Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan.
J Phys Chem A. 2021 May 6;125(17):3718-3725. doi: 10.1021/acs.jpca.1c02300. Epub 2021 Apr 27.
We performed cryogenic ion mobility-mass spectrometry (IM-MS) to study conformations of dibenzo-crown-ether complexes with Na and K ions at 86 K in the gas phase. Four dibenzo-crown-ethers (dibenzo-18-crown-6, dibenzo-21-crown-7, dibenzo-24-crown-8, and dibenzo-30-crown-10) with different cavity ring sizes were investigated. For dibenzo-18-crown-6 complexes with Na and K, only one type of conformer was assigned by comparing the experimental collision cross sections with those predicted theoretically for candidate structures. In this conformer, the distance between two benzene rings in the complexes was long due to the open form of the dibenzo-18-crown-6. This open conformer was consistent with the previous laser spectroscopic studies of the cold complex ions in the gas phase. For dibenzo-21-crown-7 and dibenzo-24-crown-8 complexes with Na and K, two types of conformers were clearly separated by IM-MS. These two conformer types were assigned to "open" and "closed" forms in which benzene-benzene distances were long and short, respectively. Observed relative abundances of the open and closed conformers qualitatively agreed with the Boltzmann distribution using Gibbs energies of the conformers calculated by quantum chemical calculations. For the Na(dibenzo-30-crown-10) complex, open and closed conformers were also observed in IM-MS. On the other hand, only the closed conformer was observed for the K(dibenzo-30-crown-10) complex. This closed conformer was similar to the "wraparound" structure, which was proposed in the previous studies in the solution. In conclusion, the closed conformers were formed by the deformation of flexible crown ethers with large cavity ring sizes. In addition, the diameter of the K ion was suitable to form the closed conformer by deformation of the molecular structure of dibenzo-30-crown-10.
我们进行了低温离子淌度-质谱（IM-MS）研究，以在气相中研究 86 K 时与 Na 和 K 离子形成的二苯并-18-冠-6、二苯并-21-冠-7、二苯并-24-冠-8 和二苯并-30-冠-10 四种不同空腔环大小的冠醚配合物的构象。对于二苯并-18-冠-6 与 Na 和 K 的配合物，仅通过将实验碰撞截面与候选结构理论预测的截面进行比较，就可以确定一种构象。在这种构象中，由于二苯并-18-冠-6 的开环形式，配合物中两个苯环之间的距离很长。这种开环构象与先前气相冷配合离子的激光光谱研究一致。对于二苯并-21-冠-7 和二苯并-24-冠-8 与 Na 和 K 的配合物，通过 IM-MS 可以清楚地分离出两种构象。这两种构象类型分别被分配为“开”和“闭”形式，其中苯-苯距离较长和较短。观察到的开环和闭环构象的相对丰度与使用量子化学计算得到的构象吉布斯能计算的玻尔兹曼分布定性一致。对于 Na（二苯并-30-冠-10）配合物，在 IM-MS 中也观察到了开环和闭环构象。另一方面，对于 K（二苯并-30-冠-10）配合物，仅观察到闭环构象。这种闭环构象类似于先前在溶液中提出的“缠绕”结构。总之，具有大空腔环大小的柔性冠醚通过变形形成了闭环构象。此外，K 离子的直径适合通过二苯并-30-冠-10 的分子结构变形形成闭环构象。
