Zhou Dexia, Zhang Miaomiao, Ma Yinhua, Mukherjee Somnath, Liu Jing, Bian Hongtao
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China.
J Phys Chem B. 2021 Nov 25;125(46):12797-12805. doi: 10.1021/acs.jpcb.1c07902. Epub 2021 Nov 11.
It is usually believed that the binding affinity and selectivity of an alkali metal ion with crown ether are defined by the size matching model. However, the underlying mechanism of the specific host-guest interactions and the structural dynamics of the metal ions confined in the cavity of the crown ethers in the solutions are still not clear. In this report, a series of alkali thiocyanate salts (XSCN; X = Li, Na, K, and Cs) complexed with 18-crown-6 (a typical crown ether) in the chloroform solutions were studied by the polarization-selective infrared pump-probe spectroscopy and the ultrafast two-dimensional infrared (2D IR) spectroscopy. The SCN counteranions were employed as the local vibrational probe to reveal the specific host-guest interactions in the crown ether complexes. The rotational dynamics and spectral diffusion of SCN vibration were both measured by ultrafast IR spectroscopy, and it was found that the metal cations hosted by the crown ethers can have a pronounced effect on the rotational dynamics of the counteranions. The reorientational time constants of the SCN vibration in the complexation follow the order Li > Na > K ≃ Cs. More importantly, the spectral diffusion dynamics of SCN, which quantifies the decay of the correlation of the frequency fluctuations in the complexation, was also affected by the metal ions but showed a different order of cationic effect. A detailed analysis of the 2D IR data showed that the spectral diffusion of SCN counteranion clearly decayed with two different time scales in the complex of 18-crown-6 with K. The 3-4-fold slowdown in spectral diffusion indicated that the fluctuation of SCN vibrational transition frequency was strongly affected by the K cation due to the geometric constraint imposed by the crown ether. The results should help the researchers to unravel the specific host-guest interactions and further reveal the origination of the binding selectivity of crown ether for metal cations in the condensed phases from the perspective of structural dynamics.
通常认为碱金属离子与冠醚的结合亲和力和选择性由尺寸匹配模型定义。然而，特定主客体相互作用的潜在机制以及溶液中冠醚腔内受限金属离子的结构动力学仍不清楚。在本报告中，通过偏振选择性红外泵浦 - 探测光谱和超快二维红外（2D IR）光谱研究了一系列碱金属硫氰酸盐（XSCN；X = Li、Na、K和Cs）在氯仿溶液中与18 - 冠 - 6（一种典型的冠醚）形成的配合物。SCN抗衡阴离子被用作局部振动探针来揭示冠醚配合物中的特定主客体相互作用。通过超快红外光谱测量了SCN振动的旋转动力学和光谱扩散，发现冠醚所容纳的金属阳离子可对抗衡阴离子的旋转动力学产生显著影响。配合物中SCN振动的重取向时间常数遵循Li > Na > K ≃ Cs的顺序。更重要的是，量化配合物中频率波动相关性衰减的SCN光谱扩散动力学也受金属离子影响，但显示出不同的阳离子效应顺序。对二维红外数据的详细分析表明，在18 - 冠 - 6与K的配合物中，SCN抗衡阴离子的光谱扩散明显以两种不同的时间尺度衰减。光谱扩散减慢3 - 4倍表明，由于冠醚施加的几何约束，SCN振动跃迁频率的波动受到K阳离子的强烈影响。这些结果应有助于研究人员揭示特定的主客体相互作用，并从结构动力学的角度进一步揭示凝聚相中冠醚对金属阳离子结合选择性的起源。
