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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Ab initio modeling of the electronic circular dichroism induced in porphyrin chromophores
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Ab initio modeling of the electronic circular dichroism induced in porphyrin chromophores

机译：卟啉生色团诱导的电子圆二色性的从头算建模

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摘要

The optical activity in porphyrins can easily be induced by a chiral environment, but it is difficult to determine the underlying mechanisms purely on an experimental basis. Therefore, in this study, magnitudes of the perturbational, dipolar, and direct covalent contributions to the electronic circular dichroism (CD) are evaluated with the aid of quantum chemical computations. Electronic properties of model porphyrin chromophores are analyzed. Time-dependent density functional theory (TD DFT), particularly with the hybrid B3LYP functional, appeared suitable for estimation of the electronic excitation energies and spectral intensities. The transition dipole coupling (TDC) between chirally stacked porphyrins was determined as the most important mechanism contributing to their optical activity. This is in agreement with previous experimental observations, where chiral matrices often induce the stacking and large CD signals. About a 10 times smaller signal could be achieved by a chiral orientation of the phenyl or similar residues covalently attached to the porphyrin core. Also, this prediction is in agreement with known experiments. Perturbation models realized by a chirally arranged porphyrin and a point charge, or by a porphyrin and the methane molecule, provided the smallest CD signals. The electrically neutral methane induced similar CD magnitudes as those of the charge, but spectral shapes were different. For a complex of porphyrin and the alanine cation, a significant influence of the solvent on the resultant CD spectral shape was observed, while for the charge and methane perturbations, a negligible solvent effect was found. Detailed dependence of the induced optical activity on variations of geometrical parameters is discussed. The simulations of the induced porphyrin activity can thus bring important information about the structure and intermolecular interactions in chiral complexes.
机译：卟啉中的光学活性很容易被手性环境诱导，但是很难仅凭实验确定潜在的机理。因此，在这项研究中，借助量子化学计算评估了对电子圆二色性（CD）的微扰，偶极和直接共价贡献的大小。分析了模型卟啉发色团的电子性质。随时间变化的密度泛函理论（TD DFT），特别是具有混合B3LYP泛函的理论，似乎适用于估计电子激发能和光谱强度。手性堆叠卟啉之间的跃迁偶极耦合（TDC）被确定为有助于其光学活性的最重要机制。这与以前的实验观察结果相符，在实验观察中，手性基质经常引起堆积和大的CD信号。通过共价连接至卟啉核心的苯基或类似残基的手性取向，可以实现约10倍的较小信号。而且，该预测与已知实验一致。通过手性排列的卟啉和点电荷，或者通过卟啉和甲烷分子实现的扰动模型提供了最小的CD信号。电中性甲烷诱导的CD量级与电荷相似，但光谱形状不同。对于卟啉和丙氨酸阳离子的复合物，观察到溶剂对所得CD光谱形状的显着影响，而对于电荷和甲烷扰动，发现可忽略的溶剂效应。讨论了诱导的光学活性对几何参数变化的详细依赖性。诱导的卟啉活性的模拟因此可以带来有关手性络合物的结构和分子间相互作用的重要信息。

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