condition of gas or liquid, while the glyphosate is a solid-state periodic system, involving intramolecular or phonon interactions. To sum up, the above said new approach proves to be deficient with the periodic boundary conditions for calculating the glyphosate being in highly the- oretical agreement.