强外电场作用下二甲基硅酮的分子结构和激发态

采用密度泛函(DFT)方法B3P86在6 311++G(d,p)基组水平上优化得到了分子轴方向不同电偶极 场(-0.04～0.04a.u.)作用下,二甲基硅酮的基态电子状态、几何结构、电偶极矩和分子总能量.在优化构型下 用同样的基组采用杂化CIS DFT方法(CIS B3P86)研究了同样外电场条件下对二甲基硅酮的激发能和振子强度 的影响.计算结果表明,分子几何构型与电场大小和方向呈现强烈的依赖,正向电场下基态偶极矩随电场强度线 性增加,分子总能量降低,当反向电场大于0.03a.u.时,偶极距方向改变,总能量增加;激发能随电场增加急剧减 小,且对电场方向的依赖呈现出不对称性,满足Grozema关系.电场对振子强度的影响比较复杂,但仍满足跃迁选 择定则.

The Molecular Structure and Excited States for Dimethyl Siloxane under the Strong Electric Field

The ground states of dimethyl siloxane under different intense electric fields ranging from -0.04 to 0.04 a.u. are optimized using density functional theory DFT/B3P86 at 6-311++G(d,p) level. The excitation energies and oscillator strengths under the same intense applied electric fields are calculated employing the revised hybrid CIS-DFT method. The result shows that the electronic state, molecular geometry, total energy, dipole moment and excitation energy are strongly dependent on the field strength and behave asymmetry to the direction of the applied electric field. As the electric field changes from -0.04 to 0.04 a.u., the bond length of Si-O increases whereas the bond length of Si-C decreases because of the charge transfer induced by the applied electric field. The dipole moment of the ground state decreases linearly with the applied field strength. However, the dipole moment of molecule changes from positive to negative as the inverse electric field increase to -0.03 a.u. Further increase of the inverse electric field results in an increase of the total energy of the molecule. The dependence of the calculated excitation energies on the applied electric field strength is fitting well to the relationship proposed by Grozema. The excitation energies of the first five excited states of dimethyl siloxane decrease as the applied electric filed increases because the energy gap between the HOMO and LUMO become close with the field, which shows that the molecule is easy to be excited under electric field and hence can be easily dissociated.