Ruhr-Universität Bochum, Fakultät für Chemie und Biochemie, Organische Chemie II, Universitätsstraße 150, 44801 Bochum, Germany.
Ruhr-Universität Bochum, Fakultät für Chemie und Biochemie, Theoretische Chemie, Universitätsstraße 150, 44801 Bochum, Germany.
Phys Chem Chem Phys. 2022 May 18;24(19):11721-11728. doi: 10.1039/d2cp01309f.
Two chiral derivatives of 18-crown-6, namely the host molecules 2,3-diphenyl- and 2-phenyl-18c6, serve as model systems to investigate whether VCD spectroscopy can be used to monitor conformational changes occurring upon complexation of guests. Host-guest complexes of both crown ethers were prepared by addition of KNO. The more bulky 2,3-diphenyl-18c6 is found to undergo major conformational changes upon encapsulation of K, which are revealed as characteristic changes of the VCD spectral signatures. In contrast, while 2-phenyl-18c6 also incorporates K into the macrocycle, strong conformational changes are not occurring and thus spectral changes are negligible. With an octyl ammonium cation as guest molecule, 2,3-diphenyl-18c6 shows the same conformational and spectral changes that were observed for K-complexes. In addition, the asymmetric NH-deformation modes are found to gain VCD intensity through an induced VCD process. An analysis of the vibrational spectra enables a differentiation of VCD active and inactive guest modes: There appears to be a correlation between the symmetry of the vibrational mode and the induced VCD intensity. While this finding makes the host-guest complexes interesting systems for future theoretical studies on the origin of induced VCD signatures, the observations described in this study demonstrate that VCD spectroscopy is indeed a suitable technique for the characterization of supramolecular host-guest complexes.
两种手性 18-冠-6 的衍生物，即主体分子 2,3-二苯基-18c6 和 2-苯基-18c6，被用作模型系统，以研究 VCD 光谱是否可用于监测客体络合时发生的构象变化。通过添加 KNO3 制备了这两种冠醚的主体-客体络合物。较大的 2,3-二苯基-18c6 在包裹 K 时经历了主要的构象变化，这表现为 VCD 光谱特征的特征变化。相比之下，尽管 2-苯基-18c6 也将 K 纳入大环，但不会发生强烈的构象变化，因此光谱变化可以忽略不计。当使用辛基铵阳离子作为客体分子时，2,3-二苯基-18c6 显示出与 K 络合物相同的构象和光谱变化。此外，非对称 NH 变形模式通过诱导 VCD 过程获得 VCD 强度。对振动光谱的分析能够区分 VCD 活性和非活性客体模式：振动模式的对称性与诱导 VCD 强度之间似乎存在相关性。虽然这一发现使主体-客体络合物成为未来关于诱导 VCD 特征起源的理论研究的有趣系统，但本研究中的观察结果表明，VCD 光谱确实是表征超分子主体-客体络合物的合适技术。
