基于密度泛函理论的对二甲苯分子电子光谱的外场效应研究

对二甲苯（PX）是化工领域一种非常重要的原料，被广泛地用于香料、医药、油墨和农药等的生产，因此研究PX分子的电子光谱和外场效应，对于它的检测和降解具有十分重要的意义。为研究外电场作用下，PX分子的紫外-可见（UV-Vis）光谱的变化，采用密度泛函理论（density functional theory，DFT）B3LYP方法在6-311++G(d, p)基组水平上，优化了不同外电场（0～0.025 a.u.，0～1.285 6×1010 V·m-1）作用下PX分子的基态几何构型，在此基础上利用含时密度泛函理论（TDDFT）计算了PX分子的UV-Vis吸收光谱，最后对PX分子紫外吸收峰和摩尔吸收系数受外电场作用的的影响规律进行了研究。结果表明：有波长为189 nm、摩尔吸收系数为35 580 L·mol-1·cm-1的强吸收峰，处于E₁带，它是环状共轭的三个乙烯键的苯型体系中的π→π*电子跃迁产生的；与苯分子相比，吸收峰出现11 nm的红移：由于两个甲基和苯环形成p-π共轭，苯环的大π键变弱，故PX分子的紫外吸收峰出现红移；当增加了外电场后，最低未占据轨道(LUMO)向外电场的反方向偏移，导致苯环上的电子密度减小，大π键变弱，π→π*跃迁需要的能量降低，电子跃迁产生的波长增大，吸收峰出现显著红移，当外电场增大到0.020 a.u.时，红移已经非常明显；外电场的引入，导致苯环上的电子密度减小，大π键变弱，π→π*跃迁的电子数减少，摩尔吸收系数降低，随着外电场的增强，摩尔吸收系数降低明显，尤其在外电场增强到0.020a.u.后，摩尔吸收系数降低非常显著。这些工作为PX的检测和降解方法研究提供了一定的理论依据，也对其他有机污染物的检测方法和降解机理的研究有启示作用。

Investigation of External Electric Field Effect on Ultraviolet-Visible Spectra of Para-Xylene Molecule by Density Functional Theory

Para-xylene(PX) is an important raw material in chemical industry. In order to study the change of ultraviolet-visible(UV-Vis) spectra of PX under external electrical field, the density functional theory (DFT) has been employed to calculate geometrical parameters of the ground state of PX molecule under different external electric fields ( from 0 to 0.025 a.u.) in this paper. On this basis, the UV-Vis absorption spectra of PX molecule were calculated by employing the time-dependent density functional theory (TDDFT). At last, by employing the TDDFT in the same fundamental group, we studied wavelength and the molar absorption coefficient of the first twenty-six excited states of PX molecule under different external electric fields. The results show that the most absorption of UV-Vis absorption spectra appears in the E₁ belt of benzene electronic transition from π to π*; the ultraviolet absorption peaks of excited states of PX are proved to appear observably red shift and the molar absorption coefficient sharply decreases with the increase of the field intensity.