National Institute for Research and Development of Isotopic and Molecular Technologies, Donat Street, No. 67-103, 400283 Cluj-Napoca, Romania.
Int J Mol Sci. 2023 Feb 2;24(3):2906. doi: 10.3390/ijms24032906.
The equilibrium and conical intersection geometries of the benzene dimer were computed in the framework of the conventional, linear-response time-dependent and spin-flipped time-dependent density functional theories (known as DFT, TDDFT and SF-TDDFT) as well as using the multiconfigurational complete active space self-consistent field (CASSCF) method considering the minimally augmented def2-TZVPP and the 6-31G(d,p) basis sets. It was found that the stacking distance between the benzene monomers decreases by about 0.5 Å in the first electronic excited state, due to the stronger intermolecular interaction energy, bringing the two monomers closer together. Intermolecular-type conical intersection (CI) geometries can be formed between the two benzene molecules, when (i) both monomer rings show planar deformation and (ii) weaker (approximately 1.6-1.8 Å long) C-C bonds are formed between the two monomers, with and orientation with respect to the monomer. These intermolecular-type CIs look energetically more favorable than dimeric CIs containing only one deformed monomer. The validity of the dimer-type CI geometries obtained by SF-TDDFT was confirmed by the CASSCF method. The nudged elastic band method used for finding the optimal relaxation path has confirmed both the accessibility of these intermolecular-type CIs and the possibility of the radiationless deactivation of the electronic excited states through these CI geometries. Although not as energetically favorable as the previous two CI geometries, there are other CI geometries characterized by the relative rotation of monomers at different angles around a vertical C-C axis.
采用传统的、线性响应含时密度泛函理论（简称 DFT）、自旋翻转含时密度泛函理论（简称 SF-TDDFT）以及考虑最小aug-cc-pVDZ 和 6-31G(d,p)基组的多组态完全活性空间自洽场（CASSCF）方法，计算了苯二聚体的平衡和锥形交叉几何形状。研究发现，由于分子间相互作用能更强，苯单体在第一电子激发态下的堆积距离减小了约 0.5 Å，使两个单体靠得更近。当（i）两个单体环都表现出平面变形，（ii）两个单体之间形成较弱（约 1.6-1.8 Å 长）的 C-C 键，并且与单体的 和 取向时，可以在两个苯分子之间形成分子间型锥形交叉（CI）几何形状。与仅含有一个变形单体的二聚型 CI 相比，这些分子间型 CI 在能量上看起来更有利。SF-TDDFT 获得的二聚型 CI 几何形状的有效性通过 CASSCF 方法得到了验证。用于寻找最优弛豫路径的推斥弹性带方法已经证实了这些分子间型 CI 的可及性，以及通过这些 CI 几何形状实现电子激发态无辐射去活化的可能性。尽管这些 CI 几何形状在能量上不如前两种 CI 几何形状有利，但还有其他 CI 几何形状的特征是单体在垂直 C-C 轴周围以不同角度相对旋转。
