Inokuchi Yoshiya, Nakatsuma Maki, Kida Motoki, Ebata Takayuki
Department of Chemistry, Graduate School of Science, Hiroshima University , Higashi-Hiroshima, Hiroshima 739-8526, Japan.
J Phys Chem A. 2016 Aug 18;120(32):6394-401. doi: 10.1021/acs.jpca.6b06626. Epub 2016 Aug 4.
We measure UV photodissociation (UVPD) spectra of benzo-12-crown-4 (B12C4) complexes with alkali metal ions, M(+)·B12C4 (M = Li, Na, K, Rb, and Cs), in the 36300-37600 cm(-1) region. Thanks to the cooling of ions to ∼10 K, all the M(+)·B12C4 complexes show sharp vibronic bands in this region. For UV-UV hole-burning (HB) spectroscopy, we first check if our experimental system works well by observing UV-UV HB spectra of the K(+) complex with benzo-18-crown-6 (B18C6), K(+)·B18C6. In the UV-UV HB spectra of the K(+)·B18C6 complex, gain signals are also observed; these are due to vibrationally hot K(+)·B18C6 complex produced by the UV excitation of cold K(+)·B18C6 complex. Then we apply UV-UV HB spectroscopy to the M(+)·B12C4 complexes, and only one conformer is found for each complex except for the Li(+) complex, which has two conformers. The vibronic structure around the origin band of the UVPD spectra is quite similar for all the complexes, indicating close resemblance of the complex structure. The most stable structures calculated for the M(+)·B12C4 (M = Li, Na, K, Rb, and Cs) complexes also have a similar conformation among them, which coincides with the UVPD results. In these conformers the metal ions are too big to be included in the B12C4 cavity, even for the Li(+) ion. In solution, it was reported that 12-crown-4 (12C4) shows the preference of Na(+) ion among alkali metal ions. From the similarity of the structure for the M(+)·B12C4 complexes, it is suggested that the solvation of free metal ions, not of the M(+)·12C4 complexes, may lead to the selectivity of Na(+) ion for 12C4 in solution.
我们测量了碱金属离子M(+)·苯并-12-冠-4（B12C4）配合物（M = Li、Na、K、Rb和Cs）在36300 - 37600 cm(-1)区域的紫外光解离（UVPD）光谱。由于离子被冷却至约10 K，所有M(+)·B12C4配合物在此区域均显示出尖锐的振转带。对于紫外-紫外空穴烧蚀（HB）光谱，我们首先通过观察钾离子与苯并-18-冠-6（B18C6）形成的配合物K(+)·B18C6的紫外-紫外HB光谱来检查我们的实验系统是否运行良好。在K(+)·B18C6配合物的紫外-紫外HB光谱中，也观察到了增益信号；这些信号归因于冷的K(+)·B18C6配合物经紫外激发产生的振动激发态K(+)·B18C6配合物。然后我们将紫外-紫外HB光谱应用于M(+)·B12C4配合物，除了具有两种构象的锂离子配合物外，每种配合物仅发现一种构象。所有配合物的UVPD光谱原点带周围的振转结构非常相似，表明配合物结构极为相似。为M(+)·B12C4（M = Li、Na、K、Rb和Cs）配合物计算出的最稳定结构在它们之间也具有相似的构象，这与UVPD结果一致。在这些构象中，即使对于锂离子，金属离子也太大而无法包含在B12C4腔内。据报道，在溶液中，12-冠-4（12C4）在碱金属离子中表现出对钠离子的偏好。从M(+)·B12C4配合物结构的相似性来看，表明是游离金属离子的溶剂化作用，而非M(+)·12C4配合物的溶剂化作用，可能导致溶液中钠离子对12C4具有选择性。
