Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan.
Chirality. 2011;23 Suppl 1:E22-7. doi: 10.1002/chir.20947. Epub 2011 Mar 23.
The electronic circular dichroism (CD) spectra of donor-acceptor binaphthyls were investigated experimentally and theoretically. The enantiomerically pure forms of 1-(2-methoxy-1-naphthyl)- and 1-(2,3-dimethoxy-1-naphthyl)-2-methylisoquinolinium tetrafluoroborates (DA and D'A) were prepared, and their UV-vis and CD spectra were compared. The donor-acceptor interaction was apparent from the absorption at longer wavelengths, whereas its strength was not very different from each other. In addition, very similar structures were obtained for the two aromatic planes in DA and D'A when the geometry was optimized by the density functional theory. The additional methoxy group in the latter spices scarcely disturbed the UV-vis spectrum but significantly affected the CD spectrum. Thus, the observed CD spectra were considerably different from each other, especially in the (1) B(b) band couplet, where the amplitude was reduced to almost one-fourth in D'A. The theoretical investigations led to the following conclusions: (1) The potential curve associated with the central C-C dihedral angle of 1,1'-binaphthyl is fairly flat at the bottom for both DA and D'A and freely rotating at an ambient temperature. The potential curve of D'A is, however, significantly different from that of DA, in which the curve is much steeper and biased to the s-cis side. As the observed CD spectrum is an ensemble of conformers of various dihedral angles, such difference in potential certainly affects the overall spectrum; (2) The additional methoxy group introduced at the 3-position effectively altered the CD spectral pattern, which was theoretically supported by the calculation at the RI-CC2/TZVPP level; (3) Consequently, the classical coupled oscillator theory, in which the angle between the transition dipole moments of two aromatic planes is solely considered, is not applicable to the quantitative evaluation of the chiroptical properties of 1,1'-binaphthyls; rather, the quantum chemical approach is preferred, permitting a direct comparison with the experiment.
对给体-受体联萘酚的电子圆二色性（CD）光谱进行了实验和理论研究。制备了对映纯的 1-（2-甲氧基-1-萘基）-和 1-（2,3-二甲氧基-1-萘基）-2-甲基异喹啉鎓四氟硼酸盐（DA 和 D'A），并比较了它们的紫外-可见和 CD 光谱。从较长波长的吸收可以明显看出给体-受体相互作用，但其强度彼此差异不大。此外，通过密度泛函理论优化几何形状时，DA 和 D'A 中的两个芳环结构非常相似。后者中的额外甲氧基基团几乎没有干扰紫外-可见光谱，但对 CD 光谱有显著影响。因此，观察到的 CD 光谱彼此差异很大，特别是在（1）B（b）带偶合中，在 D'A 中幅度减小到几乎四分之一。理论研究得出以下结论：（1）1,1'-联萘的中央 C-C 二面角相关的势能曲线在 DA 和 D'A 中底部相当平坦，在环境温度下自由旋转。然而，D'A 的势能曲线与 DA 明显不同，其中曲线更陡峭且偏向 s-cis 侧。由于观察到的 CD 光谱是各种二面角构象的集合，因此这种势能差异肯定会影响整体光谱；（2）在 3 位引入的额外甲氧基基团有效地改变了 CD 光谱模式，这在 RI-CC2/TZVPP 水平的计算中得到了理论支持；（3）因此，经典耦合振子理论，其中仅考虑两个芳环的跃迁偶极矩之间的角度，不适用于 1,1'-联萘的手性光学性质的定量评估；相反，更倾向于量子化学方法，可以与实验直接比较。
