外场作用下C12H4Cl4O2的分子结构和电子光谱研究

各种环境毒物危害着人类的生产生活，二噁英更是严重危害人类的健康.C₁₂H₄Cl₄O₂（2，3，7，8-tetrachlorodibenzo-p-dioxin，TCDD）是二噁英中毒性最强的化合物，也是目前已知毒性最强的污染物.为研究TCDD外场效应，采用密度泛函理论方法优化了不同静电场0–0.025 a.u.（0–1.2856×10¹⁰ V/m）作用下TCDD分子的基态几何结构，得到了分子总能量；在此基础上，采用含时密度泛函理论方法对TCDD分子的紫外-可见（UV-Vis）吸收光谱在不同外电场下的变化进行了研究.结果表明：分子几何构型与电场大小呈现强烈的依赖关系，分子总能量随着外电场的增强而减小；伴随着外电场的增强，分子激发态的摩尔吸收系数逐渐减小，UV-Vis吸收峰显著红移.

Molecular structure and electronic spectrum of C12H4Cl4O2 under external electric field

Various environmental poisons have caused damage to human production and life, and dioxin has seriously harmed human health. The C₁₂H₄Cl₄O₂(2, 3, 7, 8-tetrachlorodibenzo-p-dioxin, TCDD) is currently the most toxic compound. In order to study the influence of external electrical field on molecular structure and spectrum, herein the density functional theory (DFT) at a B3LYP/6-31+g (d,p) level is employed to calculate the geometrical parameters of the ground state of TCDD molecule under external electric fields ranging from 0 to 0.025 a.u. (0-1.2856×10¹⁰ V/m). Based on the optimized structure, time-dependent DFT at the same level as the above is adopted to calculate the absorption wavelengths and the molar absorption coefficients for the first twenty-six excited states of TCDD molecule under external electric fields. The results show that the most absorption band located at 221 nm with a molar absorption coefficient of 54064 L·mol^-1·cm^-1 in the UV-Vis absorption spectrum appears in the E belt, which originates from the benzene electronic transition from π to π^*. In addition, a shoulder peak at 296 nm appears in the B belt, which is the characteristic absorption of aromatic compounds' electron transition from π to π^*. Compared with the data in the literature, the wavelength of the shoulder is blue-shifted only 9 nm. The molecular geometry parameters are strongly dependent on the external field intensity, and the total energy decreases with external field intensity increasing. With the enhancement of external electric field, the electrons in the molecule have an overall transfer, which makes the big bond of benzene ring weakened, the energy of the transition decreases, and the wavelength of the transition increases, that is, the absorption peak is red-shifted. When the external electric field increases to 0.02 a.u., the electron cloud migration phenomenon of occupied and transition orbits of TCDD molecule are obvious, and the absorption peak red shift phenomenon is also very significant. With the enhancement of external electric field, the overall transfer of electrons in the molecule also reduces the density of the benzene rings and the surrounding electron cloud, reduces the number of electrons in the transition from π to π^*, and also reduces the molar absorption coefficient. When the external electric field is enhanced to 0.02 a.u., the molar absorption coefficient decreases significantly. This work provides a theoretical basis for studying the TCDD detection and degradation method, and also has implications for other environmental pollutants detection methods and degradation mechanisms.